16
Emerging Trends
16.1 Introduction
In the concluding chapter of this book we will discuss an assortment of topics representing some of the emerging trends in the field of satellite technology and its applications. The assortment of topics included in this chapter could not have been justifiably discussed as separate chapters from the viewpoint of scope of the present book. The chapter begins with a discussion on some non-conventional technologies such as space tethers and their possible use in executing propellant-less satellite orbit manoeuvres, generation of electric power for satellites and as space elevators, robotic drones for satellite stabilization and aerostats for surveillance, reconnaissance and communication relay applications. This is followed up by a detailed discussion on millimetre wave satellites, emerging concepts in space station technology and satellite services.
16.2 Space Tethers
One of the most important issues related to the development, launch and station keeping of satellite and other spacecraft missions is the economics of these operations. Scientists and technologists have been working hard to find new ways of reducing the cost to orbit and also the cost of maintaining satellites in the desired orbit during the life cycle of the mission. Efforts have also been made to develop alternative technologies to fuel rocket engines if conceived applications such as space tourism were to become a reality in the not too distant future. As a simple illustration, considering the fact that using the best available launch technology today launching 1 kg into a geostationary orbit costs about US$ 20 000 and launching 1 kg into a low Earth orbit costs around US$ 2500, one can comprehend how important it is to reduce the costs. As another illustration, if we consider that a space station such as International Space Station (ISS) needs ten tonnes of propellant per year in orbit, this would add up to billions of US dollars for the operation and maintenance of the space station during its lifetime. Space tethers drastically lower the cost to orbit by significantly reducing the differential velocity that the launch rocket would need to impart to the payload to put it in the desired orbit. Space tethers can also be used to generate power or thrust, as described in the subsequent paragraphs, to carry out orbital manoeuvres.
A space tether is a long and strong cable usually made up of thin strands of high strength conducting wires or fibres. The desired tether material properties depend on the intended applications. Some common desirable properties include high tensile strength, low density and high electrical conductivity. Space tethers are prone to damage by collision with space debris or micro meteoroids. They are also adversely affected by exposure to ultraviolet radiation and atomic oxygen. A protective coating may therefore be desirable. Some of the common materials proposed for building space tethers include Kevlar, ultra high molecular weight polyethylene, carbon nano tubes, M5 fibre and diamond. Use of space tethers for applications such as spacecraft propulsion, which can significantly reduce the cost to orbit, satellite stabilization, making use of thrust or power generated by space tether systems, and maintenance of spacecraft formations is being extensively researched and experimented worldwide.
16.2.1 Space Tethers – Different Types
There are three major categories of employment of space tethers. These include momentum exchange tethers, electrodynamic tethers and tethers for formation flying. While momentum exchange tethers are mainly employed for orbital manoeuvring, electrodynamic tethers can be used for generation of electrical power or a mechanical thrust. Tethers used in the case of formation flying, as the name suggests, are used for maintaining a fixed distance between different spacecraft in formation flying. The three types are briefly described in the following paragraphs.
16.2.1.1 Momentum Exchange Tether
Momentum exchange tethers are used to couple two objects in space in such a way that one can transfer momentum or energy to the other. The deployment of the tether takes advantage of the gravity gradient force that exists due to the differential gravitational force between the two ends of the tether. It is this differential gravitational force, called the gravity gradient force, that keeps the tether taut and the two objects tied to the two ends of the tether pulled apart. Once the tether is deployed, if there are no other forces acting on the tether, it will attain a vertically aligned equilibrium orientation. One of the main applications of a momentum exchange tether is to adjust the orbit of a spacecraft. The tether spins and the act of spinning makes the objects tied to the ends of the tether experience a continuous acceleration with the magnitude of acceleration depending on the rotation rate of the spinning system. If either of the two end objects are released during rotation, momentum exchange will occur. The transfer of momentum from the spinning system to the released object causes the spinning system to lose orbital energy and thus lose altitude. This loss of energy can be replenished by using electrodynamic thrusting without consumption of any fuel, as described in the following paragraphs. The resultant tether system is a hybrid combination of both the momentum exchange tether and the electrodynamic tether called the momentum exchange/electrodynamic-reboost system, which combines the best of both types of tethers to create an effective upper stage launch station. In the hybrid tether system, the momentum exchange property is used to catch and release spacecraft, and the electrodynamic property is used to create the thrust needed to restore its orbital energy after a payload transfer.
There are various mechanisms by which momentum or energy can be exchanged between the two objects. In one of the methods, called gravity gradient stabilization, tether is used for attitude control. The tether in this case has a small mass tied to one of its ends and the satellite whose attitude needs to be controlled is tied to the other end. Gravity gradient forces stretch the tether between the two masses. The top and bottom portions of a long object are also pulled by different forces, which help the bottom of the object to be stretched out. The result is that the satellite attitude is stabilized with its long dimension pointing towards the radius vector of the planet it is orbiting. It may be mentioned here that although the tether is stationary in the orbital reference frame, it is rotating once per orbit in the inertial reference. Figure 16.1 illustrates the concept of gravity gradient stabilization.
Figure 16.1 Gravity gradient stabilization
Another type of momentum exchange tether is the rotating momentum exchange tether, called a bolo. In this case the tether acts like a momentum energy bank and transfer of momentum and energy from the tether to and from the spacecraft with very little loss can be used for orbital manoeuvring. The mechanism can be used to either increase or decrease the altitude of the satellite orbit. The momentum exchange phenomenon could also be used to catch a payload coming in its plane of rotation from any direction at any speed less than the maximum tip speed. It could use the same momentum exchange phenomenon to launch the payload in some other direction at a different speed.
Yet another type of momentum exchange tether is a long rotating bolo called a rotovator. One common application of a rotovator tether is to lift or put payloads from/onto a planet or moon. Such a tether orbits in a relatively lower orbit around a planet or moon and is used from a space platform. Present day material strengths allow building such momentum exchange tethers for Mars, Mercury and most moons, including the Earth's moon.
16.2.1.2 Electrodynamic Tethers
An electrodynamic tether is essentially a long conducting cable extended from a spacecraft. The cable is kept stretched and oriented along the vertical direction by the gravity gradient force (Figure 16.2). The electrodynamic tether generates thrust through Lorentz force interaction with the planetary magnetic field. This thrust can be used to execute orbital manoeuvres without the need to carry large quantities of propellant into orbit.
Figure 16.2 Electrodynamic tether
An electrodynamic tether operates as follows. The tether is made up of a conducting material. The motion of the conducting tether across the planetary magnetic field induces a voltage along the length of the tether and this induced voltage could be of the order of several hundred volts per kilometre of tether length. The induced voltage drives an electric current through the conducting tether. The current carrying tether cutting across the magnetic field produces a Lorentz force, which opposes the motion of the tether and also the spacecraft attached to it. The Lorentz force can be given by the expression (J × B, where J is the current density and B is the magnetic flux density. This electrodynamic drag force has the effect of decreasing the orbit and the host spacecraft. Electrodynamic tethers can also be used to boost the spacecraft orbit in a similar manner. In this case, a source of current added to the tether system is used to drive a current through the tether in a direction opposite to motion induced EMF. The thrust generated in the process propels the spacecraft.
16.2.1.3 Tethers for Formation Flying
These are non-conducting tethers used to maintain a fixed distance between multiple spacecraft (Figure 16.3). Spacecraft formation flying is a widely researched subject and space interferometry is one of the key areas of research interest for scientists worldwide. Use of multiple relatively smaller apertures on multiple spacecraft in a precise formation to synthesize a much larger aperture using the concept of coherent interferometry is one such well established example of formation flying. A large synthesized aperture achieves higher resolution, which would otherwise be possible only with a prohibitively large single aperture. As an illustration, the resolution achievable with a 1 km wide single aperture could be realized with a few 2 or 3 m apertures. Different possible architectures that can be employed to realize this include the structurally connected interferometer (SCI), as planned for NASA's space telescope to be built jointly with Northrop-Grumman for the Space Interferometry Mission but subsequently cancelled in 2010, the separated spacecraft interferometer (SSI), as proposed for NASA's Terrestrial Planet Finder to construct a system of telescopes to detect extra solar terrestrial planets but subsequently cancelled in 2011 after several postponements, and the tethered formation flight interferometer. While the SCI architecture allows very limited baseline changes, SSI architecture requires prohibitively large quantities of propellant. Precise spacecraft formation flying has the advantages of both SCI and SSI architectures. It could be used to synthesize large apertures to build high resolution astronomical telescopes. The use of tethers in spacecraft formation flying allows the mitigating need for huge quantities of propellant that would otherwise be required to maintain the precision formation flying. NASA's SPECS (Sub-millimeter Probe of the Evolution of Cosmic Structure) mission designed to carry out studies to find answers to questions related to formation of universe is an example of tethered spacecraft flying. The mission is designed to study formation of universe, evolution of galaxies, and cosmic history of energy release and so on.
Figure 16.3 Tethers for formation flying
16.2.2 Applications
The following paragraphs briefly describe some common applications of space tethers. The applications are categorized as those related to momentum exchange, electrodynamic and hybrid tether systems.
16.2.2.1 Momentum Exchange Tether Applications
- One of the main applications of momentum exchange tethers is their use to launch payloads into a higher orbit such as from a low Earth orbit to a geosynchronous transfer orbit and then to a geostationary orbit without the use of expensive rockets. As outlined earlier, the momentum exchange tether loses some energy in the process, which can be compensated for by using a hybrid tether system. The hybrid system makes use of electrodynamic reboost properties to re-energize the momentum exchange tether system, enabling its use multiple times.
- Another application of a momentum exchange tether is a possible method for transferring payloads, for example to get back small payloads from the space stations such as the ISS without the use of rockets. This operation is carried out as follows. The payload is tied down inside a mini capsule that can be ejected downwards through a robotic airlock. The capsule deploys a tether approximately 30 km long that orientates the capsule for re-entry. The tether is eventually cut and burned up in the atmosphere. The technology was demonstrated successfully in 1993 in the NASA sponsored Small Expendable Deployment System (SEDS-1).
16.2.2.2 Electrodynamic Tether Applications
- One of the primary applications of electrodynamic tethers is in de-orbiting a satellite at the end of its useful life. When the subject satellite is to be de-orbited, a long wire antenna, which is a part of the satellite, is unfurled. A current is made to flow through the wire and the interaction of the current carrying wire with the Earth's magnetic field generates the Lorentz drag force. This drag force reduces the orbit velocity, as a result of which the satellite de-orbits to a lower orbit. It eventually enters the Earth's atmosphere and is burnt up.
- Another important application of electrodynamic tethers is in the stabilization of the orbit of the ISS. It may be mentioned here that the cost of maintaining the ISS in the desired orbit over its planned life time is estimated to be close to US$ 1000 million considering the total quantity of propellant that is required. Electrodynamic forces experienced by the electrodynamic tether, as described earlier, could be used to hold the ISS in orbit.
- Electrodynamic tethers may also be used as an economical means of generating electrical power in orbit, but only for providing bursts of energy in short duration experiments. The electrodynamic tether in this case can be used to convert some of the spacecraft's orbital energy into electrical power. This is good for generating short bursts of energy as the process of generating electrical energy is accompanied by a decrease in orbit altitude.
16.2.2.3 Hybrid Tether Applications
- The primary application of hybrid tether systems also called momentum-exchange/ electrodynamic-reboost tethers is to transfer payloads from a low Earth orbit into medium Earth and geosynchronous orbits. A typical system may be able to transfer a 2500 kg payload to a geosynchronous orbit and a 5000 kg payload to a medium Earth orbit. The main advantage of using tethers is their near zero propellant usage. The small quantity of propellant needed onboard the tether system is for the purpose of making minor trajectory corrections to ensure a precise payload rendezvous. An added advantage of using hybrid tethers for payload transfer to a geosynchronous Earth orbit is the very short transfer times, which would otherwise have required huge quantities of propellant thus increasing the size and cost of the mission. Re-usability of the tether system is yet another advantage. When the hybrid tether system is at the end of its life span, the electrodynamic part of it can be used to de-spin the system and reel in the tether. The electrodynamic drag force is then used to lower the orbit, forcing it to re-enter the Earth's atmosphere.
- Another possible application of hybrid tether systems is their use as a platform for testing momentum exchange tethers in space environment with dummy payloads before they are used for high value payloads.
- Interplanetary transfer of goods and resources could be yet another application of hybrid tethers in the future. For example, once there is a manned presence on the moon, a lunar orbiting tether along with an Earth orbiting hybrid tether may allow convenient transportation between the two. Use of tether systems could drastically reduce the cost of future space missions, making them economically viable.
16.2.3 Space Tether Missions
The following paragraphs briefly describe some important space tether missions executed worldwide in the last couple of decades and also important space tether missions planned for the coming years.
16.2.3.1 Tethered Satellite System (TSS)
The Tethered Satellite System (TSS) is a collaborative programme by NASA and Italian Space Agency (ASI) with the objective of developing a reusable multi-disciplinary facility to conduct space experiments in Earth orbit. The system consists of a satellite, a deployment system in the Space Shuttle's payload bay, an electrically conductive tether approximately 20 km long and six scientific instruments. While development of the satellite was the primary responsibility of the ASI, NASA offered the deployment system and the tether. TSS provided the capability of deploying a satellite on a long, gravity-gradient stabilized tether from the Space Shuttle for the purpose of carrying out scientific investigations in space physics and plasma electrodynamics.
The first TSS mission, called TSS-1, was conducted aboard Space Shuttle flight Atlantis (STS-46) from 31 July to 8 August 1992. Figure 16.4 shows TSS-1 deployment from the Space Shuttle orbiter's payload bay. The TSS-1 mission was twofold, first to demonstrate the feasibility of deploying and controlling long tethers in space and second to conduct exploratory experiments in space plasma physics to evaluate the efficacy of such a system. The tethered satellite system orbited the Earth at an altitude of 296 km, placing the tether system in the electrically charged atmosphere of the ionosphere. The operations lasted for about 24 hours, after which the tether was retrieved. The TSS-1 mission was followed up by a re-flight mission named TSS-1R aboard Space Shuttle flight Columbia (STS-75). The tether was deployed on 22 February 1996. The mission intended to deploy the tether to its full length of approximately 20.7 km, but the tether suddenly snapped and burnt prior to reaching full deployment of 20.7 km. Tether dynamics could be verified only up to 19.6 km. The two missions not only validated the concept of gravity gradient tethers but also the feasibility of the generation of electrical power from orbital energy.
Figure 16.4 TSS-1 deployment from Space Shuttle Atlantis payload bay (Courtesy: NASA)
16.2.3.2 Small Expendable Deployer System (SEDS)
The Small Expendable Deployer System (SEDS) was developed by NASA's Marshall Space Flight Centre (MSFC), which was primarily responsible for the development of transportation and propulsion technologies, and the Tether Application Company of San Diego. Two flight experiments, SEDS-1 and SEDS-2, were carried out in 1993 and 1994. Both SEDS-1 and SEDS-2 flew as secondary payloads on Delta-II launches of GPS satellites.
The objective of the SEDS-1 experiment was to demonstrate the use of a tether to place a payload in a de-orbit trajectory and study payload re-entry after the tether was cut. This involved a downward deployment of a 20 km long non-conducting momentum exchange tether. During the experiment, following the deployment, the tether was cut at the deployer end, forcing the payload to re-enter the atmosphere, where it was burnt. The main purpose of the SEDS-2 flight, which was also a downward deployment of a 20 km long non-conducting momentum exchange tether, was to demonstrate deployment and stabilization of the tether system along the local vertical.
16.2.3.3 OEDIPUS Tethered Sounding Rocket Missions
OEDIPUS, an acronym for Observations of Electric-field Distribution in the Ionospheric Plasma – a Unique Strategy, was a joint programme by the National Research Council of Canada and NASA. The programme included participation by the Communication Research Center in Ottawa, Canada, who were the Principal Investigators, various Canadian universities and the US Air Force Phillips Laboratory. Bristol Aerospace Ltd was the primary payload contractor. The mission consisted of two sounding rocket experiments that used spinning conductive tethers. The two experimental missions, namely OEDIPUS-A and OEDIPUS-C, were launched on 30 January 1989 and 6 November 1995, respectively, using three-stage sounding rockets called Black Brant-X. OEDIPUS-A was designed as a double probe for sensitive measurement of weak electric fields in the aurora. OEDIPUS-C also had two spinning payloads. The payloads were connected by a 1174 m long conductive tether. The mission objectives included understanding of the effect of charged particles associated with aurora on satellite transmissions and studying natural and artificial waves in the ionospheric plasma.
16.2.3.4 Plasma Motor Generator (PMG)
The Plasma Motor Generator (PMG), a 500 m long electrodynamic tether payload of a Department of Defence satellite, was launched on 26 June 1993 as a secondary payload onboard a Delta-2 rocket. The primary objectives of the PMG mission were to test the performance of a hollow cathode assembly to provide a low impedance bidirectional electrical current and demonstrate the application of an electrodynamic tether for space propulsion and conversion of orbital energy into electrical energy. The mission successfully established the use of a tether as a generator with electron current flow down the tether and as a motor with electron current driven up the tether.
16.2.3.5 Tether Physics and Survivability (TiPS)
The Tether Physics and Survivability (TiPS) experimental payload was built and operated by the Naval Centre for Space Technology (NCST) of the Naval Research Laboratory (NRL). It was launched on board the Titan-4 launch vehicle on 12 May 1996 and deployed on 20 June 1996. The TiPS payload is a free flying satellite comprising two end bodies separated by a 4.0 km long non-conducting tether. This is unlike other tether systems such as those flown onboard the shuttle where one end of the tether system was connected to the massive host vehicle. The TiPS satellite payload was jettisoned by the host launch vehicle on 20 June 1996. This was followed by separation of the end bodies by the tether. The primary objective of the TiPS experiment was to study the long term dynamics and survivability of tether systems in space.
16.2.3.6 Atmospheric Tether Mission (ATM)
The Atmospheric Tether Mission (ATM) is an experimental mission proposed to be executed from Space Shuttle in which a tethered probe will be lowered successively into different regions of the atmosphere such as the mesosphere, thermosphere and ionosphere for the purpose of understanding the atmosphere and plasma around the Earth in these regions. The measurements are proposed to be made by a set of 11 instruments housed in an end mass or spacecraft. The end mass will be lowered from the shuttle by a 90 km long tether. The instrument payload comprises an ion drift meter, a retarding potential analyzer, an ion mass spectrometer, a Langmuir probe, a neutral wind meter, a neutral mass spectrometer, an energetic particle spectrometer, E-field double probes, an infrared spectrometer, a UV photometer and a three-axis magnetometer. The experiment is proposed to be carried out over a period of six days. The orbiter will be at 220 km altitude. For the first two days of the mission, the tether is proposed to be lowered up to 170 km altitude using a 50 km tether length. Another 20 km of tether length will be lowered for the next two days and further lowered by another 20 km to its full length of 90 km and an altitude of 130 km during fifth and sixth days.
16.2.3.7 STEP-AIRSEDS
STEP-AIRSEDS is an acronym for Space Transfer Using Electrodynamic Propulsion – Atmospheric Ionospheric Research Small Expendable Deployer System. This mission satellite, developed by the Michigan Technic Corporation Holland, Michigan, USA, and weighing 1000 kg, including the tether, consists of two units tethered together by an approximately 6–7 km long electrodynamic tether. The upper and lower units drive the boost and deboost operations, respectively. The mission is intended to experiment with new and innovative methods of use of conducting tethers, solar power and the Earth's magnetic field to execute a range of satellite orbit manoeuvres such as moving the satellite up, down and across planes without using any propellants. It is proposed to perform these operations on a satellite operating in the altitude range of 350–1100 km for a minimum period of one year.
16.2.3.8 Space Tether Experiment (STEX)
The Space Tether Experiment (STEX) is a science and technology experiment of the Institute of Space and Aeronautical Science (ISAS) to be flown onboard the Space Flight Unit (SFU) mission. SFU is an unmanned, multi-purpose reusable platform that can be used for performing a range of science and technology experiments and carrying out flight tests of space and industrial technologies. The satellite weighs 4000 kg and has a payload capacity of 1000 kg. The STEX payload comprises a tether, a tether deployment and retraction system, and a 40 kg sub-satellite. The sub-satellite can be deployed up to 10 km. Deployment and retrieval of the sub-satellite can be done several times. The sub-satellite is equipped with instrumentation including a vacuum gauge, plasma probes and wave receivers to study the electromagnetic environment of SFU. One of the primary objectives of STEX is to assess the tether technology for future scientific missions.
16.2.3.9 Propellant Reboost for International Space Station
It is proposed to use a short and light electromagnetic tether as an alternative to propellant driven thrusters reboost for the ISS. The proposed electrodynamic tether system is approximately 10 km long and weighs 200 kg, which is small enough to cause an insignificant shift of less than 5 m in the centre of the mass of the space station. The tether when deployed would be capable of generating thrust of 0.5–1.0 Newton for an electrical power of 5.0–10 kW. This will compensate for the average aerodynamic drag of 0.3–1.1 Newton on the space station. To produce thrust of the order of 1.0 Newton in a 10 km long tether, the tether current needs to be of the order of 10 amperes. This is made possible by having a good part of the tether length as uninsulated or bare, unlike standard tethers, which collect electrons from the ionosphere only at the ends. Also, the tether design is such that it is insensitive to variations in electron density in the ionosphere, which also allows it to operate efficiently during night time.
16.2.4 Space Elevator
The dream of space transportation was realized back in 1981 with the successful launch of Space Shuttle mission Columbia, thereby establishing the use of a reusable spacecraft to travel to space. Since then, technology has matured a great deal and more than 100 such missions have been carried out with success. What has not changed is the cost of executing such missions. Whether it is Space Shuttle mission or a non-reusable spacecraft; the cost of launch continues to be about US$ 20 000 per kilogram of mass to be transported to geostationary Earth orbit and much more if it were to be sent to a more distant point in space.
A space elevator, which is related to the fundamentals of space tethers, provides a concept for the development of a new space transportation system. The proposed concept has the potential of making the transportation of tonnes of payloads and resources to geostationary Earth orbit and beyond almost a daily affair, albeit at an incredible cost of US$ 200 to 800.
A space elevator is nothing but a very long tether anchored to the surface of Earth or an off-shore sea platform at one end. A counterweight connected to the other end or an extension of the tether further into space ensures that the centre of mass is above the geostationary orbit altitude. A climber is the other essential component of a space elevator. It is a kind of robotic platform that is made to climb up the tether. The tether is kept stretched by the downward acting gravitational pull and upward acting centrifugal force due to the Earth's rotation. It may be mentioned here that once above the geostationary level the climber would experience an upward weight due to centrifugal force overpowering the gravitational pull. The tether consists of a ribbon-like thin cable made from a carbon nano tube composite material. Figure 16.5 illustrates the operational concept of a space elevator. The diagram is self explanatory. Once the space elevator is deployed, the climber platform could be made to repeatedly ascend and descend the ribbon-like cable.
Figure 16.5 Space elevator
Once deployed, the tether would be ascended repeatedly by mechanical means to orbit and descended to return to the surface from orbit.
One such proposed space elevator is from LiftPort, one of the many companies engaged in the development of space elevators and their components. The proposed space elevator will be 100 000 km long, close to a quarter of the distance between the Earth and the moon. The cable of carbon nano tube composite ribbon is proposed to be anchored to an off-shore sea platform. The weight of the climber may vary from 5 to 20 tonnes. A 20 tonne climber would be capable of lifting up to 13 tonnes. The robotic lifter will use the ribbon to guide the ascent into space. A 2.4 MW free electron laser located on or near the anchoring station powers photovoltaic cells that convert light energy into electrical energy. The electrical energy in turn feeds niobium magnet DC electrical motors, which drive the lifter. Figure 16.6 is a conceptual drawing of the climber. The launch is scheduled for the year 2018. Once operational, climbers will climb up the elevator almost every day at a speed of about 190 km per hour.
Figure 16.6 Conceptual drawing of climber (Courtesy: Liftport)
16.3 Aerostat Systems
An aerostat may be defined as an aircraft, balloon or dirigible, deriving its lift from the buoyancy of surrounding air rather than from aerodynamic motion. The name aerostat is derived from the fact that it operates on aerostatic lift, also called buoyant force, and not aerodynamic lift, which essentially requires at least some part of the aerial body to move through the surrounding air mass. The aerostat family includes free balloons, tethered balloons, helikites and airships, though in a narrower sense aerostats are generally identified by tethered balloons. The major constituents of an aerostat system are the aerostat, the tether, the mooring system, the payload, comprising a range of sensors depending on the application, and the command and control unit. Figure 16.7 shows deployment of a tethered aerostat system.
Figure 16.7 Tethered aerostat under deployment (GNU Free Documentation License)
16.3.1 Components of an Aerostat System
An aerostat is an unmanned, aerodynamically shaped non-rigid or semi-rigid airship tethered to the ground by a cable and kept aloft by the buoyant force. The aerostat is made up of a large fabric envelope filled with a non-inflammable lighter-than-air gas such as helium. It is this gas fill that provides the buoyant or lifting force. The tether, in addition to providing the required anchoring, also contains electrical cables to supply electrical power to the aerostat and fibre optic cables for data relay between the aerostat and the ground station. The payload comprises a wide range of sensors and other electronic systems, depending on the intended application. Some common sensor systems generally installed on aerostat systems include surveillance sensors, observation devices, communication equipment, and military intelligence systems such as ELINT (electronic intelligence) and COMINT (communication intelligence) systems. The main functions of the command and control centre include controlling the aerostat platform and the onboard payload, signal processing to build up the total situational awareness picture, and initiation and control of response functions.
16.3.2 Types of Aerostat Systems
Aerostat systems are classified as free balloons, tethered or moored balloons, helikites and airships. A free balloon is a type of aerostat that moves with the wind and remains aloft due to its buoyancy. A capsule suspended beneath the balloon with the help of cables is used to carry observation and surveillance equipment, and communication relay equipment. It also houses mechanisms used for flight control. These are hot air balloons that get their buoyancy by heating air inside the balloon, gas balloons that are hot air balloons inflated with a gas lighter than the surrounding atmosphere, such as helium, and filled with pressure equal to or slightly more than that of the surrounding atmosphere or Rozière balloons filled with both heated and unheated lifting gases. The ECHO satellite launched in 1960 for passive relay of radio communication, PAGEOS launched in 1966 for high precision calculation of different locations on the Earth's surface and Vega-1/2 satellites of the former Soviet Union launched in 1984 releasing two balloons carrying scientific payloads to perform experiments in the atmosphere of Venus are some examples of the use of balloons in the early days of satellite launches. Rozière balloons are generally used for circumnavigation.
A tethered balloon, also called a moored balloon, is not free flying like the free balloons described in the previous paragraph. Its movement is restricted by anchoring it to a ground surface or a vehicular platform by a cable or a set of cables. A tethered balloon is a structured envelope of fabric usually filled with helium gas. Traditional tethered balloons are blimp shaped, fin stabilized and use helium gas for lift. Simple round balloons do not use any fin stabilization and rely on helium alone for lift. Hybrid balloons make use of both buoyancy and aerodynamic lift to stay aloft.
The helikite is one of the most commonly used aerostat designs for all weather, high altitude applications. It is a combination of a helium balloon that uses helium gas for buoyancy and a kite that exploits wind for lift. The word helikite originates from a combination of the words ‘helium’ and ‘kite'. This kite style of aerostat has been patented by Sandy Allsopp in England. The kite-like structure provides excellent aerodynamic stabilization and the aerostat exploits both helium driven buoyancy and wind for lift.
An airship is an aerostat in the broader sense as defined earlier. In the narrower sense, where the word aerostat is generally associated with tethered or moored balloons, an airship may be considered as a kind of lighter-than-air aircraft that is steered with the help of thrust mechanisms such as rudders and propellers. Airships are being developed for a wide range of applications including passenger flight, flying cranes, scientific experimentation, surveillance, communication relay, and so on. A large number of companies worldwide are engaged in building airships. Some of the prominent names include Zeppelin NT, American Blimp Corporation, ABC Lightship, Raven Aerostar, Advanced Technology Group, Airship Industries and so on. Figure 16.8 is a photograph of an airship manufactured by Zeppelin NT.
Figure 16.8 Airship (Courtesy: [email protected])
16.3.3 Applications
Aerostats were initially considered more of a spectacle than machines that could be used for important civilian and military applications. In the early days of their development their use was limited to advertising banners or event broadcasting due to their low speed and large size. With advances in technology, particularly fibre optic technology, the application spectrum of aerostats has widened to include many new areas such as early warning, communication relay and surveillance, homeland security and law enforcement and intelligence gathering. Modern aerostats are capable of flying to altitudes in excess of 5 km, have a large detection range coverage extending as far as 500 km, have payload carrying capacity exceeding 2500 kg and continuous operation of weeks before they need to be retrieved for routine maintenance.
Figure 16.9 is a photograph of a tactical aerostat from M/S Raven Aerostar. A wide variety of payloads such as communications, intelligence gathering, surveillance and reconnaissance payloads, EO/IR sensors, communication repeaters and radar sensors can be integrated on this platform to provide a total solution.
Figure 16.9 Tethered tactical aerostat, Type TIF-75K (Courtesy: Raven Aerostar)
16.4 Millimetre Wave Satellite Communication
The requirement to have larger bandwidth for audio, video and data communication services, terrestrial or satellite based, has been on the rise with the addition of new services on a continuous basis and also with the quest to enhance the quality of existing and new services. Services such as three-dimensional television, high definition television, video on-demand, virtual reality imaging and high speed data communication demand bandwidths that are a thousand times larger than what they would be in the case of traditional voice telephony and television broadcast services, notwithstanding the fact that various digital compression techniques are in use today to conserve bandwidth while providing these services. There are a host of technologies available for delivery of the required bandwidth. One of the most prominent ones used in terrestrial systems is fibre optics. However, when it comes to the cost effectiveness of fibre optic communication systems, particularly in the case of extensive geographical coverage and the need to have easy access to remote and sparsely populated areas, it is definitely not the best solution. Millimetre wave wireless communication has the potential of offering a comparable bandwidth that is currently possible with fibre optic communication technology without the economic and logistic challenges of the latter. The following paragraphs present the fundamentals of millimetre wave communication and the associated advantages and limitations. This is followed by a brief discussion of the experimental satellite missions carried out in the last three to four decades to establish the feasibility of millimetre wave satellite communication.
16.4.1 Millimetre Wave Band
The millimetre wave band extends from a wavelength of 10 mm to 1 mm, corresponding to a frequency band of 30–300 GHz. It is also referred to as the extremely high frequency (EHF) band (Figure 16.10). The terahertz region also falls within this band, which is sandwiched between the near and far infrared bands on the shorter wavelength side and the radio and microwave regions on the longer wavelength side. Although the millimetre wave band may be considered theoretically to extend from 30 to 300 GHz, in the context of wireless communication a few spectral bands around 38, 60 and 94 GHz are considered important from the viewpoint of their propagation characteristics through the atmosphere. More recently, the propagation band from 70 to 90 GHz, also known as the E-band, has been allocated for wireless communication in the public domain. Within this band there are two sub-bands around 70 GHz (71–76 GHz) and 80 GHz (81–86 GHz,) that are used for wireless communication. Other millimetre wave bands that have been opened for commercial applications include the 59–64 GHz band, commonly known as the 60 GHz band or the V-band, and the 92–95 GHz band, also referred to as the 94 GHz band or the W-band. Because of the high level of absorption by atmospheric oxygen in the 60 GHz band, this band is best suited to very short range point-to-point and point-to-multipoint communication applications. The 94 GHz band is less spectrally efficient due to an excluded band at 94–94.1 GHz. The 70 GHz and 80 GHz (E-band) bands with the availability of a 5 GHz spectrum in each of the two bands makes them an ideal choice for wireless millimetre wave communication. In fact, the spectral bandwidth offered by these two bands exceeds the total collective bandwidth offered by all allocated microwave bands.
Figure 16.10 Millimetre wave band
16.4.2 Advantages
It is not just recently that the potential that the millimetre wave frequency band holds for a variety of applications, including their use in radio astronomy in the 1960s and military applications in the 1970s and now point-to-point and point-to-multipoint terrestrial and satellite communication systems, has been realized. The advantages that the millimetre waves would offer in terms of larger bandwidth, higher resolution, smaller antenna sizes with associated higher directive gain, reduced multipath interference, increased immunity to jamming by electronic countermeasures and so on were visualized back in the nineteenth century, as is evident from the millimetre wave experiments of J.C. Bose in the 1890s. While his contemporaries like Marconi were inventing all-important radio communications, Bose was busy experimenting with millimetre waves.
Millimetre wave research and development, however, remained confined to prominent universities and government funded research and development institutes for almost 50 years before millimetre wave applications began to see the light of the day first in the form of radio astronomy and then military applications. Subsequently, advances in the development of millimetre wave integrated circuits in the 1980s have opened up the commercial application domain in the form of automotive collision avoidance radar. Authorization of 70 GHz and 80 GHz bands by the Federal Communications Commission for licensed point-to-point communication in 2003 has helped the millimetre wave products reach industry.
Millimetre wave communication offers numerous advantages but also has some serious limitations relating to propagation through the atmosphere. Millimetre waves are particularly affected by rain, as described in the section 16.4.3, though efforts are in progress to devise new techniques to overcome these limitations with significant success. The advantages of millimetre wave communication as outlined earlier in this section are briefly described in the following paragraphs. Major advantages include increased bandwidth, smaller antennas for a given directive gain and narrower beam width for a given antenna aperture, immunity to jamming and interference, and finally inherent security.
16.4.2.1 Increased Bandwidth
Millimetre wave communication offers much higher bandwidth compared to microwave communication, for example the 5 GHz bandwidth available in each of the two sub-bands around 70 and 80 GHz of the E-band can be used as a single, contiguous transmission channel without the need for any channelization. This allows a throughput of 1–3 Gbps to be achieved in each of the two sub-bands with simple modulation schemes such as on–off keying or binary phase shift keying, which is even higher than the throughput possible with higher order modulation schemes at microwave frequencies. Using sophisticated higher order modulation schemes at E-band enables much higher throughput can be achieved.
16.4.2.2 Narrower Beam Width
The antenna beam width in both azimuth and elevation planes is inversely proportional to the wavelength. The half power beam width of an antenna with aperture diameter of D and operating at a wavelength of λ is given in degrees by 70λ/D. This implies that the beam width decreases with increase in wavelength. This further means that a given antenna will produce a narrower beam width at millimetre wavelengths than at lower frequencies or higher wavelengths. As an illustration, using an equivalent antenna, the beam width of a 70 GHz link is four times as narrow as that of an 18 GHz link. In turn, it allows as much as 16 times the density of E-band millimetre wave links in a given area. Also, a narrower beam width means a higher directive gain, which allows operation at millimetre wavelengths to compensate for some of the propagation losses at millimetre wavelengths.
16.4.2.3 Immunity to Jamming and Interference
Narrow beam width and a highly directional radiation pattern allow multiple transmission channels to operate spatially close to each other without causing any troublesome adjacent channel interference. The use of cross-polarization techniques allows even multiple channels to be deployed along the same path.
16.4.2.4 Small Antenna Size
The gain of antenna for a given aperture area is inversely proportional to the square of the wavelength. Operation at millimetre wave frequencies, which are higher than microwave frequencies, allows use of smaller and lighter antenna structures for a given gain specification.
16.4.2.5 Communication and Information Security
Millimetre wave communication is inherently secure due to narrow beam width and also because millimetre waves are blocked by many solid structures. Any attempt to sniff millimetre wave radiation would require placing the interceptor near or in the path of electromagnetic radiation, which becomes difficult when the beam width is sufficiently narrow. Also, loss of data integrity due to interception, if any, may be used to detect the interception. In addition, data encryption techniques are available to further enhance security.
16.4.3 Propagation Considerations
Every electromagnetic wave irrespective of its frequency or wavelength has to experience less or more attenuation in signal intensity as it propagates through the atmosphere. Absorption and scattering are the primary mechanisms that cause signal attenuation. Attenuation is usually expressed in decibels loss per kilometre (dB/km) of propagation distance and therefore attenuation suffered by a communication link depends upon the length of the link.
With reference to millimetre wave propagation through the atmosphere, major parameters influencing signal attenuation are atmospheric oxygen, humidity, rain and fog. Figure 16.11 shows the curves of attenuation in dB/km as a function of frequency of operation both at sea level and at an altitude of 9150 m. The atmospheric oxygen causes peak absorption, as is evident from the curves, at 60 GHz within the millimetre wave region of interest. Peak attenuation due to atmospheric oxygen at 60 GHz is approximately 3 dB/km at 9150 m altitude. Attenuation at sea level is considerably higher. These figures are at high temperature and humidity levels. There is further attenuation of signal due to fog and clouds. Attenuation caused by fog and clouds depends upon quantity and size of liquid droplets in the air. Figure 16.12 shows the curve depicting the effect of fog corresponding to 100 m visibility conditions on millimetre wave propagation. As is shown in the curve, millimetre waves suffer attenuation in the range 0.1–1.0 dB/km.
Figure 16.11 Atmospheric attenuation of millimetre waves
Figure 16.12 Attenuation of millimetre waves due to fog
Rain is the most predominant attenuating factor for both microwaves and millimetre waves, including atmospheric constituents like oxygen, carbon dioxide and water vapour, fog and clouds. The amount of signal loss depends upon the rate of rainfall, which is usually measured in millimetres per hour. Figure 16.13 shows the curve depicting the effect of rainfall on signal attenuation in the millimetre wave region. As is evident from the family of curves, signal attenuation in the millimetre wave region in heavy rain conditions (corresponding to 25 mm/hour) is in the range 8–15 dB/km.
Figure 16.13 Attenuation due to rain
16.4.4 Applications
The millimetre wave band of frequencies offers diverse applications ranging from imaging to telecommunications, and from consumer products to defence. Imaging applications mainly include check point security at airports, concerts, sports events and so on used to screen passengers and personnel, loss prevention and inventory control, through wall imaging, imaging of personnel for detection of concealed weapons and so on. Telecommunications applications mainly include campus and enterprise broadband networks, routine backhaul deployments and point-to-point communication links. The consumer market is mainly the automotive sector. Automotive radar is an established application used for adaptive cruise control and collision avoidance. There are many applications in defence and security. Some of the better known ones include millimetre wave sensors in smart munitions, satellite-to-satellite communications and radar for surveillance, perimeter protection and intrusion detection. Military intelligence is another application. Other than telecommunications, remote sensing and radio astronomy are the areas particularly important for space-based platforms, including satellites. It is not possible to discuss each and every existing and emerging application of millimetre wave bands, however some representative examples of applications relevant to satellite and space technology are presented in section 16.4.5.
16.4.5 Millimetre Wave Satellite Missions
A number of both experimental and operational space missions have been carried out since the 1970s to assess the feasibility of using millimetre wave technology and also to evaluate the performance of the technology for a variety of applications such as radio astronomy, remote sensing, imaging, meteorological observations and communication. Though the feasibility of using millimetre wave technology has been established and it has begun to be used in remote sensing, radio astronomy and meteorological applications from space-based platforms, it is not yet widely used for satellite-based telecommunications. Some representative space missions including experimental missions tried in earlier years, those carried out in recent years and also those planned for the future are briefly described in the following paragraphs.
16.4.5.1 Engineering Test Satellite-II (ETS-II)
Beginning in 1975 with the launch of ETS-I, also called KIKU-1, Japan has launched a number of engineering test satellites (ETS), with each satellite addressing the technological needs of that time. Eight satellites, ETS-I to ETS-VIII, also called KIKU-1 to KIKU-82, have been launched so far by the National Space Development Agency of Japan. ETS-II (KIKU-2) was the first geostationary satellite to carry a beacon transmitter with three coherent frequencies at 1.7 GHz, 11.5 GHz and 34.5 GHz (millimetre wave frequency) to perform propagation experiments. The signal at 34.5 GHz had 100% amplitude modulation. This mode was selected to improve rain margin. The satellite was launched in February 1977 and concluded its mission in December 1990.
16.4.5.2 Experimental Communications Satellite (ECS)
Experimental communications satellites (ECSs), also known as AYAME satellites, were experimental satellites of the National Space Development Agency (NASDA) intended to carry out communications and propagation experiments at millimetre wavelengths. Two satellites, ECS-A (also called AYAME-1) and ECS-B (also called AYAME-2), were launched on 6 February 1979 and 22 February 1980, respectively. Both satellites were lost shortly after launch during the firing of their apogee kick motors. ECS-A ceased radio transmissions 10 seconds after the apogee kick motor was fired. The last known longitude of the satellite on 13 June 1995 was 146.23°W, drifting at 33.817°E per day. ECS-B also failed in a similar manner and its last known longitude on 17 November 1988 was 146.47°W, drifting at 12.888°E per day.
16.4.5.3 FLORAD Mission
The FLORAD mission of the Italian Space Agency is a micro-satellite flower constellation of micro-satellites carrying onboard scanning millimetre wave radiometers for Earth and space observation at a regional and quasi-global scale. The research mission was carried out during 2008–2009. A flower constellation is designed using compatible orbits that allow optimization of revisiting time for the Earth regions of interest. In a flower constellation, the orbits are designed so that when a satellite leaves the petal, another satellite of the constellation takes its place. The constellation has four micro-satellites deployed in flower-like elliptical orbits and each satellite is equipped with only one payload, which is a millimetre wave radiometer. The primary objective of the mission was to study and analyze the thermal and hydrological properties of the troposphere with particular reference to water vapour profile, temperature profile, cloud liquid content, rainfall and snowfall with high spatial resolution and time repetitiveness in the terrestrial atmosphere of the Mediterranean region. In addition, the mission aimed to evaluate the performance of the flower constellation itself, passive millimetre wave sensors, advanced antenna concepts and so on. The mission investigated various configurations of millimetre wave multiband channels to find a trade-off between performance and complexity within the constraints of the micro-satellite platform.
It may be mentioned here that millimetre wave radiometers operating at 30–300 GHz score over their microwave counterparts in their reduce size and their potential in exploiting the window frequencies and different gaseous absorption bands at 60/70 GHz, 118 GHz and 183 GHz. The radiometry of the atmosphere is an established application of millimetre wave technology due to its capability to sound through clouds and detect precipitation, and it outperforms infrared sensors. The latter, however, offer higher spatial resolution but only in cloud-free areas.
16.4.5.4 Applications Technology Satellite-6 (ATS-6)
The Applications Technology Satellite-6 (ATS-6) is one of the very ambitious experimental communications satellites developed and implemented by the NASA Goddard Space Flight Centre and intended to carry out a large number of scientific and technological experiments in the geostationary space environment. The satellite carried payloads to perform a wide range of scientific experiments related to communications technology and meteorology. ATS-6, a three-axis body stabilized satellite was launched in geosynchronous orbit on 30 May 1974 from Cape Canaveral using the Titan-3C rocket. The satellite was made operational on 2 July 1974. The objectives of the scientific experiments were to study and gain a better understanding of the environment in space at the geosynchronous altitude. Figure 16.4 is a photograph of ATS-6 in orbit.
Figure 16.14 ATS-6 satellite in orbit (Courtesy: NASA)
In addition to the scientific experiments, ATS-6 also supported health and education telecommunications, satellite based air traffic control and UHF television broadcasts. ATS-6 performed about 18 scientific, communications technology and meteorological experiments, most significantly millimetre wave propagation experiments, health, education and telecommunications experiments, position, location and aircraft communications experiments, radio frequency interference experiments, tracking and data relay experiments and television relay using small terminals experiments.
With reference to the millimetre wave propagation experiment, ATS-6 provided the first direct measurements of Earth-to-space links from an orbiting geosynchronous satellite at 20 and 30 GHz. The measurements mainly focused on studies of rain attenuation effects, scintillations, depolarization, site diversity, coherence bandwidth and communications techniques. The results obtained with the direct measurements were compared with the data generated using methods of attenuation prediction with radars, rain gauges and radiometers. The experimental mission ended in July 1979. At the end of the mission, the satellite was moved 450 km out of geostationary orbit, after which it started drifting eastwards.
16.4.5.5 Communications and Broadcasting Engineering Test Satellite (COMETS)
The Communications and Broadcasting Engineering Test Satellite (COMETS), also known as KAKEHASHI, is a three-axis stabilized geostationary satellite from NASDA of Japan launched onboard the H-II launch vehicle from the Tanegashima Space Centre on 21 February 1998. The satellite had a design life of three years but its operations were terminated in August 1999. Figure 16.15 shows a photograph of the COMETS satellite. The satellite was intended to develop and evaluate futuristic communications technologies including experimenting with the use of millimetre wave frequencies.
Figure 16.15 COMETS satellite
The satellite has three mission payloads. The payload related to millimetre wave band communications is an advanced mobile satellite communications system developed by the Communications Research Laboratory (CRL). The system used a millimetre wave band at 47/44 GHz and a Ka-band at 31/21 GHz for mobile satellite communications. The two other mission payloads included a 21 GHz band advanced broadcasting system developed by CRL and NASDA, and an inter-satellite communications system developed by NASDA and operating in S-and Ka-bands.
16.4.5.6 Odin Satellite Mission
Odin is an aeronomy and astronomy mini-satellite mission funded and executed by the Swedish National Space Board jointly with the space agencies of Canada, France and Finland. The spacecraft design, development and operations were carried out by the Swedish Space Corporation (SSC).
The Odin satellite was launched on 20 February 2001 into sun-synchronous polar orbit. Figure 16.16 shows the Odin satellite deployed in orbit. The satellite mission is intended for astronomy and aeronomy applications. The aeronomy related mission objectives include observation of stratospheric ozone chemistry, mesospheric ozone science and coupling of atmospheric regions. The astronomy objectives and major scientific issues relate to star formation processes, interstellar chemistry and atmospheric ozone balance. The Odin satellite is an observatory making measurements in sub-millimetre (0.5–0.6 mm) and millimetre wave bands (2.5 mm). The Odin mission completed 12 years of operation in 2013.
Figure 16.16 Odin satellite in orbit (Courtesy: Swedish National Space Board)
16.5 Space Stations
A space station may be defined as a large spacecraft orbiting Earth usually in the low Earth orbit and used as a long term base for carrying out manned operations in space. The space stations are also designed for other spacecraft used to transport people and cargo to and from the space station to dock with it from time to time. A space station is also like a huge artificial satellite and therefore deserves a brief description in a book addressing all aspects of satellite technology and applications. It is being covered as a part of the present chapter for two simple reasons. First, it could not have been covered in any of the other chapters and a separate chapter would not be justified given the scope of the text. Second, the space station continues to be an emerging and fast growing concept despite the fact that the first space station (Salyut series) was launched as early as 1971. The following sections discuss different aspects of space stations, including their importance, missions launched during the 1970s and 1980s, currently operational space stations, space stations planned to be launched in the coming years and emerging space station concepts.
16.5.1 Importance of Space Stations
Space stations are important for a variety of reasons, including the study of the effect of the microgravity environment, scientific research, space exploration and tourism. Looking at the prolonged exposure to the microgravity environment of astronauts intending to travel to other planets, it is most important to study the effect of weightlessness on the human body when the exposure is going to last from months to years.
Space stations also provide a platform to carry out research in some cutting edge technologies in an environment that cannot be matched on Earth. There are a number of phenomena that are completely different under the influence of gravity and in microgravity conditions. As an example, in crystal growth phenomenon, near perfect crystals can be grown in a microgravity environment, which can lead to better semiconductors further leading to faster computers. There are many more such examples.
Space stations by virtue of their being above Earth's atmosphere also provide an excellent platform for building manned observatories to carry out space exploration through the use of space telescopes. The merits of unmanned observatories such as the Hubble space telescope are well known.
Another application area where space stations can play an important role is in the study of the Earth's atmosphere, its land forms, vegetation, oceans and so on. Space tourism is another application of space stations as they can be used to allow tourists to stay for a brief period or an extended period of time in the not-too-distant future.
16.5.2 Space Stations of the Past
The following paragraphs briefly discuss the space stations that were launched during the 1970s and 1980s. These include Salyut, Skylab and Mir. One or more of these space stations were operational during the period 1971–1998.
16.5.2.1 Salyut
There are two broad categories of space stations, one built as one piece on Earth and subsequently launched as a single unit and the second that are modular in nature and assembled in space bit by bit. Salyut-1, the first in the Salyut series, was the first ever space station and was put into space by the former Soviet Union on 19 April 1971. Salyut-1 in fact was a combination of Soyuz and Almaz spacecraft. Almaz was originally designed for military applications and was reconfigured as a space station for the civilian role. Soyuz was used to transport crew from Earth to the space station and back. It stayed in orbit for 175 days before it was de-orbited, forcing it to make a destructive re-entry over the Pacific Ocean. The second crew launched on Soyuz-11 remained on board the space station for 23 days after the first crew launched on Soyuz-10 failed in its attempt due to malfunctioning of the docking mechanism. The second crew members unfortunately were killed during the re-entry of Soyuz-11. The Soyuz space craft was redesigned following this failure. Salyut-1 was followed up by Salyut-2, which failed to reach orbit. Salyut-3, Salyut-4 and Salyut-5 were launched subsequently. The new Soyuz spacecraft was used to ferry crew members between Earth and space stations. These space stations were manned for longer mission periods. Salyut series space stations up to Salyut-5 had only one docking port, which was used by Soyuz spacecraft. Salyut-6 launched on 29 September 1977 had a second docking port that could be used by an unmanned space station resupply craft, Progress. Salyut-7, the last in the series of Salyut space stations, was launched in 1982. It also had two docking ports. The space station remained operational till 1982 and hosted different crews for a total of 800 days. The Salyut programme was used for both civilian and military applications. Civilian applications included the study of the long term effect of the space environment on human beings and a wide range of scientific experiments related to astronomy, Earth observation and biology. Military applications included reconnaissance missions.
16.5.2.2 Skylab
Skylab-1, the first space station from the USA, was launched into orbit on 14 May 1973. The architecture of the space station was derived from the modified third stage of the Saturn-V moon rocket. It comprised an orbital workshop that housed working and living quarters for the crew, a multiple docking adapter that allowed more than one Apollo spacecraft to dock to the station, an Apollo spacecraft that ferried crew members between Earth and the space station, an airlock module that allowed access to the outside of the station and an Apollo telescope mount carrying telescopes. The station was damaged during the launch when a micrometeoroid shield separated from the station and in the process destroyed one of the two main solar panels and jammed the other, preventing it from fully stretching out. This resulted in very little electrical power being available on the station, which further led to a rise in inside temperature, almost threatening its shutdown. The first crewed mission, Skylab-2, carried out the first ever major in-space repair and saved the space station. In fact, three manned missions, Skylab-2, Skylab-3 and Skylab-4, were carried out between 1973 and 1974. Each of these missions was launched using the Apollo Command Service Module (CSM) onboard the Saturn-IB rocket. Also, each of these missions carried a three member crew to the space station. The space station remained in orbit till 1979, spending 2249 days in orbit including 171 manned days. Skylab was abandoned at the end of Skylab-4 (third crewed mission). It re-entered Earth's atmosphere and burned on 11 July 1979 with debris falling on parts of Western Australia. During manned missions, a number of scientific studies were carried out, which included confirmation of the existence of coronal holes in the sun and Earth observation in visible, infrared and microwave bands.
16.5.2.3 Mir
Mir was the first permanent space station owned by the former Soviet Union at the time of its launch and subsequently by Russia. The core module, also called the base block, was launched aboard the Proton rocket on 20 February 1986 from the Baikonur Cosmodrome into a low Earth orbit. It was followed up by another six modules. The space station in all consisted of a core module, seven pressurized modules and several unpressurized components. It was the first ever modular space station that was constructed in orbit between 1986 and 1996. Mir represented the third generation of the Soviet Union's space station programme following the success of the Salyut space station programme. It had a entirely new docking system with six ports that enabled the creation of a far more complex space station. The space station was capable of receiving Soyuz-TM spacecraft, unmanned cargo craft and modules carrying equipment and supplies.
Major components of the Mir space station include living quarters for the crew, an assembly compartment housing rocket engines and fuel tanks, an intermediate compartment used to connect the working compartment to the rear docking port, a transfer compartment that allowed attachment of additional station modules, a docking module housing ports for Space Shuttle dockings, progress cargo craft carrying supplies from Earth and removing waste material from the space station, Soyuz spacecraft to ferry the crew, a Kvant-1 astrophysics module housing telescopes, a Kvant-2 scientific and airlock module housing equipment for scientific research, a Kristall technological module used for biological and material processing experiments and also containing a docking port for the US Space Shuttle, a Spektr module used for studying the Earth's atmosphere and monitoring its natural resources and a Priroda remote sensing module housing radar and spectrometers. Figure 16.17 shows a photograph of the Mir space station as observed from STS-89 flight of Space Shuttle Endeavour.
Figure 16.17 Mir space station (Courtesy: NASA)
The Mir space station remained in orbit for 5519 days, including 4592 manned days, during the 10 year period it was operational. It served as a permanent research station providing a microgravity environment for research activities. The Mir space station was equipped with a whole range of scientific equipment, as outlined in the previous paragraph, to perform a wide range of scientific experiments and studies in the fields of astronomy, aeronomy, meteorology, biology and physics that need to be developed for long term sustenance in space.
The Mir space station was damaged by an onboard fire in 1994. Subsequent to this incident the Russian space agency found it hard to afford its maintenance. In February 2001, the Mir space station was de-orbited. It re-entered the atmosphere on 23 March 2001 and was burned, with its debris falling over the South Pacific Ocean in Eastern Australia, marking the end of the Mir space station.
16.5.3 Currently Operational Systems
The following paragraphs discuss currently operational space stations, including the US ISS and Tiangong-1 of China.
16.5.3.1 International Space Station
The ISS is the result of former US President Ronald Reagan's proposal in 1984 to build a permanently inhabited space station jointly with other countries. The USA, in order to meet the enormous expenditure involved in building such a massive station, joined with 14 other countries, with NASA leading the coordination efforts for the construction of the ISS with participating nations, who included Brazil, Japan, Canada and 11 members of the European Space Agency (ESA), namely the UK, France, Germany, Belgium, Italy, the Netherlands, Denmark, Norway, Spain, Sweden and Switzerland. Russia joined the consortium in 1993 after the fall of the Soviet Union. Spearheaded by NASA, the ISS mission is a joint programme of five space agencies including NASA, Roskosmos, JAXA, ESA and CSA.
The construction work on the US$ 60 billion ISS programme began in 1998 with the launch of the first module, called the Zarya cargo module, on 20 November 1998 aboard the Proton rocket from the Baikonur Cosmodrome in Kazakhstan. This was followed by the launch of the first US built module for the ISS, named Unity and also called Node-1, on 4 December 1998 onboard the STS-88 flight of Space Shuttle Endeavour. It mated with the in-orbit Zarya module on 6 December 1998. Unity was one of the three connecting modules, the other two being Harmony and Tranquility. Harmony, also called Node-2, was launched on 23 October 2007 onboard the STS-120 flight of Space Shuttle Discovery from the John F. Kennedy Space Centre. The third module, named Tranquility or Node-3, was launched onboard the STS-130 flight of Space Shuttle Endeavour on 8 February 2010 from the John F. Kennedy Space Centre. A multi-purpose laboratory module with a European Robotic Arm (ERA) is scheduled for launch in 2015 onboard the Russian Proton rocket. Between 1998 and 2013 a large number of missions were carried out to build the ISS bit by bit. Some of the prominent modules launched during this period included the Zvezda service module in 2000, the Destiny laboratory module in 2001, ESA's Columbus laboratory in 2008, and multipurpose modules Leonardo and Robonaut in 2011. The funding for the ISS programme is available until 2020 and the project is likely to continue until 2028. Figure 16.18 shows a recent view of the ISS.
Figure 16.18 Recent view of the International Space Station (Courtesy: NASA)
The ISS has been in low Earth orbit maintaining an altitude between 330 and 435 km and inhabited on a continuous basis for the 13 years since the arrival of the first expedition on 2 November 2000. The space station is serviced by Soyuz spacecraft used to transport crew between Earth and the space station, Progress spacecraft to carry fuel and other supplies to the space station, automated transfer vehicles (ATVs) designed to supply the space station with propellant, water, air, payloads and experiments, and also reboost the space station to a higher orbit, H-II transfer vehicles used as a unmanned resupply spacecraft, the Dragon spacecraft also designed as a cargo spacecraft and Cygnus spacecraft designed to transport supplies.
The ISS serves as an in-space research laboratory providing a microgravity environment on a long term basis to perform a variety of scientific experiments in physics, biology, materials science, meteorology, Earth observation and astronomy. It is also an ideal platform for testing spacecraft systems and other equipment needed for planetary missions.
16.5.3.2 Tiangong-1 Space Station
Tiangong-1 is the first of the series of three Chinese space stations with the ultimate goal of setting up a large modular manned space station. Tiangong-1 was launched on 29 September 2011 onboard the Chinese Long March-2F launch vehicle from the Jiuquan Satellite Launch Centre. Tiangong-1 is intended to be both a manned space laboratory and an experimental test bed to demonstrate docking and rendezvous capabilities. The space station consists of three sections including the aft service module, a transition section and the habitable orbital module. The service module is based on Shenzhou spacecraft and provides electrical, environmental control and propulsion subsystems. The orbital module provides living and working space for the visiting crew. The transition section is an interface between the service module and the orbital module.
Unmanned docking capabilities were established first on 3 November 2011 and again on 14 November 2011 during an unmanned mission of Shenzhou-8 flight launched on 1 November 2011 on board the modified Long March-2F launch vehicle from the Jiuquan Satellite Launch Centre. Subsequent to this, a manned docking capability was established during the Shenzhou-9 mission in 2012. Shenzhou-9 was launched on 16 June 2012 on board the Long March-2F launch vehicle, from the Jiuquan Satellite Launch Centre. The first Chinese manned spacecraft docking and rendezvous was established on 18 June 2012 during the Shenzhou-9 mission. Manned spacecraft docking capability was established again during the Shenzhou-10 mission launched on 11 June 2013. Shenzhou-10, carrying three astronauts, docked with Tiangong-1 on 13 June 2013. The crew performed a series of experiments while onboard Tiangong-1 and the spacecraft returned to Earth on 26 June 2013. The Shenzhou-10 manned spacecraft mission was the last visitor that the Tiangong-1 was scheduled to receive during its mission. Tiangong-1 had a planned mission life of two years and is planned to be de-orbited in 2014. Tiangong-1 will be followed up by the launch of Tiangong-2 and Tiangong-3, which are scheduled for launch during 2015–2016. The Tiangong-3 mission will be followed by the launch of a full scale, multi-module space station in the early 2020s.
16.5.4 Planned Space Stations
The following paragraphs briefly discuss some prominent future space station programmes, including China's Tiangong-2, Tiangong-3 and the multi-module space station, the Bigelow Commercial Space Station of Bigelow Aerospace the Almaz commercial programme of Excalibur Almaz, the OPSEK programme of the Russian Space Agency and the commercial space station of Orbital Technologies of Russia.
16.5.4.1 Tiangong-2 and -3 and multi-module space station
Tiangong-2 was originally conceived as a back-up space laboratory to Tiangong-1. Subsequently, it was decided to have Tiangong-2 with an improved design featuring an orbital fuelling system, which will enable the space station to be refuelled by a cargo vehicle. The cargo vehicle will deliver both dry and wet cargo to the space station. This automated cargo spacecraft will be used to transport three types of cargo to the space station, including air, water and propellant for the maintenance of the station itself, food and other materials for the crew members on board the space station, and equipment for scientific experiments. The cargo vehicle may also be used to assist the space station for orbit maintenance. Tiangong-2 is also proposed to be used for testing a robotic arm, which will be used subsequently on future space stations. The spacecraft is scheduled to be launched in 2015.
Tiangong-3 is proposed to be a third generation project employing a modular space station concept. It is scheduled to be launched during 2015 following the launch of Tiangong-2 in 2013. Tiangong-3 is proposed to be visited by a number of unmanned and manned spacecraft missions. The space station is also proposed to provide continued habitation of a crew of three astronauts for as many as 40 days, a multi-docking and berthing mechanism enabling simultaneous docking of up to four spacecraft and a platform for testing regenerative life support technology. Docking and berthing mechanisms are used to connect one spacecraft to another spacecraft or space station. In a docking mechanism, one of the spacecraft uses its own propulsion to manoeuvre and connect to the other spacecraft. In the case of berthing, a robotic arm is used for the final few metres of rendezvous. The design of Tiangong-3 will also form the basis of China's multi-module space station, scheduled for launch during the early 2020s. The proposed multi-module space station will primarily comprise a core cabin module (CCM) analogous to the Mir core module, two laboratory cabin modules (LCM-1 and LCM-2) to be used to perform scientific experiments under microgravity conditions, a robotic resupply craft to be used to transport supplies and other resources to the space station, and a manned Shenzhou spacecraft to be used to ferry crew members between Earth and the space station.
16.5.4.2 Bigelow Commercial Space Station
The Bigelow Commercial Space Station is currently under development at a space technology start-up company in North Las Vegas, Nevada, USA. Bigelow Aerospace has already announced the development of Commercial Space Station (CSS) Skywalker, Space Complex Alpha and Space Complex Bravo. CSS Skywalker comprises multiple BA330 habitat modules. Multiple modules would be inflated and connected on reaching orbit. Space Complex Alpha (Figure 16.19) is scheduled for launch during 2014. Space Complex Alpha comprises two BA330 modules and seven nations, including the UK, the Netherlands, Australia, Singapore, Japan, Sweden and the United Arab Emirates will be using the on-orbit facilities of the commercial space station. Bigelow Aerospace has also announced the launch of another commercial space station called Space Complex Bravo in 2016. Commercial operations of Space Complexes Alpha and Bravo are scheduled to begin in 2015 and 2017, respectively.
Figure 16.19 Space complex Alpha (Courtesy: NASA)
16.5.4.3 Almaz Commercial Programme
The Almaz commercial programme of Excalibur Almaz Inc. is intended to provide safe, reliable and competitively priced space transportation services including cargo and crew delivery and return to low Earth orbit, human space flight, microgravity experimentation and trans-lunar trajectory operations using existing flight-tested hardware components. The hardware proposed to be used consists of proven Russian space modules and space stations. The first launch is scheduled for 2015.
16.5.4.4 OPSEK (Orbital Piloted Assembly and Experimental Complex)
OPSEK (Orbital Piloted Assembly and Experimental Complex) is a third generation modular space station from Russia with scheduled launches of various modules between 2010 and 2020 from the Baikonur Cosmodrome. It will be placed in a low Earth orbit at an altitude of 370–450 km. The OPSEK space station would provide a platform to assemble components of manned interplanetary spacecraft and launch missions to the moon, Mars and other planets. The OPSEK space station is also proposed to be used for recovery of crew on such interplanetary missions before they are transported to Earth.
16.5.4.5 Commercial Space Station
Two Russian aerospace companies named Orbital Technologies and RSC Energia have teamed up for the development of an orbital space station for the commercial market. This orbital space station, called the Commercial Space Station, is proposed to be launched in a low Earth orbit with an altitude of about 350 km during 2016. This new space station, rightly called a space hotel, is proposed to have enough space to house seven passengers in four cabins with huge windows to view the turning Earth beneath. In addition, the services will also be open to private and state spaceflight exploration missions. Figure 16.20 shows a photograph of the commercial space station.
Figure 16.20 Commercial space station
16.5.5 Emerging Space Station Concepts
The process of space station design and development has been continuously evolving in the last four decades beginning with the era of Salyut-and Skylab-like monolithic space stations built on Earth and launched as one unit into space and then moving onto modular space stations such as ISS of USA built by launching individual modules and assembling them together in space. There is still a long way to go before the ultimate objective of having much larger habitable space stations and space colonies becomes a reality. While space tourism, close to being a reality, is one small but significant step in that direction, a number of technologies would need to be developed and perfected before people in large numbers could actually think of living and working in space. For example, space stations would need to have artificial gravity. For this to happen, placing the space station at Larange points where Earth's gravity and the moon's gravity counterbalance each other could be a possible solution. Similarly are many more areas related to construction of large space stations and transportation of people between Earth and space in which considerable work needs to be done. According to an estimate by a company likely to offer space travel on a commercial scale, the journey per person to a space station and a stay there for five days would cost approximately US$ 1 million. The following paragraphs briefly present some emerging space station concepts. Some of them have already been put to use in recent years and are evolving, and some are still on the drawing board.
A recent concept that has allowed the construction of large space stations is that of modular space stations. In the case of modular space stations, the space station is broken down into a core module and various other modules. The modules are launched individually and are then assembled in space over a period of time that could even spread to several years. Tiangong-2, Tiangong-3 and the proposed multi-module space station of China and also the US ISS are all examples of modular space stations. Advances in launch vehicle technology will allow bigger modules to be launched in future, thereby enabling construction of much bigger space stations.
The use of space tethers, though unconventional, is another emerging concept which in addition to performing tasks of orbit stabilization, orbit re-boost and de-orbiting would also make it very convenient to transport materials and personnel to space with ease. Space tethers are discussed in section 16.2. Space elevators, based on the space tether concept and discussed in section 16.2.4, are expected to make space transportation a daily affair in the coming years.
The concept of inflatable space modules will allow the launch of space modules having a bigger inner volume for a given mass thereby providing greater living space. An inflatable module will be similar to conventional cylindrical modules but with an inflatable exterior shell. This exterior shell will be made to inflate with a breathable atmosphere within once the module is in orbit. The inflated module dimension will be much larger than the dimensions of the module on Earth at the time of launch thereby providing extra living space. The TransHab concept tried by NASA for a possible manned mission to Mars is an example. TransHab design is being further developed by Bigelow Aerospace for their commercial space station programme. The concept was first tested by them in their Genesis-I module, launched on 12 July 2006. It was the first expandable space habitat technology in orbit. Genesis-I was followed by the launch of Genesis-II (Figure 16.21) on 28 June 2007. The concept of an inflatable module is proposed to be used in Space Complex Alpha of Bigelow Aerospace.
Figure 16.21 Genesis-II space module (Courtesy: Bigelow Aerospace)
Another futuristic concept that can significantly contribute to realization of the dream of space colonies is the concept of space docking. The concept of a space dock originates from dry docks used for the construction and maintenance of ships, boats and water craft. A dry dock is a narrow vessel that allows a load to be floated in when flooded and then made to come to rest on a dry platform when drained. Space docks can be used for the construction and maintenance of space ships. A large space dock would make it possible to carry out in-orbit assembly and maintenance of large space stations. It would also allow enclosed maintenance of smaller spacecraft. In the absence of any dedicated in-orbit construction and maintenance facility, one would need to frequently lift materials from the gravity well while engaging in the construction of a large space station or make a planetary landing for the purpose of maintenance. Such an in-orbit repair facility would also be an important asset in the case of damaged spacecraft, which pose a serious risk during re-entry into the atmosphere.
The concepts briefly described in the previous paragraphs in this section are the ones that have either been realized or are close to being realized. For technologies that are not mature enough to use yet, significant progress has been made in their development towards achieving the intended objectives. In addition to these concepts, many more have been proposed and explored by science fiction writers and scientists all over the world. A large number of these concepts are proposed with the objective of achieving conditions for long term or permanent habitation of space. Overcoming weightlessness is the key requirement. Use of a rotating wheel as proposed by the Austro-Hungarian scientist Hermann Noordung in his 1929 book for his geostationary space station appears to be a simple solution to solving the complex problem. The centrifugal force associated with the rotating wheel sets up artificial gravity, which overcomes the problems of long term habitation in space due to weightlessness. The concept design for the building of the Cultural Centre of European Space Technologies has been derived from Noordung's habitation wheel. The concept of a spinning wheel has been extended further in the Stanford Torus proposal of Stanford University that examines how to build a permanent and self-sufficient orbital habitat. Some of the other well known spinning space station concepts have proposed the use of cylindrical (e.g. the O'Neill cylinder) or spherical (e.g. the Bernal sphere) structures.
Further Reading
- Edwards, B.C. and Westling, E.A. (2003) The Space Elevator: A Revolutionary Earth-to-Space Transportation System, BC Edwards.
- Ippolito, L.J. (2008) Satellite Communications Systems Engineering, John Wiley and Sons Ltd, Chichester.
- Kao Cheng, H. and Zhaocheng, W. (2011) Millimeter Wave Communication Systems, Wiley-IEEE Press, New Jersey.
- Mario, L., Cosmo, E. and Lorenzini, C. (1997) Tethers in Space Handbook, NASA Marshall Space Flight Centre.
- Pelton, J.N. and Madry, S. (2013) Sergio Camacho-Lara, Handbook of Satellite Applications, Springer, New York.
- Rappaport, T., Heath, R., Daniels, R. and Murdock, J. (2014) Millimeter Wave Wireless Communication, Prentice-Hall, New Jersey.
- Tashirogi, T. and Yoney Ama, T. (2000) Modern Millimeter Wave Technologies, IOS Press, Amsterdam.
- Van Pelt, M. (2009) Space Tethers and Elevators, Copernicus/Praxis Publishing, New York.
Internet Sites
- www.tethers.com
- www.nasa.gov/centers/marshall/capabilities/space_tethers.html
- www.spacetethers.com
- www.microwavejournal.com/.../1594-millimeter-wave-applications
- www.scifiideas.com/sfi/technology/10-space-station-concepts
- science.howstuffworks.com/space-station6.htm
- www.loeacom.com/L1104-WP_Understanding%20MMWCom.pdf
Glossary
- Aerostat:
- Aerostat is an aircraft, balloon or dirigible, deriving its lift from the buoyancy of the surrounding air rather than from aerodynamic motion. The name aerostat is derived from the fact that it operates on aerostatic lift, also called buoyant force, and not aerodynamic lift, essentially requiring at least some part of the aerial body to move through the surrounding air mass.
- Airship:
- Airship is a lighter-than-air aircraft that is steered with the help of some thrust mechanisms such as rudders and propellers.
- Applications Technology Satellite-6 (ATS-6):
- This is one of the very ambitious experimental communications satellites developed and implemented by the NASA Goddard Space Flight Centre and intended to carry out a large number of scientific and technological experiments in the geostationary space environment.
- Atmospheric Tether Mission (ATM):
- An experimental mission proposed to be executed from the Space Shuttle in which a tethered probe would be lowered successively into different regions of the atmosphere such as the mesosphere, thermosphere and ionosphere for the purpose of understanding the atmosphere and plasma around the Earth in these regions.
- Bigelow Commercial Space Station:
- A commercial space station under development at Bigelow Aerospace.
- Commercial Space Station (CSS):
- A commercial space station of two Russian companies scheduled to be launched in 2016.
- Communications and Broadcasting Engineering Test Satellite (COMETS):
- Also known as KAKEHASHI, COMETS is a three-axis stabilized geostationary satellite of the National Space Development Agency (NASDA) of Japan.
- Electrodynamic tether:
- It is a long conducting cable extended from a spacecraft and kept stretched and oriented along the vertical direction by the gravity gradient force. An electrodynamic tether generates thrust through Lorentz-force interaction with the planetary magnetic field, which can be used to execute orbital manoeuvres without the need for carrying large quantities of propellant into orbit.
- Engineering Test Satellite-II (ETS-II):
- This was the first geostationary satellite that carried a beacon transmitter with three coherent frequencies at 1.7 GHz, 11.5 GHz and 34.5 GHz (millimetre wave frequency) to perform propagation experiments.
- Experimental communications satellites (ECS):
- Experimental satellites from NASDA intended to carry out communications and propagation experiments at millimetre wavelengths. Also known as AYAME satellites
- FLORAD mission:
- A flower constellation of micro-satellites of the Italian Space Agency carrying onboard scanning millimetre-wave radiometers for Earth and space observation on a regional and quasi-global scale.
- Helikite:
- One of the most commonly used aerostat designs for all-weather, high altitude applications. It is a combination of a helium balloon that uses helium gas for buoyancy and a kite that exploits wind for lift.
- Hybrid tether:
- A momentum-exchange/electrodynamic-reboost tether used to transfer payloads from low Earth orbit into medium Earth and geosynchronous orbits.
- Inflatable space station:
- A type of expandable space module that provides a larger inner volume for a given launch mass.
- International Space Station (ISS):
- A permanently inhabited modular space station developed jointly with 14 countries with NASA as the nodal centre.
- Millimeter wave band:
- Frequency band from 30 GHz to 300 GHz.
- Mir:
- First permanent space station owned by the former Soviet Union at the time of its launch and subsequently by Russia.
- Momentum exchange tether:
- Used to couple two objects in space in such a way that one can transfer momentum or energy to the other taking advantage of the gravity gradient force that exists due to differential gravitational forces between the two ends of the tether. It is this differential gravitational force, called the gravity gradient force, that keeps the tether taut and the two objects tied to the two ends of the tether pulled apart.
- Odin:
- An aeronomy and astronomy mini-satellite mission funded and executed by the Swedish National Space Board jointly with the space agencies of Canada, France and Finland.
- OEDIPUS:
- An acronym for Observations of Electric-field Distribution in the Ionospheric Plasma -a Unique Strategy, which is a joint program of the National Research Council of Canada and NASA.
- Orbital Piloted Assembly and Experimental Complex (OPSEK):
- A third generation modular space station from Russia.
- Plasma Motor Generator (PMG):
- An electrodynamic tether payload of a Department of Defence satellite launched on 26 June 1993 as a secondary payload onboard a Delta-2 rocket with the objective of testing the performance of hollow cathode assembly to provide a low impedance bidirectional electrical current and demonstrate the application of an electrodynamic tether for space propulsion and conversion of orbital energy into electrical energy.
- Rotating wheel:
- A futuristic concept that allows long term habitation of space without the problems of weightlessness.
- Skylab:
- First space station of the USA.
- Salyut:
- A monolithic space station put into space by the former Soviet Union
- Small expendable deployer system (SEDS):
- SEDS was developed by NASA's Marshall Space Flight Centre, which is primarily responsible for the development of transportation and propulsion technologies, and the Tether Application Company of San Diego.
- Space dock:
- A type of space facility that allows in-orbit repair and maintenance of space stations.
- Space elevator:
- Provides a concept for the development of a new space transportation system. The proposed concept has the potential to make transportation of tonnes of payloads and resources to geostationary Earth orbit and beyond an almost daily affair.
- Space tether:
- A long and strong cable usually made up of thin strands of high strength conducting wires or fibres and used to generate thrust or power for satellite stabilization and maintenance of spacecraft formations.
- Space Tether Experiment (STEX):
- STEX is a science and technology experiment of the Institute of Space and Aeronautical Science to be flown on board the Space Flight Unit (SFU) mission. SFU is an unmanned, multi-purpose reusable platform that can be used for performing a range of science and technology experiments and carrying out flight tests of space and industrial technologies.
- STEP-AIRSEDS:
- An acronym for Space Transfer Using Electrodynamic Propulsion – Atmospheric Ionospheric Research Small Expendable Deployer System. It is a mission satellite developed by Michigan Technic Corporation, USA weighing 1000 kg and comprises of a tether. It consists of two units tethered together by an electrodynamic tether approximately 6–7 km long.
- Tethered balloon:
- It is not free flying like a free balloon and its movement is restricted by anchoring it to a ground surface or a vehicular platform by a cable or a set of cables.
- Tether Physics and Survivability (TiPS):
- An experimental payload built and operated by the Naval Centre for Space Technology of the Naval Research Laboratory and launched on board the Titan-4 launch vehicle on 12 May 1996.
- Tethered Satellite System (TSS):
- A collaborative programme of NASA and the Italian Space Agency (ASI) with the objective of developing a reusable multi-disciplinary facility to conduct space experiments in Earth orbit.
- Tiangong Space Station:
- A series of three space stations, namely Tiangong-1, Tiangong-2 and Tiangong-3. Tiangong-1 is the first of the series of three Chinese space stations with the ultimate goal of setting up a large modular manned space station. Tiangong-2 and Tiangong-3 are to follow Tiangong-1.