Origins of Meteors

“To wish upon a shooting star…” evokes romance, nostalgia and hope, all wrapped up into one. If you have ever been fortunate enough to witness a “shooting star,” you will likely recall your unexpected thrill at witnessing its fiery trail, brilliantly burning in and out of existence. So exactly what is a “shooting star,” and how can we increase our chances of seeing and photographing one of them?

7.1 (a) A lone Perseid meteor streaks across the sky over Badlands National Park, South Dakota, during the 2015 Perseid meteor shower. Care must be taken to distinguish meteors from (b) Iridium flare satellites and (c) the International Space Station. Other objects commonly mistaken for meteors are: satellites and airplanes, as can be seen in Figure 20.6.

To be clear, “shooting stars” aren’t stars at all. They’re small, interplanetary masses of rock and/ or ice drawn into the earth’s atmosphere through mutual gravitational attraction. They are generally much less than a kilogram in mass; most are far smaller. As they pass through the atmosphere, the particles collide with atmospheric gas molecules. These repeated collisions generate heat, just like the heat produced by vigorously rubbing the palms of your hands together. The extremely high speed of the masses results in enough collisions for the temperature of the masses to rise to the point of incandescence. Once this occurs, they become visible to observers on the ground, and become meteors.

The intense heat generated through friction generally causes the meteors to vaporize completely in mid-air, at which point they vanish. If, however, any remnant of the mass survives its descent and reaches the ground, it is designated a meteorite. If the meteor approximately matches Venus in brightness, it is termed a fireball. Fireballs often leave smoke trails visible from the ground owing to their relatively large size; some smoke trails can remain visible for many minutes. The smoke trails can even be briefly illuminated internally by the fireball’s heat! Finally, if the mass of the meteor is high enough, it may ultimately explode, at which point it is known as a bolide.

Comets and Meteor Showers

Most meteors are caused by the collision of the earth with single, lone dust particles randomly drifting through the solar system. Meteor showers, on the other hand, are clusters of meteors that occur on a single night, or over the course of just a few days. What causes them? You may be surprised to learn the meteor showers result from the collisions of the earth with the multiple pieces of rubble left behind in space by comets!

7.2 Illustrations of the relationship between comets and meteor showers. (a) Comets deposit dust and debris fields left behind in their path. When the earth intersects these debris fields, the dust particles become meteors.; their unusually large numbers result in meteor showers. Since the earth encounters the same debris field on the same date each year, meteor showers are very predictable. (b) Illustration of how a comet’s ‘tail’ always points away from the sun, regardless of its orbital position. The result that the tail can sometimes point in the direction of motion has led to the phrase, “like a comet chasing its tail.”

Comets are masses of frozen gas and ice that orbit the sun on well-established trajectories. They originate from regions of the Milky Way beyond the edges of the solar system. Comets are much larger, up to a mile or so in diameter, than the smaller pieces of matter that produce most meteors. They continuously eject gas along with small particles of matter as they pass through space, Figure 7.2(a). These mass discharges do not propel the comet along its path; rather, the comet’s inertia is solely responsible for its movement through space. They are known, however, to cause the comet to tumble and spin as it moves along its path.

As a comet comes into proximity with the sun, the sun’s heat causes the particles to “boil” from the comet substantially, akin to steam rising from boiling water. The particle ejection density thus increases as comets near the sun. The especially concentrated stream of particles immediately adjacent to the comet’s body, known as the comet’s tail, becomes visible from Earth whenever it reflects sunlight. Comets can, therefore, be quite bright, and visible even within dense metropolitan areas, as seen for the Hale-Bopp comet emerging over the New York City skyline in Figure 7.3.

7.3 “World Comet: Comet Hale Bopp visible above the United Nations Headquarters and the New York City Skyline. The variable star Algol is the bright star visible above Comet Hale-Bopp.” Dr. Donald Lubowich (Photographer)

Source: Donald Lubowich/The World At Night

The solar wind pushes the discharged particles in a direction directly away from the sun, regardless of the orientation of the comet’s movement, Figure 7.2(b). The directions of the comet’s movement and its tail, therefore, are generally misaligned. In fact, the two can be almost diametrically opposite, leading to the phrase, “like a comet chasing its tail,” as shown schematically in Figure 7.2(b).

After being ejected, the discharged particles remain suspended in space and form an enormous, permanent path of dust, or debris, like interplanetary breadcrumbs. This debris field surrounds the sun, along the comet’s trajectory, Figure 7.2(a). In a few special cases, the comet’s trajectory, and hence debris field, intersects the path of the earth’s orbit. When the earth reaches these points annually along its journey around the sun, multiple collisions between the earth and the debris particles occur and result in meteor showers. Most meteor showers only last a few days, since it only takes that long for the earth to pass completely through most debris fields. However, meteor showers always occur around the same dates each year, since those are the dates when the earth reaches the same orbital position and thus encounters the same debris field. Fortunately, there are no foreseeable instances when the earth and an actual comet are both predicted to be at the same position in their respective paths, i.e. to collide!

Meteor Showers—When and Where they Occur

The dates of each meteor shower are determined by the dates when the earth’s orbit crosses the debris field of the responsible comet, Figure 7.2(a). These intersection points lie at slightly different positions for each comet relative to the direction of the earth’s motion in its orbit. In each case, however, meteor showers appear to originate from the region of the night sky that corresponds to the primary intersection point between the earth’s orbit and the comet’s trajectory. This point is named the meteor shower radiant point. The constellation that hosts the radiant point is used as a helpful reference for the meteor shower: the radiant point of the Orionid shower is thus found within the constellation Orion; for the Perseid shower in Perseus, and so on. The major meteor showers and their annual dates are summarized in Table 7.1.

Meteor showers are so-named because of the relatively high concentration of meteors that occur compared to the rest of the nights during the year. Most meteor showers produce between several to dozens or more meteors per hour during a good show. On rare, historical occasions, much higher meteor rates have been noted; up to hundreds and even thousands per hour; wouldn’t that be a wonderful sight to see!

Best Times to Observe Meteor Showers

The best time to observe meteor showers is generally between midnight and dawn. Why? First, you may recall from the previous chapter that the direction of the earth’s rotation around its axis is in the same direction as its orbit around the sun, as seen in the top view of the earth shown in Figure 7.4. Therefore, the part of Earth whose local time is past midnight is both experiencing night and facing directly into the oncoming dust cloud. In contrast, the region of Earth whose local time is before midnight is facing away from the oncoming dust cloud; similar to looking out the rear window of a moving car. Consequently, observers on the side of the earth experiencing night after midnight have the best chance to observe the greatest number of meteors.

7.4 Illustration of why the best time to view meteor showers is after midnight. The part of Earth whose local time is past midnight is both experiencing night and facing directly into the oncoming dust cloud. In contrast, the region of Earth whose local time is night, but before midnight, is facing away from the oncoming dust cloud.

The phase of the moon is very important in determining the condition of observing meteors in general, and meteor showers in particular. The best moon phases are those that result in the darkest skies, and thus are typically immediately adjacent to the new moon: the waxing and waning crescent moon. Coupled with your knowledge of when these moon phases occur, and when it is visible during the night, you can conclude that the optimum dates and times for viewing meteor showers are between midnight and dawn on the later dates of the waning crescent moon up to, including, and shortly after the date of the new moon.

Best Meteor Shower Viewing Direction

The radiant is generally not the best direction to view meteors during a meteor shower, contrary to popular belief. A better direction is roughly between 30° to 150° away from the radiant, in order to see meteors from the “side” as they streak through the atmosphere. The paths of the meteors generally appear longer from this perspective, and thus more prominent. In fact, while the greatest density of meteors does appear to originate from the direction of the radiant, they can appear anywhere in the sky.

Asteroids

In contrast to the primarily icy composition of comets that originate from beyond the solar system, asteroids are relatively large masses of solid rock and generally lie between Mars and Jupiter. They can be hundreds of miles across—some as large as the state of Arizona! However, they are not large enough to be considered dwarf planets. Asteroids orbit the sun in much more circular orbits than comets, but their orbital planes are typically much different than the generally aligned orbital planes of the planets we saw earlier.

If an asteroid were to collide with the earth, the consequences would be severe. Indeed, the prevailing theory for the cause of the extinction of the dinosaurs originates from a direct impact between an asteroid and the earth around 65 million years ago. Fortunately, no asteroids are known to be anywhere near a direct collision with the earth at this time!