Introduction to Comets, Asteriods, and Meteors
Eight known substantial bodies orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Since its discovery, a ninth object, the Pluto-Charon system, also has been considered as a planet. If there are other planets in the Solar System besides these, they have thus far evaded the observation of watchful amateur and professional astronomers.
The planets, along with the Sun and our own Moon, capture most of the visual attention of hobby astronomers. Saturn with its rings, Mars with its ruddy glow, and Jupiter with its visible moons and surface features are perennial favorites. However, insofar as the fate of humanity is concerned, these planets have no direct effect. They stay in their orbits, and we stay in ours. Jupiter, even with its powerful magnetosphere and ferocious radiation belts, does not pose a threat to us here on Earth. Mars, named after the ancient war god, has not sent any living beings to maliciously attack civilizations on Earth (or on any other planet), although there is evidence that rocks have landed here that had their origins on Mars.
The Small Stuff
There are celestial objects that present a threat to life on Earth or, perhaps better stated, have had and will continue to have a profound effect on the way life evolves here. They are pipsqueaks in the Solar System, none of which can be recognized without a telescope and many of which remain unknown to this day. They are comets, asteroids , and meteoroids , remnants of the primordial Solar System. They are leftovers that did not congeal into the planets. Originally, according to the most popular theories, the entire Solar System consisted of objects like them.
Clouds Of Comets
Comets are one of the greatest mysteries in astronomy. They have awed and terrified people since the beginning of recorded history. Even today, some people think comets have supernatural characteristics. Yet comets are, as one astronomer has said, more numerous than fishes in the sea. They are, in the long term, more dangerous too.
A large comet smashing into the Earth would be, for the human race, the equivalent of a shark attack on an individual human being. However, the risk shouldn’t be overblown. It is not necessary to lose sleep worrying about what is going to happen when the next major comet comes down. We can say with confidence that such an event will take place, but we can’t say when, other than to note the fact that the time intervals between massive impacts are measured, on average, in tens of millions of years.
Beyond the orbit of Neptune, the Kuiper belt consists of comets that orbit the Sun at distances from 30 to 50 astronomical units (AU) (Fig. 11-1). The existence of this zone, which circles the Sun in more or less the same plane as the planets, was only a hypothesis until 1992 when the first object there, a large comet, was positively identified. More objects were found after that. Objects found between 1992 and 1994 were all gigantic compared with the comets that have made history, such as Halley’s Comet, which last appeared in 1985–1986. Since 1995, some “normal sized” comets have been found in the Kuiper belt.
Figure 11-1. The Kuiper belt consists of comets that orbit the Sun beyond the orbit of Neptune, out to approximately 50 AU.
Surrounding the Kuiper belt and extending out to a distance of about 1.5 light-years (one-third of the way to the nearest star other than the Sun) is a vast spherical halo of comets called the Oort cloud . The exact dimensions of this cloud are unknown because of its great size and distance. Nevertheless, it is believed to contain millions upon millions of comets.
Normally, the comets in the Kuiper belt and the Oort cloud stay in their parts of the Solar System, the “suburbs” and the “surrounding countryside.” All of them would remain there too if it were not for the fact that our Milky Way galaxy is an unstable and chaotic place. Once in a while, a star comes close enough to the Solar System so that its gravitation has an effect on objects in the Oort cloud, and some of these comets are deflected into paths that take them into the inner Solar System. Some astronomers suspect that there is an as-yet unknown massive planet beyond Pluto that does similar things to comets in the Kuiper belt.
The Asteroid Belt
In 1772, a German mathematician named Johann D. Titius noticed that the orbits of the planets seemed to have radii, or average distances from the Sun, that fell into a neat mathematical progression. At that time, Saturn was the outermost known planet. Titius noticed that the radii of the planetary orbits, in astronomical units (AU), could be found by applying the progression. Later, the astronomer Johann E. Bode expounded on this number sequence, which has become known as Bode’s law .
The progression is defined as follows. Start with the number 3, and double each number, getting the sequence 0, 3, 6, 12, 24, 48, 96, and so on. Then add 4 to each of these numbers so that you get 4, 7, 10, 16, 28, 52, 100, and so on. Finally, divide each of these numbers by 10. This gives the following sequence:
S = 0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0, . . .
These numbers, with the conspicuous exception of 2.8, correspond almost exactly to the orbital radii of the planets Mercury (0.4 AU), Venus (0.7 AU), Earth (1.0 AU), Mars (1.6 AU), Jupiter (5.2 AU), and Saturn (9.5 AU). The question naturally arose among scientists: Is this significant? Did the planets form at these distances from the Sun for some physical reason? Today, most astronomers doubt that Bode’s law is anything more than a coincidence. Back in the eighteenth century, however, it was suspected that there was some modus operandi to this correspondence and that an undiscovered planet must lie 2.8 AU from the Sun. Why had it escaped notice?
A search was begun for the “missing planet.” In 1801, the Italian astronomer Giuseppe Piazzi found an object orbiting the Sun at the correct distance. However, it was small, certainly not large enough to be a planet. It appeared starlike, a mere point of light, but its motion relative to the distant stars gave it away as a resident of the Solar System. It was called an asteroid (meaning “starlike”) and was given the name Ceres . More asteroids were soon found, also orbiting the Sun at distances of approximately 2.8 AU. Ultimately, thousands were discovered, and they all orbit in or near the zone corresponding to the missing slot in the Titius-Bode sequence. This zone has become known as the asteroid belt .
Most astronomers in Piazzi’s time concluded that the asteroids, also called planetoids , were objects that had failed to congeal into a planet or else were the remnants of a planet that was shattered by a cosmic catastrophe. Today, the prevailing theory holds that these objects are part of the original cloud of boulders, rocks, and dust that surrounded the Sun in the earliest part of the Solar System’s lifespan and that the gravitation of Jupiter prevented their getting well enough organized to condense into a planet.
Most of the asteroids follow nearly circular paths around the Sun and orbit between Mars and Jupiter. However, some known asteroids follow orbits that take them inside the orbit of Mars, and others wander outside the orbit of Jupiter. A few maverick asteroids cross the orbit of the Earth and, every few million years, pass even closer to our planet than the Moon. There is plenty of evidence, in the form of visible craters, that asteroids have crashed into the Moon. Such craters are erased in time by erosion when they are formed on Earth, but a few craters have been found on our planet that strongly suggest that we, along with every other object in solar orbit, are on the “asteroid visitation list.”
Myriad Meteoroids
The largest known asteroid, Ceres, measures hundreds of kilometers in diameter. The smaller we go, the more asteroids there are, in general. There isn’t any definitive limit at which we say, “This thing is an asteroid, but if it gets 1 milligram less massive, then we’ll call it a meteoroid.” In a general way, we can say that if it’s too big to be called a boulder, then it’s an asteroid; boulders and rocks in space can be called meteoroids .
Even then, it’s not that simple. Meteoroids and comets are substantially different. Comets consist of rocky and icy material combined, but meteoroids have essentially no ice. Some meteoroids are stony, something like the granite we know on Earth. Others are mixtures of stony and metallic stuff. Still other meteoroids are mostly metal, largely iron. Some resemble pieces of amber or glass.
How do we know what meteoroids are made of? They’re too small to be observed through telescopes; space probes have never been sent specifically to visit them, although some of them have struck our space vehicles. We know about meteoroids because they often fall to Earth’s surface. Technically, when a meteoroid enters Earth’s atmosphere, it becomes known as a meteor . If a meteor survives to reach the surface, either as one object or as fragments of the original meteor, then it becomes a meteorite . Meteorites are abundant on the continent of Antarctica, where the ice preserves them, and where they are easy to differentiate from Earth’s surface.
There are believed to be more rock-sized meteoroids than boulder-sized ones, more pebble-sized ones than rock-sized ones, and more sand-grain-sized ones than pebble-sized ones. Then, as we keep getting smaller, we have to call them micrometeoroids . In the extreme, they can be called interplanetary dust or meteoric dust .
The asteroid belt between Mars and Jupiter is home to a huge number of “space rocks,” but there are plenty of such objects that orbit outside this zone. Some orbit the Earth instead of the Sun. It is believed that there are myriad rock-sized objects in orbit around each of the planets, as well as around the major moons of the planets, including our own Moon. There is also evidence that some rock-sized objects are entirely independent of the Solar System and that they come into our little corner of the Universe as a result of the movement of the Sun within the Milky Way galaxy. Technically, a space rock becomes a meteoroid when it attains a solar orbit such that it has the potential to be pulled into the Earth by our planet’s gravitation.
Practice problems of this concept can be found at: Comets, Asteroids, and Meteors Practice Problems
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