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Space Topics: Asteroids and CometsComet Facts
Comets are small, irregularly shaped objects composed of a mixture of rocks, dust, and what astronomers refer to as “ice” -- frozen water, methane, and ammonia. Most have highly elliptical orbits that bring them close to the Sun and then swing them deep into space, often beyond the orbit of Pluto. When a comet is far from the Sun, it is frozen solid into a tiny nucleus. But comets put on a spectacular show when they visit the inner solar system. As a comet gets closer to the Sun, the surface of its nucleus warms. Icy materials begin to sublime into gas. As the gas boils off the comet, it can take small particles of dust with it. The gas and dust forms a cloud of diffuse material, called a coma, that surrounds the nucleus. The coma can swell to many times the size of the nucleus. If the comet gets close enough to the Sun, the solar wind can blow gas and dust away from the comet, forming elongated and often multiple tails of gas and dust. The tail always points away from the Sun -- not away from the comet’s direction of motion. Like asteroids, comets are studied through spectral analysis of the light reflected and absorbed by the comet nucleus and the spray of volatiles and gas in the comet's coma. When the nucleus is frozen, it can be seen only by reflected sunlight, and many comets are extremely dark (low-albedo) objects. When a coma develops, dust reflects still more sunlight, and coma gases absorb ultraviolet radiation and begin to fluoresce. If a comet approaches to within 5 astronomical units (750 million kilometers or 465 million miles) of the Sun, fluorescence of tail gases can become an important contributor to the brightness of the comet, together with the light reflected from the nucleus and coma. Each time a comet swings into the inner solar system toward the Sun, it loses some of its ice component. The comet Halley is believed to lose one meter of its surface each time it approaches the Sun. Eventually, comets burn out, becoming just another rocky mass in the solar system. Some objects currently classified as asteroids are probably extinct comet nuclei. Comets are divided into two groups based on their orbital periods (the time it takes them to travel around the Sun). Short-period comets have orbital periods of less than 200 years. Their elliptical orbits can take them out to the Kuiper belt, but not beyond. Long-period comets have orbital periods longer than 200 years, often many thousands of years. Their elliptical orbits are so long that they are very difficult to differentiate from parabolas. The existence of long-period comets led to the prediction of the existence of the Oort cloud, a diffuse sphere of cometary nuclei, orbiting quietly at the farthest reaches of the Sun’s gravitational influence. Passing stars may occasionally disturb the bodies in the Oort cloud, sending some on trajectories that take them into the inner solar system. Comets are traditionally named after the discoverer or discoverers. Comets are now named with a “P” if they are periodic (that is, if they have been observed on more than one trip around the Sun), and a “C” if they are not. The first documented periodic comet, Halley, is properly named 1P/Halley. Many comets cross Earth’s orbit as they approach the Sun. These near-Earth comets may be potential hazards to Earth. Near-Earth comets are thus a subset of the near-Earth objects. The Planetary Society supports many programs and projects to identify and track near-Earth objects and mitigate the risk they pose to Earth. Why Study Comets?Life on Earth began at the end of a period called the late heavy bombardment, some 3.8 billion years ago. Before this time, the influx of interplanetary debris that formed Earth was so strong that the proto-Earth was far too hot for life to have formed. Under this heavy bombardment of asteroids and comets, the early Earth's oceans vaporized, and the fragile carbon-based molecules, upon which life is based, could not have survived. The earliest known fossils on Earth date from 3.5 billion years ago and there is evidence that biological activity took place even earlier -- just at the end of the period of late heavy bombardment. So the window when life began was very short. As soon as life could have formed on our planet, it did. But if life formed so quickly on Earth and there was little in the way of water and carbon-based molecules on Earth's surface, then how were these building blocks of life delivered to Earth's surface so quickly? The answer may involve the collision of comets with Earth, since comets contain abundant supplies of both water and carbon-based molecules. As the primitive, leftover building blocks of the outer solar system formation process, comets offer clues to the chemical mixture from which the giant planets formed some 4.6 billion years ago. If we wish to know the composition of the primordial mixture from which the major planets formed, then we must determine the chemical constituents of the leftover debris from this formation process -- the comets. Comets are composed of significant fractions of water ice, dust, and carbon-based compounds. Since their orbital paths often cross that of Earth, cometary collisions with Earth have occurred in the past and additional collisions are forthcoming. It is not a question of whether a comet will strike Earth, it is a question of when the next one will hit. It now seems likely that a comet or asteroid struck near the Yucatan peninsula in Mexico some 65 million years ago and caused a massive extinction of more than 75% of Earth's living organisms, including the dinosaurs. Comets have this strange duality whereby they first brought the building blocks of life to Earth some 3.8 billion years ago and subsequent cometary collisions may have wiped out many of the developing life forms, allowing only the most adaptable species to evolve further. Indeed, we may owe our preeminence at the top of Earth's food chain to cometary collisions. A catastrophic cometary collision with Earth is only likely to happen at several-million-year intervals on average, so we need not be overly concerned with a threat of this type. However, it is prudent to mount efforts to discover and study these objects, to characterize their sizes, compositions and structures, and to keep an eye upon their future trajectories. With visits to asteroids and comets, we can also learn about which strategies may prove effective in mitigating a possible future impact threat. As with asteroids, comets are both a potential threat and a potential resource for the colonization of the solar system in the twenty-first century. Whereas asteroids are rich in the mineral raw materials required to build structures in space, the comets are rich resources for water and carbon-based molecules necessary to sustain life. In addition, an abundant supply of cometary water ice can provide copious quantities of liquid hydrogen and oxygen, the two primary ingredients in rocket fuel. One day, comets may serve as fueling stations for interplanetary spacecraft.
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