THE OORT CLOUD, from which Comet ISON emerged, and the Kuiper Belt, from which Halley’s Comet emerges, are shown. Wikimedia Commons/NASA graphic
Comet ISON, first spotted by Russian astronomers in September 2012, broke up while passing too near the sun this Thanksgiving. After billions of years of traveling through space, it now consists of little more than scattered dust and rocky debris.
While disappointing for viewers on Earth, scientists monitoring the comet’s course and eventual destruction obtained abundant information for future study. For my part, I decided to finally learn the difference between comets, asteroids and meteoroids.
Comets are frozen balls of dust, rock and ice usually less than six miles across, leftover from our solar system’s formation 4.6 billion years ago. Most of the ice consists of frozen water, carbon dioxide, ammonia and methane. Comets striking the early Earth would have added these organic compounds and water to the planet, perhaps contributing to the eventual development of life.
Short-period comets, like Halley’s comet, pass by the sun every 200 years or less and mainly come from the Kuiper belt beyond Neptune, some 50 astronomical units (AU) from the sun (one astronomical unit measures about 93 million miles, the average distance between the Earth and sun). Comet ISON, one of the long-period comets whose orbits take up to millions of years, came from the Oort Cloud, over 50,000 AU beyond the planets.
When a comet enters the inner solar system, its ice begins to vaporize and forms a surrounding cloud of gases that glow brightly in the sunlight. If the orbit comes too close to the sun, the heat and strong gravity can violently shatter the comet, leaving behind only residual dust and small rocks as happened with comet ISON.
Asteroids, like comets, consist of debris left over from the formation of the planets. But instead of icy balls, they are made of dense rock, compact rubble and metals such as nickel and iron. Asteroids number in the millions, ranging from a few feet to almost 600 miles in diameter. The largest generate enough gravity to have small moons or travel in interlocked pairs.
Asteroids follow regular orbits around the sun and are located in the inner solar system. Most are found in the “asteroid belt” between Mars and Jupiter. Because their orbital patterns are in synch with the planets, they don’t usually represent collision threats. However, asteroids can be affected by the gravity of other bodies in space and their orbits shifted to intersect Earth’s.
Despite the phrase “empty space,” the solar system contains plenty of rocks and debris. These are meteoroids, the remains of asteroid collisions, comets and planets forming. Earth collides with up to 100 tons of this rubble daily, fortunately mostly dust or sand grain sized particles. Such small objects burn up completely in the atmosphere, creating the streaks of light we know as meteors or shooting stars. The Perseid meteor shower occurs every August when Earth passes through the orbital path of Comet Swift-Tuttle on its 130 year circuit through the solar system.
Any part of a larger meteoroid that survives to reach the Earth’s surface is called a meteorite. About once a year a meteoroid the size of a car or larger strikes the Earth, creating an impressive fireball like seen on dash-cams in Russia in February. The Chelyabinsk object measured nearly 60 feet in size, and caused ground damage for over 50 miles around the area of the strike.
Roughly every thousand years a larger meteoroid, up to 1,000 yards across, reaches the Earth and causes extensive regional damage (search “Tunguska event”). Every million or so years much larger meteoroids or asteroids have struck Earth, with worldwide effects including hastening the end of the dinosaurs.
This risk has led to the NASA-sponsored Near Earth Object Program, which detects and monitors meteoroids, asteroids and comets that could potentially threaten the planet. Over 10,000 NEOs have already been identified, nearly a thousand of which are over half a mile in diameter. They have prompted scientific debate about how to avert possible collisions — but that’s a topic for another day.
What began as a simple online search about comets and asteroids soon had me reading ideas on the formation of the solar system, origin of life on Earth and events that could threaten its future. Just another example of how following science can be so fascinating.
Lifelong Oregonian Fred Schubert, a The Dalles biologist, has a lifelong interest in general science and science writing. Feel free to submit any comments on this article or suggestions for new topics to firstname.lastname@example.org.