There, the gravitational pull from the Sun and the planet are balanced by a trojan's tendency to otherwise fly out of orbit. The Jupiter trojans form the most significant population of trojan asteroids. It is thought that they are as numerous as the asteroids in the asteroid belt.
Near-Earth Asteroids: These objects have orbits that pass close by that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers. As a result, out there orbiting the Sun we have a giant space rock named for Mr. Spock — a cat named for the character of "Star Trek" fame. There's also a space rock named for the late rock musician Frank Zappa.
There are more somber tributes, too, such as the seven asteroids named for the crew of the Space Shuttle Columbia killed in Asteroids are also named for places and a variety of other things. The IAU discourages naming asteroids for pets, so Mr. Spock stands alone. Asteroids are also given a number, for example Apophis. The Harvard Smithsonian Center for Astrophysics keeps a fairly current list of asteroid names.
DS-1 flybys of asteroid Braille and Comet Borrelly. Stardust flyby of asteroid Annefrank and recent sample collection from Comet Wild 2. At the end of its mission it actually landed on Eros. The largest asteroid by far is 1 Ceres. The next largest are 2 Pallas , 4 Vesta and 10 Hygiea which are between and km in diameter.
All other known asteroids are less than km across. There is some debate as to the classification of asteroids, comets and moons. There are many planetary satellites that are probably better thought of as captured asteroids. Mars's tiny moons Deimos and Phobos, Jupiter's outer eight moons, Saturn's outermost moon, Phoebe, and perhaps some of the newly discovered moons of Saturn, Uranus and Neptune are all more similar to asteroids than to the larger moons.
The composite image at the top of this page shows Ida, Gaspra, Deimos and Phobos approximately to scale. Asteroids are classified into a number of types according to their spectra and hence their chemical composition and albedo:. Because of biases involved in the observations e. There are actually several classification schemes in use today. There is little data about the densities of asteroids.
But by sensing the Doppler effect on radio waves returning to Earth from NEAR owing to the very slight gravitational tug between asteroid and spacecraft, Mathilde's mass could be estimated. An asteroid about that large crashed into the earth 65 million years ago, killing nearly 90 percent of the animals, including the dinosaurs. Such major impacts are very rare events, but for smaller objects the likelihood of impact increases; the chance the earth being struck by an object approximately one kilometer in size is about one in 5, in a human lifetime that is, they strike every , years on average.
An object one kilometer across would still be large enough to cause a global disaster because of the enormous energy it would release upon impact, at least a million times the energy of the bomb dropped on Hiroshima in This problem and some possible solutions are discussed in a textbook I edited, Hazards due to Comets and Asteroids University of Arizona Press, , pp.
Posted August 5, Newsletter Get smart. Sign up for our email newsletter. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. This movement could have sent asteroids from the main belt raining down on the terrestrial planets, emptying and refilling the original belt.
Nearly all asteroids are irregularly shaped, although a few of the largest are nearly spherical, such as Ceres. They are often pitted or cratered — for instance, Vesta has a giant crater some miles km in diameter. The surfaces of most asteroids are thought to be covered in dust. As asteroids revolve around the sun in their elliptical orbits, they also rotate, sometimes tumbling quite erratically. More than asteroids are also known to have a small companion moon, according to NASA , with some having two moons.
Binary or double asteroids also exist, in which two asteroids of roughly equal size orbit each other, as do triple asteroid systems. Related: What can we do with a captured asteroid? The average temperature of the surface of a typical asteroid is minus degrees Fahrenheit minus 73 degrees Celsius.
Asteroids have stayed mostly unchanged for billions of years — as such, research into them could reveal a great deal about the early solar system. Asteroids come in a variety of shapes and sizes. Some are solid bodies, while others are smaller piles of rubble bound together by gravity.
One, which orbits the sun between Neptune and Uranus, comes with its own set of rings. Another has not one but six tails. Many asteroids also sport moons. They probably consist of clay and stony silicate rocks, and inhabit the main belt's outer regions.
They appear to be made of silicate materials and nickel-iron. The M-type or metallic asteroids are reddish in color, make up most of the rest of the asteroids, and dwell in the middle region of the main belt. They seem to be made up of nickel-iron. There are many other rare types based on composition as well — for instance, V-type asteroids typified by Vesta have a basaltic, volcanic crust. Ever since Earth formed about 4.
As asteroid capable of global disaster would have to be more than a quarter-mile wide. Researchers have estimated that such an impact would raise enough dust into the atmosphere to effectively create a "nuclear winter," severely disrupting agriculture around the world.
Smaller asteroids that are believed to strike Earth every 1, to 10, years could destroy a city or cause devastating tsunamis. According to NASA , space rocks smaller than 82 feet 25 m will most likely burn up as they enter Earth's atmosphere. On Feb. The space rock is thought to have measured about 65 feet 20 m wide when it entered Earth's atmosphere. When an asteroid, or a part of it, crashes into Earth, it's called a meteorite. Here are typical compositions:. Dozens of asteroids have been classified as " potentially hazardous " by the scientists who track them.
Some of these, whose orbits come close enough to Earth, could potentially be perturbed in the distant future and sent on a collision course with our planet.
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