Wikijunior:Solar System/Asteroid belt
The asteroid belt lies between the planets Mars and Jupiter. It contains lumps of rock and metal much smaller than planets. These lumps are called asteroids or minor planets. They are not visible from Earth with the naked eye, but many may be seen through binoculars or small telescopes.
|1 Ceres||933 km|
|4 Vesta||530 km|
|2 Pallas||525 km|
|10 Hygiea||407 km|
|511 Davida||326 km|
The largest asteroid in the solar system is called "2001 KX76." In the asteroid belt, the four largest bodies are Ceres (a dwarf planet named after the Roman goddess of agriculture), Vesta (the Roman goddess of the home), Pallas (the granddaughter of Poseidon), and Hygiea (the Greek goddess of health). These four bodies make up about half of the asteroid belt.
Some asteroids are less than a kilometer across. Unofficially the limit has been set at 50 meters, and anything smaller than that is going to be simply called a meteoroid. With advances in telescopes and particularly for objects that travel close to the Earth, some objects smaller than 50 metres have indeed been seen passing nearby the Earth.
There are probably several million asteroids in the solar system. Over 96,000 asteroids have been given numbers. Almost 12,000 of them have names. But even though there are a lot of asteroids, the asteroid belt is mostly empty space. Traveling through the asteroid belt in a space ship would not be very much like what you see in a science fiction film. This would not be the same in the first 100 million years of the solar system's history (had humans evolved and discovered film). The asteroid belt lost 99.99% of its mass in the first 100 million years of the solar system.
|The Solar System|
What are they named after?
The first asteroids were named after mythical heroes and gods much like the major planets. The first to be discovered was named Ceres after the Roman goddess of growing plants (particularly grain) and of motherly love. The second asteroid discovered was called Pallas and was named after one of the Greek gods of wisdom. Asteroids are also given a number in the order of their discovery, so Ceres is 1, Pallas is 2, and so forth.
As the number of known asteroids increased, they ran out of mythical names so other names were used instead. Some asteroids were named after countries. For example asteroid number 136 is named Austria. Others were named after plants, for example 978 Petunia. 1620 Geographos was named after the National Geographic Society, in recognition to their efforts at sharing knowledge about the Solar System. Many are named after people, both alive and dead. In a couple of cases, like 2309 Mr. Spock, asteroids were named after the discoverer's pet cat. This last type of naming is discouraged, but it still happens from time to time. Even fictional characters have been used.
Today, names for asteroids can be suggested by the people who discover them. The names become official after a group of people reviews them to make sure they are not offensive or too much like another name. Because so many asteroids are now being found, most new asteroid discoveries are not even getting a name at all, but rather a numbered code. It is not likely that they will ever be given a formal name, at least in this century.
The actual word Asteroid can refer to any member of the family of Asteroidea; in other words, a starfish. The etymology of the word comes from Ancient Greek ἀστεροειδής, literally meaning star form. The Ancient Greeks probably must have mistook the asteroids for stars.
Most of the asteroids have symbols representing them; for example, Ceres has ⚳, representing Ceres's scythe, and Pallas has ⚴, reperesenting Athena's spear.
Who discovered them?
The first asteroid to be discovered was Ceres, on 1 January 1801, by Giuseppe Piazzi accidentally. At first he thought this was a comet, then later a planet! When it was realised it was too small Sir William Herschel (the astronomer who discovered Uranus) made up the word "asteroid" to describe it, using the Ancient Greek word aster, meaning star, and the -oid, meaning form or shape. In other words a star-like planet, because he couldn't see any details due to the small size of the object.
By 1807 another 3 asteroids were discovered, but no more were found until 1845 when a persistent asteroid hunter named Karl Ludwig Hencke found a fifth, and sixth asteroid in 1847. Ever since then at least one new asteroid has been found each year.
In 1891, the first pictures of the night sky were taken to find more asteroids. This led to the discovery of many more asteroids. A picture of the same part of the sky is taken on two different nights. When the two pictures are lined up, the stars will be in the same places but an asteroid will have moved.
In our modern times, over 280,000 asteroids have been discovered. Many more are being found all the time. Some of these asteroids pass near the earth and astronomers want to find any that come close to our planet. Large numbers of asteroids are now being discovered by machines.
What are they made of?
Three out of four asteroids are made of rock that is rich in carbon. The rest are made of the metals iron and nickel. About half of these are pure iron and nickel; the rest are mixed with compounds of silica, the element that makes up rocks. Each of the larger metal asteroids contains huge amounts of iron: much more than is mined every year on Earth. Vesta, the brightest asteroid, has a very unusual composition - being made of high-density volcanic rocks.
Scientists are very interested in what asteroids are made of because it can help them learn how the solar system was formed. Several spacecraft have visited asteroids to learn more about them.
Are there asteroids outside of the asteroid belt?
Most asteroids are found in the asteroid belt, but not all. Some asteroids orbit closer to the Sun, and many asteroids orbit beyond Neptune. Asteroids that closely approach Earth are called Near-Earth Asteroids. Sometimes pieces of asteroids strike the Earth, burning in the atmosphere as a meteor. If they are large enough, they might actually hit the surface and become meteorites.
A centaur is a mythical beast having a horse's body with a man's head and torso in place of the head and neck of the horse. There are some asteroids in the outer solar system that are called Centaurs. It is hard to tell whether any one Centaur is an asteroid, comet, or Kuiper Belt object. For example, the first Centaur to be discovered was Chiron(In Greek Mythology, an important centaur). But some scientists think it is a comet, not an asteroid. Officially it is called both the asteroid 2060 Chiron and the comet 95P/Chiron!
In most cases when an asteroid's orbit crosses the path of a planet such as Jupiter, at some point the asteroid will either hit the planet or else be hurled into another orbit. Many of the small moons of some planets may have once been asteroids that were captured by the planet's gravity when they came too close. However there are two points along the orbit of a planet were an asteroid can safely linger. These are found at a point one-sixth (or 60°) of an orbit ahead of the planet, and the same distance behind the planet. These sites are called Lagrange points, and they are found where the force of gravity from the Sun and the planet balance out with the motion of the asteroid's orbit. The asteroids found in these spots are called Trojans, and they move around the Sun at the same velocity as the planet.
Ceres is a large Dwarf Planet like Pluto located in the asteroid field. The difference between Ceres and most other dwarf planets is that it is also an asteroid. Ceres is one of the few asteroids that are shaped like a sphere, and it may have some ice just underneath its dusty surface.
Some famous asteroids
Ceres the first discovered and biggest asteroid in the solar system.
The asteroid Ida has its own moon named Dactyl. A number of asteroid moons have now been found.