The first asteroid was discovered on New Year's Day in 1801.
Giuseppe Piazzi discovered the object that he named Ceres. It was at the right distance from the Sun to be Bode's “missing planet”, so everyone accepted it as a new planet. The four biggest asteroids were often called planets until nearly the end of the 19th century.
Asteroids have also been called planetoids, minor planets and small solar system bodies.
When Pallas was discovered the year after Ceres, William Herschel thought it odd that two planets were in similar orbits. It seemed to him that they were a new type of object, and he suggested the name “asteroid” (starlike) because through a telescope they looked more like stars than planets. In 2006 the International Astronomical Union (IAU) officially classified them as “small Solar System bodies”. At the same time Ceres also became a dwarf planet.
Asteroids are some of the leftovers from making the Solar System.
Astronomers think the planets formed by accretion, which means matter clumping together into bigger and bigger objects. The material in the Asteroid Belt started to accrete, but couldn't hold together as a planet because Jupiter's gravity disrupted the process. The asteroids are very interesting to astronomers because they contain material that can tell us a lot about the early Solar System.
There are over a quarter of a million known asteroids, over 12,000 of the have been named.
Ceres is the biggest asteroid and it's only 940 km (580 miles) in diameter. Most are much smaller. There could be billions of them since the size goes down to the size of a dust grain. But even if you could collect all the material in the asteroid belt, it wouldn't make much of a planet. You'd need 25 asteroid belts to get something as massive as the Moon.
The asteroid belt isn't really as crowded as it seems.
In a diagram of the inner Solar System they try to give you an idea of the numbers of asteroids. However the belt is more than 1 astronomical unit (AU) across and its circumference is bigger than the orbit of Mars. An AU is the distance from the Earth to the Sun, 150 million km (93 million miles). There is plenty of space for all those rocks.
Except for Ceres, the asteroids are all sorts of irregular shapes.
Ceres has enough mass for gravity to pull it into the shape of a ball. That's why it's a dwarf planet. You can see what some of the others look like in this collection of pictures that Emily Lakdawalla put together.
Almost all asteroids are made of rock, but about 5% are metallic – iron and nickel.
The meteorites on Earth have mostly come from the asteroid belt. The iron meteorites were highly prized by ancient peoples who didn't have the technology to get iron from iron ore.
There are asteroids outside the asteroid belt.
We are very interested in the asteroids that cross Earth's orbit. They are called Near Earth Objects, or NEOs. More than 600 of these are all classified as Potentially Hazardous Asteroids, or PHAs. The Minor Planet Center of the International Astronomical Union keeps a close watch on these, and there are other projects searching for new ones. Unfortunately, we don't yet know what to do if we find one. Millions of years ago an asteroid hitting Earth seems to have led to wiping out the dinosaurs.
NASA's Dawn mission is visiting the asteroid belt.
NASA launched Dawn in 2007 and since July 2011 it has been studying the asteroid Vesta. It will be orbiting Ceres in 2015.
The person who discovers an asteroid is usually allowed to name it, as long as it doesn't break the rules of the Minor Planet Center.
The first asteroids discovered were named for goddesses. But by now the list includes the names of the discoverers and their families, of scientists, writers, artists, movie stars and many more. The names can't be duplicates, offensive, or living political or military figures.
The youngest person to discover an asteroid was Luigi Sannino in Italy.
In September 1999, 18-year-old Sannino was observing with P. Pietrapiana at the Monte Viseggi Observatory when they found the asteroid which was later named Palmaria.
References:
NASA asteroids page, http://solarsystem.nasa.gov/planets/profile.cfm?Object=Asteroids
Minor Planet Names: Alphabetical List, http://www.minorplanetcenter.net/iau/lists/MPNames.html
