Asteroids used to annoy astronomers by making streaks their photos and hiding more interesting things. But now they are the interesting things. Here is their story.
The first asteroid was discovered on New Year's Day in 1801.
Giuseppe Piazzi discovered the object that he named Ceres. It was between Mars and Jupiter where astronomers thought there was a "missing planet". They assumed Piazzi had found it. 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. 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 Ceres as a dwarf planet and the other asteroids as small Solar System bodies.
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 it couldn't hold together as a planet because Jupiter's gravity kept breaking it up. The asteroids are very interesting to astronomers because they contain material from the early Solar System.
There are over a quarter of a million known asteroids, over 12,000 of them named.
Ceres is the biggest asteroid and it's only 940 km (580 miles) in diameter. Most are much smaller. Since they can be as small as a dust grain, there could be billions of them. 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.
Click to see a diagram of the inner Solar System. They try to give you an idea of the numbers of asteroids, but they make it look crowded. The belt is more than 1 astronomical unit (AU) across and its circumference is bigger than the orbit of Mars. One AU is the distance from the Earth to the Sun, 150 million km (93 million miles). There is lots 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 asteroid pictures.
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're called Near Earth Objects. More than 600 of these are also classified as Potentially Hazardous Asteroids. 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 may have caused the dinosaurs to die out.
NASA's Dawn mission is visiting the asteroid belt.
NASA launched Dawn in 2007 and from July 2011 to August 2012 it studied the asteroid Vesta. Dawn has been studying Ceres since 2015.
The person who discovers an asteroid usually gets to name it, as long as it doesn't break the rules of the Minor Planet Center.
The first asteroids 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 of 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.