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Atmosphere
As they passed through the inner Solar System, the scattered planetesimals began to interfere with the orbits of the planets already forming there. As a result, the inner planets and the planetesimals collided. Most were smashed into fragments by the collisions, but their remains still orbit the Sun as the asteroid belt, occasionally colliding with each other and breaking into more fragments.
Some asteroids were big enough to begin to heat up as a result of the collisions, just like Earth, and in these cases the rocks melted, iron sank to the core, and molten lava flowed out on the surfaces, again, just like Earth.
The largest asteroid, called Ceres, is over 1,000km across. It is thought that there may be half
a million asteroids with diameters bigger than one kilometer and countless numbers that are smaller. The smallest asteroids are just a few meters across. Together they have a mass of about a twentieth of that of the Moon. The large asteroids are shaped like a ball, suggesting that they have built up by accretion, while the smaller ones are very irregular.
On average, a house-sized Apollo asteroid hits the Earth every century; the last one burst over Russia in 1908. An asteroid big enough to threaten life on Earth hits
the planet only once every 50 million years or so. The last time this happened may have been the cause of dinosaurs becoming extinct on Earth (although many other scientists believe the extinction of the dinosaurs
was due to massive volcanic eruptions).
Some of the largest asteroids even have their own satellites.
The biggest asteroid fragment that has landed on the Earth’s surface in the last few hundred million years produced Meteor Crater in Arizona in the southwestern United States. (See also: Asteroid number; Crater; Minor planets.)
 Astronaut—An astronaut is a person who travels in space, whether inside a spacecraft or, as here, using a spacesuit to repair equipment in space.
Astronaut
A person who travels in space. The Russian word for astronaut is cosmonaut.
Astronomical unit (AU)
The average distance from the Earth to the Sun. One AU is 149,597,870 kilometers.
Astronomy
The science of studying space.
Atmosphere
A shell of gas that envelops a planet, moon, or sun. (See also: Atmosphere, the.)
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