The Solar System
Scientists believe the solar system began about 5 billion years ago, perhaps when a nearby star exploded and caused a large cloud of dust and gas to collapse in on itself. The hot, central part of the cloud became the sun, while some smaller pieces formed around it and became the planets. Other fragments became asteroids and comets, which also orbit the sun. The early solar system was a turbulent mix of hot gas and rocky debris.
In the solar system everything is affected by the sun's gravity. The planets and a variety of other objects, including comets, move the way they do because of the sun's gravitational attraction. Our planet, Earth, is the third out from the sun. The planets are all different. Their differences are largely the result of their different distances from the sun.
We call the planets that are closer to the sun, including the Earth, the inner planets. They are small rocky worlds. The outer planets are much larger and are made from much lighter materials. All but two planets, Mercury and Venus, have moons in orbit around them.
The explorers found a dead world. There is no air and no water. Without an atmosphere, the sky is black at midday and the temperatures swing by hundreds of degrees from day to night. No rivers or oceans have eroded the surface; no volcanoes are rebuilding the boring landscape.
The surface has survived unchanged for billions of years. Smooth plains and lava flows that froze on the Moon long before life arose on the Earth. Elsewhere, the Moon still bears the scars of every rock that has hit it from space, both large and small. The footprints of the visiting astronauts will survive for millions of years.
Astronomers now believe the Moon is the remains of a giant cosmic traffic accident. In its very early days, the Earth was hit by a runaway planet the size of Mars. White-hot molten rock were splashed into space, and solidified into a ring of rocks around the Earth. These then came together to make up the moon.
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