Humans live on a small planet in a tiny part of a vast universe. This part of the universe is called the solar system, and is dominated by a single brilliant star-the sun. The solar system is the earthís neighbourhood and the planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are the Earthís neighbours. They all have the same stars in the sky and orbit the same sun.
(more content follows the advertisement below) A D V E R T I S E M E N T
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 sunís gravity. The planets and a variety of other objects, including comets, move the way they do because of the sunís 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.