Rocks from space fall to Earth all the time. Meteorite fragments are so common that they turn up in jewelry. Some of these fragments have been found to come from the Moon and Mars, giving us a very cheap form of sample retrieval. They're blown off the surface of these worlds when a large object impacts the surface, creating a massive explosion.

Drifting through space for eons, they eventually come into Earth's gravity well, and fall to the ground. The most infamous example of this was a small rock found in Antarctica in 1984, which was later discovered to be Martian.

Analysis of this meteorite led to the controversial suspicion that it contained microfossils, possibly our first evidence of life beyond Earth. The Martian fossil theory has now been generally discredited, but it pointed to the importance of studying material from other worlds.

There's another side to the rocks from space story that isn't as well known. Some fragments of Earth are believed to have been deposited on other worlds.

Earth has erased most of its impact scars through erosion, but experienced a massive level of bombardment in the solar system's angry youth phase. Chunks of our home were sent into space. Scientists want to see them, and think it's worth going to the Moon to find them.

Why go looking for fragments of Earth in space when it's so easy to find Earth rocks on Earth? Because the Moon is actually a time machine.

The Earth has been into recycling for much longer than environmentalists. It recycles its water, atmosphere and rocks. It's hard to find truly old rocks on Earth because the surface has changed so much through geological processes.

A fragment of the early Earth on the Moon could have been protected from weathering for eons, especially if it has been buried. It's like visiting the Earth billions of years in the past.

This provides insight into the ancient history of the Earth and of the rest of the solar system. It could also yield evidence of some of the earliest lifeforms to inhabit Earth.

It's generally accepted that the Moon itself was formed when a planet-sized object collided with the early Earth, scattering material into space that later coalesced into the Moon. Both of these twin worlds now hold fragments of the original Earth and the small planet.

It's another reminder that collisions in space, and the transfer of fragments, is a key to understanding the universe.

Dr Morris Jones is the author of "The New Moon Race", available from Rosenberg Publishing (www.rosenbergpub.com.au)