For the second time in a month, a massive satellite is due to fall to earth. Up to 30 separate parts weighing a total of 1.6 tonnes could come crashing down at speeds of up to 450km/h (280mph).

There’s no need to panic, though, as the chances of being hit by a part of the satellite are unbelievably tiny. The German Aerospace Centre says that the chances of someone being hit are one in 2,000. Although that sounds a bit unsafe, there are seven billion people in the world, which means your chances of being struck drop to one in 14 trillion!

Also, because of the satellite's orbit, it is not possible for it to fall anywhere north of a line running below Liverpool and Sheffield. One expert has even said that the chances of being hit by ANY falling satellite in your lifetime is around one in one billion. To compare that, you're 60,000 times more likely to get hit by lightning!

The Roentgen Satellite (ROSAT) is an old model, meaning that it is bigger and heavier than most modern ones. Because it doesn’t have its own propulsion system, ROSAT is at the mercy of drag caused by the Earth’s upper atmosphere. Since this makes the satellite slow down in orbit, gravity is gradually causing ROSAT to spiral back down to earth.

ROSAT was launched on 1 June 1990, with the aim of studying the high-energy X-rays that are emitted by many objects in the universe, including stars and black holes.

ROSAT was originally meant to operate for 18 months, but the data it captured was so useful that it was kept in use for eight and a half years. It stopped operating when its tracking system failed, making the telescope point at the sun, which caused irreversible damage to the satellite.

It is not possible to work out exactly where or when ROSAT will come down. Controllers are not in contact with ROSAT anymore, and the fact that it is travelling at 28,000km/h on a constantly changing path makes predictions tricky.

The main reason why it is hard to predict is because of changes in the sun's activity. The sun heats up the Earth's atmosphere, which has a large effect on the drag forces on the satellite. The latest estimate is that ROSAT will fall to earth sometime between 22-23 October.