Mir re-entry is unprecedented

By BBC News Online's science editor Dr David Whitehouse

The final command in the Mir drama will be issued on Friday.

The platform's final moments will be as a swarm of incandescent fragments hurtling into the water at near-sonic speeds

Thrusters on the Progress spacecraft that is docked with the platform will be fired three times to adjust the station's orbit. The final burn will take the platform into a fatal encounter with the Earth's atmosphere.

The de-orbiting of Mir would be a routine procedure if it were not for platform's size. At 135 tonnes, Mir is far larger than any other manmade object brought to Earth before.

Since 1978, mission controllers have used the same technique to bring home about 80 Progress spacecraft and several space stations, the largest being the 40-tonne Salyut 7 in 1991.

Most of the Progress craft came back down without any problems, but this has not been the case with anything larger.

Skylab experience

The re-entry of Salyut 7, Mir's predecessor, did not go entirely to plan. Its controllers wanted to dump it in the same isolated region of the Pacific as they want to put Mir.

For a while, Soviet controllers lost track of the returning craft. They were not sure if it had come down or not - they maintained that it was still in orbit.

The solar panels and antennae will the be first things to go

In reality, it had struggled on a little further in space and had struck South America. It was the same with the re-entry of the Mars 96 spacecraft that failed shortly after take-off.

The main problem with bringing Mir down is its sheer size. The only comparable event was the 1979 re-entry of US Skylab. The American craft had a mass of about 70 tonnes.

Skylab hit the outback of Australia. Radar observations indicated that it had come down in one big piece. This is not expected to happen with Mir as, unlike Skylab, it is made of modules.

To ensure that Mir does splash down in the right place, the Progress will make a late, 10-minute burn, to force the Russian platform into the zone designated as the "landing footprint". When Mir does break up, it will be swift.

Violent tumble

In 2000, the 14-tonne Compton Gamma-ray Observatory was brought back to Earth by US controllers. They observed that Compton started to burn up at an altitude of 80 kilometres (50 miles) and broke apart at 70 km (42 miles). Mir is expected to do much the same.

The fragments that survive re-entry will hit the water at near-sonic speeds

Mir will begin to glow red as it dips into the atmosphere heading south as it passes about 1,500 km (932 miles) northeast of Australia. First, its flimsy solar panels will be torn away and then its maze of exterior antennae will go. It will be tumbling violently.

It is possible that Mir's five pressurised modules could break away and explode.

The platform's final moments will be as a swarm of incandescent fragments hurtling into the water at near-sonic speeds. The flying debris will leave streaming trails of smoke that will persist for several minutes after they have dived into the ocean.

It would be a spectacular sight if you were in the vicinity. However, it is likely that no-one will witness Mir's last moments. The plucky space station that circled the Earth so many times will be lost at sea.

Destruction simulation images by Analytical Graphics, Inc