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The final frontier: A box of chocolates and three carnations mark the occasion as Atlantis and Mir linked in silent darkness

TOM WILKIE

Science Editor

Every day since he went aboard the Mir space station on 5 September, the German astronaut Thomas Reiter has been beaten 500 times on the heel - at the behest of scientists from Bristol University.

Yesterday, however, he got a break from his daily "bastinado" while astronauts and cosmonauts shook hands in orbit as the space shuttle Atlantis joined the Russian space station in orbit 392 kilometres (245 miles) above Earth, for the second time in four months.

About two and a half hours after steering Atlantis to a flawless docking with the 125-ton station, the shuttle commander Kenneth Cameron opened the hatch separating the spacecraft and greeted his opposite number, the Mir commander Yuri Gidzenko, with a box of chocolates and three flowers: peach-coloured carnations. On the shuttle is the Canadian astronaut Chris Hadfield, and the combined crews represent the United States, Russia, Canada and the European Space Agency.

The mix of nationalities is a record for a single spacecraft, and represents four out of the five participants in the planned international space station Alpha, which is due to start in 1997. Japan is the other contributor to the space station project, for which the present mission is a pathfinder.

This second link-up between the two craft was considered trickier and more dangerous than the first, in late June, because of a 4.5m (15ft) docking tunnel jutting out of the shuttle cargo bay. The shuttle crew's first job during this mission is to deliver the tunnel, so the next five dockings will be easier and safer. The tunnel will remain part of Mir after Atlantis leaves on Saturday.

Reiter and his Russian colleagues, however, will continue in orbit until 29 February next year. The mission has been extended by 44 days so that Reiter will spend 180 days in space - a record for a European astronaut. For most of the time, he will have been a human guinea pig, with all his bodily functions being monitored to assess the effects of prolonged weightlessness on the body.

The results could hold the key to understanding down-to-earth diseases such as osteoporosis, according to Professor Allen Goodship, of Bristol University. People may feel that space is a luxury, he said, "but there are things you can do in space that give information on conditions people experience on the ground".

Together with colleagues from the engineering department at nearby Bath University, Professor Goodship has built a machine which mimics theshock that bones endure as the heel strikes the ground repeatedly while walking.

Professor Goodship said that although the cosmonauts exercise for a minimum of two hours everyday, on bicycles and rowing machines, the bones in their legs still get thinner and weaker as a result of weightlessness. The process appears similar to that of post-menopausal osteoporosis.

The skeleton is not just an inert piece of scaffolding supporting the rest of the body, said Professor Goodship, but a dynamic tissue which responds to mechanical stresses. He hopes that his machine will stimulate the bone cells to maintain bone density during the flight.

One heel is being hit 500 times during the daily 10-minute sessions while the other is left alone. The thickness of Reiter's bones was measured before he took off, is being measured during the flight and will be measured again when he lands, to see if there is any difference between the two legs.

Another British experiment, originating from the Hammersmith Hospital in London, requires the hapless Reiter to wear a tourniquet round one ankle, to which pressure is intermittently applied for one hour each day.

Researchers will also peer at his chromosomes to see if any aberrations develop during his sojourn in space. The state of his lungs and of his cardiovascular system will be assessed and the levels of hormones in his blood and urine will be monitored. He will also take Vitamin K tablets, as part of another experiment on bone density loss.