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Monday, 10 January, 2000, 14:02 GMT
Nasa considers back-to-back shuttle missions

The first two components of the ISS are already in space The first two components of the ISS are already in space

By BBC News Online Science Editor Dr David Whitehouse

The American space agency Nasa is considering flying the same crew on two successive missions for only the second time in the space shuttle programme.

The back-to-back launchings of the space shuttle Atlantis would involve two trips to the under-construction International Space Station (ISS).

Nasa has seven space shuttle missions planned for this year - all but one are ISS assembly missions. They include delivering into orbit docking ports, solar panels and a US lab called "Destiny".

But the major ISS component to be launched this year will be Russian, and it is already two and a half years behind schedule.

Called Zvezda (Star), it will be the centrepiece of the ISS. According to Russian officials, it is ready to go but its delivery rocket, the normally reliable proton rocket, has had problems of late.

Proton failures

Two recent failures, the last one in October, have left officials nervous and they say they want three successful proton flights before they risk Zvezda on the vehicle.

But this poses a problem. The component designed to tide the ISS over until Zvezda is in place, a module called Zarya (star), launched in late 1998, is nearing the end of its nominal design lifetime in orbit and has been experiencing some problems.

Two of its complement of six batteries have failed, and although officials believe it has enough power to continue working, there is concern Zarya may start to misbehave unless it is given a service.

This means the shuttle crew that was to have been launched after Zvezda will keep that mission but will fly an additional mission to maintain Zarya before Zvezda arrives.

By using the same crew, space shuttle and computer software, Nasa would save tens of millions of dollars in hardware, development and training costs.

Such a back-to-back flight has only been done once before in April 1998, when a fuel-cell problem cut short a microgravity research mission on the space shuttle Columbia. The same crew replayed the mission three months later.

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See also:
18 Feb 99 |  Sci/Tech
Space station 'not worth' joining
05 Oct 99 |  Sci/Tech
Space Station delayed again
05 May 99 |  Sci/Tech
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25 May 99 |  ISS
New test for space 'lifeboat'

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