Shuttle faces return delay

Heading home: Susan Helms, James Voss and Uri Usachev

Astronauts on the space shuttle Atlantis may be delayed for a day after a fault was found in a battery on the International Space Station (ISS).

A fourth and final dying battery in the Russian control module was replaced on Wednesday.

I'm very comfortable that when we leave on this flight, we're still going to have a very robust spacecraft

Paul Hill, lead flight director

But ground controllers testing the batteries, which will restore the space station to full electrical power for the first space station residents later this year, found minor fluctuations with one.

Astronauts will check it on Thursday and if it cannot be fixed immediately, Atlantis could stay linked to the station for an extra day. They are currently due to leave at 00:07GMT on Saturday.

Inside looking out: View from the shuttle's cargo bay

Paul Hill, the space station's lead flight director, said: "We knew going into this flight that with this level of refurbishment and all the equipment we're pulling out of all these systems, we knew this wouldn't be easy, so having one or two things slow us down ... isn't particularly unexpected.

"I'm very comfortable that when we leave on this flight, we're still going to have a very robust spacecraft."

The seven-strong crew of Atlantis, which has been docked with the space station since Sunday, have boosted the sagging ISS into a higher orbit, fixed a wobbly construction crane, replaced a broken communications antenna , installed four new cooling fans and 10 new smoke detectors. They will also have hauled 1.5 tonnes of equipment into the ISS by the time they leave.

The $60bn ISS, being built more than 322 km (200 miles) above Earth by the United States, Russia, Canada, Europe and Japan, had been suffering from problems to do with the build-up of carbon dioxide. This had made previous visitors nauseous.

Orbit boost

But by switching the position of a few valves, rearranging airflow ducts between the linked shuttle and station and adding some fans, the air quality has improved.

Shuttle flight director Paul Engelauf said: "Even when we collected a large number of crew members for a press conference, we took some CO2 readings afterwards.

"Everything seems to be going great and air quality is no problem for us."

All systems go: Jeff Williams assembles storage racks

The astronauts also said the space station - in particular the Russian-made Zarya control module - is not as noisy as they expected. The din was lessened one year ago with the installation of sound-proofing.

"It's just kind of like motors running in the background - machinery," pilot Scott Horowitz said. "All the insulation wrap they put on has helped a lot."

A Russian service module, due to fly in July, will have to undergo repairs in orbit to make it quieter.

Nasa hopes to have hoisted the space station into an orbit 370 km (230 miles) from the Earth's surface by the time the astronauts leave.

The space station had sunk to an orbit as low as 325 km (202 miles) because of increased solar activity, which causes the atmosphere to expand and spacecraft to sink.

The astronauts spent much of Wednesday transferring supplies ranging from rubbish bags and smoke detectors to clothing and an exercise treadmill from the shuttle's pressurised cargo hold to the station's storage racks.

Work to finish the giant space laboratory will require some 40 space missions between now and 2005.

The ISS will eventually house six and seven-member crews who will rotate after stays of about five months each, with the first crew of one US astronaut and two Russians, to arrive in November.