Astronauts begin key space station repairs
Two astronauts are undertaking a crucial spacewalk to make urgent repairs to a cooling system on the International Space Station.
Douglas Wheelock and Tracy Caldwell Dyson are carrying out the first of two walks that will be needed to repair a broken ammonia coolant pump.
There are two cooling units on the ISS needed to deal with temperatures that can range from 121C (250F) to minus 157C (-250F).
The second unit is so far unaffected.
Nasa has said the three Russian cosmonauts and three Nasa astronauts aboard the station are not at any risk but the functioning coolant system is having to do all the work.
The right-hand side system failed dramatically last week, forcing crew to reduce power on the space station and halt experiments.
'Extra credit?'
The two astronauts slept in the Quest airlock module to acclimatise to pressure differences and began the seven-hour walk at 1119GMT.
ISS crew members
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Tracy Caldwell Dyson Shannon Walker Doug Wheelock Fyodor Yurchikhin Mikhail Kornienko Alexander Skvortsov Tracy Caldwell Dyson asked "Do I get extra credit?" as she prepared for the walk.
Douglas Wheelock inspected the failed pump and said there appeared to be no damage from debris.
Astronauts back in Houston had simulated the repair walk in Nasa's massive training water pool.
The complicated nature of the repairs delayed the start of the first walk.
The first spacewalk will remove the failed unit. Once it has been removed, the two astronauts will have to move a 355kg (780lbs) spare unit about 10m (30ft) in order to insert it into the gap.
The second walk, scheduled for Wednesday, will connect the ammonia fluid lines.
If the second of the two cooling units were to fail - said to be a highly unlikely scenario - then the astronauts would no longer be able to cool most of the components.
The crew would not be in immediate danger, however, as they could move to the Russian segment of the ISS, which has its own cooling system.
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