Image caption The weekend's repairs to the station were only a 'partial success'

Nasa will conduct a third spacewalk on Wednesday to repair vital cooling units on the International Space Station.

A first attempt on Saturday failed, despite two astronauts undertaking one of the longest spacewalks in history.

During the eight-hour mission, Douglas Wheelock and Tracy Caldwell Dyson managed to detach the unit's hoses but could not replace its large pump.

The coolant units allow the station to cope with external temperatures ranging from 121C (250F) to -157C (-250F).

The unit is one of two that cools the station's systems.

There were originally two spacewalks scheduled, but because of Saturday's failure, astronauts will need a third to complete repairs. Additional repairs are now scheduled for Wednesday and Sunday.

Nasa said the three Russian cosmonauts and three US 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 and halt experiments.

Nagging problem

The astronauts found their Saturday mission tougher than expected.

One of the unit's four hoses was stubbornly jammed and Mr Wheelock had to use brute force to detach it.

"Wow! That thing is not budging," he told mission control at one point.

At times, the astronauts worked so closely that their helmets bumped. When Mr Wheelock eventually managed to release the jammed hose, mission control erupted in applause.

ISS crew members Tracy Caldwell Dyson

Shannon Walker

Douglas Wheelock

Fyodor Yurchikhin

Mikhail Kornienko

Alexander Skvortsov

However, exuberance quickly dampened when the pair were sprayed with ammonia and had to abandon their work to clean off their suits.

Michael Suffredini, the space station manager, said that they were now "looking at every possible option" to ensure a rapid resolution to what has become a nagging problem on board the orbiting station.

Over the next two spacewalks, astronauts will attempt to remove the failed unit and move a 355kg (780lbs) spare unit about 10m (30ft) in order to insert it into the gap. The ammonia fluid lines will then have to be re-connected.

If the second of the two cooling units were to fail - said to be a highly unlikely scenario - the crew would not be in immediate danger as they could move to the Russian segment of the station, which has its own cooling system.

The space station "is a living, breathing beast," commented Mr Suffredini recently. "Some days everything doesn't exactly work perfectly."