Astronauts will launch into space this week aboard the space shuttle Discover to install the last four solar arrays on the International Space Station (Image: NASA)

Update Wednesday 1840 EDT: NASA called off Wednesday’s launch attempt after a slight leak was detected in a gaseous hydrogen vent line outside the external fuel tank. The system is used to carry excess hydrogen safely away from the launch pad. Officials are discussing how to proceed, but the next launch attempt will not occur before Sunday, 15 March.

The space shuttle Discover is set to blast off from Florida on Wednesday night on a mission to install the last set of solar arrays on the International Space Station.

Launch is planned for 2120 EDT (0120 GMT on Thursday) from Florida’s Kennedy Space Center. The chance that the weather will be good for launch is 90%, says Kathy Winters, NASA’s shuttle weather officer.


If all goes well, the mission will provide the station with a power boost that will enable it to expand science activities in two new service modules and double its crew, to six, in May.

Astronauts will perform up to four spacewalks to install four 35-metre-long solar arrays on the starboard side of the station and perform other tasks. The full complement of arrays will provide up to 120 kilowatts of power to the station, enough to power 40 average US homes.

To accomplish the task, the crew will use both the station and shuttle robotic arms to manoeuvre the 14.5-tonne solar panel truss into position.

Water recycler

Discovery will also carry replacement distillation equipment to fix the station’s $250 million water recycler, which is supposed to convert urine to potable water.

The space shuttle’s fuel cells have supplied most of the outpost’s water, but the shuttle is set to retire in 2010.

To fill in the gap, astronauts installed the water recovery system in November 2008, but ongoing glitches in the system have slowed the testing process needed to get the system ready for a crew of six.

Cracked valve

The launch was repeatedly delayed as programme managers worked to sort out potential problems with engine valves that are used to maintain pressure in the vehicle’s external fuel tank.

One of the three fuel valves aboard the previous shuttle mission, Endeavour, cracked during launch in November.

Although the shuttle reached orbit without incident, agency officials wanted to be sure that a similar problem aboard Discovery would not cause catastrophic damage.

For example, they were concerned that a stray fragment from a cracked valve could rupture one of the shuttle’s hydrogen fuel lines, resulting in a loss of pressure that could shut down one of the engines and potentially prevent the shuttle from reaching orbit. Damage might also cause hydrogen fuel to vent into an oxygen-rich area, creating a potential fire hazard.

Valve replacement

NASA ordered engineers to remove and inspect Discovery’s three valves, which had each flown 12 times, in February. Electromagnetic tests revealed one valve was damaged, and it was replaced.

NASA also replaced another of the three valves, since the cracking problem might result from the fatigue of repeated use. That means the shuttle will fly with only one of the valves it was originally intended to use for this flight.

Agency officials have calculated that the chance of a catastrophic problem occurring as a result of the valves is low, clearing the way for the upcoming launch.

“We showed that if [a stray fragment] got in the plumbing it’s very unlikely to cause damage and then we showed that even if it does cause damage, that damage is not something that we needed to worry about,” shuttle program manager John Shannon told reporters Friday, according to CBS News.

“The vehicle’s in great shape and we’re ready to pick up with our normal countdown,” Shannon said.

GPS antenna

If time permits, the Discovery crew will also install a GPS antenna to help guide Japan’s new cargo supply vessel, the H-II transfer vehicle, whose first flight is targeted for September 2009.

Discovery will also deliver Japanese astronaut Koichi Wakata to the space station for a six-month stay aboard the outpost. Wakata will replace NASA astronaut Sandra Magnus as flight engineer for the station.

NASA plans to send seven more shuttle missions to the station before the shuttle fleet is retired in 2010.