Mission specialists Rick Mastracchio and Clay Anderson finished their seven-hour, 26-minute walk in open space at 8:56 am EDT (1256 GMT), after installing a new ammonia tank on a space station truss, NASA officials said.

Due to a "troublesome bolt," NASA said the walkers ran out of time and were unable to complete all their scheduled work, including retrieving two micrometeoroid debris shields that were to return to Earth.

The spacewalk -- on the 40th anniversary of the launch of Apollo 13, the famously aborted 1970 mission to the moon -- was the second of the three scheduled walks for this mission, and was the 142nd in service of the orbiting outpost's construction.

During the first spacewalk on Friday, Anderson and Mastracchio retrieved an experiment from the Japanese Kibo Laboratory on the station's porch and replaced a Rate Gyro Assembly, part of the station's navigation system.

The final spacewalk of the mission is scheduled for Tuesday.

The Discovery, which blasted off from Kennedy Space Center at Cape Canaveral, Florida on Monday, docked on Thursday with the International Space Station.

Eight tonnes of supplies, gear and racks of science experiments were transferred from the shuttle to the space station in an Italian-made module known as Leonardo.

Besides the replacement ammonia tank, the gear included a freezer to preserve samples of blood, urine, saliva, plants or microbes used in micro-gravity experiments for later analysis back on Earth.

Meanwhile, the US National Aeronautics and Space Administration announced Saturday that Discovery's mission had been extended by an extra 24 hours for routine heat shield inspections.

Landing was now scheduled for Monday, April 19, at 1254 GMT (8:54 am local time) to cap what will be a 14-day mission.

"Since Discovery's Ku-Band communications system is not functioning correctly, the space station's Ku system will transmit the heat shield video and laser scan to Mission Control for imagery experts to analyze," NASA said in a statement.

Discovery's Ku-Band system has been down since the shuttle reached orbit April 5.

Normally the shield inspection is done before reentry but after undocking from the ISS. It is aimed at making sure heat shields have not been damaged by space debris or small meteorites.

The shuttle's heat shield is subjected to temperatures as high as 1,500 degrees Celsius (2,732 Fahrenheit) due to friction on reentry.

earlier related report
Discovery astronauts undertake second spacewalk
Washington (AFP) April 11, 2010 - Two astronauts from the US shuttle Discovery undertook a second spacewalk Sunday as part of efforts to replace a depleted ammonia coolant tank outside the International Space Station.

Mission specialists Rick Mastracchio and Clay Anderson started their work in open space at 1:30 am EDT (0530 GMT), or around three quarters of an hour earlier than had been initially planned.

They have since installed two radiator grapple fixture stowage beams on a space station truss, NASA officials said.

The procedure has allowed station robotic arm operators Jim Dutton and Stephanie Wilson to attach a new ammonia tank to the truss, replacing a depleted one.

The spacewalkers were due to complete the installation of the tank by engaging four bolts and connecting electrical cables as well as nitrogen and ammonia plumbing lines.

Then they will prepare the depleted tank for its return to Earth.

The spacewalk, scheduled to last six and a half hours, was being managed by Discovery crewmember Dottie Metcalf-Lindenburger.

During the first spacewalk on Friday, Anderson and Mastracchio retrieved an experiment from the Japanese Kibo Laboratory on the station's porch and replaced a Rate Gyro Assembly, part of the station's navigation system.

They began work to withdraw batteries from the four solar antennae on the ISS. The batteries will be replaced on a future mission.

A third and final spacewalk of the mission is scheduled for Tuesday.

The Discovery, which blasted off from Kennedy Space Center at Cape Canaveral, Florida on Monday, docked on Thursday with the International Space Station.

Eight tons of supplies, gear and racks of science experiments were transferred from the shuttle to the space station in an Italian-made module known as Leonardo.

Besides the replacement ammonia tank, the gear included a freezer to preserve samples of blood, urine, saliva, plants or microbes used in micro-gravity experiments for later analysis back on Earth.

Meanwhile, the US National Aeronautics and Space Administration announced Saturday that Discovery's mission had been extended by an extra 24 hours for routine heat shield inspections.

Landing was now scheduled for Monday, April 19, at 1254 GMT (8:54 am local time) to cap what will be a 14-day mission.

"Since Discovery's Ku-Band communications system is not functioning correctly, the space station's Ku system will transmit the heat shield video and laser scan to Mission Control for imagery experts to analyze," NASA said in a statement.

Discovery's Ku-Band system has been down since the shuttle reached orbit April 5.

Normally the shield inspection is done before reentry but after undocking from the ISS. It is aimed at making sure heat shields have not been damaged by space debris or small meteorites.

The shuttle's heat shield is subjected to temperatures as high as 1,500 degrees Celsius (2,732 Fahrenheit) due to friction on reentry.

Mission Control in Houston also noted that a false smoke alarm had sounded aboard the space station.

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