Casting Off: Shuttle Discovery Undocks from Space Station

The space shuttle Discovery sits poised to undock from the International Space Station on July 15, 2006 as NASA's STS-121 mission nears completion. CREDIT: NASA TV.

This story was updated at 8:18 a.m. EDT.

HOUSTON - Six NASA astronauts began their homeward trek Saturday as their Discovery orbiter cast off from the International Space Station (ISS) following eight days of orbital work.

Discovery's STS-121 astronauts took some final parting snapshots before they shut the hatches linking their spacecraft to the ISS at about 4:00 a.m. EDT (0800 GMT).

"This moment came so fast," ISS Expedition 13 commander Pavel Vinogradov told flight controllers earlier this morning. "It's already undocking."

Discovery pulled away from its berth at the end of the space station's U.S.-built Destiny laboratory at about 6:08 a.m. EDT (1008 GMT) to begin a planned two-day trip back to Earth.

"We have physical separation," Discovery's commander Steven Lindsey said as the 100-ton orbiter pulled away from the 200-ton space station 223 statute miles (358 kilometers) up and off the coast of New Zealand.

Aboard the ISS, three astronauts watched their departing comrades fly off after a busy week of reunions and resupply.

"Have a safe trip, soft landings, and we'll see you on the ground in a few months," said ISS Expedition 13 flight engineer Jeffrey Williams as the shuttle pulled away.

"And thanks Jeff, we enjoyed it tremendousely and we'll see you back on the ground," Lindsey replied.

Mission's end approaches

Discovery is slated to land in Cape Canaveral, Florida at NASA's Kennedy Space Center on July 17 at about 9:14 a.m. EDT (1314 GMT). The spaceflight - NASA's second shuttle mission since the 2003 loss of the Columbia orbiter and its seven-astronaut crew - launched toward the ISS on July 4 and docked two days later.

"We've got a great ship," STS-121 pilot Mark Kelly, who deftly guided Discovery away from the ISS today, said Friday. "It's ready to come home and we're going to do that on Monday."

In addition to completing NASA's return to flight objectives - which included testing an orbital inspection boom for use as a repair platform and evaluating a heat shield crack repair method - Discovery's STS-121 mission also delivered about 7,400 pounds (3,356 kilograms) of supplies, science equipment and other necessities for ISS crews.

Discovery also ferried European Space Agency (ESA) astronaut Thomas Reiter to the ISS. Reiter joined the Expedition 13 mission as a third crewmember, marking the first time an ISS crew has reached its three-person capacity since the Columbia accident. Previous crews had been limited to two-astronaut teams.

"I am in great spirits," Reiter told reporters in his homeland of Germany Friday. "I'm in a very good mood."

STS-121 spacewalkers Piers Sellers and Michael Fossum staged three spacewalks outside the ISS during Discovery's eight-day stay, two of which were primarily aimed at testing shuttle heat shield repair tools and techniques.

But a July 10 spacewalk featured a critical repair to the station's Mobile Transporter, a railcar-like carrier that moves large components and a robotic arm along the orbital lab's major truss. The fix opens the gate for the next 15 shuttle missions that will complete the ISS, NASA has said.

"I wish I was part poet, and not mostly engineer, because the words escape me," Fossum said Friday as he tried to describe the spacewalks and shuttle mission, both firsts for his astronaut career.

Final heat shield inspections

Discovery will hover about 40 nautical miles (74 kilometers) from the ISS today until after STS-121 mission specialists Lisa Nowak and Stephanie Wilson, as well as Lindsey and Kelly, complete a final check of the orbiter's nose cap and starboard wing leading edge.

The astronauts guided Discovery's sensor-tipped inspection boom through a similar scan of the heat-resistant reinforced carbon carbon panels along Discovery's port wing on Friday, and will finish the survey today. The two-part scan is similar to the intensive Flight Day 2 inspections Nowak and Wilson conducted to verify Discovery's health for reentry.

Engineers will sift through the imagery for any signs of damage from orbital debris or micrometeorites before giving the orbiter a complete pass to return to Earth, NASA officials said. If image analysts find any concerns, Discovery and its crew could return to the ISS for safekeeping, they added.

"When we went through the risks to the shuttle, it was number two on the list," NASA's deputy shuttle program manager John Shannon said Friday of micrometeorites and orbital debris. "I feel, for the program, very encouraged that we're taking risks that we used to think we just had to live with, and are doing positive things to get us in a better configuration."

Shuttle heat shield follow-up inspections were set to begin about 8:58 a.m. EDT (1258 GMT), with the 50-foot (15-meter) inspection boom to be replaced in its flight berth at about 12:28 pm. EDT (1628 GMT), NASA said.

Meanwhile, engineers continue to study a potential hydrazine fuel leak in one of Discovery's three auxiliary power units. Flight controllers plan to test the small leak - which could be either hydrazine or harmless gaseous nitrogen - on Sunday to determine if the APU should remain in operation during Monday's planned landing.

"I think it's highly likely we'll end up flying the plan we planned preflight," Shannon said.

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