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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