Two
astronauts living aboard the International Space Station (ISS) are gearing up
for the final spacewalk of their six-month mission.
With just
over one month left in their spaceflight, ISS Expedition 10 commander Leroy Chiao
and flight engineer Salizhan Sharipov plan to step outside the station on March
28 to install a batch of antennas and hand-toss a small satellite into space.
"Their
attitude continues to be super and they are certainly looking forward to this
spacewalk," said Derek Hassman, NASA's Expedition 10 lead spacewalk director,
of the ISS crew during a mission briefing Friday at Johnson Space Center.
Chiao and
Sharipov are expected to install a series of space-to-space and global
positioning system antennas to the Russian-built Zvezda service module during their
spacewalk. The antennas will be used by a new European cargo ship - the
Automated Transfer Vehicle Jules Verne - for navigation during docking
maneuvers.
Sharipov
will also toss the 11-pound (five-kilogram) Nanosat satellite into space with
both hands the same way you would pass a basketball, said Scott Bleisath, NASA's
lead spacewalk officer for Expedition 10.
The 12-inch
(30-centimeter) long, transmitter-equipped Nanosat is an experiment designed to
study small satellite control methods and operations.
Dealing
with drift
Chiao and
Sharipov will don Russian-built Orlan spacesuits and begin their spacewalk at
1:30 a.m. EST (1830 GMT) on March 28. The extravehicular activity (EVA) is
scheduled to last about five hours and 40 minutes, ISS officials said.
During that
time, the space station is expected to drift freely in space for about three
hours - enough time for the ISS to circle the Earth twice - as its
two functioning gyroscopes become overloaded due to an expected torque.
The torque
has appeared in several of the last few spacewalks, though ISS flight controllers
have not pinned down its source. They can, however, prepare for it hence the
free-drift plan.
Russian
thrusters will not be able to correct for the drift during that three-hour
period because Chiao and Sharipov will be working near thruster nozzles at the
time, NASA officials said.
A power failure to
one of three stabilizing gyroscopes earlier this week has cut in half - from one
hour to 30 minutes - the amount of time the U.S. attitude control system will function
before the phantom torque overpowers it during the upcoming spacewalk, Hassman
said.
"We feel
comfortable that even with two [control moment gyroscopes] we can do the work,
tolerate the drift, have the crew come back from their work and clean [the drift]
up with Russian thrusters," Hassman added.
A new set
of communications protocols should ensure that Expedition 10 spacewalkers are
nowhere near the thrusters when they fire, ISS flight controllers said. During
the last Expedition 10 spacewalk,
a miscommunication
between ground-based flight controllers and the ISS crew resulted in Chiao
getting too close to the Russian thruster system while it was firing.
Chiao and
Sharipov have spent 155 days living in space and are due to return to Earth on
April 25, following six months of space station living. The upcoming spacewalk
will mark the sixth EVA for Chiao and the second for Sharipov.
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