21:10
23 August 2006
NewScientist.com news service
Kelly Young
With the space shuttle Atlantis scheduled to launch on Sunday,
construction of the unfinished International Space Station (ISS) is
set to pick up again after a 3.5-year hiatus.
The mission marks the beginning of a packed launch schedule for the
shuttles the only vehicles large enough to carry up station
components because they will be retired in 2010.
"Now we're going to get back to the business of expanding the space
station," says Paul Dye, lead flight director for the upcoming
shuttle mission.
Before the Columbia accident in 2003, the ISS was growing at a
steady rate. But construction was put on hold when the shuttles
were grounded during the accident investigation and the subsequent
push to improve the shuttles' safety.
Then, the first two shuttle missions after Columbia were devoted to
testing out new safety measures and stocking the station with
supplies. There was not enough room on those shuttles to deliver
major parts of the station.
Massive expansion
Now, a massive expansion to the station is about to begin, starting
with Atlantis's flight on 27 August.
The agency expects to increase the ISS crew from three to six
people by 2009. Before that, they will need more living space and
an extra bathroom and kitchen. The ISS will eventually have the
volume of a five-bedroom house.
Fifteen elements have been sitting on the ground, awaiting launch
to the ISS. These include components provided by NASA's
international partners such as laboratories for Japan and Europe
and a two-armed robot named Dextre from Canada.
"We called this the International Space Station for many years, and
we've talked about all the partners that are going to participate,"
says Phil Engelauf, chief of NASA's office of flight directors.
"Happily, now that is going to come to fruition."
Wiggle room
The components are set to be launched on at least 15 and possibly
17 shuttle flights. Another shuttle mission might be sent to
service the Hubble Space Telescope.
"You can imagine in 18 shuttle flights, that's going to be a very
busy period of time for the shuttle programme," says Mike
Suffredini, NASA's ISS manager. "But it's why we're here."
One of the consequences of the shuttle's impending retirement in
2010 is that all of the shuttle flights are packed. If something on
the ISS fails and needs to be replaced immediately, NASA would have
to juggle its scheduling of cargo deliveries to the station.
"There's very little wiggle room," says Suffredini. "A major
problem with one of the elements would cause us a major change in
the final configuration of the ISS."
Heavy load
Barring any unforeseen problems, the next three shuttle flights
will be devoted to expanding the station's power system.
Atlantis will begin by delivering a segment of the station's
backbone-like truss something they were originally meant to do
just three months after Columbia's last flight in February
2003.
Three years later, they will finally be able to deliver the P3/P4
truss segment. Weighing in at 15,824 kilograms (34,885 pounds), the
P3/P4 truss is one of the heaviest elements ever flown to the ISS.
Three spacewalks during this shuttle mission will be needed to
attach it and prepare it for use.
It will be connected to the station's P1 truss segment ("P" stands
for port, the left side of the station; truss segments on the other
side are denoted with an "S" for starboard).
Sticky coating
The P3/P4 truss provides a framework for the distribution of power
and a track for the railcar that moves the station's Canadian
robotic arm. It will also provide a base for the station's third
and fourth solar arrays, which will be carried up on
Atlantis.
The arrays each contain 32,800 solar cells and together stretch 73
metres long. When they are activated in several months, they will
double the station's power supply to 40 kilowatts of power.
Installing the arrays, which are folded up like an accordion in the
shuttle, will be tricky. When the station's only other pair of
arrays was unfolded during a previous shuttle mission in 2000, one
array had trouble deploying because of a sticky coating on parts of
it.
Atlantis astronauts Joe Tanner and Brent Jett were both on that
shuttle flight and were chosen for this mission in part because of
that experience. "I think Joe was the first one starting to see the
arrays stick I think his comment was, 'That doesn't look good,'"
Jett told New Scientist.
Pulled taut
The partially unfurled arrays began to sway, causing cables
attached to the arrays to come loose. The cables later had to be
restrung by astronauts on a spacewalk.
NASA will put lessons learned on that flight to use on Atlantis.
This time, the arrays were not folded tightly on the ground before
launch.
And during their deployment in space, the crew will partially
extend the arrays. That will allow them to warm up reducing their
stickiness before they are stretched to their full length. The
cables attached to the arrays will also be pulled tightly during
the deployment to restrict unintended movement of the arrays.
"I have a high degree of confidence in the arrays, largely because
we've done it before," Suffredini says.
After this mission, the space shuttle Discovery is set to take up
the next truss segment to the ISS. On a flight scheduled for no
sooner than 14 December, it will cart up the P5 truss, which will
attach to the newly installed P4 segment.
