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 expansionNow, 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 roomThe 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 loadBarring 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 coatingThe 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 tautThe 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.
