A planned July 12 launch, however, was pushed back to early August to give ground engineers more time to sort out problems with a balky shoulder joint on the stations $600 million Canadian robot arm.
Also delayed: The planned June 20 launch of shuttle Atlantis and a specially designed airlock that will serve as a staging area for spacewalks at the outpost.
The 57.7-foot (17.5-meter) station crane is required to mount the $164 million airlock to the outpost because the shuttles shorter robot arm isnt long enough to do the job.
Considered the centerpiece of Canadas $900 million station contribution, the new robot arm was delivered to the outpost by a visiting shuttle crew in April. Extensive testing of the device began in early May, and since then two separate problems have cropped up.
On May 17, the arms wrist joint locked up during a checkout session. Engineers think a computer control box near the elbow of the arm might not have been working properly but they have not been able to duplicate the problem.
The wrist joint since then has been working fine. Even so, Usachev and Voss might be asked to replace the suspect computer control box during a spacewalk now scheduled for June 8.
A more serious problem cropped up six days later when the arms shoulder joint began acting up. A software glitch initially was thought to be the problem, but engineers now suspect that a backup electronics unit on the shoulder joint might have failed.
"The exact problem is not yet known. Weve got specialists on the ground who are obviously looking at this very closely," Helms said in a separate interview.
"But I think we can nail the problem down to a general statement [to] a problem with the electronics of one of the joints, and that would be the shoulder pitch joint."
Like most space station systems, the arm is equipped with both prime and backup computer control boxes and electronics units. In order to operate the arm, computer commands are sent from consoles inside the station to the computer control boxes, which then relay commands to the electronics units on the arms wrist, elbow and shoulder joints.
If any of the joints fail to work properly, then the arm puts itself into what NASA calls a "safe mode" which means that it automatically stops operating.
The prime units on the arm all have been working as advertised, and in that mode the construction crane is operating as intended. Both problems cropped up during tests designed to show that the backup units are working properly.
NASA flight rules, meanwhile, call for both the prime and backup computer control and electronics units to be operating properly before the arm is used to install the 12-ton airlock.
The flight rule is intended to avoid a scenario during which an arm failure could leave the airlock stranded at the end of the crane while astronauts are trying to mount it to the station.
NASA engineers, meanwhile, are working on a software patch that would essentially mask the problem encountered with the shoulder joint when backup units are being used to operate the arm. The idea is to design a patch that would enable the arm to continue operating even if a backup computer control box or electronics unit failed.
The software "work-around" would enable NASA to press ahead with the airlock installation mission. The other option is to perform shoulder joint replacement surgery. That would involve launching a new shoulder joint and then installing it during a spacewalk.
As it stands, the launch of Atlantis and the airlock now have been pushed back to no earlier than July 2. Discovery and a new resident station crew which will include U.S. astronaut Frank Culbertson and Russian cosmonauts Vladimir Dezhurov and Mikhail Turin -- then would follow a month later, with launch tentatively slated for no earlier than Aug. 3.
Should the arm problems persist, NASA officials might reverse the order of the shuttle missions, with the airlock installation flight slipping until September.
Culbertson and his station crew, however, are not yet trained to perform the airlock installation job, so NASA officials would prefer to launch the shuttle missions in their current order.
Usachev and his crew, meanwhile, trained extensively for the airlock installation job and think that the robot arm problems will be resolved in a timely manner.
"Im confident that [engineers] will find a way to work around it," Helms said. "They also do have a way to fix it. And all we need to do is wait until they make their decision."
In the meantime, an extra month in orbit is something the veteran astronaut and her colleagues are looking forward to.
"The shuttle [schedule] always had the potential to slip to the right, so we were just mentally prepared for that," Helms said. "We expected it, and were going to enjoy the extra time that we have."
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