NASA's next seven astronauts to fly aboard a space shuttle have a bit of orbital hot potato in store for them when their spacecraft reaches the International Space Station (ISS) later this year.
Using the space shuttle Discovery's robotic arm alongside its ISS counterpart, astronauts with NASA's STS-114 mission - the agency's first since the Columbia accident - will perform a meticulously choreographed operation to hand off a new sensor boom and resupply the space station.
"We have flight rules in place that say 'Thou shall not use both robot arms at the same time,'" said Jeffrey Sugar, a NASA shuttle robotics instructor, in an interview. "So during the hand-off, it's very clear in the procedure that you have to get a verbal 'Go' before you can make a move."
At the center of the tricky move is a 50-foot (15-meter) orbital boom tipped with a camera and a laser ranging system designed to help STS-114 astronauts evaluate the integrity of their spacecraft. The boom connects to the end of the Discovery's robotic arm, just about doubling its reach.
While the arm-boom combination will have an ample range of motion in the early days of the STS-114's flight, once Discovery docks at the ISS on mission Day 3 connecting the two may be a bit more difficult.
"Because of where we're docked, and the fact the docking mechanism is in the forward part of our payload bay, you actually can't take the shuttle arm and reach across to grapple the inspection boom without hitting the space station structure," said STS-114 mission specialist Wendy Lawrence, who will be operating the ISS arm during the hand-off. "So we're going to space station's arm."
Lawrence and the rest of the STS-114 crew are currently scheduled to launch aboard Discovery no earlier than May 15. As NASA's first return to flight mission, STS-114 will mark the first test of the orbital boom system, which flight controllers hope will be able to detect any damage to Discovery's heat-resistant tiles and reinforced carbon carbon (RCC) panels.
The STS-114 crew is scheduled to rehearse the inspection process and go through the robotic arm motions required for the three spacewalks currently set for the mission.
Passing the boom baton
STS-114 flight director Paul Hill said that while unusual, using both shuttle and ISS arms for a specific task is not unprecedented.
The two robotic manipulators were a put in simultaneous play previously during NASA's STS-104 flight in 2001, when the shuttle Atlantis delivered the U.S.-built airlock and four high-pressure gas tanks for assembly to the ISS.
"While we don't like it, we understand how to plan for this," Hill said of the double-arm operations.
For STS-114, Lawrence will use the ISS arm to pluck the orbital boom from the shuttle payload bay, then hold it up for her fellow astronauts to grab with Discovery's own arm. Once the shuttle's arm has a good hold, the station's manipulator will loosen its grip and the Discovery's arm-boomassembly will be positioned to get a good camera view of docked operations and spacewalks, NASA officials said.
"We're going to move [the arms] one at a time," Lawrence said. "It keeps it simple."
With the orbital boom safely removed from Discovery's payload bay, Lawrence and STS-114 pilot Jim Kelly will use the ISS manipulator to pull the Raffaello cargo module from the shuttle payload bay and connect it to the space station for delivery.
A dynamic boom
Before Discovery arrives at the ISS, shuttle astronauts will have already put the orbiter's arm-boom combo through its paces.
On Day 2 of the spaceflight, mission specialists Andrew Thomas, Charles Camarda and Kelly will maneuver the boom along Discovery's wing leading edges and nosecap to inspect heat tiles and RCC panels.
"You've got to be doing fairly careful, hand-eye coordinated motions at a time when you might not be at your best," said Thomas, a veteran of tree spaceflights, adding that shuttle astronauts are often still acclimating to weightlessness on the second day of a mission. "What it's all going to be about is clearance monitoring."
While the inspection run is automated for the most part, Thomas will have to pre-position the boom at the start of every scan and make fine adjustments as needed while the sensor package sweeps with two feet of the orbiter's surface at times. During each sensor sweep, Thomas, Camarda and Kelly will scrutinize television camera views to make sure the entire assembly doesn't stray too close to their spacecraft.
"There is no collision avoidance software, so it is up to the operator to make sure the boom doesn't hit the shuttle," Sugar said.
Thomas has also trained for the possibility that Discovery's orbital boom sensors malfunction or a repair is needed, requiring a spacewalk in which a crewmember would mount the end of the boom and make a visual survey of the orbiter.
"He's on the end of a 50-foot arm which is on the end of a 50-foot arm," Thomas said of the scenario. "If he pushed with a lot of give, there can be a lot of flexing on the boom....those dynamics you have to worry about."
Flight controllers are also still analyzing another contingency - the failure of the space station arm during the hand-off operations, Lawrence said. One potential alternative could include undocking Discovery, pre-positioning the shuttle arm-boom combination, and then redocking with the ISS, though both the orbiter and station remote manipulators are highly robust systems, she added.
"It's a rare day that either arm will be completely incapable of accomplishing our needs," Lawrence said.
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