Space station set to welcome Japanese visitor Thursday
BY STEPHEN CLARK
Posted: September 16, 2009
Having received an exemplary grade for its performance since launch last week, Japan's new cargo freighter is closing in on the International Space Station for its most challenging test on Thursday.
But the space station program hopes to pass a significant milestone during Thursday's rendezvous because the HTV will be the first spacecraft to be plucked from open space using the complex's robot arm.
Such techniques will be crucial in the future because NASA has contracted with two U.S. space companies to begin supplying the station in 2011. Both spacecraft will employ the free-flyer capture technique to be demonstrated Thursday.
"All of those are techniques we're going to need for other vehicles that are going to come. So not only are we going to have a new system, we're going to test techniques that we need for future systems," said Mike Suffredini, NASA's space station program manager, in a pre-launch press conference.
One major HTV system also still untested is the HTV's laser rendezvous sensor, a futuristic device that shoots a laser beam toward reflectors mounted on the bottom of the station's Kibo laboratory module.
The laser beams will bounce off the reflectors and back to receivers on the front end of the HTV, allowing the ship to determine its exact position relative to the station.
That navigation data will be fed into the HTV's flight computers to send commands to the freighter's propulsion system to fine-tune the approach.
"We have a laser head installed on the HTV side which will shoot the laser beam to the reflectors. By measuring the distance and also the angle of the returned signal, we'll know the XYZ position relative to the ISS," said Hiro Uematsu, a senior engineer on the HTV spacecraft.
The European Space Agency's Automated Transfer Vehicle used a similar optical instrument during its successful docking to the outpost last year.
The HTV will begin relying on the laser system once the ship closes within around 500 meters, or 1,640 feet, directly below the station at a location called the rendezvous insertion point.
Before the switch to the laser radar, the HTV will use data from its relative GPS navigation system that compares satellite-determined positions of the cargo craft and the space station.
Since its launch last Thursday, the HTV has completed five abort demos and a series of engine firings to raise its orbit to the altitude of the space station.
Japan Aerospace Exploration Agency officials say the vehicle is working beautifully, and the space station's senior management met earlier this week to approve the spacecraft's rendezvous.
Engineers designed the rendezvous sequence with a number of gates, or decision points, when controllers will pause to analyze the performance of key systems.
"On the actual rendezvous and capture day, there are a handful of additional demonstrations that we'll be doing, and we have very specific success criteria associated with those. So that will be evaluated in real time," said Dana Weigel, NASA's lead flight director for the HTV mission.
The HTV is built with backups in critical systems such as propulsion, navigation and computers.
"The overall philosophy is that at any point that I get down to zero-fault tolerance in any given critical system, the vehicle will automatically do an abort," said Dana Weigel, NASA's lead flight director for the HTV mission.
Thursday's major rendezvous operations will begin in the morning, U.S. time, when the HTV arrives at an approach initiation point 3 miles behind the orbiting outpost.
A burn called Height Adjustment Maneuver 2 around 1226 GMT (8:26 a.m. EDT) will put the cylindrical spacecraft on an automated course to intercept the point.
When the HTV stops at the initiation point, flight controllers in Houston and Japan will evaluate the spacecraft before committing to the next phase.
Already in proximity communications range of the complex, the 33-foot-long barrel-shaped ship will depart the initiation point at about 1529 GMT (11:29 a.m. EDT) and move to a position 500 meters, or 1,640 feet, below the station using its relative GPS navigation system.
After arriving at the rendezvous insertion point around 1633 GMT (12:33 p.m. EDT), the HTV will switch to the laser sensor and press closer to the station before stopping approximately 1,000 feet below the complex.
The HTV will hold at the 300-meter point for almost an hour. The ship will yaw 180 degrees during the pause to better position the spacecraft for potential abort maneuvers.
Once the cargo carrier leaves the hold point, the station's crew will test its ability to send commands to the ship by sending orders for the craft to retreat back to a range of 1,000 feet.
If the demonstration goes as planned, ground controllers will give the "go" for the HTV to resume its approach to the station shortly after 1800 GMT (2 p.m. EDT).
The HTV will halt again around 100 feet below the station for final checks and to receive approval to proceed to the capture box, an imaginary three-dimensional location about 30 feet directly beneath the Kibo laboratory module.
"It's basically autonomous, but we have a lot of interaction from the ground," Uematsu said. "We have predefined hold points at 300 meters and 30 meters below the ISS. The vehicle will automatically stop at those hold points and unless the ground gives a go to the HTV, it will never start from those hold points."
During the final moments of the rendezvous, the HTV will be moving toward the station at a glacial relative speed of about 1 inch per second.
After stopping in the capture box, the HTV will be ordered to free drift by the station astronauts, beginning a 99-second clock to capture the spacecraft.
"When the vehicle arrives at the capture point, it's controlling with its thrusters. Then the crew takes it to free drift, which means we shut off all of its thruster systems and it's just drifting," Weigel said.
Nicole Stott, positioned at the robotic arm control panel, will guide the crane-like appendage to grapple the free-floating spacecraft at about 1950 GMT (3:50 p.m. EDT) as the station begins a nighttime orbital pass.
"You can get a build-up of residual rates if we had a leftover from its controlling rates, and then also orbital mechanics will start taking the vehicle away. One of the big challenges that we face with a free-flyer capture versus a fixed capture is that vehicle typically has some small rotational and translational rate," Weigel said.
Stott will try to exactly match those undefined residual movements as she moves the robot arm over the HTV's grapple fixture.
Canadian astronaut Bob Thirsk will take over the robot arm controls later Thursday to position the HTV for inspections as Japanese engineers begin putting the ship's systems in dormant mode for its stay at the complex.
Thirsk will attach the HTV to the Harmony module's downward-facing berthing mechanism beginning around 2200 GMT (6 p.m. EDT) Thursday.
Officials have drawn up plans for Stott and astronaut Mike Barratt to stage a contingency spacewalk to fix potential problems after the ship is grappled.
Electrical and data lines will be connected before the crew goes to sleep, allowing ground controllers in Houston and Japan to activate the newly-arrived ship overnight.
The station residents will open the hatches and enter the HTV on Friday to begin up to one-and-a-half months of logistics transfers and stowage.
Thirsk will hand off the carrying crate to Japan's own robot arm, which will be operated by European flight engineer Frank De Winne. The Japanese arm will place the pallet on the Kibo lab's newly-installed exposed facility and move each experiment from the hauler to permanent positions on the complex's science deck Monday.
The 839-pound NASA payload, called HREP, holds two experiments to study the oceans and atmosphere. HREP's ocean sensor will focus on coastal features, and the atmospheric ultraviolet and visible instrument will look at the ionosphere and thermosphere.
JAXA's SMILES experiment will detect trace gases in the ozone layer using a submillimeter sounder. The 1,049-pound instrument will help determine the extent of human activity's affects on ozone.
HREP and SMILES are the station's first significant science payloads devoted to Earth science.
"It's good to be able to finally start having this kind of research on board the ISS," Suffredini said.
If all goes as planned, De Winne will remove the empty pallet from the station on Tuesday and hand it off to the outpost's primary arm controlled by Thirsk, who will deftly put the carrier back into the HTV.
"The pallet is removed and inserted a lot like a common dresser drawer. It's got wheels along the edges of the pallet and there are guide rails inside the HTV," Weigel said.
The crew will spend the rest of the HTV's stay unpacking almost 5,500 pounds of logistics stored inside bags and racks mounted in the ship's pressurized section. Mission planners expect the process will take about 70 crew hours.
According to a payload manifest provided by NASA, the HTV is carrying 1,890 pounds of food, 1,249 pounds of pressurized payloads, 1,149 pounds of JAXA equipment, 384 pounds of crew provisions, and 215 pounds of computer hardware.
Once the cargo is transferred to the station, the residents will load the HTV with trash for disposal as the spacecraft is destroyed as it enters the atmosphere.
The mission's duration is still pending a final decision, but the stay is expected to last between 30 and 45 days, leading to unberthing in late October or early November.
Two days after departing the complex, the HTV will conduct its final engine burn to slow itself enough to drop from orbit and burn up in the atmosphere over the remote South Pacific.
Six more HTV spacecraft will be launched at a rate of about one per year through the middle of the next decade.
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