Two former teachers set for today's spacewalk
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 23, 2009
Astronauts Richard Arnold and Joseph Acaba, both former school teachers, are preparing to venture back outside the international space station today for a planned six-and-a-half-hour spacewalk. The primary objectives are to complete the deployment of an external cargo mounting mechanism; to extend another cargo carrier on the far side of the station's solar power truss; to lubricate the grappler on the station's robot arm; to reconfigure a wiring patch panel; and to reposition an equipment cart.
Floating in the Quest airlock module, Arnold and Acaba plan to switch their spacesuits to battery power at 11:43 a.m to officially kick off the Discovery crew's third and final spacewalk. This will be the 123rd spacewalk devoted to station assembly and maintenance since construction began in 1998 and the second for Arnold and Acaba.
The first item on the agenda is to relocate one of two CETA carts, small rail cars coupled to the mobile transporter on the front side of the station's solar power truss that moves the lab's robot arm to various worksites. The CETA carts currently are on the left side of the transporter. Acaba, anchored to the end of the station arm, will pick up one cart and move it to the starboard side of the transporter so the arm can reach the proper left-side worksite in June to install a Japanese experiment platform.
"The first task right out of the hatch is to get Joe on the robotic arm," Arnold said. "We have these CETA carts, they're crew and equipment transfer vehicles. They're like little train cars that ride up and down the truss and we need to move one from one side of the station to the other so Joe gets a really exciting ride on the robotic arm. He gets to grab this thing and go around. I'll translate along and the arm will put him back on the other side and we'll mount it back on the track on the other side where it needs to be."
Acaba said today's spacewalk "is going to be a special EVA. ... We're classmates so it's pretty neat for us both to go out the door at the same time."
With the CETA cart transfer complete, Arnold and Acaba will make their way to the partially deployed unpressurized cargo carrier attachment system, or UCCAS, on the underside of the port 3, or P3, truss segment. Acaba and Steven Swanson attempted to swing the mechanism out during a spacewalk Saturday, but they were unable to pull it into place.
Engineers speculated that a variable-diameter locking pin in a clamp that was mistakenly installed in the wrong orientation was causing physical interference. But it now appears that might not be the case. Engineers suspect the mechanism will deploy if a bit more force is applied.
"Our leading theory was the adjustable diameter pin, because of the way it was installed, might have been causing some interference with the mechanism," said Flight Director Kwatsi Alibaruho. "Based on a thorough analysis of schematics, as well as CAD models for that mechanism, we think that is probably unlikely. After some conversation with the EV crew on orbit, we came to the conclusion that it is highly likely the detent for that mechanism is simply very stiff, much stiffer than we expected.
"We train our astronauts not to apply excessive forces on any kind of rotating mechanism. So when the mechanism did not rotate freely with a slight application of force, we investigated the possibility there was some binding or some interference with the mechanism. So the fix for it may be as simple as pulling on it harder to free it from the detent.
"But this is again a theory, we won't really know until we get the spacewalkers at the site. And then we'll have some idea about how to go forward. Of course, our first order will be to try to pull harder to try to get the mechanism to its nominal position. If we're not able to free the mechanism ... we will employ a long-duration tie down, where we simply attach some very sturdy straps and tie that swinging platform into a configuration we can leave it in for an extended period."
After the UCCAS work is complete, Arnold and Acaba will make their way to the starboard 3, or S3, truss segment and deploy another built-in cargo support mechanism that will be used later to hold spare parts and other equipment.
The spacewalkers then will split up. Acaba will lubricate the gripping snares on one end of the station's robot arm while Arnold attempts to re-wire a patch panel on the zenith 1, or Z1, truss segment to restore full redundancy to the station's gyro control circuits.
Four control moment gyroscopes in the Z1 truss are used to maintain the station's orientation. One gyro was tied to another gyro's circuit breaker during an earlier mission and astronauts have not been able to unlatch the connector to plug the gyro back into its own control circuit.
If all goes well, the astronauts will begin repressurizing the Quest airlock around 6:13 p.m., bringing today's spacewalk to a close. A mission status briefing is planned for 7 p.m.
Here is an updated timeline of today's activity (in EDT and mission elapsed time; includes revision K of the NASA television schedule):
EDT........DD...HH...MM...EVENT 03/23/09 06:43 AM...07...11...00...Crew wakeup 07:18 AM...07...11...35...EVA-3: 14.7 psi repress/hygiene break 08:08 AM...07...12...25...EVA-3: Airlock depress to 10.2 psi 08:28 AM...07...12...45...EVA-3: Campout EVA preps 08:43 AM...07...13...00...ISS daily planning conference 09:58 AM...07...14...15...EVA-3: Spacesuit purge 10:00 AM...07...14...17...ISS-19/Soyuz TMA-14 pre-launch b-roll 10:13 AM...07...14...30...EVA-3: Spacesuit prebreathe 11:13 AM...07...15...30...EVA-3: Crew lock depressurization 11:43 AM...07...16...00...EVA-3: Spacesuits to battery power 11:48 AM...07...16...05...EVA-3: Airlock egress 12:13 PM...07...16...30...EVA-3: Setup 12:33 PM...07...16...50...EVA-3: CETA relocate 01:33 PM...07...17...50...EVA-3: P3 nadir UCCAS deploy attempt 03:58 PM...07...20...15...EVA-3: S3 outboard zenith PAS deploy 04:58 PM...07...21...15...EVA-3/Arnold: Z1 patch panel reconfig 04:58 PM...07...21...15...EVA-3/Acaba: SSRMS LEE-B lube 05:38 PM...07...21...55...EVA-3: Cleanup and ingress 06:13 PM...07...22...30...EVA-3: Airlock repressurization 06:28 PM...07...22...45...Spacesuit servicing 07:00 PM...07...23...17...Mission status briefing 07:48 PM...08...00...05...Evening planning conference 08:13 PM...08...00...30...Crew choice downlink 09:43 PM...08...02...00...ISS crew sleep begins 10:13 PM...08...02...30...STS crew sleep begins 11:00 PM...08...03...17...Daily video highlights reel