Today's EVA to attempt repairs on another instrument
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: May 17, 2009
Pressing ahead with a complex overhaul of the Hubble Space Telescope, astronauts Michael Massimino and Michael "Bueno" Good are gearing up for a planned six-and-a-half-hour spacewalk to install insulation and repair a sophisticated spectrograph that broke down in 2004, the victim of a blown power supply.
In some ways similar to an attempt to repair the Advanced Camera for Surveys during a spacewalk Saturday, Massimino and Good must open up an instrument that wasn't designed to be serviced in orbit - the Space Telescope Imaging Spectrograph - and replace a circuit board, installing a new power supply in the process.
But unlike the ACS repair, which was devised on short notice and which included somewhat experimental steps to "back power" one camera channel through the electronics of another, the STIS repair is considered more mature and straight forward.
Hubble Program Manager Preston Burch said before launch the odds of a successful repair were "better than 50-50 for ACS and I think they're much better than 80 percent for STIS."
As it turned out, the ACS repair was only partially successful but it accomplished the science team's primary objective. The more heavily used wide-field channel appears to be working again - a major success for the Hubble team - as is the so-called solar blind channel. But tests indicate the camera's high-resolution channel still has problems with its power system.
"The folks had a chance to analyze the data that was dumped during the night," astronaut Dan Burbank told the crew from Houston early Sunday. "I'm happy to report that although the high-res channel is still not functional, the wide-field camera, which is the real workhorse, carrying around 95 percent of ACS's science output, appears to be fully functional now. Nice job."
"Thanks very much, I really appreciate that," replied John Grunsfeld, who carried out the ACS repair Saturday. "Great job to everybody on the team who worked that."
"And just so you know, we also ran a functional test on the solar blind (channel)," Burbank continued. "We didn't expect any issues there and as expected, it's fully functional as well."
"Fantastic! Power is restored to ACS," Grunsfeld said.
Engineers have higher confidence Massimino and Good can restore STIS to operation.
"Over the past two-and-a-half years leading up to this flight, we've kind of ironed out little changes in how we want to do things and practiced over and over again," Massimino said in a NASA interview. "For STIS, we have a bit of an advantage in that we have an actual trainer that they built for us. It's a little mockup of the area we're going to be working in and we've spent a lot of time together, all of us, inside of that trainer.
"There are a lot of steps and a lot of things that can go wrong during that procedure. But together, all of us working as a team and practicing, practicing, practicing, we're whittling that down. We feel pretty confident we're going to be able to do this task and pull it off and get the electronics board replaced. But it's going to be very challenging and a real interesting task."
Today's spacewalk is scheduled to get underway around 9:16 a.m. Massimino, call sign EV-3, will be wearing a suit with broken horizontal stripes around the legs. Good, call sign EV-4, will be wearing a suit with barber pole stripes.
This will be the fourth of five planned EVAs for othe Atlantis astronauts and the 22nd spacewalk devoted to Hubble servicing. The Atlantis astronauts have logged 21 hours and 52 minutes of EVA time during their three previous spacewalks while total Hubble EVA time now stands at 151 hours and two minutes.
The STIS repair is the first item on today's agenda and it will take up most of the crew's time. Massimino will be the designated "free floater" while Good will be anchored to the end of the shuttle's robot arm. Massimino is responsible for the bulk of the STIS repair.
STIS broke down in August 2004. To fix it, Massimino must remove a handrail, a cover held by 111 screws and then replace a circuit board that is locked in place.
"In order to get at a failed electronics board inside the STIS main electronics box, we need to take the cover off the box," Burch said. "We're very fortunate in that when the astronauts open the doors to the aft shroud and look at this instrument, that cover is sitting right there in front of them. The challenge is the 111 screws that are holding it on. The screws are not captive. So they have to go in there and take all these screws out. You can imagine what went through a lot of people's minds when we first started thinking about this, you know, 111 screws floating around all inside Hubble. That was unacceptable.
"So, we came up with a very clever device called the fastener capture plate, which is basically made out of a Lexan-type material. This plate goes over the top of the MEB (main electronics box) cover, it's aligned and fastened on there. And then this fastener capture plate has a series of little holes in it that line up with all the screws. The holes are small enough to allow the tool bit to go in so you can turn the screw, but they're small enough to keep the screw from falling out. So once you get all 111 screws taken care of, the cover stays attached to the fastener capture plate and you move the whole thing out. So all the debris and all the screws are captured in there."
An astronaut-friendly replacement cover was developed that will be installed in place of the main electronics box cover that was removed.
"Once we're done servicing, we take the new cover and put it on," Burch said. "There are two latches, you just throw the latches and bingo, it's on there. And then there's a third latch they throw that has some fingers that grab the electronics boards and mate them to the cover."
That was one challenge. Another was making sure the astronauts could replace the circuit card with the failed power supply.
"If you've ever fooled around with your desktop computer, those things usually aren't much of a challenge," Burch said. "But the way these instruments are built on Hubble, these boards slide into slots in the box but they're held in place by things called wedge locks. And the wedge locks are designed to keep the boards from rattling around and they also provide a heat path to reject waste heat out to the sides of the box so things stay nice and cool.
"Unfortunately, these wedge locks have a property like these Chinese finger handcuffs you may have played with as a kid. You put them on and the harder you pull, the tighter it gets. Well, the wedge locks have this kind of a property and when you loosen the bolts on them sometimes you can slide the board right out and sometimes you have to wrestle with it for a half hour or an hour to get it out.
"We obviously needed a tool to overcome this problem. So we have a card extraction tool that was developed. We went into a small research program to see even if these wedge locks jammed in their worst possible way could we pull the board out without having the board disintegrate and leave a pile of debris. I'm happy to report we've come up with a tool that enables us to do exactly that. So those were the major challenges."
The card extraction technique was successfully demonstrated during the ACS repair Saturday using a slightly different tool.
Here is an updated timeline of today's activity (in EDT and mission elapsed time; includes revision E of the NASA television schedule):
EDT........DD...HH...MM...EVENT 05/17/09 03:31 AM...05...13...30...HST: SSR engineering playback 05:31 AM...05...15...30...Crew wakeup 06:46 AM...05...16...45...EVA-4: Preparations begin 08:16 AM...05...18...15...EVA-4: Spacesuit purge 08:26 AM...05...18...25...EVA-4: Spacesuit pre-breathe 09:06 AM...05...19...05...EVA-4: Airlock depressurization 09:16 AM...05...19...15...EVA-4: Spacesuits to battery power 09:21 AM...05...19...20...EVA-4: Airlock egress and setup 09:46 AM...05...19...45...EVA-4: STIS repair 01:26 PM...05...23...25...HST: STIS aliveness test 01:56 PM...05...23...55...HST: STIS functional test 02:16 PM...06...00...15...EVA-4: NOBL 8 03:01 PM...06...01...00...EVA-4: Cleanup and airlock ingress 03:46 PM...06...01...45...EVA-4: Airlock repressurization 03:56 PM...06...01...55...Spacesuit servicing 04:45 PM...06...02...44...Mission status briefing on NTV 05:01 PM...06...03...00...LIOH and battery config 05:01 PM...06...03...00...EVA-5: Tool config 05:21 PM...06...03...20...Spacesuit swap 06:01 PM...06...04...00...HD downlink opportunity 06:16 PM...06...04...15...EVA-5: Procedures review 09:31 PM...06...07...30...Crew sleep begins 09:45 PM...06...07...44...HST update on NTV 10:00 PM...06...07...44...Daily highlights reel on NTV 11:01 PM...06...09...00...HST: SSR engineering playback 12:31 AM...06...10...30...HST: Bay 3 battery discharge