Hubble Upgrade Report #4: A Power Trip

Astronauts with Hubble
Astronauts John Grunsfeld (center) and Richard Linnehan work to replace the Hubble Space Telescope's power control unit during a March 6th space walk. One of the observatory's new solar-cell arrays, installed earlier in the week, looms at right.
Courtesy NASA.
EVA Day 3 was PCU day, the power control unit changeout. The PCU is the main power relay box for the whole Hubble Space Telescope. A problem with the old one threatened to end the telescope's life early, so it was decided by NASA to change it out. The interesting part is that the box is not really designed for changeout by space walkers.

Early this morning (space morning) Rick Linnehan, my space-walking partner, and I got into our space suits. This was the third EVA ("extravehicular activity") of the flight, the first two being the changeout of the solar arrays with new, more powerful arrays. After my space suit was started it began to leak water out of the cooling water tank. We quickly changed out the upper part of the space suit with one of the other team's suits, and off we went, just a couple of hours late.

Finally out in the clear vacuum of space, Rick and I started working on preparing the telescope for a complete power-down, the first time in 12 years on orbit! I put thermal covers on some of the temperature-sensitive bays (it gets cold in space without heaters). Rick began disconnecting batteries, and I lowered light covers over the star-tracker cameras.

Then, the meat of our task at the PCU worksite. In one of the electronics bays is the PCU. It has 36 circular connectors, one after another, on the left side of its large black box. Inside are electronics and relays to power the telescope. Our job was to disconnect all of the connections, and then swap the PCU for a new one, called PCU-R (for replacement), and then connect all the wires back up. Sounds easy, except that we are in the very bulky, stiff space suits, and the connectors are too close together to use your hand.

We have a special wrench/connector tool developed just for this task, and one after another, Rick disconnected the wires. With just a few to go, we swapped out positions on the shuttle's robotic arm, and I finished off the last few. After driving the bolts to remove the old PCU from HST, I met Rick in the payload bay, picked up the new PCU, and left Rick to stow the old one. Off to HST, riding on the robot arm driven by Nancy Currie, I put the new PCU in the Hubble.

For the next two hours I attached one connector after another. Because of their location on the side of the box, I often only had a view of the interface with my left eye. Parallax is a significant aid, which I had to do without. Each connector took a couple of minutes to do, with inspections of the little pins and sockets, alignment and then turning the collar. Many of the wires were quite stiff, and I had to simultaneously align and turn. While it doesn't sound hard in principle, in practice it was really hard in the space suit with the stiff, bulky gloves.

During training I joked that my task was "Zen and the art of connectors," in that to perform this task Rick and I needed extreme concentration, patience, and a little bit of skill. At one point I looked at one particular connector and started laughing, thinking, "This is it, the PCU task ends here." I didn't think I could get access and align and mate the connector with my big gloves on it. After trying a couple of different approaches I finally used my connector tool, and zip, it went on. Seven more connections and I was all done.

We reconnected the HST batteries, powered the telescope back on, and went back to the barn, in this case the barn being the airlock on the space shuttle Columbia.

Overnight the ground commanded the Hubble systems back online, and with a day of work in space we now have a Hubble power system that will allow the telescope continued operation well into the future.

Onboard Columbia I went to sleep satisfied I did an honest day's work, and very tired.

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