EVA Day 3 --
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 change-out 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 of the flight,
the first two being the change-out 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 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 Shuttle
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.
Overnight the
ground commanded the Hubble systems back on line, 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.
On board Columbia
I went to sleep satisfied I did an honest day's work, and very tired.
