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>>KATRINA: One of the Hubble
Space Telescope’s most

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memorable moments was observing
the fragments of Comet

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Shoemaker-Levy 9 impact Jupiter
in July, 1994. This was a huge

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astronomical event that captured
the attention of the public

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across the globe. But what most
people don’t know is that Hubble

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came close to not getting those
observations. The telescope

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encountered some problems that
might normally take over a month

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to solve just days before the
comet impact.

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[music]

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>>HEIDI: Shoemaker-Levy 9 was
a comet that was discovered by

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David Levy, Caroline Shoemaker,
and Gene Shoemaker. We had

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about nine months of warning
that this comet was going to hit

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Jupiter. I knew from the
calculations we had done
beforehand that if

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Hubble was working at the peak
of its game, that we would get

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images that far surpassed images
that could be taken by any other

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telescope on the Earth. >>BRIAN
V: So, so we knew that Comet

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Shoemaker-Levy was going to
impact Jupiter on July 16th, so

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we wanted to make sure that
Hubble was ready for that.

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>>BRIAN R: July 5th, 1994
started out as an ordinary day

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until we were called and told
that the Hubble had been

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commanded into inertial hold
safemode because it appeared

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that it was misbehaving and not
executing the proper commands.

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>>BRIAN V: When they command
inertial hold, all those

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commands that were loaded stop
executing. We tell it to don’t

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look at those any more and just
hold steady.

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>>HEIDI: You know your first response is sort of an empty feeling in the pit of

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your stomach, like oh no, we
have this huge event coming up,

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and the telescope’s not working.

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>>CHARLIE: We had about a week

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to resolve the vehicle issue and
bring it back to normal mode.

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>>BRIAN R: The spacecraft was
not executing the commands in

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the stored command sequence it
was supposed to be, it looked

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like it was doing something
completely random. So we

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immediately suspected that there
was a memory unit problem.

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>>CHARLIE: We were lucky
because, the previous mission we

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had done we installed a
coprocessor.

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>>HARRY: During the first
servicing mission

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in December of ’93, the
coprocessor

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was basically a memory upgrade,
an additional memory to augment

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the DF-224 flight computer.

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>>BRIAN V: Now this coprocessor has what we call a shared memory
that both computers can use.

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So the shared memory wasn’t really
configured yet to be useable,

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but we knew that it was there.
>>CHARLIE: We verified different

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configurations and different
architectures that we could test

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out, and we found that it was no
issue to swap out the memory.

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>>BRIAN R: So we started doing
that, and things were going

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extremely well. And suddenly
while we were in the middle of

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reconfiguring it, we were told
that the spacecraft had entered

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zero-gyro-sunpoint, which is a
more serious version of

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safemode. What it looked like is
that we had just lost two gyros

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simultaneously, which that can’t
happen. We had just replaced all

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six of them in the first
servicing mission along with the

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coprocessor; this just couldn’t
be happening. >>BRIAN V: I

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remember just going, “What’s
going on?” You know, I didn’t

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know what was happening. Because
the hardware guys looked at the

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gyros — they were fine. But the
software said that they had a

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problem with two of them.
>>BRIAN R: Here we are halfway

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through the reconfiguration, and
we were in deeper trouble than

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we had thought. >>BRIAN V: I
started looking at the time

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between the two events. Started
figuring out, well okay, it’s

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two and a half days, what is
that in hours, minutes, and

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seconds. I saw the number, and
it was, you know it was really

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obvious at that point what had
happened. There was an overflow

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in the computer. Finally
discovered that we had this

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thing called a “gyro good count”
that counts up how many seconds

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has there been since there was
the last, you know, “bad gyro

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count” if you will. >>BRIAN R:
When the gyros change mode, this

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counter gets reset. Typically
this will happen several times a

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day. Well because of the work we
had been doing for the past two,

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two and a half days, this
counter never got reset. And

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when it overflowed, or when the
counter value went past the

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highest value it could count to,
bad things happened in the code.

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>>BRIAN V: Once we discovered
that, and knew there was an

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overflow in the software, we
knew we didn’t have our computer

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problem. It was easy; now it was
just, now we just have to

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recover. So by the time we
finished that and got back up

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into science operating mode, it
was probably the middle of, end

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of July 9th. So in plenty of
time for the observations.

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>>HEIDI: Well as soon as we
heard that Hubble was back on

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track, we were like, “Yes! Ready
to go!” >>HARRY: It allowed the

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Shoemaker-Levy campaign to go
forward, and I’d hate to think

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what would happen if we hadn’t
gotten the coprocessor in and

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got it checked out. >>BRIAN R:
We retreated to our offices and

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waited for the impact, wanted to
see the pictures just like

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everybody else did, so we were
watching it on NASA TV just like

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the rest of the world was. And I
remember seeing a press

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conference at the Science
Institute announcing it and they

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were waiting for the pictures,
when Heidi Hammel came in waving

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the picture, the first picture
from Hubble. >>OFF-CAMERA VOICE:

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Prepper shot, prepper shot,
prepper shot! She’s going in!

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Camera 3! She’s bursting in,
she’s bursting in! Go, Heidi!

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[noise of press conference]

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>>GENE: ...that probably
indicates

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we’re dealing with larger
objects than was concluded by

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Asphaug and Benz. And I think we
may have some up-to-date

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information from Heidi Hammel!
[laughter] >>HEIDI: Eugene

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Shoemaker said he would be
personally astonished if we saw

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nothing. Well he’s not going to
be astonished. We actually saw

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some amazing things. The comet
delivered. It delivered

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big-time. It had big black
spots, and if you looked at

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certain colors of light it had
white spots, and it had the

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rings, and it had plumes, and it
had big giant storms! And I was

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really proud to be a part of
that. And I was really proud

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that all the engineers and
scientists could pull together

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and make that happen.

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[music]

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>>KATRINA: Comet
Shoemaker-Levy 9

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added to our knowledge
of Jupiter’s atmosphere
and the

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physics of atmospheric
explosions, and even influenced

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some space policies on finding
near-Earth objects that could

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impact us. Thanks to the entire
Hubble team, the telescope was

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able to make these impactful
observations, and perform all of

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its incredible science over the
past 25-plus years. The stories

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don’t end here. Stay tuned for
more Hubble Memorable Moments.

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[music ends]

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[spacecraft swooshes by]