>>KATRINA: One of the Hubble Space Telescope’s most
memorable moments was observing the fragments of Comet
Shoemaker-Levy 9 impact Jupiter in July, 1994. This was a huge
astronomical event that captured the attention of the public
across the globe. But what most people don’t know is that Hubble
came close to not getting those observations. The telescope
encountered some problems that might normally take over a month
to solve just days before the comet impact.
[music]
>>HEIDI: Shoemaker-Levy 9 was a comet that was discovered by
David Levy, Caroline Shoemaker, and Gene Shoemaker. We had
about nine months of warning that this comet was going to hit
Jupiter. I knew from the calculations we had done beforehand that if
Hubble was working at the peak of its game, that we would get
images that far surpassed images that could be taken by any other
telescope on the Earth. >>BRIAN V: So, so we knew that Comet
Shoemaker-Levy was going to impact Jupiter on July 16th, so
we wanted to make sure that Hubble was ready for that.
>>BRIAN R: July 5th, 1994 started out as an ordinary day
until we were called and told that the Hubble had been
commanded into inertial hold safemode because it appeared
that it was misbehaving and not executing the proper commands.
>>BRIAN V: When they command inertial hold, all those
commands that were loaded stop executing. We tell it to don’t
look at those any more and just hold steady.
>>HEIDI: You know your first response is sort of an empty feeling in the pit of
your stomach, like oh no, we have this huge event coming up,
and the telescope’s not working.
>>CHARLIE: We had about a week
to resolve the vehicle issue and bring it back to normal mode.
>>BRIAN R: The spacecraft was not executing the commands in
the stored command sequence it was supposed to be, it looked
like it was doing something completely random. So we
immediately suspected that there was a memory unit problem.
>>CHARLIE: We were lucky because, the previous mission we
had done we installed a coprocessor.
>>HARRY: During the first servicing mission
in December of ’93, the coprocessor
was basically a memory upgrade, an additional memory to augment
the DF-224 flight computer.
>>BRIAN V: Now this coprocessor has what we call a shared memory that both computers can use.
So the shared memory wasn’t really configured yet to be useable,
but we knew that it was there. >>CHARLIE: We verified different
configurations and different architectures that we could test
out, and we found that it was no issue to swap out the memory.
>>BRIAN R: So we started doing that, and things were going
extremely well. And suddenly while we were in the middle of
reconfiguring it, we were told that the spacecraft had entered
zero-gyro-sunpoint, which is a more serious version of
safemode. What it looked like is that we had just lost two gyros
simultaneously, which that can’t happen. We had just replaced all
six of them in the first servicing mission along with the
coprocessor; this just couldn’t be happening. >>BRIAN V: I
remember just going, “What’s going on?” You know, I didn’t
know what was happening. Because the hardware guys looked at the
gyros — they were fine. But the software said that they had a
problem with two of them. >>BRIAN R: Here we are halfway
through the reconfiguration, and we were in deeper trouble than
we had thought. >>BRIAN V: I started looking at the time
between the two events. Started figuring out, well okay, it’s
two and a half days, what is that in hours, minutes, and
seconds. I saw the number, and it was, you know it was really
obvious at that point what had happened. There was an overflow
in the computer. Finally discovered that we had this
thing called a “gyro good count” that counts up how many seconds
has there been since there was the last, you know, “bad gyro
count” if you will. >>BRIAN R: When the gyros change mode, this
counter gets reset. Typically this will happen several times a
day. Well because of the work we had been doing for the past two,
two and a half days, this counter never got reset. And
when it overflowed, or when the counter value went past the
highest value it could count to, bad things happened in the code.
>>BRIAN V: Once we discovered that, and knew there was an
overflow in the software, we knew we didn’t have our computer
problem. It was easy; now it was just, now we just have to
recover. So by the time we finished that and got back up
into science operating mode, it was probably the middle of, end
of July 9th. So in plenty of time for the observations.
>>HEIDI: Well as soon as we heard that Hubble was back on
track, we were like, “Yes! Ready to go!” >>HARRY: It allowed the
Shoemaker-Levy campaign to go forward, and I’d hate to think
what would happen if we hadn’t gotten the coprocessor in and
got it checked out. >>BRIAN R: We retreated to our offices and
waited for the impact, wanted to see the pictures just like
everybody else did, so we were watching it on NASA TV just like
the rest of the world was. And I remember seeing a press
conference at the Science Institute announcing it and they
were waiting for the pictures, when Heidi Hammel came in waving
the picture, the first picture from Hubble. >>OFF-CAMERA VOICE:
Prepper shot, prepper shot, prepper shot! She’s going in!
Camera 3! She’s bursting in, she’s bursting in! Go, Heidi!
[noise of press conference]
>>GENE: ...that probably indicates
we’re dealing with larger objects than was concluded by
Asphaug and Benz. And I think we may have some up-to-date
information from Heidi Hammel! [laughter] >>HEIDI: Eugene
Shoemaker said he would be personally astonished if we saw
nothing. Well he’s not going to be astonished. We actually saw
some amazing things. The comet delivered. It delivered
big-time. It had big black spots, and if you looked at
certain colors of light it had white spots, and it had the
rings, and it had plumes, and it had big giant storms! And I was
really proud to be a part of that. And I was really proud
that all the engineers and scientists could pull together
and make that happen.
[music]
>>KATRINA: Comet Shoemaker-Levy 9
added to our knowledge of Jupiter’s atmosphere and the
physics of atmospheric explosions, and even influenced
some space policies on finding near-Earth objects that could
impact us. Thanks to the entire Hubble team, the telescope was
able to make these impactful observations, and perform all of
its incredible science over the past 25-plus years. The stories
don’t end here. Stay tuned for more Hubble Memorable Moments.
[music ends]
[spacecraft swooshes by]
[spacecraft beeps]
[beep beep... beep beep...]