|
   |
 |
|
U P D A T E # 6 PART 1: Correction
from update #5
In the last message, the report "DOING THE VARIED WORK CALLED SCIENCE" was actually written by Dr. Glenn S. Orten. The credit was mistakenly given as Joya Gorton. My apologies to Dr. Gorton. To learn more about Dr. Gorton's background, please visit the web site and look at the people section. His bio has recently been added. Lou D'Amario (Galileo Deputy Navigation Team Chief) October 13, 1995 Normally, I would not have made another journal entry so soon. The last one was only a week ago. However, because of the Galileo tape recorder problem, I felt I should put my current thoughts into a journal. I am hopeful that the problem can be solved. In any case, we can accurately navigate Galileo through the orbital tour without the tape recorder. The reason for this is that optical navigation (OPNAV) pictures taken with the Galileo camera during the orbital tour are not stored on the tape recorder (as science pictures are). The information we need from each of the OPNAV pictures is extracted by a new program in the main computer; this takes only about 20 minutes. Then only the extracted information is immediately returned to Earth. It takes no more than about one hour for the data to get from Galileo to the Earth. The new program will be sent to Galileo's computer in April 1996. There are three OPNAV pictures taken during Jupiter approach to improve the navigation accuracy at the Io flyby (which occurs about four hours before Jupiter closest approach). The processing of these OPNAV pictures cannot take advantage of the new method described above. These pictures are first stored on the tape recorder. Then pieces of each picture are read off the tape recorder and returned to Earth over a span of about two weeks. Losing these three OPNAV pictures would decrease the accuracy of the Io flyby somewhat, but only in the latitude direction. It would have no effect on the Probe mission or getting Galileo into orbit about Jupiter. Todd Barber
October 17, 1995
I have been continuing my work in trying to understand what the
propulsion system data can tell us as we move ever closer to Jupiter
orbit insertion. In my last journal entry, I mentioned that it is
possible that an oxidizer check valve in the propulsion system has
failed open, which could have large implications for the Jupiter
orbital tour. The past few weeks have been devoted primarily to
continuing to ask the "what if" questions, if the valve really is
stuck open.
The possibility of a stuck-open oxidizer check valve has dictated
very tight control over the temperature of the Galileo propellant
tanks. Galileo is powered by Radioisotope Thermoelectric Generators
(RTGs), rather than solar panels since it is traveling to a point five
times further from the sun than the Earth (physics students might
recognize here an application of the inverse square law; that is, the
amount of solar energy reaching Galileo is not just 1/5 as much as
reaching the Earth, but is 1/(5x5) = 1/25, or only 4%).
RTGs supply electrical power by turning the heat energy from the radio-
active decay of plutonium into electricity. They are not nuclear reactors
and were verified (by analysis) to be safe for Galileo's launch on the space
shuttle Atlantis in October, 1989, even given a shuttle accident
similar to the Challenger disaster. Even though proven not to be a
health hazard, there were some concerned citizens who tried to block
the launch of Galileo because of its radioactive power source. This
is another example of some of the politics that can influence our
missions.
Anyway, I digress from my point. The power supplied from
the RTGs is used throughout the spacecraft for all spacecraft power
needs; for example, radio transmitters, science instruments, heaters,
etc. Now here is where propulsion comes in! The excess power not
being used by the spacecraft (called power margin) is dissipated in
heaters that are connected to the propellant tanks. So the propellant
tank temperatures are really at the mercy of what electrical power
loads are powered and unpowered on the spacecraft! This is a very
interesting design, and, in the mission to date, has meant pretty
large variations in the Galileo propellant tank temperatures.
It is precisely these temperature changes that we are now trying to
avoid! We do know that if the oxidizer check valve is stuck open,
large temperature changes in the propellant tanks can cause oxidizer
vapor to migrate to the fuel plumbing, which could cause undesirable
chemical reactions. So, given our uncertainty on whether or not the
oxidizer check valve is open, we have to "play it safe" and try to
keep the propellant tank temperatures constant as much as possible.
This is a large effort between now and orbit insertion, but so far it
has been handled expertly by my colleagues. You might imagine that
every time you turn a science instrument on, for example, that you
would have to turn an equivalently sized heater off to keep the power
margin nearly constant. In fact, this is just precisely the kind of
action that we have been taking.
One possible way to help alleviate this problem could be attempted
next March or April. After we get into Jupiter orbit on December 7 of
this year, we are on a very large orbit around Jupiter that takes over
200 days for us to come back into the vicinity of Jupiter again.
About halfway through this first orbit, at the so-called apojove
("apo" = furthest point of the trajectory from and "jove" = Jupiter),
there is to be another (the final!) use of the main rocket engine in
March, 1996. This maneuver is called the perijove raise maneuver, and
it does exactly what it says--it raises the perijove ("peri" = closest
point of the trajectory to and "jove" = Jupiter). In other words, we
are performing a rocket firing in order that we do not come so near
Jupiter during our next close pass by the giant planet. This is
necessary because repeated passes so close to the intense radiation
belts near to Jupiter (like Earth's Van Allen belts, but MUCH stronger)
could damage the spacecraft.
Anyway, that is a another digression. Basically, getting through the
perijove raise maneuver will be much cause for celebration, since that is
the last required use for the main rocket engine. Incidentally, after this
point there is very little propellant left on-board the spacecraft (only
about 135 kg out of a launch load of 925 kg), but this is ample to perform
the 2-year, 10-orbit tour of Jupiter, its moons, and magnetosphere.
Back to the check valve issue. One way to make sure that no
oxidizer could be transported to fuel plumbing would be to isolate the
high pressure helium source after the perijove raise maneuver and then
intentionally spill some oxidizer overboard through the main engine to
get the oxidizer tank pressures down! This is a neat solution, but
certainly might pose some risks to the spacecraft (for example, the
oxidizer is nitrogen tetroxide, a very corrosive chemical that can
attack metals and the Galileo thermal protection blankets!).
Very difficult decisions like this one are often made after putting
together what's called a "trade study." This is basically a list of the
pros and cons of performing a given action and it helps in the
difficult decision process. A lot of our jobs here really entail the
proper balancing of risk. That was difficult for me to comprehend
coming out of school, since I was used to there being only one right
answer. But this a very rewarding part of the job, too, using
so-called "engineering judgment" to arrive at an acceptable solution.
Duane Bindschadler.
October 26, 1995
It's 7:15 in the morning, and I'm sitting in the JPL cafeteria
having some breakfast and trying to write this log entry. As you
might well imagine, things here have happening very quickly over
the past week while the Galileo Project tries to grapple with the
serious problem that appeared in the spacecraft tape recorder just
2 weeks ago today. In that time, the project has tested the tape
recorder, made a preliminary diagnosis, and set in motion a plan to
allow us to fulfill our mission at Jupiter without using the
recorder, should that become necessary.
I've been spending a great deal of time in meetings during the past
week. The purpose of these meetings is to create what is called a
"point design" for "Phase 3." This may sound a bit mysterious, but
what it means is that the scientists, engineers, and managers on
Galileo are trying to define exactly how we can redesign Galileo's
software (that's the "point design" part) to return data without a
tape recorder (a no-tape-recorder mission would use the "Phase 3
point design."). Of course, we can't alter the mechanical part of
the spacecraft -- Jupiter is a little too far for a mechanic to make a
service call! But it is possible to rewrite the software that runs
Galileo's computer. This has already been done once before ("Phase 2"),
after Galileo's high-gain antenna failed to open.
For me, these meetings are often as much a learning experience as
they are an opportunity to contribute to fixing the spacecraft. My
primary contribution is to keep track of what is going on in these
meetings and to keep Carol (my Team Chief, Carol Polanskey)
informed. So it sometimes feels like I'm back in school, taking
notes and listening as carefully as I can.
Besides all those meetings, I'm also trying to help with the
building of a "strawman" sequence. For each of Galileo's ten orbits
around Jupiter, the science coordinators build a sequence of
commands to the spacecraft that tell it which instruments to use,
and when to use them. This would be relatively easy to do, except
that there are eleven instruments onboard Galileo. Each instrument
may or may not interact with any other instrument. Each instrument
requires a certain amount and type of power from the spacecraft.
Each requires particular kinds of processing of their data by the
onboard computer. Most of the remote sensing instruments use the
scan platform -- what if one of those instruments wants to take a
picture of Io at the same time that two of the other remote sensing
instruments want to look at Ganymede?
The answer, of course, is that we have to work out all the conflicts and
then put all the individual instrument plans together to integrate the
sequence.
Getting back to the "strawman" sequence, the idea is that we try to
put together an example of a Phase 3 command sequence for an orbit
-- one that doesn't rely on the tape recorder to store data. Even
though we don't know all the details of what the Phase 3 software
will or won't do, we can still use the basic ideas and principles
that go into a no-tape-recorder mission to build a model of a Phase
3 sequence.
It usually takes a couple of months to build an orbital sequence --
we started on Friday of last week and have to be finished by this
Friday. Because we are using one of the sequences that was
already done (the first orbit, called "G1" because it includes a
close encounter with Ganymede) as a starting point, we'll get the
job done. But time is of the essence.
Yesterday, Carol came into my office and asked me to take the
orbital sequence command file, pull out only the commands to SSI
(the Solid State Imaging subsystem, Galileo's primary camera), run
those commands through one of our software programs that simulates
the Galileo spacecraft's instruments, computers, and data flow, and
find out how much data these commands were sending back and whether
or not the SSI alone was monopolizing the data link to Earth.
Without a tape recorder, Galileo must send all of its data back to
Earth immediately. But without the high-gain antenna, it can only
do so slowly.
"And can you have that for the folks working down in the SIWR*
(pronounced "sewer") before your meeting at 1:00?" No problem, it
was only noon. What I didn't know was that (1) the file containing
the sequence of commands for G1-Phase 3 was very large, (2) there
were a lot of individual SSI observations in the sequence (I lost
count at about 70), and (3) there were other people running the
spacecraft modeling program that I needed and so the computers were
a little bit slower than normal. So it was only after a very busy
hour and 15 minutes that I could show the members of several
instrument teams that although the SSI images would demand a
significant part of our resources, there was also room for sending other
kinds of data to the ground.
One of the things that the past 35 years of space exploration has
taught us is that there is a strong synergy in making complimentary
observations with different instruments. What that means is if we
take two different kinds of data for the same object or phenomena,
we don't learn twice as much as we would have with only one kind of
data. Instead, we learn three or four or five times as much.
Everything we are discovering about Phase 3 and the sequences we
can build seems to show that even if we can never use the tape
recorder again, we will still be able to return a set of data that
will revolutionize the way that scientists think about Jupiter, its
satellite, and its magnetosphere. That means that in a few years,
many of the textbooks that we all use today will need to be updated
and corrected because of what Galileo will teach us. And we will
know a little bit more about the universe around us, and about how
to face the inevitable challenges that it will present us with.
Online from Jupiter, this is Duane Bindschadler.
NOTES:
* SIWR is the Sequence Integration Work Room. It includes a
large-screen display that can display whatever is shown on a
computer workstation's display, and allows us to see the effects
of changes we make to sequences as soon as we make them.
|
||
|
|
|
