First of all, my apologies for missing Monday's message. Every effort 
will be made to stay on our established Monday/Thursday schedule in 
the future.

The people of Galileo have been very generous with their journals. As 
a result, we've accumulated a bit of a backlog. One goal of ours is to 
deliver these messages in a timely fashion. So in order to get caught 
up and to stay caught up, you will be receiving some extra messages 
over the next week and each message may have additional journals. 
The good side of being caught up is that you will receive insider 
reports closer to when the events described actually happened. The 
bad side is that there may be occasional dry spells that surface 
since we'll longer maintain an inventory of journals (to fill in these 
dry spells). Such is the penalty for real news as it happens.  

During past projects, we have received comments that some of the
updates are too long or that some vocabulary/concepts are too difficult
for the average middle schooler. So for this project, in addition to the
regular Field Journals, we will be offering an easier-to-read version
geared towards an average 5th/6th grader's interests and vocabulary.
These messages will be distilled from the regular messages. I am looking
for a few volunteers who would be willing to produce these reports.
These folks should have a clear understanding of 5th/6th grade reading
and comprehension skills. I expect to begin these reports in about two 
weeks. Volunteers would be expected to write no more than one report per 
week from an existing Field Journal. If you are interested, please send a 
note to me at marc@quest.arc.nasa.gov. Thank you so much.

Directions for receiving these so-called Junior Journals will be provided 
in a week or so.

To recap, we suggest that after each Probe Squash installment, you 
and your students make a prediction about how long the Probe's 
mission will last.

Installment  #2: The Probe's Entry Into Jupiter: Trial By Fire

Jupiter's gravitational pull is so immense that the Galileo Probe's speed 
on entering Jupiter's swirling cloudtops will be around 170,000 km/hr 
(106,000 mph); a speed equivalent to flying from San Francisco to 
Washington D.C. in 100 seconds! As the spacecraft strikes the 
atmosphere it will experience a force up to 345 times Earth's gravity 
and searing temperatures in the shock wave in front of it as high as 
28,000 degrees Fahrenheit (F). To survive entry, the Probe must be
strong enough to withstand these severe temperatures and pressures, as 
well as the mechanical erosion of its surface caused by the incandescent 
shock layer ahead of it.

Never before has a spacecraft experienced such intense conditions, and to
simulate the entry environment and the response of the Probe, scientists at
NASA's Ames Research Center had to build special high-speed arcjet and 
laser facilities. In addition, a complex computer code was developed by 
NASA-Ames, NASA-Langley, and contractors to determine response of the 
Probe to severe entry temperatures. What resulted was a spacecraft 
composed of two sections:  a virtually impenetrable outer shell 
(deceleration module) for protection during entry and an inner capsule 
(descent module) containing the delicate electronics and scientific 
instruments.

The outer shell, which will surround the capsule through entry and then
drop away, includes thick heat shields and their supporting structure, the 
thermal control hardware that will be used through entry, and a pilot 
parachute. Nested inside of this shell is the inner capsule, that carries the 
payload and which alone will descend through Jupiter's atmosphere. The 
payload carries the main parachute and the science instruments, plus the 
systems that support the experiments and transmit their data back to the 
overflying Orbiter for relay to Earth (kind of like an outfielder hitting the 
cutoff man in the infield).

By comparing test results to the calculations and allowing a  30 to 44 
percent safety margin at various places along the Probe's body, scientists 
are confident that the shield is thick enough (about 6 inches at the nose) to 
withstand these severe entry conditions. The total weight of the forebody 
heat shield is 335 pounds, of which 193 pounds are expected to be vaporized 
during entry. What remains of this heat shield after entry will separate 
from the Descent Module when the main body of the outer shell drops away.

Additional detailed technical information on the heat shield is available at 
the end of this section.

The Probe is aimed to strike the atmosphere at an angle of 8.5 degrees to 
the horizontal. If that entry angle was a mere 1.5 degrees shallower, the 
Probe would skip off back into space; increasing the angle by 1.5 degrees 
means that the entry would overheat the spacecraft and destroy it.

In the next installment of "Will the Probe Get Squashed?", we'll talk about 
how the Probe slows itself down even further. Plus, it's not only students 
who take tests--the Probe had its own tests to pass!

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Heat Shield Technical Information

In about 2 minutes of deceleration, an ablative heat shield of carbon 
phenolic material dissipates the enormous kinetic energy of entry, reducing 
the Probe velocity to Mach 1. For the expected entry conditions, the 
maximum dynamic pressure will be about 5.0 x 10^5 N /m^12 (1.0 x 10^4 
lb/ft^2) and the maximum deceleration level will be about 225 g. At a Mach 
1 altitude of about 49 km (29 miles), the Probe deploys a parachute and 
jettisons the heat shield.

Thermal protection during entry is provided by a carbon phenolic forebody 
heat shield and a phenolic nylon afterbody heat shield. Although these 
materials have been used extensively for Earth re-entry vehicles, on the 
Galileo Mission they will be subjected to environments never before 
experienced in flight. Entry velocity relative to the atmosphere is 48 
km/sec, far higher than any atmospheric entry attempted to date.

The shield is also subjected to mechanical erosion. The shield is subjected 
to a hot atmospheric shock layer (16,000 K, or 28,000 degrees F). The heat 
transfer at the nose of the vehicle at peak heating exceeds 42 kW/cm^2. The 
approximate mass of the forebody heat shield is 152 kg (334 pounds), of 
which about 87 kg (191 pounds) is expected to be lost by ablation during 
entry. 

13 Oct 1995
These past few weeks I have been working on figuring out the best 
telecom (telecommunications) configuration for the first time we 
try sending down the atmospheric probe data. This event is 
scheduled just after arrival at Jupiter in December of this year.  
Usually, telecom configurations are quite standard but, in this case, 
special analysis is required due to the position of the spacecraft, 
the Earth and the Sun. The spacecraft is very close to being on the 
opposite side of the Sun as seen from Earth and the spacecraft's 
radio signal is passing very close to the Sun (within 7 to 4 degrees).  
At this angular separation, the solar activity could disrupt the radio 
signal and, if intense enough, could cause the loss of Probe data. The 
correct telecom configuration can reduce the effect of the Sun on 
the radio link.  

What is this telecom configuration stuff all about? Let me give you 
an example.  We use deep space antennas (70 meters in diameter-
about the length of a 747 jet!) to listen to Galileo's radio signal.  
These antennas can be configured in what is called two-way mode 
or one-way mode. In one-way mode, the antennas are simply 
listening to the radio signal. Two-way mode, however, allows us to 
listen to the radio signal as well as send commands to the 
spacecraft and collect "two-way Doppler" data for navigation. From 
that description, you might think - why wouldn't you be in two-way 
mode always? Well, like all good things, there is a cost to being in 
two-way mode. The additional equipment that is used while in two-
way mode reduces the end-to-end strength (from Galileo to the radio 
signal processors) of Galileo's radio signal. A weaker signal means 
that the rate at which Galileo can send data to the ground goes down, 
which means that we get less data back. Part of my job is to 
schedule two-way and one-way time so that we maximize data 
return while ensuring that commanding and navigation needs are 
being met.

So, why is this important? Why don't we wait until the spacecraft 
comes out from behind the Sun to return the data? Well, the 
atmospheric probe's mission is the first of its kind. It is the first 
time an outer planet's (those beyond the asteroid belt) atmosphere 
will be studied directly. So, it's tremendously important, and we 
want to get the data back safely to Earth as soon as possible.

What we plan to send back in this first attempt is the Probe "symbol 
data set" which comprises approximately half of all the Probe data.  
It includes all of the Probe data down to where Jupiter's 
atmospheric pressure is 10 times that of the Earth (the 10 bar level. 
The full data set is expected to go down to the 20 bar level or 
greater).  

We have scheduled other opportunities to return this same data set, 
but these opportunities are not until January 1996 (about 3 weeks 
later). This first return is important because it will provide the 
Probe principal investigators with a quick look at the Probe data set 
and they will be able to determine whether the Probe mission was 
successful or not.

Away from work, I've been playing a lot a sports lately. This past 
Wednesday our volleyball team played its first (and last) playoff 
game. The playoffs were single elimination, best of 3 games. We lost 
our first game by about 10 points, but played much better in the 
second game. At game point, we were losing by, again, about 10 
points, but rallied behind killer serving. We managed to get within a 
few points before losing the game and the match. Oh well, we will 
get them next season.  

I also had a basketball game this past Thursday. Lost that one also, 
but this one was more disheartening than the volleyball loss. We 
were up by 10 points with 1:47 on the clock. The opposing team 
started fouling us to stop the clock and we started making lots of 
mistakes. They managed to catch up and tie and we went into 
overtime.  n overtime, it was back and forth and we kept making 
mistakes. With 10 seconds left in overtime, we were down by three 
and I took a three point shot. My chance to be the hero or the goat (I 
was 2 for 4 from behind the 3 point line)..... GOAT!  
We will get them next time.

I am looking forward to this weekend. I plan to hit the beach. The 
forecast is for shoulder to head-high waves AND Santa Ana winds 
(off-shore flow). It should be pretty good!

Week of 10/16
Monday we spent the day in a science review. The morning was 
dedicated to prioritizing (saying how important something is) all the 
current problems in the "phase 2" software. The phase 2 software is 
used after the spacecraft is placed in orbit and performs data 
compression on the science data before sending it back to earth. It's 
called phase 2 because phase 0 was original operations, phase 1 was 
modifications to perform Jupiter arrival, and phase 2 was for after 
arrival. The prioritizing is to let the software programmers know 
which problems are most serious and should therefore get fixed 
first.

The second part of the day was a workshop for the Principal 
Investigators (our scientists) to get together and think about what 
the spacecraft could usefully do in the event that our tape recorder 
was permanently broken. They discussed possible further changes to 
the compression software (which would be called phase 3), and 
identified what data we could still get.

Thursday we approved a rebuilt early Jupiter approach sequence of 
computer commands (JAA - Jupiter Approach A) which removed all 
tape recorder use. Until we know that the tape recorder can be used, 
we can't risk scheduling it for use by the sequences. The modified 
sequence of commands will be sent to the spacecraft on Sunday.

Throughout the week we've been busy re-planning what to do during 
the rest of Jupiter approach and arrival day. We've been asking 
ourselves "what-if" questions. What if we can't use the recorder?  
Can we protect the relay of probe data? What if we can use the 
recorder? What is the safest way to use it?  Safety of the 
spacecraft, and then safety of the probe data is uppermost in 
every one's mind. Friday afternoon we will begin troubleshooting the 
tape recorder to provide some answers.

                            Jim Erickson

1995 October 13
This week I've been attending the annual meeting of the American 
Astronomical Society's Division for Planetary Science, preceded by a 
week of vacation which took place (quite coincidentally) less than a 
couple of miles south along the Kohala (northwest) coast of the big 
island of Hawaii. This is the annual meeting which is the most 
important for me, as I get to interact with a large number of 
colleagues in similar directions of work. I also get to hear some of
"the latest" and most important news from other fields in review 
sessions.

The vacation was nice and relaxing (well, as relaxing as it gets with 
squabbling 7 and 9 year olds), and it occasionally gave me a sense of 
cognitive dissonance. I could see the summit of Mauna Kea (where I 
usually observe Jupiter etc.) from the back balcony of the condo. I 
could see Jupiter and the moon, as well. My brain said, "I know where 
I am - I've been here before", but then the palm trees swayed and I 
heard the ocean and realized that I wasn't at the summit but down at 
1 atmosphere pressure, and I'm supposed to be enjoying myself and 
not thinking about planets except in science fiction.

Still, I knew that Hubble Space Telescope was taking images of 
Jupiter, and with that the knowledge that the NASA Infrared 
Telescope Facility was working on an automatic program which 
would image Jupiter and its innermost Galilean satellite, Io, 
whenever its facility near-infrared camera (operating at
wavelengths between 1 and 5 microns) was being used. How were the 
images? For a while, I didn't have any electronic access to the data, 
although - unlike being at home in Arcadia, California (one town east 
of JPL's home of Pasadena) - at least I knew what the weather at the 
summit was.

I had spent the month before the meeting frantically getting 
graphical and photographic results out from a half year of observing 
Jupiter in a really intensive Galileo-supporting program. This was 
generally done between 9 PM and 3 AM, while my day job centered on 
serving on Jury Duty in the Los Angeles County Superior Court 
system, finally ending up as a juror for someone who was fighting 
his conservatorship by the Veterans Administration Hospital 
psychiatric lock-up ward - and in that rewarding experience, we 
ended up with a hung jury. While I was to be on vacation, proposals 
for renewing time on JPL's Cray supercomputer were due, as were 
proposals for telescope time at Palomar and at the NASA IRTF; of 
course, I had to get THOSE all done before I left, too.

So the long-anticipated vacation (it had been delayed a year as a 
result of my wife's hospitalization) was REALLY appreciated. Yet it 
was hard to turn off my brain. At least I got to show my mother, who 
went along with us, the summit of Mauna Kea and the 10 or so 
telescopes operating or under construction up there. She didn't have 
a problem with the altitude (nearly 14,000 feet and only about
63% of sea level pressure). About a decade ago, I brought my wife, 
Dr. Linda Brown, a spectroscopist at JPL, to the summit where she 
succumbed immediately to altitude sickness and swore that she'd 
never return EVER!

I had a Sun workstation at the DPS which I'd requested as a way to 
show the results of the NSFCAM program to a meeting of the 
International Jupiter Watch Atmospheres Team: these are 
professionals who are interested in examining the time-dependent 
variability of physical and chemical conditions in Jupiter's 
atmosphere. (I should note that some dedicated amateurs ARE on the 
electronic mailing list: if you want to be on it, contact me at 
go@orton.jpl.nasa.gov). I found out, then, that the NSFCAM images 
supporting the shorter-wavlength HST observations were pretty 
good. I also had two colleagues, (Dr.) Jim Friedson and Joe Spitale, 
who were at the IRTF from Wed. through Fri. nights (Oct. 11 - 13).
Jim is a research scientist, as I am; Joe is a recently graduated 
Caltech student taking a one-year hiatus before returning to 
graduate school in planetary science. He had worked for me as an 
undergraduate student starting in the spring of 1994, and he 
developed a menu-driven program for efficient reduction of our 
astronomical data, particularly infrared imaging. It goes by the 
unattractive name of DRM, for Data Reduction Manager.

Jim and Joe were using NSFCAM over an extended set of wavelengths 
than the automated program (which only records 5), and they were 
working with MIRAC2 - the Middle-Infrared Array Camera, Version 2 - 
one of the first of a new generation of 128 x 128 array cameras 
working between 5 and 25 microns. This region is one in which I'm 
particularly interested, as it gives us direct information on the 
temperature structure of the planet in both the stratosphere and the 
troposphere. This, in turn, is our primary clue to what is driving the 
circulation system.  Before last summer, we'd been more routinely 
mapping Jupiter's temperature field by using a single-element 
detector, a facility instrument on the IRTF which was available to 
anyone, and literally scanning in a regular pattern ("raster scanning") 
over the planet in somewhere between 20 to 50 minutes. This was 
effective, as it gave us a chance to see a great deal on the planet, and 
we published two major articles in the well-known scientific
journal Science which is particularly choosy about the types of 
articles that it will print. On the other hand Jupiter rotates 
significantly in this time (it has a 10-hour period), forcing us to 
make odd corrections for constant longitude lines which were being 
swept around the planet as we took the data. Finally for the 
Shoemaker-Levy 9 Comet Crash campaign at the IRTF, MIRAC2 was
commissioned for the first time and worked almost without flaw, 
allowing us to do in a minute or two what took a major fraction of 
an hour with raster scanning, as Jupiter fit entirely inside its field 
of view.

On Oct. 11, Jim and Joe got a very short briefing from another 
astronomer on MIRAC2's current method for operation, and they got 
some standard star data and just a little bit on Jupiter before it set 
in the west. The following night, they got a little bit of NSFCAM data 
at the same longitude as the Galileo SSI experiment (a CCD camera) 
in the later afternoon before doing a little bit more on MIRAC2. On 
the final night, MIRAC2 had not been cooled with liquid helium 
properly by the IRTF day crew and it was warm and unusable, so only 
NSFCAM data were obtained. On the other hand, the seeing that night 
(the "jitter" of the atmosphere) was quite low, and the near-infrared 
images were quite good.

1995 Oct. 16
We had an emergency meeting of many people on the Galileo project 
this afternoon. A malfunction of the Data Management System (DMS), 
which is a tape recorder on the spacecraft failed to center itself 
properly at the start of the three SSI images last Wednesday. It 
subsequently failed to stop its rewind at the end of the tape track 
and kept on running for 16 hours before controllers could send out a 
"manual" stop command.

We learned that, while the issue is far from resolved, a hardware 
error is apparently more likely than a software error. We fear that 
the tape is entirely wound on one spool and is unusable for the rest 
of the mission. While we won't have more information until further 
tests are in on Thursday or Friday, we started to plan what sort of 
mission we could mount using only the 100-kbyte memory buffer 
which was to service the original mission - the one with a 
functioning high-gain antenna.

To mount new software for getting the data from the instruments 
into a buffer from which it would be transmitted to the ground will 
take several months into 1996 to create. This means while most of 
the Probe data are safe, the remote sensing data which will track 
the Probe entry site in the atmosphere and establish the 
correspondence between remote sensing and directly measured 
results is lost.

This, for me, is pretty devastating news, as this has been the 
central goal of my Interdisciplinary Investigation since 1978 when I 
was chosen.

Whatever correlation is done, should the DMS really be unusable, will 
have to be done from ground-based data. These, in turn, are going to 
be very difficult to get, as Jupiter is only 9 degrees from the Sun on 
December 7, and most telescopes just won't point that close to the 
sun in order to avoid damage to the telescope from focused sunlight. 
The IRTF is one exception where we practiced last year with NSFCAM 
and MIRAC2, using a thin film of polypropylene
(which you last saw lining the inside of a potato chip bag). We 
stretched this material over the 3-meter IRTF primary mirror and 
were able to observe wavelengths of 10 microns and longer with no 
difficulty, 5-10 microns with a little more time needed to get 
decent signal, and nothing shortward of 5 microns.  Time will tell.

1995 Oct. 17
The Atmospheric Working Group, representatives of the four remote 
sensing instruments whose science goals are focused on 
atmospheric objectives, met and determined our most likely 
operating plans. We can probably recover 20 - 25% of the amount of 
the data which we had planned for the DMS-aided (phase 2) mission.  
What is a more daunting task, however, is the fact that all the plans
we made to create command sequences for the phase-2 mission must 
be scrapped, as we now have new rules. For example, it's going to be 
unlikely that we can obtain images of the planet right next to one 
another, as we'll have to play the first set (in at least two colors) 
back to the earth before we can take more and fill the memory 
buffer again. On the other hand, we can try for targets on opposite 
sides of the planet, with something like 5-hour separations.

On everyone's minds is the cost-cutting mind-set of the current 
Congress and its likely predisposition to cancel funds for the entire 
mission, if they have the impression that it's going to fail.

1995 Oct. 18
A subdued celebration of the 6th anniversary of Galileo's launch.  
News is that we'll know by Friday whether certain tests were 
positive for hardware failure, but if they were negative, we still 
may not know whether the problem is hardware or software related.  
I have to decide whether to re-submit my Palomar time proposal 
with more than just the 3 days per encounter (for early July to 
support the first encounter and early September to support the 
second encounter) to five or more; I want to cover the time, and the 
value of ground-based support is infinitely higher if we have no 
operating DMS on the spacecraft.

I began to work on an insidious problem, trying to match the 
computer results of my colleagues, (Drs.) Larry Sromovsky and 
Andrew Collard at the University of Wisconsin, in simulating the 
results of the Probe Net Flux Radiometer. I'm down now to nuts and 
bolts of the program to see where the differences lie.

I also began to work on reworking simulations of the atmospheric 
structure which I worked on last in 1979. This was hard, and I didn't 
succeed in replicating the results; I couldn't find all the 1979 
software, so I used the current versions of the software. I succeeded 
in reproducing the pressure-temperature curve, but I think that I'm 
having the same difficulty in replicating the altitude scale as (Dr.) 
Al Seiff (NASA Ames Research Center) did when he asked me to look 
them over again. The original difference is now most probably lost 
to history, but the Probe engineers designed to the specification of 
those models. So, it's more than just a little disconcerting to me.  
Still, the altitude scale is just off by about 4%. It reminds me that I 
probably want to update the model with a whole lot more recent 
information.

Email from (Dr.) Bob Joseph, the director of the IRTF. He's heard 
rumors that the Galileo tape has failed and asks what the IRTF can 
do to help. I want to print it out and frame it, God bless him. So now 
we're brainstorming on what we can do more than what we're doing 
now. Can we "stop down" the mirror by placing an opaque annulus 
which covers the outer area of the telescope where the sun will be 
shining on the primary and work (very carefully!) with the shorter 
wavelengths with NSFCAM? Can we use the off-axis CCD guide 
camera and get images, with similar "stopping down"? Do we want 
to try to use the near-infrared spectrometers (a crude spectral 
capability of NSFCAM or higher-resolution capabilities for another 
IRTF facility, CSHELL) in the 5-micron region which would work OK 
with the polypropylene cover? Ditto with the riskier opaque annulus, 
with much less noise?