Charles Whetsel is a spacecraft systems engineer on the Mars Global
Surveyor Project. Currently he is at the Kennedy Space Center preparing
for the upcoming MGS launch. He will be available in the Live From Mars
chatroom this coming Wednesday, October 30 from 9-10AM Pacific
(noon-1PM Eastern).

Consider joining us to ask Charles about:
- the latest status on MGS (the launch is scheduled just one week after
  the chat)
- what a spacecraft systems engineer really does
- how he got from rural Tennessee to NASA's Jet Propulsion Lab
- the bike rides he and his wife Anne go on
- or other things your students are interested in

To best prepare, please have your students read Charles'
biography before the WebChat session. It is at:

To virtually meet Charles, point your Web browser to

and follow the
links to the chat room for experts. If you plan to participate in this
event, please RSVP to Andrea by sending a brief Email note to
andream@quest.arc.nasa.gov. This RSVP is very important, since it will
allow us to ensure that the chatroom does not become too crowded.

Within the next week, a more complete schedule of future guests will be
announced. If you want to participate, but the time selected is a problem,
please summarize your schedule issues to Andrea so we can try to
accommodate your class in future events.

Last week, we asked:

     Let's say you have just been appointed Baseball Commissioner
     for Mars. You would like the game to be similar in difficulty to
     the game as played on Earth. With that in mind, how far back
     should you place the center field fence (so that it is just as
     hard to hit a home run).

     Assume that a center field fence on Earth is 410 feet from home
plate.

ANSWER from Alan Federman:
As a first approximation, we need to look at the relevant equation:

      F = MA Force is equal to Mass times Acceleration.

The acceleration we are interested in, is due to the gravity field of
Mars.
Mars gravity is equal to 0.38 of Earth's, so as a first approximation, the
"A"
on Mars is .38 * 980 cm/s/s = 370 cm/s/s. If the Force of Gravity were the
only effect on the ball, 410ft / 0.38 = 1079 feet (or 323 meters).

To make the game "play the same" Other factors need to be considered.
For example, atmospheric effects. The thin atmosphere means less air
resistance so balls will carry further. How fast people can run wearing
space suits, would also be a problem. Maybe changing the mass of the
players and their equipment is an option.

While rain-outs are not going to be a problem, games may need to be
called on account of wind or sandstorms!

Good Luck, Commish!

ANOTHER ANSWER from Bryan Glenn:
The old baseball Commish will have quite a problem on his hands placing
that fence in the right location. There are actually 2 variables he will
have to consider, gravitational differences and atmospheric differences.
Both will have a significant impact, but the latter will be much less
predictable that the first.

When gravitation is compared, Earth's would be +/- 978 cm/sec2, while
Mars' is estimated @ 371cm/sec2. 978/371= 2.64, so the
410 ft x 2.64 = 1081ft. That would seem a mighty drive for anyone,
if the two atmospheres were comparable. But they are anything but!

Earth's gravity and Venus' gravity are almost identical, but if we were
putting up a fence on Venus, a 410 ft fence might as well be 2 miles
away. Atmospheric pressures on Venus are 100 times that of Earth, so
driving a ball through that layer of carbon dioxide smog would require a
mighty, mighty bat.

Mars' atmospheric pressure is estimated at .005% that of Earth's. Again,
some quick calculations should yield the lower atmospheric drag on the
bat and ball to determine the "atmospheric" adjustment. But it is not so
simple; here again we cannot think of this in Earthly terms. The extreme
thinness of the atmosphere and the generally colder temperatures will
produce some very "Mars Only" considerations. This thin atmosphere is
easily varied by minor climatic events that would produce far less change
in Earth's heavier atmosphere. Martian temperature changes could easily
produce sudden gusty winds roaring over 100 miles/hour. As winter
approaches and more of the CO2 becomes crystallized at the poles, the
already thin atmosphere will become even thinner. Parks near the poles
will play far differently than those near the Martian equator. Home runs
will be even easier to hit then, unless the ball runs into an unexpected
200 mile/hr blast of wind on its way to the fence!

Good luck commissioner. Your Martian game will add elements never
dreamed of back on good ol' Earth!

A special thanks to these folks who sent in their best guesses:
Philip and DMackson (Dad)
Chris Rowan's students
Roxanna Muniz
Susan Rico,
Mr. Grott's students
Mrs. Phaneuf's students
Mrs.Grady's students
Sandy from St. Anne School
Mr.Makar's 3rd Graders, Alden Place Elementary School, Millbrook, NY
DMLedet
Evan (from Janet Cook's class)

Their complete answers will available online on the Web

Here is this week's Challenge Question:

The Valles Marineris is much larger and deeper than the Grand Canyon in
Arizona. Yet, if you stood at the rim of the Valles Marineris, it probably
wouldn't seem as impressive to the eye. Why?

You are invited to send original student answers to us. We will list the
names of these folks online and token prizes will be given out to a small
number of the students with the best answers. Send your answers to
Jan Wee at jwee@mail.arc.nasa.gov.
PLEASE include the words "CHALLENGE QUESTION" in the subject of the
email. The deadline for this question is Halloween (October 31).

[Editor's note: Guy Beutelschies is a test director for the Mars
Pathfinder project. He leads a team that tests the spacecraft to
make sure everything works. Here he describes his activities at the end of
the summer]

TESTING THE PATHFINDER AT KENNEDY SPACE CENTER
Guy Beutelschies - http://passporttoknowledge.com/lfm/team/beutelschies.html

Week of August 12th
The spacecraft arrived at Kennedy Space Center. It was pouring
rain. We waited until the rain stopped to roll the container that the
spacecraft is in into the airlock. We then wiped down the exterior of the
container with alcohol to clean off any dirt and to kill any biological
material. Mars Pathfinder has to be very clean from a biological
standpoint so it does not contaminate Mars. Spores from Earth life are
actually hardy enough to withstand the flight through space. After the
container is cleaned, the top was taken off and the spacecraft was
moved to a workstand using an overhead crane.

Meanwhile, the rest of the electronic equipment used to test the
spacecraft was craned up to the test complex, which is on the second
floor. It is very nerve racking watching million dollar equipment swinging
in the air 20 feet from the ground. This equipment used to create
commands,
process telemetry, and provide power to the spacecraft. It is connected
via long cables which pass through the wall and down into the cleanroom.
This allows us to do most of our electrical testing without having to put
This allows us to do most of our electrical testing without having to put
on cleanroom clothing, which are called bunny suits (because they make
you look like a big bunny without ears).


Week of August 19th
Testing started. We found several problems with our Flight
Software during our testing at JPL so we decided to repeat our complete
Mission Mode test down here in Florida. This test started with launch,
went through the cruise to Mars, the descent to the surface, and then the
Day 1 activities. This went pretty smoothly until the (simulated) entry to
the Mars atmosphere. We were feeding data into the accelerometers to make
the spacecraft think that it was entering the atmosphere and slowing down.
There is software onboard that is supposed to read this data and figure
out
when to fire the parachute. This software never produced an answer. The
telemetry showed that the parachute fire signal was sent based on the
backup timers and not the software algorithm. We spent a couple of days
(and nights) troubleshooting this before we found the bug in the software.
The rest of the test went smoothly


Week of August 25th
The mechanics then opened up the Lander and took the petals off.
We are doing this so that we can install fresh batteries for launch. We
also have to put in the Radio-isotope Heater Units (RHUs) on the Rover.
These are devices containing a very small amount of Plutonium that give
off heat to keep the Rover warm. We also installed a small amount of
Radioactive Curium in the Alpha-Proton X-Ray Spectrometer (APXS)
instrument on the Rover. This instrument uses the radioactivity in the
Curium to give off alpha particles. When placed against a rock, these
particles will hit the molecules in the rock, which in turn will release
x-rays, Protons, and other alpha particles. The instrument looks at all
three of these and determines what elements are in the rock.

We also took off the thermal enclosure on the Lander so that we
could replace an antenna switch. During one of the last tests at JPL, we
broke it due to a bug in the software which applied power to it for much
longer than it was designed for. We had a spare so we swapped it for the
broken one. We then ran a series of tests to make sure that everything
under the thermal enclosure was working properly before we put the
enclosure back on.

[Editor's note: Greg Wilson is a planetary geologist who studies the 
effects that wind creates on the landscape of other planets. He
helps run special wind tunnels (unique in the world) that simulate
the atmospheres and surface interactions on Mars and Venus. Some
tests on the Mars Pathfinder spacecraft were run last year in the
Mars wind tunnel.]

SAND DUNES ON MARS
Greg Wilson - http://passporttoknowledge.com/lfm/team/wilson.html

October 10, 1996
After two weeks of summer vacation in Germany and England, it was time
to get back to work and the Planetary Aeolian Laboratory. I really like to
travel, but it was time to get back to science and a full summer of
research, presentations, and proposal writing.

Upon returning home and getting over jet-lag, I had to prepare the Mars
Wind Tunnel for a series of experiments involving wind flows of sand
dunes. Dr. Haim Tsoar, a geography professor from Israel, and a world
authority on sand dunes, would be spending two months at the laboratory,
and together we were hoping to explain why we don't see a particular sand
dune type on Mars. The dune type we were interested is called a linear
dune, and they happen to be the most common form here on Earth, but we
don't see them on Mars. Why? We think is has something to do with the
thin atmosphere of Mars, so with the help of Dr. Bruce White, a fluid
mechanics professor at University of California, Davis, we developed a
matrix of experiments where we simulated different Martian atmospheric
conditions and wind directions. Hopefully this would answer our
questions, but developing the instrumentation need to measure the flow
field in all the locations was going to be difficult. We started by
attaching in a grid pattern very pieces of light string (tuffs) to the
dune model in the tunnel. When the tunnel was turned on, we could see
the flow directions and Haim explained to me the mechanism for how
linear dunes were formed. If these mechanisms were not present on
Mars, it would explain why we don't see them.

While this was going on, I had to prepare at talk on the physics of wind
erosion and dust generation for a presentation in Lubbock, Texas. Not
only did this take time away from the dune experiment, but I would be
traveling for a whole week. I was nice in Lubbock. I spent 5 years
there during my graduate studies, and it was really great to see my old
instructors and advisors. The talk went well, but I couldn't help but
worry about the experiments going on at the lab, and how we were going to
get the measurements we wanted. I think being a scientist is a lot
different than other jobs because you are always thinking about science
and how to solve problems.

When I got back to the lab, Haim was tired of the "tuffs" and want to
make measurements of the shear stress at different locations on and
around the model. You see, it is the shear stress (energy of the wind)
that moves sand grains and make sand dunes, and measuring this would
help prove our theories. We normally measure shear stress by measuring
the wind velocity gradient near the surface with a pitot tube, but this
technique does not work in separated flow, which was occurring down-
wind of the dune model. I said, "Houston, we have a problem!" Until one
day,
I was walking down the hall of this building, and I saw a poster on the
wall of this super sonic (faster than the speed of sound) airplane wing
and a simple technique of measuring skin friction on it. After doing
some math, I learned that shear stress can be calculated from skin
friction measurements. So I went to talk with Dr. David Driver, the
author of the poster, and he was more than happy to help me. The
technique worked really well, except for the fact that the small dust
particles really mess up the measurements. Solution, wash out the entire
wind tunnel and put a big air filter on it. This was not a fun job. But
it was well worth it. And while the experiment is not complete yet, and
Haim is back in Israel, we are really excited about the things we are
going to learn and the many applications we have for this new technique.
While all this was going on, proposals to do science with Mar Pathfinder
were quickly becoming due. I worked on the wind tunnel experiment
during the day, my nights and weekends were spent working on my proposal.
I think it would be really exciting to operate a spacecraft on the surface
of another planet, so I didn't mind giving-up a lot of my personal time to
try to make it happen.

Well, as of October 10, the proposal is all done and in the mail. It is
time to sleep and relax a little. I still have the dune experiment to
complete, other proposals to write, and other trips to take. As for now,
I'm going to go backpacking in Kings Canyon, California this weekend with
another planetary geologist here at Ames and my girlfriend.

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