Thursday, May 15, 9-10 a.m., Pacific: Phil Christensen.
Phil is the principal investigator for the Thermal Emission Spectrometer
instrument onboard the Mars Global Surveyor spacecraft. In addition, Phil
is also a professor at Arizona State University where he works as a
planetary geologist. He enjoys teaching courses on the solar system, the
geology of Mars, and the use of satellite images and data to study
planetary surfaces. Please prepare for the WebChat by having your students
read Phils bio at: http://quest/lfm/team/christensen.html

MARS TEAM JOURNAL #1: JUST HOW BIG IS THIS MANEUVER, ANYWAY?

by Pieter Kallemeyn

[Part two of a four-part series on the design and execution of Mars
Pathfinder's third trajectory correction maneuver (TCM-3). In last weeks
journal I reported about what is involved in determining the orbit of Mars
Pathfinder by examining tracking data, the first step in planning a TCM.
The next step is determining the characteristics of the maneuver itself,
specifically the size and direction of the maneuver. - PHK]

April 29, 01:00-05:00 PM

Okay, a few hours ago we determined where the spacecraft is through orbit
determination. It is now after lunch and we start the next stage. We can
take the orbit solution and project it forward into the future to predict
where the spacecraft will be when it gets closest to Mars. This gives us
our best guess of where Mars Pathfinder will be two months from now. This
prediction shows that on its current course, the spacecraft will come
closest to Mars on July 4, 1997 at 16:52:52 GMT, and at that time it'll be
169.3 km from the surface. Since the atmosphere of Mars is less than 150
km, this means we're on what we call a flyby trajectory. If we did nothing
else for the rest of cruise, we would fly right past Mars, never entering
the atmosphere and never landing. Clearly we have to do *something* at
TCM-3 in order to land successfully.

But what exactly do we do? First, the Navigation Team must determine what
the target is for this maneuver. The target is simply the coordinates
required to enter the atmosphere at the right time and at the right angle
so that the entry vehicle will slow down in the atmosphere, the parachute
deploys on time, and we land where the mission needs us to land. To find
this target, we run a program called AEP, short for the Atmosphere Entry
Program. Our local expert on atmosphere entry is David Spencer, a member
of the NAV team, so he performs this analysis. David takes the desired
landing site along with a computer model of the Martian atmosphere and the
entry vehicle, and computes the target at the top of the atmosphere. After
about an hour of computer time and some consultation with another
atmospheric flight expert, (Bobby Braun in Langley, Virginia, who is a
part-time member of the NAV team) we have our answer. The spacecraft must
enter the atmosphere at an altitude of 125 km and flight path angle of
14.2 degrees below the horizon at exactly 16:51:49 GMT.

Now we have the target, so we combine it with our current orbit solution
in a program that determines the size and direction of the maneuver. We
have to input the time of the maneuver, which we know will be May 7 at
01:00 GMT, based on the tracking schedule. The answer is (...drum roll
please...) a maneuver size of 10.5 centimeters per second. This is a very
small maneuver, approximately 1/4 of a mile per hour. Despite its small
size, this maneuver will alter our trajectory by over 200 kilometers over
the next two months.

The results of the maneuver calculations are passed along to Sam Sirlin,
who will determine the parameters needed for the commands we'll send to
the spacecraft. It's his job to take this maneuver size and direction and
convert it into a language the spacecraft will understand. This is what
the 'language' looks like:

1997-127T00:15:49.000 J2000_TRACK 0.956677 -0.265867 -0.118676 1200
1997-127T01:00:00.000 LATERAL_TCM 37.9436 -0.348121 -0.828953 -0.437778
0.956677 -0.265867 -0.118676 AB 450
1997-127T01:26:54.831 J2000_TRACK 0.956677 -0.265867 -0.118676 800
1997-127T02:02:45.831 AXIAL_TCM 20.6263 0.956677 -0.265867 -0.118676
MINUSZ 885
1997-127T02:35:22.457 J2000_TRACK 0.956677 -0.265867 -0.118676 800
1997-127T03:11:13.457 LATERAL_TCM 46.8061 0.231928 0.85967 0.455167
0.956677 -0.265867 -0.118676 AB 495
1997-127T03:39:19.875 J2000_TRACK 0.956677 -0.265867 -0.118676 800

These are command parameters and they're part of a group of commands
called a sequence.  Don't worry about understanding this... I'll explain
it later.

April 30, 1997 - TCM-3 Design Status Meeting

At 10:00 today, members of the Flight Team involved with the design of
TCM-3 gathered in a conference room to go over the game plan for the
maneuver, now less than eight days away. The person in charge of the
meeting is Guy Beutelschies, who is the flight engineer responsible for
building and testing the maneuver sequence. The rest of the people in the
room represent Navigation, the various spacecraft subsystems, and the
mission manager Richard Cook. This meeting is our first chance to discuss
the TCM-3 plan in detail, ask each other questions, and obtain a consensus
on the sequence of events for this activity.

The meeting starts with an explanation of the orbit determination, which
Robin Vaughan presents. Then Guy describes the sequence of events.
Normally, we would execute this maneuver with a combination of two
maneuvers. The first would be along the direction of Earth (called the
axial mode) and the second would be along a direction at nearly a right
angle to the Earth direction (called the lateral mode). However, we want
to do something new with this maneuver. We want to test the capability to
perform a 0.4-meter-per-second maneuver in the lateral mode. Why? Because
on July 4 there is a slight possibility we may need to do such a maneuver
to correct our flight path, and we haven't yet tested that large a
maneuver in the lateral mode. In order to assure ourselves that we can
rely on such a maneuver in the last few hours before approach, we want to
test it out while we perform TCM-3, 60 days before we land. This will give
us plenty of time to understand how the spacecraft behaves in such a test,
and to debug any problems we may encounter. A good deal of spacecraft
operations involves testing the capabilities we later need to rely on for
doing the real mission.

Therefore, the team decides to do the maneuver as follows: We will first
update the attitude of the spacecraft so we are pointed 5 degrees from
Earth, a routine activity we've done many times in the past. About an hour
later, we will perform a lateral mode maneuver of 0.4 meters per second.
An hour and a half later, we do a .1-meter per-second axial maneuver
(toward Earth), and finally another lateral maneuver of 0.5 meters per
second, an hour and a half after that. The last maneuver is in the
opposite direction from the first one, so this is what I like to call a
'zig-zig-zag' maneuver. We 'zig' a bit to the left, 'zig' a little bit
forward, and finally 'zag' a bit more to the right, so to speak.

The rest of the status meeting is devoted to other concerns: What data
rate shall we be at? Who will notify the ground stations? What emergency
actions should we be ready to perform if something goes wrong? Etc., etc.
Since we've done two maneuvers before (on Jan 10 and Feb. 4) we know a lot
more about this type of activity, so the meeting goes pretty fast. The
meeting ends after an hour. The next step is for Guy to build the sequence
of commands that will tell the spacecraft what to do on May 7, and to test
it on the ground before sending it to the spacecraft.

Next journal..... Building and Testing a Maneuver Sequence.

MARS TEAM JOURNAL #2: A THIRD GRADER'S PERSPECTIVE

by Katie Herleman My name is Katie Herleman. I'm nine years old and am in third grade at Pineview School in Sarasota, Florida. I love rockets and my dream is to some day work at NASA as a scientist. I went to the Kennedy Space Center to see the Mars Pathfinder launch. It was one of the most exciting times of my life. I often think that in the years to come, as we continue to explore the planets and the universe, I will always remember that I was there during a very big moment in NASA history, watching the launch. I look forward to going to many more launches and I hope to go to Space Camp this summer. My dream is to go to the Air Force Academy, become a pilot and then ON TO NASA! When I got to see the Mars Pathfinder lift-off, it was one of the most exciting experiences in my life. It was like one big party at the Kennedy Space Center, and even in the nearby town of Cocoa Beach and Cape Canaveral, where everybody seems to be a space exploration addict. I have seen photos of launches but they are nothing like the real thing. When the Pathfinder took off, I felt like jumping out of my seat, it was so exciting. Even from a distance, the noise and the light were amazing. I felt almost like I was going up with it. All of us sitting in the bleachers jumped up together and cheered. I had a chance to meet the people who make the space program happen, and they were all really, really nice to me. They gave me pins and souvenirs and stuff, including a great T-shirt that showed a Martian and the Sojourner Rover that says "The Earthlings are coming. There goes the neighborhood!" I even met Buzz Aldrin, one of the most famous astronauts of all times. Wow, what a neat man! And real friendly to everybody. Mr. Hugh Harris of the Public Relations Department at the Kennedy Space Center was my hero. He introduced me to all sorts of people working with NASA and the Jet Propulsion Laboratory. A nice man named Dan Woods from the Washington, DC NASA office made me feel so special, too. I have never seen a launch with real astronauts, but I can tell you this: I will go back to Cape Canaveral every chance I get. It's wonderful to be a part of history - even a tiny little part. Just think... the rocket I saw take off will reach Mars on July 4, 1997 if all goes well! We are planning a Mars Pathfinder party out in the country where we moved to so I can launch my rockets and build experiments without the neighbors calling my parents very time I shoot off rockets in the backyard. I got a telescope for my birthday and we live out where you can see the stars so clearly. I fell in love with the Sojourner Rover and learned a lot about it at speeches and press conferences I was invited to when I got to go to the Kennedy Space Center. I am working very hard on making Sojourner Rover for my school science fair. It is an annual science fair and I won a big prize last year for my experiments with rockets where I found a lot of fun experiments and information that NASA had on the Internet. I made a hero engine out of Coke cans, a balloon rocket car and matchstick rockets where I tested Newton's Third Law of Motion. My school was very proud that a second grader could win a big science contest like I did and they are looking forward to my entry this year to represent our school. Now that I am older and in third grade I think my trip to NASA and all of the neat engineers I got to sit with at the Mars Pathfinder lift-off have helped me come up with my science project. I actually met the real-live engineers who put the Mars Pathfinder into orbit and who built Sojourner. I am studying solar energy and I think I can build a small demonstration of the Sojourner Rover after seeing what I saw at the Kennedy Space Center. Plus, I found some really good information on the Internet to help me with the dimensions for Sojourner. I also found a nice engineer named Geoffrey Landis at the NASA Lewis Research Center who is an expert on solar energy and even invented the dust measurement device that is attached on the Sojourner Rover. Who knows? Someday I might even be at NASA, too, and some kid could watch me take off and say to her parents, I want to be an astronaut just like Katie when I grow up! I got to meet Donna Shirley who works at JPL and Wesley Huntress from NASA Hqs. at the beach party I was invited to at Patrick Air Force Base. They gave me their personal pins... Donna gave me her Sojourner Rover pin and Mr. Huntress gave me his Mars Pathfinder pin. I have a coat from the U.S. Space Camp gift store I got as a memory for my most favorite trip in the world to NASA and to see the Mars Pathfinder lift-off. I got patches on the coat and I put those special pins from Donna and Mr. Huntress on my jacket. At my school they call me "Rocket Girl" and "NASA Girl" because I wear mostly NASA shirts to school and I write all of my reports about NASA. We had a contest at our school called the Nobel Prize Competition and I won the Physics prize for my project on rockets. I am so glad that I got to go to the Mars Pathfinder lift-off. It is my goal to see a manned lift-off someday at the Kennedy Space Center. I wish all kids could see a lift-off, too. I will never be the same!

MARS PATHFINDER MISSION STATUS

[Editor's note: This status report was prepared by the Office of the Flight Operations Manager, Mars Pathfinder Project, NASA Jet Propulsion Laboratory.] Friday, May 2, 1997 The spacecraft remains in good health and is currently about 98 million kilometers from Earth. No significant spacecraft activities were performed this week. The total flight time since launch is now 149 days, and we have 63 days until Mars arrival. The EDL planning team completed a very successful Sequence of Events Peer Review. This review covered the detailed flight software sequence used during EDL, recent changes in the EDL software, robustness and regression testing performed since launch, and detailed operational plans for TCM-5 and EDL approach. The review board, led by Jim Marr, agreed that no significant holes exist in the EDL sequence and that we are well on our way to being prepared for EDL operations. For further information, please visit our website at: http://mpfwww.jpl.nasa.gov

MARS GLOBAL SURVEYOR FLIGHT STATUS REPORT

[Editor's note: This status report was prepared by the Office of the Flight Operations Manager, Mars Surveyor Operations Project, NASA Jet Propulsion Laboratory.] Friday, May 2, 1997 No major activities took place this week. For the past three weeks, few activities have occurred because the Surveyor spacecraft has been configured in a quiet state for a search campaign to detect gravity waves. According to theoretical physics, these waves are gravitational disturbances emitted by all objects in the universe. However, because gravity is a relatively weak force, detection of these waves is almost impossible unless they are generated by massive objects such as black holes and matter at the center of the Milky Way Galaxy. To date, nobody has ever detected a gravity wave. If Surveyor encountered these waves, the spacecraft would experience an extremely small jolt. This tiny bumping motion would cause a tiny shift in the frequency of the spacecraft's radio signal transmitted to Earth. Analysis of the data generated by this experiment will take six months or more. After a mission elapsed time of 176 days from launch, Surveyor is 92.74 million kilometers from the Earth, 37.03 million kilometers from Mars, and is moving in an orbit around the Sun with a velocity of 23.89 kilometers per second. This orbit will intercept Mars 132 days from now, slightly after 6:00 p.m. PDT on September 11 (01:00 UTC, September 12). The spacecraft is currently executing the C7 command sequence, and all systems continue to be in excellent condition.

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