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How Pathfinder Moves Through Spaceby Rob Manning
June 24, 1997
A couple weeks ago we finished up with something that has been haunting us a little bit. When we launched Pathfinder, we didn't have enough time in the one testbed that we have, to test all the software that does all the controls, the whole process as it comes from outer space to the surface of Mars. We tested virtually everything else, but not the software. We went through and did a lot of testing in January, February, March and April, and then just a couple of weeks ago we finally finished testing all the changes we made and we sent the software to the spacecraft this last week!
The last couple of weeks we've been watching very carefully to see where the spacecraft is headed. We follow its current position and we try to project into the future its path to Mars. But remember, Mars is a moving target! We're basically rendezvousing with each other in space! The further out in time and distance we project, the fuzzier the understanding is. In come our navigators, who are very smart people, all trained in mathematics, space physics and space science astrodynamics; they have special software tools that help them do this work. They watch very carefully and project the trajectory over and over again, virtually everyday, trying to figure out where the spacecraft is. That's kind of a trick because there are no cameras on the spacecraft to see where we're going. However, we can actually measure how far the spacecraft is from Earth by bouncing our radio signal off our spacecraft. We basically send a signal up to our spacecraft, the spacecraft hears that tone exactly and sends it right back to the Deep Space Network.
There are three deep space stations placed strategically around Earth: one near Madrid, Spain; one in the desert of Goldstone, CA; and another near Canberra, Australia. At any time, day or night, one of these stations can see our spacecraft because it rises and sets just like a star and just like Mars. When one station is done it passes the tracking responsibility onto the next one. At any given moment, not only are they listening to the spacecraft, but they're also sending this tone up to the spacecraft and when it comes back to Earth we can actually measure how long it takes for the radio waves, which are just like light, to go from Earth to the spacecraft and back. We're basically getting a detailed record of how Pathfinder moves in space and how far it is at any given moment. This is called ranging.
Think of the spacecraft as a train whistle that's producing a tone. As the train comes toward you, you hear at one frequency a higher pitch. After the train passes by you it produces a lower pitch. This is called the Doppler effect. We can use the Doppler effect to tell us how fast the spacecraft is moving away from us. The combination of Doppler and ranging - ranging tells us how far, Doppler tells us how fast - we can figure exactly where the spacecraft is relative to the deep space stations.
So now we know where the spacecraft is and we know how fast it's going. We know where Earth is in the solar system. We know where the sun is and we know where our spacecraft is. But why do we not know more precisely where it will be on July 4 at 10 a.m.? The answer to that is, we don't know exactly where Mars is! We don't know where Mars is even right now and the reason is because it doesn't follow a perfectly circular orbit around the sun. The reason it doesn't follow a perfectly circular orbit is because of Jupiter.
Jupiter is an enormous planet out there. We knew where Mars was 20 years ago when the Viking spacecraft landed. We were able to figure out exactly where the spacecraft were on the surface of Mars, therefore, we knew exactly where the center of Mars was. But since then, we've been watching Mars go around and around, all the while Jupiter has been pushing and tugging on it. By today, nearly 21 years later, Mars' orbit is uncertain by about 10-18 km. Now that's not very much, but it makes a difference to us! Because we're hitting Mars at a glancing angle, 18 km makes a big difference of whether we land in the east by north-east corner of our landing ellipse, vs. the south by south-west corner.
As Pathfinder gets closer and closer to Mars, it starts to feel the effect of gravity due to Mars and starts falling in, actually starts speeding up toward Mars. As soon as we start to see that happening in the Doppler data, we know almost exactly where the center of Mars is because we know exactly how much the gravity of Mars is. We don't expect to know until about a day and a half before we get to Mars. We're going to be watching very carefully. By about midnight July 3, we should have a pretty good idea if we have to do a course correction. We have two opportunities: one at 10 p.m. and the other at 2 a.m. the morning of landing, to do a contingency course correction and aim more toward Ares Vallis. I think the odds of us doing that are kind of low because even right now we're aimed pretty close, we think to Ares Vallis. We're going to do a really tiny trajectory correction maneuver, or TCM, tomorrow. It's our fourth and smallest one we've ever done. It's hardly anything, but it's enough between now and July 4 to move us very slowly in the right direction so that we'll be aiming right to the center of Ares Vallis.
We're still trying to decide exactly how much tomorrow's burn is going to be. We're going to have a meeting to talk about that in half an hour from now. Another thing that has been going on, and this is really kind of fun because we really haven't done as much of this, is that the press have arrived. We have "Good Morning America" here today. We've had lots of newspaper reporters here. I saw myself on page 14 of the "Daily News." I went and bought a bunch of them, cut out the articles and sent them to my family!
When Pathfinder is descending to Mars, I will have a fun job. It turns out that just after cruise-stage separation and all the way through landing I'm going to be flight director. This is not really an important job because we're not going to direct anything but the spacecraft, and it's doing that all on it's own. But I will be doing a vocal commentary play-by-play. At 10 a.m., Pacific, on July 4 when we land, you will be hearing my voice! After that we don't hear much from the spacecraft so I'm basically biting my nails and hoping for our first digital data downlink session, where we actually get ones and zeroes, not just a tone. This should happen at a little after 2 p.m., Pacific when hopefully, we'll get our first real data from the spacecraft.
Pathfinder will not be using its high-gain antenna, the lollipop-shaped fixture that can actually point at Earth, because we won't yet know where Earth will be. We'll use the other antenna that looks like a piece of aluminum pipe sticking up next to the camera. We will use this antenna, which will squirt radio signals out in all directions, in hopes that Earth will catch a few of those video photons and we'll see data at about 40 bits per second. With that data rate being so slow, if you wanted to load a picture using Netscape or Microsoft Explorer, you'd have a long wait before you got a picture back! So we decided we're not sending any pictures back, we're not using the camera at all then. We'll just look at the actual health of the spacecraft: Did it make it? Is the rover still alive? Is the lander still alive? Did anything break? How hard was that bounce anyway? At 3:30 p.m. there will be a little press conference where I will give a quick verbal description of what we saw.
Then I'm going to leave it to some other folks to deal with the daytime operations. I will have been up for about 14 hours at that point and I'm going to be really tired. I'm not a very good person to ask to make big important decisions. So I'm going to leave it to our flight director, Jennifer Harris, who is going to be responsible for all the operations on the first day on Mars. I'll get a good night's sleep, finally, when it's all over. I'll watch the second day and on the third and fourth days I'll be the flight director for the surface operations. We take turns.
It's amazing to me that we can even get to this point. There is a lot of responsibility to deal with here. All of us who've been involved in this for so long keep looking at each other and saying, "Did we forget anything?" I haven't had any nightmares recently. But in one that I did have, I launched my dog Scooter up in the spacecraft and it landed on Mars. I realized, too late, how was I going to get him home? I was really upset with myself for letting him go! I haven't had any nightmares since then! At this point it's kind of a relief because there's so little we can do. Any mistakes that were caused by us were made sometime ago and it's not likely that we'll be able to uncover them. Quite frankly, if it doesn't work it's mostly likely because we messed up, forgot something, or didn't visualize something right, or we just didn't know how the thing really worked when we thought we did.
The only thing I can do to convince other people that we did our job right, that I did my job right, is by going through and showing them exactly what we did and why. I've done that a twice now with some top people in the space industry--people who helped invent the Apollo program, people who helped lead the Viking lander design.
This is a nerve-wracking experience and I don't mean to say we're perfect. It is really tough to go to another planet. I've worked on the Galileo, Magellan and Cassini missions and I know how tough it is to build these big spacecraft to go other planets in orbit. But building a lander is a whole other animal because you're subject to so much more of the vagaries of the nature of Mars and nature everywhere.