OFJ Field Journal from Steven Tyler - 12/2/95
AIMING THE SPACECRAFT 1000KM ABOVE IOOur big day, Thursday December 7, is less than five days away. Five days was a special number for me. That's when we would do our last course correction maneuver before reaching Jupiter, "Trajectory Correction Maneuver 28A" (or TCM-28A for short).
We planned to do a correction maneuver 20 days out. But the one we did back in August was so good that we didn't need to. At 10 days out, we planned to do another correction. However, once again, we were so close to being on target that there didn't seem much point to it: we would probably wind up further from our target if we did the maneuver! So we canceled that one as well.
Since we are not taking any close-up pictures of Io as we fly past, the main issue is: what will our altitude be at Io? At 10 days out, it looked as though we were right on target: 1000 kilometers above Io's surface.
If we do TCM-28A, the Navigation Team will design the maneuver on Friday morning, December 1. By 11 AM they will give the Orbiter Engineering Team (that's us!) the change in velocity (or delta-v) that's needed. We'll start working on how we need to fire the spacecraft's thrusters in order to get that delta-v. At noon there will be a maneuver design meeting where we'll get approval to continue working on the maneuver. By this time, we'll be just about through with our implementation. By the end of the meeting, the Attitude and Articulation Control Team and Retro Propulsion Module Team (the people in charge of the thrusters) will have finished their work and we'll be ready to generate a sequence of events. We'll all look at it to see if it's right and by 4 PM the Sequence Team will have our product. They will have until 9 PM to produce a final sequence for everyone to review. At 1 AM Saturday (yes, that's right, 1 AM--in the morning), we'll have a final approval meeting, and by 2:30 AM the sequence may be on its way to our spacecraft. The maneuver will start at about 3:30 PM on Saturday. We call this schedule our "24- hour template" ... it is the equivalent of a football team's 2-minute offense. By the way, when I add it up, it comes to a little less than 24 hours for us, just like a football team sometimes has a little less than 2 minutes. Unlike a football team, we have no opponent who is actively trying to stop us. Our team is ready, and we're becoming increasingly confident.
Of course, there is no point in doing an unnecessary maneuver. Why should we tire ourselves by working all night when our biggest week is coming right up? It is becoming clear as the Friday noon meeting approaches that this will be the time for a decision to be made. Will we do the maneuver or not?
The argument for doing the maneuver is straightforward. New data are showing that we are coming into Io a little low. Instead of 1000 km, we'll probably be somewhere between 900 km and 975 km. If we do a maneuver to raise our aimpoint up by about 60 km, we will be closer to our target. The time to do this maneuver is now!
However, there is a counter argument. Even if we do nothing until January about the error, the total cost in propellant will be less than 8 kilograms. We can afford that. If the big Jupiter Orbit Insertion burn works perfectly, this will be a small insurance premium for us. It isn't true that we have fuel to waste: it is just that we budgeted for losing about this much propellant due to an imperfect big burn. If we are wrong by a little bit about the 937 km altitude at Jupiter, or if the accelerometers (which measure how the burn is progressing) are just a little bit off and our big burn is off by, say, half of one percent, our insurance will pay off. If the errors add up to between a third and two-thirds of a percent, the propellant penalty will be about 3 kilograms of propellant.
Doesn't this sound strange? It is easy to see that if we aim low and our aim is off and we wind up higher than we expected, aiming low was a good idea. But what is so good about aiming low and then winding up lower than we aim? Why does that save us anything?
The answer is complicated but interesting. We are trying to get into the proper orbit for our tour. Our first destination is Ganymede. Ganymede takes about a week to orbit around Jupiter. If we are exactly on target, we'll get to Ganymede in July of 1996. But if we are too low at Io, we'll slow down more than expected, go into a tighter and faster orbit around Jupiter, and get to Ganymede's orbit early. If our Io altitude and big burn are perfect we'll be about half a week early. That's no good because Ganymede won't be there: it will be on the other side of Jupiter. If we are higher than expected at Io, we'll be closer to getting to Ganymede at the right time. But here is the trick: if we are lower than expected at Io, we'll be closer to getting to Ganymede's orbit a full week early, and if we do that, Ganymede will be there for us! All we will need to do is be sure to choose a different latitude and higher altitude for our encounter with Ganymede, to get us back on track for the rest of our scheduled Tour of the Galilean moons.
Now it is clear that aiming a little low at Io is a form of insurance. But which is less dangerous anyway, being too low at Io or being too high? We conclude that being too low is less dangerous. That just puts us in too tight an orbit, and we can make up for that by getting to Ganymede two, three, four, or even more weeks early. However, if we are too high at Io, we won't slow down enough, and it won't be easy to recover from that. We may be able to get to Ganymede a week late. After that, we'd need to spend extra fuel to get back to our Tour. The problem is that if we used a strategy to get to Ganymede two weeks late, we would have to reduce our altitude so much that we'd hit Ganymede. Even at one week late, we might get too close if we miscalculated a little.
Today is Sunday. Since we were already aiming a little low at Io, we did not do the maneuver yesterday. Many of the people on the Orbiter Engineering Team have a feeling that we'll be be right on our aimpoint at Io and get the delta-v right on our big burn as well. We'll be pretty happy if this happens, especially if Probe Relay works well.
In a few days, we'll find out.