Date: August 12, 2011
Title: Orbiting Between the Fire and Frying Pan
Podcaster: Bob Hirshon
Organization: American Association for the Advancement of Science (AAAS)
Link: www.aaas.org
Description: Science Update host Bob Hirshon speaks with MESSENGER Mission Specialist Eric Finnegan at the Johns Hopkins Applied Physics Laboratory about the challenge of putting a spacecraft in orbit around the planet Mercury and keeping it there.
Bio: Bob Hirshon is Senior Project Director at the American Association for the Advancement of Science (AAAS) and host of the daily radio show and podcast Science Update. Now in its 24th year, Science Update is heard on over 300 commercial stations nationwide. Hirshon also heads up Kinetic City, including the Peabody Award winning children’s radio drama, McGraw-Hill book series and Codie Award winning website and education program. He oversees the Science NetLinks project for K-12 science teachers, part of the Verizon Foundation Thinkfinity partnership. Hirshon is a Computerworld/ Smithsonian Hero for a New Millennium laureate.
Sponsor: The Education and Outreach team for the MESSENGER mission to planet Mercury. Follow the mission as the spacecraft helps to unlock the secrets of the inner solar system at www.messenger-education.org.
Transcript:
365 Days of Astronomy Podcast
August 12, 2011
Orbiting Between the Fire and Frying Pan
Podcaster: Bob Hirshon
Organization: American Association for the Advancement of Science (AAAS)
Hirshon:
Welcome to the 365 Days of Astronomy Podcast. I’m Bob Hirshon, host of the AAAS radio show and podcast Science Update. The Greek hero Odysseus had to guide his ship between the monstrous shoals of Scylla and the consuming maelstrom of Charybdis. It’s an apt analogy for the engineers guiding the path of MESSENGER, the first spacecraft to orbit Mercury. The spacecraft has to thread the needle between the searing anvil of Mercury and the gravitational vortex of the sun. But unlike Odysseus, MESSENGER must not only pass through this strait unharmed, but pass through it again and again with each orbit, and explore everything in the vicinity. It’s as if Odysseus had not only to pass by Scylla’s razor sharp teeth, but also conduct a complete dental exam. Eric Finnegan is a MESSENGER Mission Specialist.
Finnegan:
MESSENGER is, out of all the spacecraft I worked on to date, MESSENGER is one of the most constrained missions I’ve worked on. Cause you do have all these competing requirements: you’ve got the thermal environment just from the sun itself; you’re, you know, three times closer to the sun than we are here at the Earth, which gives you ah, you know, energies that are on the order of nine, ten times higher, so, you’ve got this planet that’s also very, very hot, cause it’s also three times closer to the sun than we are here at the earth, so you’re seeing that environment; you’re trying to take all these measurements looking at the planet, so you want to be looking at this hot surface.
Hirshon:
On top of the thermal constraints, the team also has to contend with gravitational constraints. They’re keeping the craft in orbit around a tiny, low mass planet while being pulled away by the gravity of the sun.
Finnegan:
Once you’re in orbit, the solar gravity, so the gravity of the sun tugs on the orbit and tries to pull, basically, the satellite out of orbit, so it wants to pull the altitude of the satellite at its minimum point, which we call periapse, we want to pull it—the scientists want to be at about 200 km altitude but the sun wants to pull it all the way out towards it, so it pulls it up and within about 44 days, about 88 days, it’s up about 500 km, and we have to do a maneuver to push it back down again. So every 88 days we wind up firing the main engines of the spacecraft to push that orbit, that orbital periapse back down to 200 km.
Hirshon:
And what is that maneuver, what do you do?
Finnegan:
Well, it’s a maneuver we do at what we call apolapse, this is the opposite part, so when we’re furthest away from the planet, we turn the spacecraft such that the thruster is perpendicular to the radius of the planet, and we fire the thruster to lower the periapse.
Hirshon:
And in addition to the gravitational pull of the sun, the team also has to contend with the pressure of the solar photons pushing on the craft and potentially destabilizing the orbit.
Finnegan:
So the satellite as it’s orbiting around, again the sun pushes on the satellite on all of its surfaces, on its solar arrays, on the front of the sunshade, and then we rotate the spacecraft around to take pictures, or whatever we’re doing in terms of an observing plan. Well, the spacecraft, in order to maintain the stability, has a bunch of, basically, spinning wheels inside. It looks like your bicycle wheel spinning very fast. And those spinning wheels try to absorb any jitter or reorientations on the spacecraft such that it maintains a nice stable orientation. Well, the force of the sun is pushing on the spacecraft and forcing it to move, and those wheels have to keep spinning up and spinning up in order to absorb that energy. At some point in time, they get spinning too fast and you have to slow them back down again. And that’s what we do with a momentum dump, we take that energy out of the spinning wheels, with thrusters, and we spin them back down again, so that they can reaccumulate the momentum back up again.
Hirshon:
I didn’t know those existed in there. How big are they?
Finnegan:
The wheels are about a foot, maybe a foot and a half in diameter. They’re made out of metal, so they have a lot of mass to them. And you know if you’re spinning something very massive, like a big weight or something, it’s got a lot of speed to it, and we can absorb a lot of momentum. The momentum would be a combination of the mass spinning and the rate of its spinning. So we have a fixed mass and we just spin it faster to accumulate a lot more momentum, and we spin it slower to release that momentum.
Hirshon:
Funny, you’re building this spacecraft and the whole idea is to make it as light as possible, and then you have these components and you say “we want them to be as dense and heavy as they possibly can be.”
Finnegan:
Yeah, for MESSENGER, though, we had to again conserve mass, so we tried to choose the smallest wheels and actually, we were a little concerned that they would accumulate momentum too fast and we’d have to do these momentum unloads, or momentum dumps very frequently. But the solar pressure on the spacecraft has been pretty balanced and it hasn’t accumulated as much momentum. So we’ve been able to do momentum unloads about once a week when we’re in what we call the “new midnight orbit.” So, if you will, you’re in the shadow of the planet, so you’re not getting a lot of solar pressure, so there’s not a lot of momentum building up, and then you come over the top and you get sun on you and you get a lot of momentum buildup and it kind of goes in a cyclical orientation, that winds up storing a lot more momentum on the spacecraft. But we’re in the dawn dusk orbit so now, you can always see the sun, you never go into an eclipse orientation. You’ve got a constant pressure on the spacecraft from the sun and that’s a much more benign state so we don’t have to unload the wheels quite as frequently, and we can do that every couple of weeks. Actually, I think we’re going now around three weeks without having to unload the wheels.
Hirshon:
He says the momentum dumps have gone extremely well, and so have the two orbital correction maneuvers, or OCMs, conducted so far.
Finnegan:
And we’ve been very very successful in orbit. We’ve been able to execute burns to a tenth a percent error. So out of the hundreds of meters per second, we might execute only a couple millimeters per second in error, so it’s a pretty good maneuvering performance.
Hirshon:
And when you get that good performance, what’s the result? You save fuel?
Finnegan:
Well, you do save fuel but from the science team perspective, there’s no interruption to operations. So in many cases when you do maneuvers, the science team would have to wait, figure out, you know, what happened and re-plan all their observations relative to where you are. Not big re-plans, but enough that it would affect operations. The last two maneuvers we’ve executed, the science team probably didn’t even know we executed a maneuver. Their science operations continued without any interruption at all. So getting contiguous science operation is very, very important to them.
Hirshon:
So perhaps I should ask the trajectory people this, but the orbit that you chose, was it optimized to be as efficient as possible? To get as close to the sort of orbit that people might be familiar with, where you go into orbit and just stay there for years and years until it decays? Or was it not optimized for that?
Finnegan:
Well, our trajectory design team, their bottom line is how much delta V can they squeeze out of the system, so those guys are always looking at the bottom line.
Hirshon:
“Delta V,” for our audience?
Finnegan:
Ah, delta V is change in velocity. Every orbit can be defined as velocity at a certain point. So we want to change the orbital velocity of the spacecraft we call it delta V. A convenient term. But our trajectory design engineers, they are, they have DNA that says I want to minimize the amount of delta V it takes to do this maneuver. They’re always looking for the optimum place to put it. In most cases they can do that. For MESSENGER unfortunately, due to some requirements in order to monitor burns from the earth, which means we can’t execute one of these large burns when we’re not looking at the spacecraft. That tends to place some of these maneuvers in off-optimal positions in order to do that. So it causes our trajectory design engineers a little angst to, you know, “I could do a great maneuver here, but you’re telling me I got to be off a little bit.”
Hirshon:
They’re like hyper-milers!
Finnegan:
Exactly. They’re trying to get the best fuel efficiency out of the system as they can. And sometimes there’s other requirements that cause them from not being able to do that. But they’re very efficient in planning those burns. And we do squeeze out a lot of performance.
Hirshon:
And then the science team has their requirements. So there might be a great orbit that would be good from the point of view of the trajectory, but it may not be good for the instruments, it may not be good for what the science team wants to gather.
Finnegan: Oh, yeah, absolutely, I mean our imaging team would love to be low all the time, and some of our particle measurement folks would like to be at 200 km, you know, circular orbit. Unfortunately, the spacecraft wouldn’t be able to stand that; it would get too hot or probably melt or burn. So we have to get at a very high altitude away from the planet for a certain amount of time in order to cool the spacecraft off. That’s a trade between getting the science the people want and making sure the spacecraft will last a long enough time to keep going.
Hirshon:
So the success so far you’ve had with these orbital maneuvers, has it had a long range result on the mission, or can’t you say yet?
Finnegan:
Oh, absolutely, again, we measure our performance in delta V, and our delta V margin, the amount of delta V we have left at the end of our one-year operations is fairly large. To the extent that we could operate the same mission over again for another year and possibly two more years. So conserving that delta V, being efficient, allows us to be there longer and take more science observations over a much longer time frame.
Hirshon:
It’s not known yet whether NASA will approve an additional year or more of data gathering for MESSENGER once it completes its planned one-year mission in March. But if they do, Finnegan and the rest of the team are confident the craft will have plenty of gas in the tank to keep exploring. For the 365 Days of Astronomy Podcast, I’m Bob Hirshon.
End of podcast:
365 Days of Astronomy
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