Events of October 24, 2012
Is it really November already? After Columbus Day, everything just seemed to whiz by me in a blur of midterms, essays and hurricanes... but I was able to take some time out to travel across the river to MIT for an appointment I'd made several weeks before.
Back in January, I talked about all of the events I was looking forward to this year in space and astronomy news, starting with the twin GRAIL probes entering orbit around the moon on New Year's Eve and New Year's Day. The mission certainly didn't disappoint -- my father and I were huddled around the computer watching the live animation of the probes' movements, provided by the excellent Eyes on the Solar System program, watching the "distance" and "speed" figures shrink, and listening to the communications chatter from the control room. When the GRAILs were safely in their orbits, we celebrated.
A few weeks later, I found myself in a rather unenviable position, out getting lunch at the Student Union without my laptop just minutes before the press conference announcing the winners of the contest to name the probes that had been held for schoolchildren across the country. I got the press conference up on my smartphone, but without headphones, I couldn't hear a thing in the noisy cafeteria! I hurried downstairs and found a quiet spot in a room that wasn't in use, just in time to hear the announcement.
The winning class held up cards, revealing their chosen names letter-by-letter, cheerleader-style:
"What does that spell? EBB and FLOW!"
Like most of the space fans watching, I approved of the names -- since the probes were mapping the moon's gravity, and the moon's gravity creates tides on Earth, the tidal theme was very appropriate. The names were opposite, but connected, perfect for twin spacecraft and in addition -- they just sounded kind of cute!
Back in September, when I went to hear Dr. Charles Elachi speak, I ran into Maria Zuber, the top scientist on the GRAIL mission, and asked her if it would be possible to meet up and do an interview for my blog at some point in the future. The 24th was the first day she was available, as her work keeps her pretty busy (as you'll see), so I packed my tape recorder and caught a cab to MIT. After a little trouble finding the building, we sat down and started to talk. Our conversation went on for about 45 minutes, so I had to split it into two parts. The first part appears here...
Me: I feel bad that I wasn't able to interview you last time [we first corresponded last fall, when the GRAIL probes were launched], but now the mission has actually been going on, so I'm sure you'll have more to talk about.
Maria Zuber (MZ): Well, some people like to talk about what they hope they're going to do. I tend -- when I have something, that's when I'm ready to talk. (laughs)
Me: So this is more comfortable for you?
MZ: Well, it is, yeah, because I've never been one to toot my horn unless there's something to toot about. So when you hear from me, we've got the stuff.
Me: OK, so why don't you start off by stating your name and what you do?
MZ: OK -- Maria Zuber, I'm a Professor of Geophysics at MIT and I'm the Principal Investigator on the GRAIL mission. That stands for Gravity Recovery And Interior Laboratory. The mission launched last September 10th -- successfully, thank goodness. When the Apollo missions went to the moon, they went on a three-day trajectory to get to the moon. And what we decided to do was something called a low-energy trajectory. So we went out to the Earth-Sun Lagrange Point, which is the place where gravity --
Me: It's cancelled out.
MZ: -- yeah, gravity vanishes, it's balanced between the earth-moon system and the sun. And the reason that we did that is that we were able to use very little fuel to get into lunar orbit. So we launched [both spacecrafts] on a single rocket and then each spacecraft only had to slow down by 190 meters per second to get captured by the moon's gravity. And so, that meant we could launch two spacecrafts on a single rocket rather than using two rockets, and so we actually saved 150 million dollars by going to the moon in three months instead of three days. That was a very clever thing by our mission design team.
Me: How does that compare to the looping that the LRO [Lunar Reconnaissance Orbiter] did, the LRO and LCROSS missions? I remember that they went very far away from the earth and moon and then came back in.
MZ: They got into a big, looping orbit around the moon. This is a much lower one. Essentially, we went into a parking orbit around the Earth and just gave the spacecraft a little nudge to get them going towards the Lagrange Point.
Me: Because it's space.
Me: [In the microgravity of space] things just keep going where they're pushed.
MZ: Yes. The way that you get into orbit around a planet is that you slow down enough so that the gravity field of the planet can capture you. So to only have to slow down 190 meters per second is pretty good. We could have a nice small fuel tank.
Me: I think it's incredible that we know orbital mechanics well enough now to do all of these different things, these maneuvers and to have so many different ways to do these things based on the mechanical and mission requirements.
MZ: For the mechanical design of our mission, we were just so fortunate to have really, really good mission designers. These two spacecrafts together have so far done 55 propulsive maneuvers since they were launched, which is (laughs) --a lot! Most spacecrafts will do just a handful. We've done 55 in the course of about a year. When a spacecraft first gets into orbit, it will often do just a big elliptical orbit, which is what LRO did around the moon, and then you do a burn, to take energy out of the orbit, and that brings the orbit down to the point we want to map at.
We did eight burns per spacecraft, to bring these spacecraft down to their lower orbits, and then we had to do a bunch of tiny burns to line them up. We could have done big burns, but by doing these little burns it built in some resiliency, because we could miss any given one and it wasn't critical, we could make it up the next time if we missed one. We didn't miss any, but because we were doing it so slowly, we could assess very accurately what our orbit was. We didn't waste any fuel, we could do it extremely precisely by doing all of these little burns.
Personally, I've been very involved in every aspect of the mission, so I either go out to Lockheed, where they operate the spacecraft, or to JPL, where they run the mission, or I'm on the phone with them while all of this is going on, so during our extended mission right now, I'm on the phone three-to-five hours a day, either doing maneuvers or planning maneuvers or assessing maneuvers. And I think to myself, "Was it really a good idea to do this?" (Laughs)
But it was! It was! It's taken a lot of time, but we're getting absolutely the most out of this mission that we can.
Me: What have you learned so far from these probes?
MZ: Well, I have to be careful about what I say here, because we have three papers in review in Science [magazine] and they're embargoed, so I can't talk to the press about results too much yet. Although, I'll be happy to talk to you about results in a couple of months. (Smiles)
Me: Oh, sure, sure, I understand. I just meant vaguely.
MZ: Well, I can tell you generally what we've done in terms of gravity field research, and then I'll tell you what we're doing right now. We haven't produced our best [map of the moon's] gravity field yet, but so far we've produced the best gravity field map for any planetary body in the solar system, and the best one that there will be for any known time to come.
This is because we can go low. In the primary mission, the two spacecrafts -- it's probably worth stopping to explain that these two spacecrafts measure gravity by measuring the distance change between them. So that's the reason we need two spacecrafts. The way we normally track the gravity field of planets is to track one spacecraft in orbit around that planet from earth, but we don't see the back side of the moon from earth, so we have to have two spacecrafts that can track each other like that.
We orbited at an average altitude of 55 kilometers above the moon, and you can't do that with earth, because the atmospheric drag doesn't allow you to orbit at that distance.
Me: Fifty-five kilometers up isn't even space! [The Kármán Line, the edge of Earth's atmosphere and the beginning of space, is defined as 100 kilometers, or 62 miles, above sea level. This means that from my dorm room here in Boston, outer space is actually closer than Cape Cod. Something to think about.]
MZ: Right, you have to be at at least 200 kilometers to orbit the Earth, often higher. In the primary mission, we produced a gravity field map with a spatial resolution size on the surface of 13 kilometers, which is pretty amazing for a global model. And right now, we're in an extended mission that's going to end in December. We have learned the gravity field well enough that in the extended mission we can operate at an even lower altitude.
I asked my mission designers, "How low can we go and still map safely?" We're currently mapping the moon at half that altitude, so 22-and-a-half kilometers average altitude above the moon. Last week, we got within 7.6 kilometers of the moon. So while the average altitude is 22-and-a-half kilometers, the orbit goes elliptical very quickly because the moon's gravity field is so bumpy, especially when you get down low. You get bounced all over the place by the gravity field. So the high point of the orbit is about 28 kilometers and the low point was 7.6. And it will get lower as the course of the mission goes on. [Astute aerospace fans will have noted that 28 kilometers is not only far short of the Kármán Line but actually lower than Felix Baumgartner's recent stratospheric skydive.]
So the fact that you can orbit around a planet --
MZ: -- with two spacecrafts flying in precise formation and measure the distance changes between them to better than a tenth of a micron per second, which is about the same as a human blood cell. We can do that extremely precisely, and we do it five times per second.
Me: When they're at that lowest point now, if you were on the surface of the moon, would they be visible overhead?
MZ: Oh, hmmm, well, let's see... so the two spacecrafts are about the size of an apartment-sized washer and dryer, so they're small. So when they're at seven kilometers, that's about the height that commercial airplanes fly on Earth. So you probably couldn't see each one of them, but they're getting close to the size you could see.
At the very, very end of the mission, I have further challenged my engineering team to see if we can go lower, and we're in the process of planning that. The goal here was to get enough gravity information at as low an altitude as we could, so that we could study the mass distribution inside the planet. By flying really low, we can measure very small mass variations. On Earth, if you wanted to measure similar things, you would have to fly over with an airplane or tow similar equipment on the ground.
Some of the early surface gravity measurements on Earth were taken by the army as they trudged around with a hand-held gravimeter, and they were measuring that kind of thing. By doing it from orbit, you can measure much more precisely than you can by dragging something around in the field.
Me: I'm an archeological remote sensing major, so this is very, very relevant. I think it's really great that there are techniques and technologies that are developed for one branch of science that are so transferable to others. And it's so useful, to be able to apply that to other fields and to other questions that you might have in the future.
MZ: One of the really exciting things about this is that in the Earth Sciences, we study the interior of the planets using geophysical techniques. We do seismology, we do magnetics, we do gravity, and that kind of information is usually at a much larger scale than when we study the surface. People go out and do fieldwork, they lug equipment around, and by getting these measurements at low-altitude, we can map at high-resolution and the geophysics is really entering the realm of geology.
In fact, we gave an update on the data we're collecting at a lunar meeting this week, and Jack Schmidt, the astronaut from Apollo 17, he was --
Me: A geologist who was on the moon.
MZ: -- he was the only geologist who walked on the surface of the moon. And he said, "Hey, we collected some surface gravity data at the Apollo 17 landing site, we should compare our data to your data from orbit!" (laughs) And I said, "You know what, we could do that now." It makes sense to do this now, so that's something that we're really looking forward to doing.
Me: Do you think that there would be changes over forty years? Or would that only be on a planet that was geologically active?
MZ: The moon is not geologically active. I would not expect to see changes, but one thing I would say is never say never. There could be changes on the moon, they would be very tiny, so there are moonquakes that occur every month. The moon raises tides on the earth and we see the results on earth, but the earth raises tides on the solid moon, and that produces moonquakes.
So we consider the moon to be a geologically inactive planet, but recent orbiters have detected that there's a monthly water cycle on the moon. There's a micron-thick layer of hydroxyl that gets activated when the sun shines on it and the molecules bounce around and then when the Sun goes down, they drop to the surface. So with remote sensing in orbit, you would see a very, very tiny signal of water on the surface that disappears when the sun is shining on it. So we think of the moon as being inactive, but there are some things that change.
So one of the things that has been exciting about studying the moon is the discovery of water, particularly in the Polar Regions. And we're down low enough that we can actually try to look for subsurface ice signatures in some of the polar craters. If there's enough water there, we would see something unusual in those craters.
That'll be something that we're looking for, but it takes very long to process this data.
Me: I know that on earth they've used gravity data to find places where there is water underground. With the GRACE mission, they did that.
MZ: Sure. The GRACE mission on Earth, which is also a dual-spacecraft mission, looks at river discharge and aquifers. You can actually see water replenishing in the aquifer during the rainy season and being discharged in the dry season. We don't think the moon has nearly that much water, so we wouldn't expect to see changes in ice content, but what we would be looking for is anomalously low densities in polar craters that are much different than similar craters away from the poles.
Stay tuned for Part 2 of our interview, in which Professor Zuber describes the joy of discovering "Something No One Else Has Ever Known"...
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