NASA Probes Set To Orbit The Moon Over New Year's

Twin GRAIL spacecraft on a mission to study lunar gravity are nearing the end of their almost four month journey. The probes are expected to reach the moon on New Year's Eve and New Year's Day. GRAIL's principal investigator, Maria Zuber of MIT talks about the data they hope to collect.

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IRA FLATOW, HOST:

This is SCIENCE FRIDAY. I'm Ira Flatow. The New Year's countdown has begun, not at the clock in Times Square but this one for two NASA probes set to orbit the moon this weekend. The twin spacecraft, GRAIL-A and GRAIL-B are expected to enter lunar orbit 24 hours apart, one on New Year's Eve, the other on New Year's Day.

This marks the end of a three-and-a-half-month journey for the washing-machine-size crafts launched from Florida in September. Just what does NASA scientists, what do they hope to do and accomplish? What are the pair of probes, what are they going to reveal about the Earth's closest neighbor, four decades after man first landed on the moon?

And how can twin spacecrafts chasing one another in circles help reveal the inner secrets of the moon? Why do we need two of them to do it? Yeah, all interesting questions, all here. We're going to try to get them answered by Dr. Maria Zuber, she's principal investigator for NASA's GRAIL mission. Dr. Zuber is also a professor of geophysics, head of the Department of Earth, Atmospheric and Planetary Sciences at MIT. She's at WBUR in Boston. Welcome back to SCIENCE FRIDAY, Dr. Zuber.

MARIA ZUBER: Thank you, Ira. It's great to be back.

FLATOW: It's always good to have you. Tell us about the purpose? Why now after all these years do we send two of these probes to the moon?

ZUBER: Well, the country, the various countries of the world have sent over 100 spacecraft of the moon, and there's still a lot of fundamental things that we don't understand about the moon. Most of these missions have studied the surface of the moon in various capacities, whether it be imaging, craters and mountains or learning about composition, studying lunar samples.

But fundamental things, such as why the near side of the moon is different from the far side of the moon still aren't very well-understood. And so it occurred to us that the answer might not be found on the surface, the answer might be inside of the moon, and that's what this mission proposes to study.

FLATOW: And what part of the inside of the moon are you trying to study?

ZUBER: From crust to core, we'll be producing a very precise map of the gravitational field of the moon. In fact, when we're finished with this mission, the gravity field of the moon will be known better than the gravitational field of the Earth. So it will be quite accurate.

FLATOW: And why do we need to do that? I mean why specifically the gravitational field?

ZUBER: Gravity is one of the fundamental things that we try to study of the inside of a planet. Gravity studies the distribution of mass in the interior. So if we know something about what the composition is on the surface, if we know the size of the body, and if we know the mass, then we can understand what the - what I'll call radial distribution of mass is in the interior.

So we can learn about the crust, the mantle, and if the moon has a core, we will study that, as well.

FLATOW: You do say that the moon is a mysterious place, the side facing us is different than the side further - on the other side. It's also mysterious because we're not quite sure how the moon was created, are we?

ZUBER: Well, the - I will say the only theory for the formation of the moon that doesn't have a fatal flaw is one where the moon was formed due to the impact of a Mars-sized body into the Earth. This impactor hit the Earth at a relatively high angle, and the core of this impactor is thought to have wrapped itself around the core of the Earth, and parts of the crust and mantle of the impactor and the Earth were thrown off into orbit and were thought to have formed the moon or perhaps even more than one moon, as a recent theory has suggested.

FLATOW: And will these studies with these twin satellites, satellites of the moon, will they tell us any more about that process?

ZUBER: Well, it's certainly hoped that that would be the case. The recent study suggesting that the Earth may have in fact had more than one moon back in time after this gigantic collision took place came out relatively recently. It came out this spring in a paper in nature, and it wasn't something that we had conceived of when we had written the proposal to do the GRAIL mission.

But the idea shows some plausibility, did some calculations to demonstrate plausibility, and it laid out specific ideas that can be tested by understanding things about the inside of the moon. So the idea behind this second moon theory is that a moon about a third of the size of our current moon was in the same orbit around Earth as our moon was.

And because it was in the same orbital plane, it wasn't moving very fast in comparison to our moon. And so it was stable for some period of time, but eventually it became unstable and drifted towards our moon and impacted it.

But because it was moving at about the same - very close to the same velocity of our moon, it behaved very differently than normal collisions that we have on planets. Normally when we have a collision on a planet, what happens is a great deal of kinetic energy gets thrown into the planet, and a big hole gets excavated, which forms an impact crater or an impact basin.

And if you look at the surface of the moon, Mercury, any of the solid planets, you will see a heavily cratered surface. Well, because of the fact that this collision of the Earth's - or the moon's purported companion was at a very low velocity, it actually adds more material to the moon than it excavates. So instead of forming a hole on the moon, it formed a mountain.

And one of the puzzling things about the back side of the moon is that there is a huge highland area in the equatorial region of the moon, which is - which does not occur on the moon's front side. There have been a lot of ideas about what this mountain might have been due to, what would produce a mountain on one side of a planet and not another, and one of the ideas is that it could be the remnants of this collision of an early moon.

FLATOW: Fascinating. Would some pieces have drifted away, or would they all still be on the moon, the remnants of that second moon there?

ZUBER: Certainly I think some parts of the moon would have in the collision, are lost from the moon, but the bulk of it, the preponderance of the material actually sticks with the moon and adds to its mass. So it's a very remarkably creative idea, and actually the simulations that were shown demonstrated that it's plausible.

I mean, it doesn't prove that it was the case, but it lays out some things that one can look for in the inside of the moon to test whether or not this was the case.

FLATOW: So your twin GRAILs, A and B, can help solve that problem?

ZUBER: They certainly can. They will provide information that will allow us to evaluate that hypothesis. So we can either at that point reject it, or it may need to be modified on the basis of what we find or demonstrate that it was correct.

FLATOW: Well, is it sort of serendipitous that you designed this way before this whole hypothesis came up?

ZUBER: That's correct. We had plenty of other interesting things that we wanted to look at, but it's remarkable that, you know, having studied the moon for so long, that there are still great new ideas about the moon and the Earth-moon system.

I mean, the Earth is - you know, the Earth is involved in this whole process of what was going on back then.

FLATOW: Wow, and I understand that even though you've dubbed these satellites GRAIL-A and GRAIL-B, their names are going to change.

ZUBER: The names are going to change soon. So...

FLATOW: Do we know what they are yet?

ZUBER: We - I know what they are.

(SOUNDBITE OF LAUGHTER)

FLATOW: Tell me, OK?

(SOUNDBITE OF LAUGHTER)

ZUBER: You and all the many, many listeners of SCIENCE FRIDAY, you will all learn soon enough. After the successful launch of the two spacecraft in September, we announced that there would be a naming contest, and we permitted schools and other student organizations to propose names for the spacecraft.

And they needed, when they proposed these, they had to write an essay with the idea that the name should in some way reflect something about the mission, something about the moon, something about the fact that the students realized that there are two spacecraft operating in a certain way.

And so a group of us evaluated those names, and after the two spacecraft are successfully in orbit, we'll announce the winners.

FLATOW: We have a winner. There's also a moon-cam, right? Kids can look at the moon from the spacecraft?

ZUBER: Yes, this - so this - in terms of the scientific payload, it's two spacecraft with the same instrument on each spacecraft because the spacecraft make their measurement by ranging to each other. But what we decided to do is put cameras on the spacecraft but not to give them a scientific purpose.

The experiment is called MoonKam, and it's being run by Sally Ride, America's first woman in space, and each spacecraft has four cameras on it, which are pointed at different - two are looking at nadir, which is directly below. One looks a little bit in front of, and one looks a little bit below.

And these cameras have no scientific purpose. The purpose of them is totally education and outreach, and students and schools can sign up to be a part of the MoonKam project, it's targeted for middle-schoolers, and students will meet - in collaboration with their teachers, learn how to use the software.

I imagine it'll be the students teaching the teachers how to use this software in a lot of cases. That's the way it often works, where they can - students will - schools will be assigned a period of time, and they will study the trajectory of the spacecraft and what parts of the moon they'll be passing over and propose to take images.

And that'll be run - operated out of the University of California San Diego by Sally Ride Science.

FLATOW: And we'll - it sounds very excited. There's a lot of stuff going on up there. Dr. Zuber, I want to thank you for taking time to be with us and making this educational opportunity available.

ZUBER: Yeah, thank you very much, Ira.

FLATOW: You're welcome. Dr. Maria Zuber is principal investigator for NASA's GRAIL mission. And she is also a professor of geophysics and head of the Department of Earth, Atmospheric and Planetary Sciences at MIT. We're going to take a break, and when we come back, we're going to - we're going to make a phone call, get a phone call, take a phone call from Antarctica, yeah, live from Antarctica. We'll talk about research. You know, it's summertime down there, just beginning to heat up with the research. So we'll talk about what's going on down there. Stay with us. We'll be right back after this break.

(SOUNDBITE OF MUSIC)

FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY from NPR.

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