NASA sends GRAIL shaped beacon to the Moon

By Phil Plait | January 1, 2012 3:14 pm

Mynd you, Møøn bites Kan be pretti nasti…

Today, NASA successfully put a new mission into lunar orbit: GRAIL, for Gravity Recovery and Interior Laboratory. Great acronym, weird name, right? What this mission will do is map the gravity field of the Moon, and use that to probe the interior composition. The basic idea isn’t all that complicated: fly a probe around the Moon. If it goes above a region where the density is higher, there will be a slightly stronger gravitational pull, and the spacecraft will accelerate a bit. By carefully measuring the spacecraft position and velocity, you can make the lunar gravity map.

In detail, that’s a bit tougher! What NASA has done is launch two probes, GRAIL-A and GRAIL-B, that will fly in the same orbit, one behind the other*. They’ll stay in constant communication, sending radio pulses to each other. The timing of these pulses allows an extremely accurate determination of their separation: their distance will be known to an accuracy of about a micron: that’s a hundredth the width of a human hair, or the size of a red blood cell!

So how does that help? If one of the two probes speeds up or slows down, the radio signal timing will change, taking more or less time to get from one probe to the other. The amount of change is related to the force of gravity felt by the probe, and that in turn is related to the density of the material below. In practice, making a gravity map this way is extremely complex, but it’s been done before here at Earth using probes like GRACE and GOCE. It’s tried and true.

By mapping out its gravity, we’ll be able to essentially probe the material inside the Moon. This will help scientists understand the composition and structure of the Moon’s interior, which will in turn help them understand how the Moon formed and perhaps how it’s changed over time. We have a pretty good general idea of how the Moon came to be, but more details are always needed for better understanding. GRAIL should provide them.

I’m also personally excited about MoonKAM, a set of four cameras on each probe. These will make high-resolution maps of the lunar surface… but not strictly for science. This is actually part of the Education and Public Outreach for GRAIL designed for middle school (grade 6 – 8) students. They can set up mini-control centers in their classrooms and track where the two GRAIL spacecraft are, getting precise position data. They can then see if the probes will fly over any interesting areas the students want to know more about. They can then write proposals and request the data from NASA itself!

That’s very cool. I love the idea of letting students get their hands on the actual mission data, and going through the actual process to get it. When they receive those images, they’ll know it’s because they did the work to obtain them, and their sense of ownership will increase. That’s a fantastic motivator to get them interested in the science. And no matter what, I bet it’ll be an experience they’ll remember their whole lives.

And who knows? A future lunar colonist may get their start in the next few weeks, because they happened to be in a classroom with a direct connection to the Moon.

* As I write this, GRAIL-A is in orbit around the Moon, having successfully arrived on New Year’s Eve. GRAIL-B is literally firing its engine right now, and is expected to achieve orbit soon. [UPDATE: GRAIL-B engine firing was a success, and both probes are orbiting the Moon! Congrats to NASA and everyone involved!]

CATEGORIZED UNDER: Cool stuff, NASA, Science, Space
MORE ABOUT: GRAIL, gravity, Moon

Comments (39)

  1. ian

    Very exciting stuff.

  2. So those lucky kids will play with cameras orbiting the fraking MOON! Man, I love the future! My science classes were all about growing beans in a cotton-filled plastic cup.

    That´s a nice way of making unmanned exploration still look cool.

  3. Dave Scruggs

    It is against the rules to light the Grail Shaped Beacon. NASA will be soundly spanked.

  4. Dragonchild

    They didn’t light it. They just sent it.

    What’s the penalty for sending a Grail-shaped beacon?

    Oh, same as the others, I’m sure. . .

  5. DrFlimmer

    Mynd you, Møøn bites Kan be pretti nasti…

    The people responsible for the subtitle have just been sacked!

  6. Daniel J. Andrews

    What’s the penalty for sending a Grail-shaped beacon?

    Don’t know, but it sounds like a very perilous journey. Hopefully NASA can handle just a little bit of peril.

    How do they measure the distance between Grail A and B so accurately? Do they use two different set of distant stars? If so, I assume those stars are so distant their relative positions to the satellites won’t be affected by earth swinging from one end of its orbit to the other every 6 months. Or this is taken into account. How do they recalibrate or do they need to recalibrate? Is the solar wind effect far too small over the surface of the satellite to produce any measurable changes that might be misread as a gravity effect? Or do they have a way to filter out the noise from the gravity signal?

    Perhaps Phil could post an explanation as a future post?

  7. OtherRob

    My understanding is that these probes were launched back in September. Why did it take so long for them to get to the moon? Isn’t it usually a 3–day trip?

  8. Paul D.

    OtherRob: the satellites took a jaunt out to one of the Earth-Sun Lagrange points before swinging back. This ended up saving propellant over a direct trajectory.

  9. Robby

    @Daniel: The radio pulses Phil mentions are what is helping the two satellites know how far apart they are. A big part of the run-up to launching GRAIL was ensuring that the instruments on the pair were properly calibrated and synchronized. Running two satellites means you don’t have to have any external method of determining positions to compare, you can just have the probes “ping” each other with the radio signals and they’ll know exactly the distance between them by timing the delay between ping and response.

  10. OtherRob

    @Paul D.: Thanks for the explanation.

    Though I must add that it seems counter-intuitive that a much longer journey would take less fuel.

  11. Paul D.

    OtherRob: I think that’s the lowest delta-V way to put payloads into polar lunar orbits. Inclination changes are easier to do at great distances.

  12. Aw, Phil! I already have the “Grail in a cloud” image in Keynote for Wednesday’s meeting!
    Now it’s ruined! (sarcasm/)
    I also have a dented trumpet bell image in there. Any of you BAblogees wanna take a guess as to why?

  13. John Sandlin

    So Robby, you’re saying they sent the “machine that goes ping” too.

    I’m sure Zoot and Dingo have something to do with all this.


  14. Floyd

    cardoso: hey–my daughter got an award for growing beans in an array of plastic cups with a variety of soils. She did well because she documented what she did, not because her experiment was all that unique.

  15. Daniel J. Andrews

    Thanks Robby. I must still be missing something obvious though. They’re using radio pings to know their location respective to each other, but if the radio pings come faster or slower how do they know it is gravity that has changed the distance between the two and not something else affecting the distance?

    I suppose since they’re flying one right behind the other as one goes over an area of different gravity the distance will change (e.g. speed up and further apart). When the second one goes over the same area it should also speed up at the same rate the first one did. But how do they counter orbital drift or decay and do they need to recalibrate the distance to reestablish a baseline by which to measure changes?

    Maybe my confusion or overlooking something obvious is due to being tired (and not advancing age, sigh).

  16. Don

    Daniel, there was an article at last September that explained it in a bit more detail. They even used the extra journey time to calibrate the two satellites, measuring the solar wind’s effects. They expect some extraneous results, but gravity by far will be the strongest force affecting the measurements.

  17. Robert

    Daniel: Well, it’s the moon, with no atmosphere, so orbital decay shouldn’t be an issue. Drift is something we know about, so we can predict and subtract it. That close to the moon, local gravity is the only unknown, so its a “simple matter of mathematics”.

  18. Messier Tidy Upper

    Congratulations to the GRAIL team – good to hear that GRAIL-B has followed its twin into lunar orbit. :-)

    Wondering what’s happened to the idea mentioned here :

    to re-name each probe separately & properly?

    Late entry if I may (if someone aged appropriately wants to submit it) for teh craft to be named ‘Most’ & ‘Holy’ – but with those having to be growled in a thick Welsh (?) accent! ;-)

    Why? Because of this :

    especially at the 2 minute 45 second mark.

    It also plays on the purpose of the mission too – which areas have the *most* gravity and which are *hole-y* to some extent! ;-)

    @ 4. Dragonchild : January 1st, 2012 at 4:14 pm

    They didn’t light it. They just sent it.

    Oh I dunno ’bout tha’, they lit that candle when the rocket went up didn’t they? ;-)

    What’s the penalty for sending a Grail-shaped beacon?

    Well, I’m guessing that this :

    WARNING – NOT Safe For Work

    might answer that question? Especially at the 5 minute mark. ;-)

    Hmm .. Zoot and Dingo could work as GRAIL names too as I think others have already suggested.

    I also kind of like ‘Hound’ & ‘Hare’ or more seriously ‘VonBraun’ and ‘Korolev’ GRAIL names~wise too.

  19. JMW

    My prediction for 2012: both probes will be drilled by space debris, and become Holey Grails.

  20. CJ Nerd

    I went to KSC to watch the launch, and was lucky enough to talk to one of the project scientists afterwards.

    He explained that one big benefit from the slow journey out is that it allows time for outgassing.

    Even a smooth piece of metal is, if you look closely enough, porous enough to retain molecules of oxygen and nitrogen. Even in a vacuum, it takes time for them to jiggle out of the metal and leave the spacecraft. Every time a molecule leaves, there’s a Newtonian action-and-reaction, and the spacecraft gets jiggled a bit. With the sensitivity of the distance measurements involved, that can introduce an error into the data.

    The 16-week cruise allowed time for most of those residual molecules to detach.

  21. Dr.Sid

    In vacuum there is no friction, so just flying somewhere takes zero fuel. What costs fuel is acceleration and changes of direction. And for that there is a lot of tricks. Generally shortest road is the most fuel expensive in space. When people went to the Moon, they took much shorter road. They had to carry supplies, and they had to minimize exposure to radiation. Satellites do not have such limitation.

  22. Pete Jackson

    @18 JMW: GRAIL to Moon: “You are my density”!

  23. Joden

    If only NASA has two spare to send to Europa.

  24. Messier Tidy Upper

    Hmm … all these ‘Monty Python & the Holy Grail’ references and no comment from Kuhnigget? Kuhnigget, are you still here and okay? I hope so.

  25. Daniel J. Andrews

    Daniel, there was an article at last September that explained it in a bit more detail. They even used the extra journey time to calibrate the two satellites, measuring the solar wind’s effects.

    Thank you, Don. I will look that up. So they did measure the solar wind effect during the journey! (little light bulb goes on over my head).

    Thanks to Robert as well. It occurred to me this a.m. that they don’t need the exact position of the satellites in reference to, say, stars, but the relative position of the satellites to each other; or at least, the relative position is probably more important–e.g. according to fact sheet, the distance between them will vary from 175 to 225 km. I imagine if the distance is slowly increasing, analysts can subtract that known change from the gravity changes and it would be, as Robert said, “a simple matter of mathematics”.

    Anyway, I’m starting to understand. Thanks for your patience with my questions.

  26. Chris the Canadian

    If I were a kid in those classrooms I would ask NASA to do a flyover of one of the Apollo landing sites and take pictures!!! That would shut most of the ‘conspiracy’ theorists right up about man not actually landing on the moon and that it was all a big hollywood spectacle!!!

  27. ASFalcon13

    @Messier Tidy Upper

    “Wondering what’s happened to the idea mentioned here :
    to re-name each probe separately & properly? ”

    That’s still going on. The plan was to get them around the Moon first, then announce the winning entry shortly afterward. Now that they’re in lunar orbit, expect an announcement sometime soon, probably within the next few days if I had to guess.

  28. @Chris the Canadian

    You must realize that any processing of images that leads to a image that does not support my point of view, is a ‘cover up’ – but any such processing that reveals what I believe, is simply ‘enhancing’ the image!

    [no humour nor irony tags added, due to budget constraints!]

  29. Ari

    If these two satellites are able to know their locations with great precision, are they able to use their cameras for interferometry?

  30. Nigel Depledge

    Daniel J Andrews (15) said:

    But how do they counter orbital drift or decay . . .

    IIUC, because the moon has effectively no atmosphere, there will be no orbital decay within the lifespan of the mission (or at least, as near to nothing as makes no odds). Orbits in LEO decay because of the outer wisps of Earth’s atmosphere. Even the most rarefied gas exerts drag when you plough through it at orbital velocity.

  31. Messier Tidy Upper

    @28. ASFalcon13 : Thanks for that. I look forward to herring what names they’ve finally chosen. :-)

  32. Number 6

    Thank you, Phil, for answering my question as to why there are two probes — one following the other in lockstep. When I first saw this story on the national news and in other places, this question was never covered, and I was suprised by that. The Bad Astronomy web site always comes through!

  33. Calli Arcale

    I understand orbital decay actually is a problem in lunar orbit. Atmosphere isn’t everything. There are tidal interactions, and then there’s one of the main reasons we need GRAIL in the first place — the Moon is not a perfect sphere of uniform density. Far from it. It’s actually extraordinarily lumpy. During the Apollo program, there was a great deal of concern about “mascons”, denser areas (mass concentrations) where the lunar gravity is stronger, so much so that it affects the paths of orbiting spacecraft even over short periods of time. It was playing havoc with landing calculations (though they got good enough by Apollo 12 to make the pinpoint landing next to Surveyor 3).

    Many spacecraft sent to orbit the Moon are not orbiting it any longer, having impacted the surface eventually, deliberately or otherwise.

  34. RobT

    RE: how to measure the change in velocity using radio signals between the 2 probes.

    Wouldn’t they use the Doppler effect to measure a change in velocity relative to the 2 probes? As one speeds up in relation to the other the frequency it sends out would appear to change based on the difference in speed. As probe 1 speeds up the wavelength would appear, to probe 2, to lengthen (lower frequency). As it slows down the frequency would seem to get higher (shorter wavelength).

    That would be my impression, of course, they could be using some other method.

  35. Messier Tidy Upper

    The Moon does indeed have a very tenous atmosphere :

    I wonder if they’ve taken this into account? (Okay, I’m pretty sure they would’ve.)

    Come to think of it that might be another area where the GRAIL spaceprobes can help improve our Selenological understanding too. :-)


    “This is surreal, how each grain of moondust falls into place in these little fans, almost like rose petals.”
    - Buzz Aldrin (during his first Moonwalk July 1969), Page 38, ‘Magnificent Desolation’, B. Aldrin, Bloomsbury, 2009.

  36. Phil

    Tracking a single lunar satellite from earth to determine the lunar gravity field works great for mapping the near side. But we’ve been doing that for decades, so it’s not the real problem.

    The real challenge is to determine the lunar gravity field on the FAR side — where we can’t track satellites because the moon itself is in our way. We can estimate far side gravity by the effect it has on satellites when they come back to the near side, but that’s just it — the measurements are only estimates, so they’re inaccurate.

    Having two satellites ranging each other will provide excellent data on the far side of the moon where we can’t track from earth. Each satellite continuously tracks the other, whether or not the earth can see them.

    The Japanese just did a somewhat similar experiment as part of Kaguya; they launched a small relay satellite into high lunar orbit so as to allow Kaguya itself to be tracked when it was on the far side. I didn’t hear how that experiment turned out. Anybody know?


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