GRAIL closing in on Moon

The Gravity Recovery and Interior Laboratory, which will neither be recovering gravity or being inside the moon but “GRAIL” apparently sounds good, is coming into Lunar Orbit as I write this.

As you know from watching Apollo 13 the travel distance to or back from the moon is a matter of several days … Apollo crews got there in about three Earth Days. But the GRAIL mission has taken months to get there, because of the kind of orbit they wanted to achieve. Also, the long duration allows for other things to happen, like temperatures to stabilize within the machinery.

GRAIL is two space ships flying together and will achieve a more or less polar orbit around the moon, going around the moon ever 11.5 hours. Over time, the period of the orbit will be reduced to about 2 hours. Then, GRAIL will start mapping the Moon’s mass.

When science collection begins, the spacecraft will transmit radio signals precisely defining the distance between them as they orbit the moon. As they fly over areas of greater and lesser gravity, caused both by visible features such as mountains and craters and by masses hidden beneath the lunar surface. they will move slightly toward and away from each other. An instrument aboard each spacecraft will measure the changes in their relative velocity very precisely, and scientists will translate this information into a high-resolution map of the moon’s gravitational field. The data will allow mission scientists to understand what goes on below the surface. This information will increase our knowledge of how Earth and its rocky neighbors in the inner solar system developed into the diverse worlds we see today.


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9 thoughts on “GRAIL closing in on Moon

  1. That’s interesting – so they’re not using a gravity sensor like the one used on that ESA satellite a year or two ago? I wonder what the expected magnitude of anomalies are.

  2. But the GRAIL mission has taken months to get there, because of the kind of orbit they wanted to achieve.

    Is it that or is it more due to their weaker propulsion method and absence of a Saturn V rocket launcher?

  3. Steve, good question. I’m going by what they said in the press release, though I’m not sure if they directly address that. Perhaps they are using the time well, but stuck with it for some reason having to do with propulsion.

  4. I suspect that they are using ion drives which are low thrust but high specific impulse. Once you are in space all that matters is the total impulse, not the thrust. It takes a lot of thrust to get clear of the atmosphere and into orbit and high thrust means high fuel consumption.

    They need to be able to adjust the orbits of each one, so as to keep them flying together. Using the same ion drive to get to the moon makes a lot of sense because it conserves mass for payload. They need to fly very “cleanly”, where all non-gravitational forces can be accounted for. That includes things like radiation pressure from the sun and from thermal radiation from the satellites themselves. Off gassing is important too, and that is very hard to predict, so letting them sit in vacuum for a while is a good idea.

    The GRACE experiment was able to measure things like the change in mass of ice in Greenland from its gravity. Grail will be a lot closer to the moon (the moon has no significant atmosphere), and the moon has a lower gravity field so deviations should be more apparent.

    I don’t think that it is expected that there are any dynamic processes on the moon to actually measure, but if there are, this might be able to see them. If it could measure voids, like empty lava tubes, that would be very interesting and important.

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