Understanding how the moon is developed would really help us in understanding how the other planets in the
inner part of the solar system, the rocky planets, have also developed. The purpose of the GRAIL mission
is to investigate the interior structure of the moon. To achieve that, we have to measure the gravitational
field of the moon. So we're all familiar with tides on the Earth that are caused by the moon.
The moon actually has similar tidal response, and when the moon changes its shape a little bit due to
those tides, just like the Earth does, we can actually measure that. That affects the gravity field.
As the lunar shape varies, its gravity field changes and we are able to observe that. We are using a very
simple measurement, which is gravity. It's a little bit like taking a CAT scan of the moon, but using
waves instead of acoustic ones to unlock the secrets of the interior of the moon. We have two spacecraft that
are in a polar orbit around the moon, one essentially chasing the other around the moon. There's a part of the
Earth's moon, a very significant pant, the far side of the moon that does not present its face to the Earth.
So we can never directly observe the spacecraft as it goes across the far side of the moon using Earth–based
tracking antennas. So the solution to that was, rather than track the motion of the spacecraft as it's
orbiting the moon from the Earth, track it with another spacecraft also orbiting the moon. It's
essentially the same technology as the GPS in your mobile phone. And in fact, The two orbiters together
form a sort of a mini GPS and receiver constellation except using different frequencies from normal GPS.
The first thing we do, actually, once the map is in our hands is compare it with a topography map, hopefully
of comparable resolution. And from there, we can look at specific features such as mountains and basin
craters and compare the gravity and the topography. This is where it gets really interesting.
You would normally think that if there is a mountain present on the planet there's additional mass and
additional gravitational signal as the spacecraft flies over it. But if it does and there's no
additional gravity signal, then something on the inside is going on that's particularly interesting
and that's what we want to know. When I was just getting into middle school, I had to rely on my
information on the moon watching the Apollo missions on television. We wanted students to have a hands-on
role in exploring the moon. It's experiment called Moonkam. We will have up to four cameras on each GRAIL
spacecraft. Students will be able to look at the ground track of where GRAIL will be at going around
the moon on a daily basis and to propose images that will be taken.
It's going to go a long way towards helping students understand just how much fun science and
engineering is as a career.
NASA Jet Propulsion Laboratory, California Institute of Technology
