Jeffrey Andrews-Hanna

And this is a mission that I was fortunate enough to play a small role in.

But this was a designated mission to study the gravity field of the moon.

What was unique about this mission is two things.

One is that it consists of not one, but two satellites, two spacecraft orbiting the moon in tandem, basically just measuring the distance between those two satellites.

The reason why you do that is because with these two satellites orbiting, one a little bit in front and one a little bit behind, that front spacecraft encounters a little gravity anomaly before the back one does, and it speeds up or it slows down, and that back spacecraft either catches up a little bit or falls further behind.

So if you're always measuring the distance between these two satellites, you're really measuring variations in the gravity field, and they can keep tracking one another even when they're on the far side of the moon.

So that's one of the really important things about GRAIL.

And the other thing is its orbit altitude.

It orbited incredibly low to the surface of the moon.

This is a map of the minimum altitude of the spacecraft as a function of location on the lunar surface.

The orbit was changing now and then, so you see a lot of variability.

But what you'll notice is that a lot of the moon is covered in these deep shades of blue orbits less than about eight kilometers.

Now, if anybody pays attention to satellite orbits, that is incredibly low.

That is much lower than any satellite has ever orbited another body before.

For comparison,

The Mars Reconnaissance Orbiter, an orbiter of Mars from which we have gravity data from Mars, orbited at about 300 kilometers above the surface.

GOCE, which is a real cutting edge gravity mission to the Earth, orbited at an altitude of about 220 kilometers.

747 cruising altitude, somewhere around 13 kilometers, about 40,000 feet.

GRAIL, getting much of the moon's surface, about eight kilometers, that's lower than a typical airliner is flying.

So picture next time you're flying in an airplane, just look out the window and imagine looking down below you and seeing two spacecraft going zipping by at a speed fast enough that you probably wouldn't see them zip by.