Jeffrey Andrews-Hanna

If this has a higher density than the rock around it, it should give you a positive gravity anomaly that with a sensitive enough instrument, something better than a plumb bob and a string, you could actually measure.

All right, let's turn to lunar science now.

So the first gravity observations of the moon actually occurred more than 50 years ago.

So the lunar orbiter spacecraft were in orbit, the first orbiters of the moon.

When scientists on the Earth were tracking that orbit, what they found was the lunar orbiter was never quite exactly where they expected it to be.

The orbit of the satellite was being perturbed by something, and that something was subtle variations in the gravity field of the moon.

So back as early as 1967, we were able to detect variability in the moon's gravity field because of its interior structure.

Fast forward, this is a gravity map of the moon circa late 90s, early 2000s.

This is from the lunar prospector orbiter, where now with more sensitive measurements tracking an orbiter around the moon, we can create a full global gravity map where reds are positive high gravity anomalies from excess mass and blues are negative anomalies.

And this is an incredible tool for probing the structure of the moon.

However, if you look at this,

This is centered on the far side of the moon.

Here's the near side.

Notice on the far side, it gets all stripey and kind of ugly looking.

It's looking pretty good on the near side, pretty bad on the far side.

This is because this data is taken from tracking the orbit of a spacecraft.

Well, what happens when the spacecraft goes over the far side?

You can't see it anymore, and you can't track it anymore.

So up until very recently, our gravity data from the far side of the moon was really quite poor.

And that's where a recent NASA mission called GRAIL, the Gravity Recovery and Interior Laboratory, comes in.