# How NASA Finds the Mass of the Dirt Grabbed From an Asteroid

When you think of space missions, you might think of humans walking on the moon or rovers rolling around on Mars. But there's a whole bunch of other awesome space missions, too. One of these events was the NASA OSIRIS-REx spacecraft touching an asteroid . It didn't just touch the asteroid named Bennu, it also picked up some material from the surface. It will then return that material to Earth so that humans can study it.

Now for the cool physics. How does OSIRIS-REx determine how much material it collected? After it uses a robotic arm to "poke" the asteroid, it backs up and then spins. Yup. By looking at the change in rotational rate for the spacecraft, you can determine the amount of extra mass in the collector arm. Let me go over all the important pieces to this physics puzzle.

Moment of InertiaThe key to this whole thing is the moment of inertia. But to really understand the moment of inertia you need to look at mass. You can think of mass as the amount of "stuff" in an object—but it also has to do with the change in motion (inertial mass). Let's try a simple experiment. Grab a full bottle of water in one hand and an empty water bottle in the other. Now shake them both back and forth.

Surely you can feel a difference. It's not about motion, it's about CHANGE in motion. Consider the time when a full bottle of water is moving to the left. In order to move it back to the right, it has to first slow down and stop, then speed up in the opposite direction. This change in velocity is an acceleration where the acceleration (in one dimension) is defined as: