On Monday, InSIGHT (Interior exploration using Seismic Investigations, Geodesy, and Heat Transport) should land on Mars if all goes well. InSIGHT’s stated mission is to study Mars as a body, telling us of its makeup and structure. But it will also tell us of other bodies- small ones.
InSIGHT carries two major experiments, SEIS and HP3, and two less-promoted ones, RISE and a ‘weather station.’ SEIS is a seismometer, which will feel for ‘marsquakes’. HP3 is a burrowing probe, to measure the heat response of the ground. RISE will track the lander’s motion; since landers are fixed, that will tell us Mars’ motion. The weather instruments are a good thing to have anyway, and will check the SEIS and HP3 data.
In the most direct application, RISE (Rotation and Interior Structure Experiment) will weigh asteroids. That’s right, InSIGHT (like prior Mars missions) can actually weigh passing asteroids. As a body passes another, their gravities pull each other. The tug on Mars is small, but our radio measurements are very precise. Yes, we can gauge Mars being scooted one way, then back as asteroids pass. InSIGHT, solidly planted at a spot, has been wanted for this job. The rover missions span ~15 years, which would be nice. But their roving confounds our assay of Mars’ scoots. Just correcting for wheel slip in the Martian ‘dirt’ is an ongoing topic.
Today, we have the mass of Vesta, the biggest body in the inner Belt, and that of Ceres, the biggest of them all in the middle Belt. The Dawn probe orbited both; its radio gave us mass numbers out to many decimal places. As asteroids slip between Mars and Vesta, and to a lesser extent Mars-Ceres, we will doubly-constrain their gravities, and thus masses. Mass, with size, gives density. Density then tells us what that body is made of- rock, metal (more than twice as dense as rock), or ice (less than half rock’s density).