New in Space Science Reviews (2025, vol. 221):
Tortora, P. Granmiga, E. Lasagni Manghi, R et al. The Radio Science Experiment on Hera, Juventus and Milani Art 124 s11214-025-01253-8
The Hera mission contains the CubeSats Juventus and Milani. The two daughter probes will be deployed, primarily to do close-up science that might be risky for the main Hera spacecraft. Ultimately, we would like to have one or both of the ‘disposable’ craft approach and land on Dimorphos. But while all three are operating independently, there’s an experiment they can do together.
Radio science is the study of bodies using the transmitter (sometimes receiver) on a spacecraft. At Dimorphos (as well as Didymos), we will have multiple spacecraft, plus the Earth-to-Hera link. One of the results we can get from radio science is a gravity measurement, by detecting how a spacecraft gets pulled (or not) as it approaches a space body. In the case of Hera, a Hera-Earth downlink provides one gravity number. But, as above, the main spacecraft is large, complete, and valuable; we would rather not risk it if we don’t have to. Therefore, Hera will keep its distance from Didymos and Dimorphos, at least during the main mission. Sending the CubeSats, then, allows far closer approaches to the asteroids, and therefore much better gravity resolution- if we have a CubeSat-Hera radio link.
We have never had an asteroid mission deploy a sub-spacecraft with a precision radio… now, we will have two. Unlike the MASCOT lander at Ryugu, the Juventus and Milani craft will complete multiple passes of the asteroids, giving us multiple samples of the gravity fields in 3D. And now the arrival at the Dimorphos-Didymos system is barely a year away… can’t wait!