Mass Determination of Small Bodies in the Solar System

The masses and gravity fields of the planetary bodies were determined by radio tracking of spacecraft flying by or orbiting that body at a suffiently close distance. Small bodies (asteroids, cometary nuclei...) of the solar system pose certain challenges in order to reveal their masses and gravity fields. Those challenges mostly concerns spacecraft safety and/or optimal instrment operations. In order to resolve an acceptable Doppler shift with regard to the frequency noise, a spacecraft shall flyby at close distances, at slow speed and at an optimal flyby geometry for a given body mass. This cannot always be achieved. The flybys of Mars Express at Phobos, the flyby of Rosetta at asteroid Lutetia, its orbiting about the nucleus of 67P/Churyumov-Gerasimenko shall be reviewed. The prospects and challenges of future flybys like New Horizons at 2016MU69 and Lucy at the Trojan asteroids shall be presented.