The interior structure of asteroids and planets gives insight into their formation and evolution. This knowledge is especially crucial for asteroids that could become a threat to life on Earth. Interior composition and mechanical properties must be known to develop effective countermeasures, and to safely perform proximity operations on asteroids with spacecraft. As for terrestrial planets, the interior structure has strong implications on habitability considerations.

In order to understand the interior structure, the ground motion is investigated by rotational seismology. The surface of celestial objects can move because of seismic sources, global rotation and tidal deformations.
The PIONEERS project will allow Europe to enter a new realm of planetary exploration with an innovative ground motion instrumentation concept relying on high precision sensors based on optical interferometry. Innovative data analysis methods, as well as Earth analogue field testing will validate the expected increase of performance level by two orders of magnitude. The new instrumentation measures all six degrees of freedom (6-DOF): x, y, z translations and ωx, ωy, ωz rotations. This returns information equivalent to small seismic arrays.