Ramses Plasma Spectrometer (RPS) onboard the Ramses mission to characterize the environment of Apophis and its surface properties

The flyby of asteroid 99942 Apophis in 2029 offers the unique opportunity to study the interaction of an asteroid with the Earth magnetosphere.  By measuring the spatial and energy distribution of positive and negative particles in close proximity to the asteroid it is possible to get significant information about the object without landing on it or without returning a sample from it. By characterizing the environment of an asteroid, we can also learn if the interaction is similar or totally different compared to the processes near the Earth’s moon. Full information is revealed if the particle properties impacting the surface of the object as well as all the various types of particles released from the surface e.g., backscattered and sputtered particles, photoelectrons, secondary electrons, charged dust, and neutral particles are measured. There is evidence that the number density of those particles and their energy distribution reveal information on the structure of the surface regolith. Together with computer simulations on similar types of asteroids the obtained information is essential for the study of Near Earth Objects like Apophis.
The flyby of Apophis near Earth is special in many ways. First, the flyby geometry is unique with a closest approach of about 31000 km at the edge or inside the outer radiation belt which can vary between 13000 km to 60000 km dependent on solar cycle and solar wind activity during the flyby period. This offers the unique opportunity to study the interaction of Apophis with the solar wind before and after the closest approach as well as the interaction with conditions inside the Earth’s radiation belts around closest approach. The unique possibility of the RAMSES mission compared to all the other missions currently planned to fly by Apophis is that it will carry the RAMSES Plasma Spectrometer RPS. We will describe in detail the scientific goals of RPS: 1. How does Apophis interact with its surrounding space plasma environment? 2. What plasma physics processes govern Apophis’s near-surface environment? 3. What is the surface composition of Apophis? and discuss the additional science added.