The ESA -JAXA RAMSES Mission to Apophis: Investigating Asteroid Response to Planetary Tidal Forces

The ESA–JAXA RAMSES (Rapid Apophis Mission for Space Safety) mission has been designed to investigate the near-Earth asteroid (99942) Apophis during its exceptionally close encounter with Earth on 13 April 2029. During this event, Apophis will pass at a distance of about 32,000 km from Earth’s surface, well within the geostationary orbit region, providing a unique natural experiment to observe how a small body responds to strong planetary tidal forces.

RAMSES will rendezvous with Apophis several weeks before the closest approach and monitor the asteroid before, during, and after the encounter. This temporal coverage will enable the first direct investigation of tidal-induced modifications affecting the asteroid’s rotation state, surface morphology, internal structure, and physical properties.

The mission concept builds upon the heritage of ESA’s Hera mission and includes a main spacecraft supported by two CubeSats, enabling a distributed measurement strategy that combines remote sensing and in-situ observations.

From a scientific perspective, the Apophis encounter represents a key opportunity to address fundamental questions on the mechanical behavior of rubble-pile bodies. In particular, RAMSES will assess whether tidal forces can trigger surface processes such as landslides, regolith mobilization, and exposure of fresh material, as well as possible internal reorganization.

A major strength of the mission lies in its ability to combine complementary measurements. High-resolution imaging and spectroscopy will detect compositional and mineralogical changes, while a low-frequency radar will probe the internal structure. Radio science experiments will provide precise constraints on Apophis’ mass, gravity field, and rotational state. For the first time on an asteroid, a lander carrying a seismometer will attempt in-situ measurements, providing unique constraints on mechanical properties and internal response to tidal forcing. In addition, a plasma spectrometer and a magnetometer will investigate the interaction of Apophis with the solar wind and Earth’s magnetospheric environment. These measurements, coupled with dust detection and analysis by one of the CubeSats, will offer a new perspective on dust activity and surface–environment coupling during the encounter.

In parallel, the Apophis 2029 encounter will be observed within a broader international framework. The JAXA DESTINY+ mission, launched on the same H3 rocket provided by JAXA, will perform a flyby of Apophis before the arrival of RAMSES, while NASA’s OSIRIS-APEX mission will rendezvous with the asteroid after the encounter. This coordinated international effort will provide a unique multi-mission temporal coverage.