THE ASTEROID PROBE EXPERIMENT ( APEX ) : SEISMOLOGY AT 99942

Introduction: The ~400 m diameter asteroid 99942 Apophis will make a close approach to Earth on April 13, 2029, passing at a distance of only 36700 ± 9000 km (or 5.7 ± 1.4 Earth radii) [1] – closer than geosynchronous satellites. While the approach is not close enough to disaggregate the asteroid, it is expected to produce changes in the rotational state, solid body deformation, and surface morphology [2–6]. Apophis’ encounter with the Earth presents a unique opportunity for a detailed study of a Near-Earth Asteroid (NEA) with a small spacecraft. The Asteroid Probe Experiment (APEX) mission is to characterize the internal structure, rotational dynamics, and surface morphology as Apophis passes the Earth in 2029. This mission would place a seismometer on the surface to monitor seismic signals generated during the Earth/asteroid encounter. The seismometer mission requirements for APEX are to record seismic energy on 3 orthogonal components and across a range of high frequencies (10–500 Hz), and amplitudes (> 10–100 ng), spanning several weeks. The seismometer concept has been developed by Hongyu Yu at Arizona State University with APL participation under the NASA MATISSE program [6]. This instrument senses seismic displacement via electrolyte flow through a sensor plate, instead of the more conventional "mass on a spring" designs. As there are no moving parts, a critical aspect of the sensor is that it is orientation independent (i.e., it does not need to be leveled), greatly simplifying deployment. The seismometer would be deployed by the spacecraft pushing it into the surface.