We have developed a concept to place a series of seismometers on a small body (asteroid, comet) to conduct an active seismic experiment. The objective of the experiment is to determine the internal structure of an asteroid and understand the extent to which these bodies are rubble piles, solid, or have a debris mantle overlying an interact interior. Similarly for a comet, the objective would be to understand the extent of layering and a core. Developing an understanding of the internal structure of a small body is critical to unraveling its history. In addition, understanding the internal structure is relevant to assessment of impact events on the Earth. A rubble pile body would be weak and is more likely to be disaggregated by tidal forces or by an atmosphere compared with a solid body.
The mission uses a small spacecraft launched on an Athena rocket to reach the target body. The spacecraft is based on early work conducted for the AIDA mission and is a hexagonal design about 1 m across and 1 m tall. Overall dry mass is about 200 kg (excluding contingency). Solar panels are mounted to the sides along with a series of arms to deploy the surface seismic instrumentation. An imager is the only other instrument and is mounted on the spacecraft bus.
Once a rendezvous is accomplished, the spacecraft conducts station keeping while the body is surveyed to determine the position for the surface packages. In the current analysis each surface package is a combination source and receiver (although this is not a requirement - the source and receiver could be separate).
The receiver is a seismometer that employs a fluid-filled sensing element can be placed in any orientation on the surface. The source is a NASA Standard Initiator (NSI). NSIs were used during the Apollo missions to conduct seismic profiling experiments.
After the sites have been selected, the spacecraft maneuvers to the body and presses the instrument package onto the surface. Passive anchoring is used to ensure coupling with the surface. The arm releases the instrument package and backs away to deploy another package.
The sensors passively record for several days to define the ambient noise environment. The passive recording could be extended for a full revolution around the sun to search for tidally induced seismicity. The active experiment begins with the detonation of one of the sources. Energy is recorded by the stations and transmitted to the spacecraft. After preliminary analysis, a second source is detonated and the procedure is repeated until all but one of the sources have been used. If the sources and receivers are packaged together then each time a source is detonated, it destroys the receiver. If they are packaged separately then all of the sources can be used and the complete receiver array remains intact for passive monitoring.