An electric field sensor to measure charged dust on the Marco Polo asteroid sample return mission
- Paper number
IAC-08.A3.I.9
- Author
Dr. Karen Aplin, Rutherford Appleton Laboratory, United Kingdom
- Coauthor
Dr. Neil Bowles, University of Oxford, United Kingdom
- Coauthor
Mr. Eric Sawyer, Rutherford Appleton Laboratory, United Kingdom
- Coauthor
Mr. David Parker, Rutherford Appleton Laboratory, United Kingdom
- Coauthor
Mr. Martin Whalley, Rutherford Appleton Laboratory, United Kingdom
- Year
2008
- Abstract
The Marco Polo mission has been selected by the European Space agency as a candidate for launch under their 2015-2025 Cosmic Vision programme. The mission ultimately aims to understand the origins of the planets and even life itself, by returning a sample of material from a primitive asteroid, representative of the early Solar System. The dust on the surface of the asteroid is readily charged and one of the proposed in situ instruments, described here, is an electric field sensor to measure charged dust particles on an asteroid for the first time. The instrument concept is two separated electrodes that are electrically isolated from each other. The electric field can be determined from the differential potentials of the electrodes, caused by the impact of charged particles. Local conductivity can also be measured by the rate of decay of a small bias voltage initially applied to the electrodes. Physical mechanisms controlling dust transport on asteroids are poorly understood, and by analogy with the Moon, there is likely to be considerable electric charging of the surface due to photoelectron emission. Electrostatic dust levitation has been proposed as a possible method to redistribute particles, and also a loss mechanism for smaller particles which are not bound by the small gravitational field of the asteroid. Asteroid electric charge has never been measured, but models predict that an electric potential (~1 kV) can be attained on the dark side compared to the sunlit side, which becomes slightly positively charged by photoelectron emission. These differences are enhanced further by local geometry at the terminator (the day/night boundary), when fields could reach ~100-300 kV/m. The geometric location of the instrument is an important issue, as an orthogonal pair of booms are ideally needed to measure vertical and horizontal electric fields. An alternative accommodation option is for the electrodes to be mounted on two of the legs of the lander. This would involve geometric screening of the electric field by the spacecraft, but the screening effect would be constant, and could be modelled and measured. However, it could have some effect on the instrument specification (sensitivity, resolution). An accommodation study is required to compare these two options. There may also be a case for measuring the entire charge on the asteroid during approach. This could be determined by mounting a small metal plate on the outside of the spacecraft, facing the asteroid, and measuring the Maxwell displacement current generated at the plate by the change in electric field. This would add a few grams of mass and minimal extra power (e.g. logarithmic current amplifier) to the payload. The scientific motivation for this additional part of the instrument would be that during approach, the difference in potential between the day and night sides can be detected remotely, which would back up the more detailed in situ measurements of electric fields from dust charging.
- Abstract document
- Manuscript document
IAC-08.A3.I.9.pdf (🔒 authorized access only).
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