Measurements of Charged Dust on the Marco Polo Asteroid Sample Return Mission
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MEASUREMENTS OF CHARGED DUST ON THE MARCO POLO ASTEROID SAMPLE RETURN MISSION K L Aplin, N E Bowles*, D J Parker, E C Sawyer and M S Whalley Space Science and Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK *Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK Introduction 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 detect the electrostatic levitation and transport of charged dust particles. ****** Scientific Background 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 [e.g. Lee, 1996; Colwell et al, 2007], and also a loss mechanism for smaller particles which are not bound by the small gravitational field of the asteroid [Lee, 1996]. 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 kVm-1 [Lee, 1996]. An instrument is proposed to measure horizontal and vertical electric fields on the surface of the asteroid. This instrument is similar to the Atmospheric Relaxation and Electric field Sensor (ARES) [Berthelier et al, 2000] which will measure the Martian atmospheric electric field on the ExoMars mission, due for launch in 2013. The instrument concept is two separated electrodes which 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. Instrument concept The instrument will consist of conducting electrodes mounted at different heights on a deployable boom of length ~1 m. The boom is required to make the measurements away from the geometrical field distortion of the spacecraft. The electrodes could be made of conducting tape wrapped around the boom to reduce the mass loading. The electric field is measured from the differential potential acquired by the electrodes. Local conductivity can also be measured by the rate of decay of a small bias voltage initially applied to the electrodes. A high-resolution, high data rate, low gain mode will be required for terminator crossings when the electric field is expected to be at its highest and most rapidly changing, e.g. 0-500 ± 1 kVm-1. On the dayside, electrostatic levitation of dust is expected even though the electric fields are lower, which is likely to require a high-resolution, high gain mode e.g. 0-500 ± 1 Vm-1 with a resolution of 0.1 Vm-1 (16-bit). Sampling frequencies are to be confirmed pending calculations of the time of asteroid terminator crossing and dust charging. For 2-D electric field measurements, a two-axis boom is required to cover the variety of conditions expected at the dayside, night side and terminator. Horizontal electric fields across the terminator region are especially relevant as it is thought that they control the motion of dust particles across the asteroid’s surface [e.g. Colwell et al, 2007]. The geometric location of the instrument is an important issue. As described above, an orthogonal pair of booms would be ideal. The booms are likely to be a one-segment version of the two-segment deployable ARES boom for Exomars and, as on ARES, could readily be shared with other instruments. However, 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 this would be constant, and could be modelled and measured. This screening could have some effect on the instrument specification (sensitivity, resolution). An accommodation study is required to compare these two options. Discussion An electric field sensor for the surface of an asteroid has been proposed and several aspects identified for future study. In particular, specific calculations needed to predict electric field magnitudes on the four candidate asteroids identified for the mission (e.g. terminator crossing time) need to be carried out. Electrostatic modelling of the instrument’s position with respect to the spacecraft is also required. 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 [e.g. Bennett and Harrison, 2006]. 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. References Berthelier J.J., Grard R., Laakso H. and Parrot M., ARES, atmospheric relaxation and electric field sensor, the electric field experiment on NETLANDER, Planet. Space Sci., 48, 1193-1200 (2000) Bennett A.J. and Harrison R.G. Surface determination of the air-earth electrical current density using co-located sensors of different geometry, Rev. Sci. Instrum., 77, 066104; DOI:10.1063/1.2213210 (2006) Colwell, J. E., M. Horanyi, S. Robertson, X. Wang, A. Haugsjaa, and P. Wheeler, Behavior of Charged Dust in Plasma and Photoelectron Sheaths. Proc. ‘Dust in Planetary Systems’, Kauai, Hawaii, USA. 26-30 September 2005 ESA Special Publication 643 (H. Krager and A. Graps, Eds), 171-176 (2007) Lee P., Dust levitation on asteroids, Icarus 124, 181–194, (1996) .