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Monitoring the side effects of electric propulsion

Some of the SMART-1 instruments will monitor the effects of the solar electric propulsion system.

The demonstration of the use of solar electric propulsion is the principal objective of the SMART-1 mission. As a consequence, two of the experiments on the spacecraft are devoted to monitoring how well it functions in space and to observing all its consequences.

The main effects of the operation of a Hall thruster are physical, mechanical, thermal and electrical. The plasma can cause erosion and redeposition of eroded material on surrounding surfaces. The beam impact on spacecraft surfaces can produce torque and variations in the thrust vector. Surface temperatures can rise. Surface potentials may change and electromagnetic effects can be induced by the on-off operation of the thruster and produce radio-frequency interference to, for instance, the spacecraft antennae. Ground measurements of such effects in vacuum chambers do not fully reflect space conditions and the true spacecraft geometry cannot be simulated. Real spaceflight data are therefore essential.

EPDP: monitoring solar electric propulsion

The Electric Propulsion Diagnostic Package (EPDP) will monitor these effects. The package consists of a number of sensors, mounted on the bottom panel of the spacecraft, approximately 80 cm from the thruster. It represents approximately 2 kg of instrumentation and electronics. A Retarding Potential Analyser will measure the ion energy and current density distribution. The ions that will be measured are those of low energy mainly responsible for the backflow contamination. With the resulting data, designers of future space science missions will be able to optimise the position of thrusters and payload instruments for spacecraft of different shapes.

A Langmuir Probe will measure the plasma potential and the electron density and temperature. It will give information on the plasma conditions at one side of the spacecraft, whilst another probe will give similar information on the opposite side. Using single point measurements, 3D representations of the environment will be possible.

EPDP also includes a solar cell, placed away from the spacecraft's solar array, and a Quartz-Crystal Micro-balance which will both be used as deposition material sensors, providing real data on the extent of contamination.

Complimentary information on the consequences of the solar electric propulsion will be obtained notably from the spacecraft's Attitude Control System, from the telemetry of the onboard antennae, house keeping data from the solar arrays, from the operation of the spacecraft's hydrazine thrusters and from disturbances to the payload data that is obtained when the electric propulsion is functionning.

The main contractor for EPDP is LABEN/PROEL (Florence, Italy) whose Giovanni Noci is the EPDP technology investigator. Scientific co-ordinator is Josi Gonzales, from ESA's Electric Propulsion Unit at ESTEC, Noordwijk, the Netherlands.

SPEDE: Piggyback plasma science

The EPDP package also supports the science investigations of the Spacecraft Potential, Electron and Dust Experiment (SPEDE - 'speedy' for which A. Malkki, from the Finnish Meteorological Institute (FMI), Helsinki, is the principal investigator.

Diagram indicating location of EPDP/SPEDE detectors around the thruster.

The Geophysical Research unit at FMI does research in the fields of space plasma physics, geomagnetism, aeronomy, and planetary research. It has contributed to the SOHO satellite providing (together with Service Aeronomie, Paris, France) the SWAN instrument which since 1996 has been observing solar Lyman-alpha radiation, the heliopause signatures and solar corona, and which has monitored several comets.

SPEDE represents but 800 g of equipment. The experiment is built jointly by investigators from FMI (Helsinki, Finland), ESA/SSD (Noordwijk, the Netherlands), IRFU (Uppsala, Sweden) and KTH (Stockholm, Sweden).

SPEDE consists of two electric sensors mounted on the ends of 60 cm booms. These sensors can be driven either as a Langmuir probe or as an Electric Field probe. These sensors will also monitor the effects of the solar electric propulsion on the spacecraft. In addition, SPEDE will be used for some space plasma physics.

During SMART-1's cruise phase, the experiment will map the plasma density distribution around the Earth and when SMART-1 is in lunar orbit, it will study the lunar plasma environment and notably how the solar wind is coupled to the Moon.

Last Update: 16 August 2006

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