No abstract

Space-borne physics experiments involving the measurement of small motions of test bodies are likely to be limited by disturbance forces. Of particular concern are forces arising from electrostatic charging of the test body due to interactions with particle radiation. Estimates of charging rates have been computed using Monte Carlo particle-transport codes in combination with semi-empirical particle flux models. Results are presented for the STEP and LISA geometries, and are extrapolated for GP-B. The consequences of the charging are assessed for each experiment, and a method for alleviating the problem is discussed which uses the photoemission technique already in the hardware development phase for GP-B.

The build-up of electrical charge on the proof masses is an important disturbance for LISA. The charging is due to penetrating particle radiation (primarily protons) from cosmic rays, and from the Sun during solar flares. Estimates of charging rates have been computed using the GEANT Monte Carlo particle-transport code in combination with realistic proton flux models. The consequences of the charging are discussed.

Computational results are presented for the flow through a helium microthruster. This device is to be used for fine adjustments in attitude control for a proposed space experiment. The mass-flow rates used by the thruster are very low giving Knudsen numbers at the nozzle throat between 0.01 and 1 based on the stagnation conditions and the nozzle throat diameter. These conditions indicate that low-density effects will dominate the fluid mechanics. Therefore, the flows are computed with a particle simulation scheme [the direct simulation Monte Carlo method (DSMC)]. This study presents an application of the DSMC technique to the complete expansion process of a real thruster: from the stagnation chamber of the thruster, to the far-field expansion of the plume. The numerical approach is evaluated by comparison with existing experimental data taken in the expansion plume. The computational results are employed to assess the effect of varying the mass-flow rate on the terminal state of the gas. In addition, the effect of including the background chamber pressure measured in the experimental vacuum facility is investigated and found to be significant. KM) P Nomenclature= Mach number at nozzle exit = mass-flow rate = background pressure in vacuum tank = Reynolds number at the nozzle throat = radial distance from the nozzle exit = flow velocity = axial distance from the nozzle throat -ratio of specific heats = flow angle = nozzle exit half-angle = maximum plume turning angle = Prandtl-Meyer expansion angle at Mach number M = density

LISA will be the first spaceborne gmvitational wave observatory. It aims to detect gravitational waves in the 0.1 mHz+l Hz range from sources including galactic bharies, super-massive blacl-hole binaries, capture of objects by super-massive blackholes and stochastic backgmnnd_ LlSA is an ESA approved Come&one Mission foreseen as a joint ESA-NASA cndeavour tobelaunchadin2010-11.~principlcofo~~ofLlSAisbkpedonlrurarangingoftcst-masrpesrmderpmc~ic motion. Achieving pure gc0dcsic motion at the level requested for LISA, 3x10~'s ms~z/JHz at 0.1 mHz, is considenxl a challenging technological objective. To reduce the risk, both ESA and NASA = pursuing an in-flight test of the relevaut technology. The goal of the test is to demonstmte geodetic motion within one order of magnitade fhm the LlSA performancc.ESAhasgiventhistestasthcprimerygoalofits~~gydadics~missionSMART-2withalannchin 2006. This paper describes the basics of LISA, its key technologies, and its in-fight precwsor test on SMART-2.