Radiometer effect in the μSCOPE space mission

Space experiments to test the Equivalence Principle (EP) are affected by a systematic radiometer effect having the same signature as the target signal. In [PhRvD 63 (2001) 101101(R)] we have investigated this effect for the three proposed experiments currently under study by space agencies: muSCOPE, STEP and GG, setting the requirements to be met-on temperature gradients at the level of the test masses-for each experiment to reach its goal. We have now re-examined the radiometer effect in the case of muSCOPE and carried out a quantitative comparative analysis, on this issue, with the proposed heliocentric LISA mission for the detection of gravity waves. We find that, even assuming that the muSCOPE spacecraft and payload be built to meet all the challenging requirements of LISA, temperature gradients along its test masses would still make the radiometer effect larger than the target signal of an EP violation because of flying in the low geocentric orbit required for EP testing. We find no way to separate with certainty the radiometer systematic disturbance from the signal. muSCOPE is designed to fly a second accelerometer whose test masses have the same composition, in order to separate out systematic effects which-not being composition dependent like the signal-must be detected by both accelerometers. We point out that this accelerometer is in fact insensitive to the radiometer effect, just as it is to an EP violation signal, and therefore even having it onboard will not allow this disturbance to be separated out. muSCOPE is under construction and it is scheduled to fly in 2004. If it will detect a signal to the expected level, it will be impossible to establish with certainty whether it is due to the well known classical radiometer effect or else to a violation of the equivalence principle-which would invalidate General Relativity. The option to increase the rotation speed of the spacecraft (now set at about 10(-3) Hz) so as to average out the temperature gradients which generate the radiometer effect, is allowed in the GG design, not in that of STEP and muSCOPE. (C) 2002 Elsevier Science B.V. All rights reserved.