Radiometer effect in space
missions to test the equivalence principle
- (Nobili et al. Phys. Rev. D, Rapid Communications
Vol 63, 101101 May 15 2001)Full paper ready to download and print:
radiometer.ps.gz (postscript, g-zipped, 93 KB), radiometer.pdf (85 KB)
Experiments to test the equivalence principle in space by testing the universality of free
fall in the gravitational field of the Earth have to take into account the radiometer
effect, caused by temperature differences in the residual gas inside the spacecraft as it
is exposed to the infrared radiation from Earth itself. We report the results of our
evaluation of this effect for the three proposed experiments currently under investigation
by space agencies: µSCOPE, STEP, and GG. It is found that in µSCOPE, which operates at
room temperature, and even in STEP, where the effect is greatly reduced by means of very
low temperatures, the radiometer effect is a serious limitation to the achievable
sensitivity. Instead, by axially spinning the whole spacecraft and with an appropriate
choice of the sensitivity axes—as proposed in GG—the radiometer effect averages
out and becomes unimportant even at room temperature. ©2001 The American Physical Society
Radiometer effect in the Microscope space
mission
- (Nobili et al. submitted, November 2001; revised
December 2001)
Full manuscript ready to download and print:
radiometer_microscope_revised.pdf (67
KB)
In space missions to test the Equivalence Principle (EP) the
disturbing radiometer effect has exactly the same frequency and phase as the target
signal. In [1] we have evaluated the radiometer effect for the three proposed experiments
under investigation by space agencies: m SCOPE, STEP and GG.
Motivated by a recent presentation of m SCOPE [2] (due to fly
in 2004), we have re-examined the radiometer effect for this experiment carrying out a
comparative analysis, on this issue, with the proposed heliocentric LISA mission for the
detection of gravity waves. Should the spacecraft and payload be built to meet all the
challenging requirements of LISA, but flying in the geocentric orbit of m SCOPE instead, temperature gradients along its test masses would
still be so high as to make the radiometer effect larger than the target signal of an EP
violation. We find no way to separate the radiometer systematic disturbance from the
signal. Even the additional m SCOPE accelerometer, with test
masses made of the same material for checking purposes, will not allow the radiometer
effect to be distinguished from the signal because both of them vanish for test cylinders
of equal composition and density. The option to increase the rotation speed of the
spacecraft (now set at about 10-3 Hz) so as to average out temperature
gradients and cancel the radiometer effect, is not viable in m
SCOPE (neither in STEP). The axis of rotation not being the axis of symmetry of the test
cylinders, they are constrained to move in one dimension and this makes them highly
unstable if rotating faster than their (very low) natural frequencies. This analysis
causes us to question the possibility for the m SCOPE space
experiment, as currently designed, to achieve its goal of testing the equivalence
principle to 1 part in 1015. The issue is relevant, although to a less extent,
also for STEP.
Anna Nobili – nobili@dm.unipi.it
Last Updated: January 2002