MICROSCOPE Mission: First Results of a Space Test of the Equivalence Principle

According to the weak equivalence principle, all bodies should fall at the same rate in a gravitational field. The MICROSCOPE satellite, launched in April 2016, aims to test its validity at the ${10}^{-15}$ precision level, by measuring the force required to maintain two test masses (of titanium and platinum alloys) exactly in the same orbit. A nonvanishing result would correspond to a violation of the equivalence principle, or to the discovery of a new long-range force. Analysis of the first data gives $\delta (\mathrm{Ti},\mathrm{Pt})=[-1\pm 9(\text{stat})\pm 9(\text{syst})]\times {10}^{-15}$ ($1\sigma$ statistical uncertainty) for the titanium-platinum Eötvös parameter characterizing the relative difference in their free-fall accelerations.

Figure 1

Left: The four test mass orbiting around Earth (credit CNES, Virtual-IT 2017). Right: Test masses and satellite frames; the ($X_{\text{sat}}$, $Y_{\text{sat}}$, $Z_{\text{sat}}$) triad defines the satellite frame; the reference frames ($X_k$, $Y_k$, $Z_k$, $k=1$, 2) are attached to the test masses (black for the inner mass $k=1$, red for the outer mass $k=2$); the $X_k$ axes are the test-mass cylinders’ longitudinal axis and define the direction of WEP test measurement; the $Y_k$ axes are normal to the orbital plane, and define the rotation axis when the satellite spins; the $Z_k$ axes complete the triads. The $7\mu \mathrm{m}$ gold wires connecting the test masses to the common Invar sole plate are shown as yellow lines. $\overrightarrow{\mathrm{\Delta }}$ represents the test-mass off-centering. The centers of mass correspond to the origins of the sensor-cage-attached reference frames.

Figure 2

Square root of the measured PSD of the differential acceleration along $X$ during the scientific session 218 with SUEP (left) and during the scientific session 176 with SUREF (right); on left, $f_{\mathrm{EP}}=3.1113\times {10}^{-3}\mathrm{Hz}$, $f_{\text{orb}}=1.6818\times {10}^{-4}\mathrm{Hz}$ and satellite $\text{spin}=2.9432\times {10}^{-3}\mathrm{Hz}$; on right, $f_{\mathrm{EP}}=0.9250\times {10}^{-3}\mathrm{Hz}$ and satellite $\text{spin}=0.5886\times {10}^{-3}\mathrm{Hz}$; the gravity gradient effect are clearly observed at $2f_{\mathrm{EP}}$. The red line is a power-law fit to the spectrum.

Figure 3

Evolution of the mean amplitude of the FFT of the differential signal along $X$ at $f_{\mathrm{EP}}$ as a function of integrating times (on left, from 12 to 120 orbits for the session 218 with SUEP and on right, from 11 to 62 orbits for SUREF). The mean of the FFT is computed as the average of the Fourier amplitudes over a narrow band of ${10}^{-4}\mathrm{Hz}$ around $f_{\mathrm{EP}}$. For SUEP, the black dashed line shows the estimated upper bound of the systematic errors given by the error assessment of Table 3.