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.

• Received 12 May 2017

DOI:https://doi.org/10.1103/PhysRevLett.119.231101