Regular Article

Testing gravity with cold-atom clocks in space^★

The ACES mission

¹ European Space Agency, Noordwijk, The Netherlands
² Airbus Defence and Space, Friedrichshafen, Germany
³ CNES, Toulouse, France
⁴ Orolia Switzerland (Spectratime), Neuchâtel, Switzerland
⁵ Timetech, Stuttgart, Germany
⁶ Czech Technical University in Prague, Prague, Czech Republic
⁷ Technical University of Munich, Munich, Germany
⁸ SYRTE, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, LNE, Paris, France
⁹ Bureau International des Poids et Mesures, Sèvres Cedex, France
¹⁰ Laboratoire Kastler Brossel, ENS-Université PSL, CNRS, Sorbonne Université, Collège de France, Paris, France

^a e-mail: Luigi.Cacciapuoti@esa.int

Received: 18 March 2020
Received in final form: 1 May 2020
Published online: 4 August 2020

Abstract

Atomic Clock Ensemble in Space (ACES) is a mission designed to test Einstein’s theory of General Relativity from the International Space Station (ISS). A primary frequency standard based on laser cooled caesium atoms (PHARAO) and an active H-maser (SHM) generate a clock signal that is distributed to a network of clocks on the ground to perform space-to-ground comparison. With a fractional frequency stability of 1 × 10⁻¹⁶ after 10 days of integration time and an accuracy of 1 – 2 × 10⁻¹⁶, ACES will provide an absolute measurement of the gravitational redshift, it will search for time variations of fundamental constant, and perform Standard Model Extension (SME) tests. The ACES payload is currently completing its qualification tests before flying. The mission status, the latest test results, and the ACES performance for testing General Relativity are discussed.