A Drag-Free Torsion Balance in Low Earth Orbit Searching for a Spin-Dependent Differential Acceleration

This paper presents a space mission design, computed from stated assumptions and not measured, built to measure one quantity: the differential acceleration attributed to a spin-dependent torsion coupling, for which the design case adopts 6.50e-15 metres per second squared. The instrument is a drag-free satellite carrying two torsion pendulums whose bobs are made of materials with opposite net nuclear spin polarisation, suspended on fused-silica fibres inside a common vacuum housing, with the whole payload slowly rotated so a genuine differential signal appears at the rotation frequency. It operates from a 700-kilometre sun-synchronous orbit chosen for thermal stability and a near-constant eclipse pattern for 2 years. The one-day measurement uncertainty is 3.00e-14 metres per second squared; averaged over the full mission of 730 days it falls to 1.11e-15 metres per second squared, giving a signal-to-noise ratio of 5.86 for the design case and a five-sigma detection limit of 5.55e-15 metres per second squared. The spacecraft has a dry mass of 740 kilograms, a wet mass of 860 kilograms including propellant, and a total electrical load of 1120 watts supplied by a solar array of 3.23 square metres at the operating distance. Every performance figure in this paper comes from the deposited computation; the design is simulated, not measured, and no hardware described here has been built or flown.