A Heliocentric Laser Interferometer with Five-Million-Kilometre Arms Tuned to the Spectral Turnover of a Torsion-Regulated Bounce

This paper presents a space mission design, computed from stated assumptions and not measured, built to measure one quantity: the characteristic strain of the bounce background at 3 millihertz, for which the design case adopts 2.40e-21 in characteristic strain. The instrument is three spacecraft in a triangular formation with five-million-kilometre arms, each carrying two free-falling gold-platinum test masses and a 2-watt laser, combining time-delay interferometry to cancel laser frequency noise while retaining the gravitational-wave response in the millihertz band. It operates from an Earth-trailing heliocentric orbit, the triangle inclined to the ecliptic for 4 years. The one-day measurement uncertainty is 1.20e-20 in characteristic strain; averaged over the full mission of 1461 days it falls to 3.14e-22 in characteristic strain, giving a signal-to-noise ratio of 7.64 for the design case and a five-sigma detection limit of 1.57e-21 in characteristic strain. The spacecraft has a dry mass of 1930 kilograms, a wet mass of 2550 kilograms including propellant, and a total electrical load of 1870 watts supplied by a solar array of 5.39 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.