An Ultraviolet Spectrograph Satellite Measuring the Fine-Structure Constant Along Sight Lines Selected by the Reported Axis

This paper presents a space mission design, computed from stated assumptions and not measured, built to measure one quantity: the spatial variation in the fine-structure constant across the selected sight lines, for which the design case adopts 4.20e-06 in delta alpha over alpha. The instrument is a 1.2-metre space telescope feeding a cross-dispersed echelle spectrograph at resolving power 60,000 covering 115 to 320 nanometres, with a wavelength calibration system based on a laser frequency comb referenced to an onboard atomic standard. It operates from a halo orbit about the Sun-Earth L2 point, giving uninterrupted long exposures on faint quasars for 3 years. The one-day measurement uncertainty is 2.60e-05 in delta alpha over alpha; averaged over the full mission of 1096 days it falls to 7.85e-07 in delta alpha over alpha, giving a signal-to-noise ratio of 5.35 for the design case and a five-sigma detection limit of 3.93e-06 in delta alpha over alpha. The spacecraft has a dry mass of 1030 kilograms, a wet mass of 1205 kilograms including propellant, and a total electrical load of 1451 watts supplied by a solar array of 4.27 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.