Satellite laser guide stars for large aperture segmented ground telescopes

In this work we consider using satellite laser guide stars to enable high contrast imaging with extremely large telescopes (ELTs). In 2010, Traub and Oppenheimer calculated that a natural or artificial guide star of magnitude -4.2 +/- 0.2 (in BV, RI, J, H, and/or K band) is required to enable a 30-m ground-based telescope to image exoplanet systems with contrast ratios better than 10⁻¹⁰. Telescopes with 100-m diameters require guide stars of magnitude -1.4 +/- 0.2. Typical sodium laser guide stars are much dimmer, magnitude 9 or 10. The angular size of a natural guide star modeled after a sun-like star at 10 pc is 0.9 mas (4.5 nrad), much smaller than a 10 m spot at the 80 km sodium layer, which is about 25 arcsec (125 µrad). Satellite laser guide stars were introduced by Greenaway and Clark in the early 1990s but were not considered cost-effective at the time. With recent advances in both satellite laser communication and the miniaturization and reduced-cost access to space of nanosatellites, we revisit the impact and value of a satellite laser guide star system. A satellite laser guide star with a 4 cm diameter spot at geostationary altitude (35,786 km) has an angular size of 0.2 mas (1 nrad), which is comparable to a natural guide star, but the satellite laser guide star has the potential to be several magnitudes brighter. For example, the 1064 nm GEO laser communications terminal Technology Demonstration Payload 1 (TDP1) that flew on Alphasat has a nominal 0.1 W transmit power and 1.5 arcsec (7.1 µrad) half-angle beam divergence with a magnitude brighter than -2. At maximum power of 5 W, the Alphasat terminal has magnitude brighter than -6. In the past year, flight and in-development nanosatellites have demonstrated laser communications downlinks as well as interplanetary operations. Leveraging nanosatellite technological advancements and electric propulsion systems, several cost-competitive satellite laser guide stars can be developed for specific targets of interest. The satellite laser guide stars can fly in coordination with ELT observational campaigns and enable high contrast ground-based imaging.