SEL2 servicing: increased science return via on-orbit propellant replenishment

Spacecraft designers are driving observatories to the distant Sun-Earth Lagrange Point 2 (SEL2) to meet ever-increasing science requirements. The mass fraction dedicated to propellant for these observatories to reach and operate at SEL2 will be allocated with the upmost care, as it comes at the expense of optics and instrument masses. As such, these observatories could benefit from on-orbit refueling, allowing greater dry-to-wet mass ratio at launch and/or longer mission life. NASA is developing technologies, capabilities and integrated mission designs for multiple servicing applications in low Earth orbit (LEO), geosynchronous Earth orbit (GEO) and cisluner locations. Restore-L, a mission officially in formulation, will launch a free-flying robotic servicer to refuel a government-owned satellite in LEO by mid 2020. This paper will detail the results of a point design mission study to extend Restore-L servicing technologies from LEO to SEL2. This SEL2 mission would launch an autonomous, robotic servicer spacecraft equipped to extend the life of two space assets through refueling. Two space platforms were chosen to 1) drive the requirements for achieving SEL2 orbit and rendezvous with a spacecraft, and 2) to drive the requirements to translate within SEL2 to conduct a follow-on servicing mission. Two fuels, xenon and hydrazine, were selected to assess a multiple delivery system. This paper will address key mission drivers, such as servicer autonomy (necessitated due to communications latency at L2). Also discussed will be the value of adding cooperative servicing elements to the client observatories to reduce mission risk.