THEMIS Reaction Control System: From I and T Through Early Mission Operations

THEMIS is a NASA Middle Explorer (MIDEX) program, managed in a “Principal Investigator (PI) mode” by the University of California at Berkeley (UCB). THEMIS employs five identical micro-satellites (otherwise called “probes”) launched by a single, Delta II/7925 rocket into a highly eccentric, 14.6 Earth radii (RE), orbit and equipped with hydrazine Reaction Control Systems (RCS) to attain their final orbits ranging from 10RE to 30RE. ATK was contracted by UCB to build the five Probes and the Probe Carrier. Aerojet-General Corporation, located in Redmond, WA, was selected by ATK Aerospace to build the RCS systems for the probes. The objective of the THEMIS mission is to place the five probes in one, two and four-day period orbits so that once every four days the probes align near their apogees along the Sun-Earth line, and capture correlated measurements of energy flow in Earth’s magnetosphere in order to pin-point the origin of magnetospheric sub-storms – a fundamental space weather process. The probes were successfully launched on 17 February 2007 out of Cape Canaveral and have been placed in their initial coast-phase positions in anticipation of their final placements in winter of 2007. This paper describes some of the challenges presented to the program by the I&T process, safety issues, and mission operations.