THEMIS is a five-spacecraft constellation launched in 2007 to study magnetospheric phenomena leading to the aurora borealis. During the primary mission phase, completed in the fall of 2009, all five spacecraft collected science data in synchronized, highly elliptical Earth orbits. For an ambitious mission extension, the Project proposed to split the constellation into two parts-THEMIS-Low and ARTEMIS. THEMIS-Low includes the three spacecraft on the inner orbits with approximately one-day periods, continuing their study of the magnetosphere in a tighter formation. ARTEMIS involves transferring the outer two spacecraft from their Earth orbits with two and four-day periods into lunar orbits to conduct measurements of the interaction of the Moon with the solar wind and of crustal magnetic fields. This transfer was initiated on July 21, 2009 and follows low-energy trajectories with Earth and lunar gravity assists. The THEMIS mission is controlled from the highly automated multi-mission operations center at the University of California, Berkeley and was originally designed to be supported by 11-m class ground stations and NASA's Space Network. To increase the telemetry bandwidth for science data return at lunar distances, the mission network was expanded to also include the 34-m subnet of NASA's Deep Space Network (DSN). This paper discusses all aspects of the process to seamlessly integrate the new DSN interfaces into the THEMIS/ARTEMIS mission control network, and describes challenges and lessons learned with the implementation of real-time telemetry and command data transfer using the CCSDS Space Link Extension protocol. It also includes on-orbit characterization of the transponder ranging channels, orbit determination results using two-way Doppler and range data from a combination of conventional ground stations and DSN stations, as well as pass scheduling via the DSN Resource Allocation Planning Service and via automated, electronic data exchanges. All of these tasks were accomplished within a compressed schedule of one year, with very limited staffing resources, and on a tight budget.
THEMIS, a constellation of five spacecraft, referred to as probes, was launched in 2007 to study the physical processes leading to the aurora. In 2009, THEMIS successfully completed its primary mission phase. As an ambitious mission extension, the constellation was then split into two new missions -THEMIS-Low and ARTEMIS. THEMIS-Low refers to three of the five probes that continued magnetospheric observations in Earth orbits while the remaining two probes started a new lunar mission called ARTEMIS. The two ARTEMIS probes were transferred from Earth to lunar orbits via low-energy trajectories with Earth and lunar gravity assists. The complex mission design and navigation operations took the two probes on trajectories along weak stability boundary manifolds, venturing out as far as 1,500,000 km and 1,200,000 km from Earth, respectively. Upon arrival in the lunar environment, both probes were first inserted into libration point orbits where they spent up to ten months collecting science data. Periodic stationkeeping maneuvers were executed to ensure the two probes would not be ejected from these unstable orbits. In 2011, both probes were successfully inserted into stable, retrograde and prograde lunar orbits, respectively. We report on the challenges with executing the complex navigation plans, discuss experiences and lessons learned from operating two spacecraft in lunar libration point orbits for the first time ever, and finally cover mission planning and science operations in the lunar environment.
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