THEMIS, NASA’s fifth Medium Class Explorer (MIDEX) mission will monitor the onset and macro-scale evolution of magnetospheric substorms. It is a fleet of 5 small satellites (probes) measuring in situ the magnetospheric particles and fields while a network of 20 ground based observatories (GBOs) monitor auroral brightening over Northern America. Three inner probes (∼1 day period, 10 RE apogee) monitor current disruption and two outer probes (∼2 day and ∼4 day period, 20 RE and 30 RE apogees respectively) monitor lobe flux dissipation. In order to time and localize substorm onsets, THEMIS utilizes Sun–Earth aligned conjunctions between the probes when the ground-based observatories are on the nightside. To maintain high recurrence of conjunctions the outer orbits have to be actively adjusted during each observation season. Orbit maintenance is required to rearrange the inner probes for dayside observations and also inject the probes into their science orbits after near-simultaneous release from a common launch vehicle.We present an overview of the orbit strategy, which is primarily driven by the scientific goals of the mission but also represents a compromise between the probe thermal constraints and fuel capabilities. We outline the process of orbit design, describe the mission profile and explain how mission requirements are targeted and evaluated. Mission-specific tools, based on high-fidelity orbit prediction and common magnetospheric models, are also presented. The planning results have been verified by in-flight data from launch through the end of the first primary science seasons and have been used for mission adjustments subject to the early scientific results from the coast phase and first tail season.
[1]
James R. Wertz,et al.
Mission geometry; orbit and constellation design and management
,
2001
.
[2]
Harald U. Frey,et al.
The THEMIS Array of Ground-based Observatories for the Study of Auroral Substorms
,
2008
.
[3]
J. D. Means,et al.
THEMIS Ground-Based Magnetometers
,
2008
.
[4]
Vassilis Angelopoulos,et al.
The THEMIS Mission
,
2008
.
[5]
David G. Sibeck,et al.
THEMIS Science Objectives and Mission Phases
,
2008
.
[6]
Nikolai A. Tsyganenko,et al.
Modeling the Earth's magnetospheric magnetic field confined within a realistic magnetopause
,
1995
.
[7]
Martin Connors,et al.
THEMIS Ground Based Observatory System Design
,
2008
.
[8]
C. M. Hammond,et al.
Imaging the effect of dipole tilt on magnetotail boundaries
,
1994
.
[9]
THEMIS Reaction Control System: From I and T Through Early Mission Operations
,
2007
.