The five THEMIS spacecraft (probes) are placed in highly elliptical orbits where the spacecraft will line up at apogee every four days. The apogee rotates slowly around the Earth to cover the dayside, dawnside, nightside, and duskside of the magnetosphere. Inititally, right after launch, the 5 THEMIS spacecraft are lined up in the same orbit. THEMIS orbit parameters:
The different orbit configurations define the five stages of the mission:
Stage 1: Injection or “Coast” Phase
2/15/07 - 9/15/07
Right after launch all spacecraft are lined up in the same orbit with a 15.4 Re apogee.
Stage 2: Orbit Placement Phase
9/15/07 - 12/15/07
The orbit placement phase is also called “Dawn phase” because the apogee of the orbits are on the dawn side of the magnetosphere. Probe 1 apogee is at 30 Re, Probe 2 at 20 Re, Probes 3 and 4 at 12 Re, and Probe 5 at 10 Re.
Stage 3: Tail Science Phase
12/15/07 – 4/15/08
In the tail science phase the apogee of the orbits are in the magnetotail. Probe 1 apogee is at 30 Re, Probe 2 at 20 Re, Probes 3 and 4 at 12 Re, and Probe 5 at 10 Re.
Stage 4: Radiation Belt Science Phase
4/15/08 - 6/15/08
The radiation belt science phase is also called the “Dusk phase” because the apogee of the orbits are on the dusk side of the magnetosphere. Probe 1 apogee is at 30 Re, Probe 2 at 20 Re, Probes 3 and 4 at 12 Re, and Probe 5 at 10 Re.
Stage 5: Dayside Science Phase
6/15/08 - 10/15/08
In the dayside science phase the apogee of the orbits are on the dayside of the magnetosphere. Probe 1 apogee is at 30 Re, Probe 2 at 20 Re, Probes 3 and 4 at 12 Re, and Probe 5 at 10 Re.
