Orbit/Navigation

Operational Orbit

The operational orbit for LISA Pathfinder will be a halo orbit around the first Lagrange (L1) point, about 1.5 million kilometres from the Earth in the sunward direction. This orbit was selected from four possible options:

• Geo-synchronous Orbit
• Heliocentric Earth Trailing Orbit
• Highly elliptical orbit around the Earth
• Orbit around L1 or L2

The L1 Halo orbit has been chosen among the above orbits, because it is an intrinsically "quiet" place in space, far away from massive bodies, which induce tidal forces on the spacecraft, has constant illumination from the Sun, and has a quasi-constant Earth distance for communication. This orbit fulfils the stringent requirements of LISA Pathfinder concerning thermal and gravitational stability.

The baseline halo orbit has a typical potato chip shape, which can be enveloped in a parallelepiped centred on the L1 point with sides X (Earth-Sun line) 0.6 million km, Y (out of ecliptic) 1.6 million km and Z 1.2 million km. The distance between the spacecraft and the Earth varies between 1.2 and 1.8 million km.

This halo orbit, with period of 180 days, is unstable and periodic station keeping manoeuvres are required of about 1.8 ms^-1 per year to be performed with the FEEPs.

LISA Pathfinder transfer and operational orbit

Transfer orbit

The LISA Pathfinder transfer trajectory is based on a launch on board a Rockot vehicle from the Plesetsk Cosmodrome in Russian Siberia. A compatibility study with a possible launch with the newly developed VEGA launcher, from the European Spaceport in Kourou, French Guiana is ongoing.

The Rockot upper stage, named Breeze KM, will inject the LISA Pathfinder spacecraft into an elliptical orbit inclined at 63° to the equator, with apogee near 600 km and a perigee at 200 km. The argument of perigee depends on the exact day of the launch and will be selected to get the right orbit orientation for that day of the year. The LISA Pathfinder propulsion module, loaded with 1100 kg of bipropellant fuel, will be used to transfer the spacecraft to the operational halo orbit at L1.

The velocity increment required for apogee raising and injection into the operational orbit is about 3100 ms^-1. After the transfer to the operational orbit, the propulsion module separates from the science spacecraft prior to drag free operations to prevent disturbances that would be generated by the residual propellants acting on the inertial sensors.

Ground Segment

The ESA Mission Operations Centre (MOC) at ESOC, Darmstadt in Germany will control the mission. The communications to the spacecraft will be performed in X-band through a network of ground stations (including Kourou, Maspalomas and Perth) during the Launch and Early Operations, and through a single 35m X-band deep space antenna located in Cebreros, Spain during nominal science operations. Through this single station, the communications link will be established for 8 hours per day. During each pass the data stored on-board will be retrieved, the spacecraft tracking will be performed and the commands timeline will be uplinked.

A Science and Technology Operations Centre (STOC) will be located in ESTEC. This centre will plan and coordinate all the operations performed with the LTP and the DRS and perform quick look data analysis and archiving. The STOC also interfaces with all the scientific institutes involved in the mission.