Orbit

The tests to be conducted with STE-QUEST require a highly elliptic orbit with the following characteristics:

• Large variations in the gravitational potential
• Long contact times at perigee to compare the clocks at the ground stations with the STE-QUEST clock in space
• Long common-view durations to simultaneously compare two ground clocks separated by intercontinental distances with the STE-QUEST clock in space.

An orbit with a 16-hour period, a minimum perigee altitude of ~600 kilometres, and an apogee altitude of ~51 000 kilometre is being studied for the mission. The orbit would evolve freely and would have the following characteristics:

• A first phase characterised by a low perigee altitude (between 600 and 800 kilometre) and long visibility durations around perigee
• A second phase characterised by long common-view contacts.

The ground track can be optimised to maximise visibility at the STE-QUEST ground stations, which may be located in Boulder (USA), Turin (Italy), and Tokyo (Japan). These locations are particularly favourable because of their geographical distributions and vicinity to research laboratories operating highly stable and accurate atomic clocks.

Top: Evolution of STE-QUEST's perigee ground track. Bottom: Altitude of the STE-QUEST spacecraft along the orbit. Credit: ESA

Measurement Strategy

The primary data product of the STE-QUEST mission will be:

• Space-to-ground comparisons between the STE-QUEST on-board clock and clocks on the ground;
• Atomic interferometry measurements of the differential acceleration between ultra-cold samples of rubidium isotopes (⁸⁵Rb and ⁸⁷Rb).

Space-to-ground clock comparisons will be performed all along the orbit, and in particular, while the spacecraft is at apogee and perigee. In this way, Einstein's prediction of the gravitational frequency shift will be verified both by an absolute measurement between space and ground clocks and by examining the modulation of the redshift effect on the STE-QUEST clock between perigee and apogee.

STE-QUEST will also allow common-view comparison of terrestrial clocks, which can be used to measure the periodic effect of the gravitational frequency shift induced by the Sun.

The atom interferometer will primarily perform differential acceleration measurements while the spacecraft is around perigee (spacecraft altitude below 3000 kilometres), thus maximising the signal-to-noise ratio of a possible violation of the Weak Equivalence Principle.

Ground Segment

The STE-QUEST ground segment is composed of:

• the Mission Operations Centre, which is responsible for performing all spacecraft, payload, and ground segment related operations and receiving, processing, storing and distributing the complete payload telemetry.
• the Science Operations Centre, which is responsible for the definition of the STE-QUEST scientific operations (both payload and ground segment), for the analysis and processing of the science data, for the generation and archiving of mission data products.
• a Distributed Network of Users' Ground Stations which is made up of scientific institutes participating in STE-QUEST and responsible for
  • comparing high-performance atomic clocks on ground with the signals from the STE-QUEST clock in space;
  • analysing STE-QUEST scientific data.

Close monitoring of the spacecraft will be necessary during the on-orbit characterisation phase which is expected to last for the first 6 months of the mission.

The payload will be operated on the basis of scheduled sequences of commands which will be uploaded to the spacecraft and executed on board

The estimated data volume is compatible with small ESOC ground stations.