Observations of PHEBUS around Mercury
The PHEBUS instrument has the peculiarity of having its own pointing mechanism (scanner) allowing it to choose its line of sight regardless of the orientation of the satellite. In order to meet its scientific objectives, the following different points or modes of observation have been defined:
⟨ 1 ⟩ Scanning the Interplanetary Background for the detection of chemical species (hydrogen, helium) by rotating the pointing mechanism from the North to the South ecliptic pole *.
⟨ 2 ⟩ Observations of Mercury’s poles to detect water ice inside the craters that are permanently in the shade. The line of sight points to “nadir” (i.e. to the center of the planet).
Observations of the dayside ⟨ 3 ⟩ or nightside ⟨ 4 ⟩ exosphere at different altitudes above the surface to determine its composition and vertical structure (spatial distribution of detected chemical species).
⟨ 5 ⟩ Observations of stars at the Apoapsis (point of orbit that is the furthest from the center of the planet) dedicated to the calibration and monitoring of the performance * * of the instrument.
* The Earth describes an elliptical orbit around the Sun, always remaining on the same plane called “ecliptic”. The ecliptic coordinate system is a coordinate system adapted to the objects of the solar system: it uses the plane of the ecliptic as a reference plane, the Sun being in the center of the reference system.
* * Since the performance of the instrument can deteriorate over time, the reference measures acquired throughout the mission will allow us to calibrate the instrument, i.e. to translate the quantities measured by the instrument into exploitable physical quantities. This in-flight validation of the instrumental response is essential to have confidence in the scientific results.
What is the ground segment?
The Ground Segment comprises all the human and material ground resources necessary for the operation of the mission and consists of several entities spread over several geographical locations:
• The MOC (Mission Operations Centre) at the European Space Operations Centre (ESOC) in Darmstadt, Germany. It is responsible for controlling BepiColombo from the ground, sending and receiving information to and from the satellite.
• The SOC (Science Operations Centre) at the European Space Astronomy Centre (ESAC) in Villafranca del Castillo, Spain. It is responsible for the planning of observations, archiving and distribution of all mission data (see “How are data processed?”).
• The PHESOC (Phebus Science operations Centre) at LATMOS. It is responsible for proposing to the SOC the observation plans (pointings, time) of the PHEBUS instrument, one month before their execution, in accordance with:
• The mission’s scientific activity plan;
• MPO (Bepi) resources (power consumption, data flow);
• The predictive data (orbit/attitude of the satellite) provided by the SOC.
The PHESOC prepares and submits the telecommands sequences of the instrument one week before the implementation of the observation plan validated by SOC. The PHESOC must also develop and provide SOC with the software for processing the data acquired by the instrument. Thereafter, the PHESOC analyses and validates the data quality.
What types and volumes of data are transiting?
• Data sent from Earth to the satellite and used to control the satellite or instruments are called telecommands (TC). The TC are recovered by the satellite’s central onboard computer, which then distributes them to satellite subsystems and instruments.
• Data that are generated by the instrument and sent to Earth are called telemetry (TM).
These data are of different kinds:
• Science data, such as the rate of photon counts emitted by the chemical species present in the observed medium,
• Monitoring data of the proper functioning of the instrument, such as the temperatures and voltages of the instrument’s subsystems,
• or error or event reports, such as a clock synchronization problem with the platform or an operational mode change.
• The MPO (Bepi) probe will rotate around Mercury in only 2.3 hours and will thus complete 10 orbits per day. PHEBUS will observe an average of 30 minutes per orbit, or 5 hours per day. With a 16 kbit/s telemetry stream for compressed data, an annual volume of 80 Gbit will be transmitted.
How are data processed?
Each instrumental team develops a software (pipeline) to process the data acquired by its instrument. A pipeline is a sequence of tasks, each of which applies a specific treatment or correction to the data output of the instrument in order to make them usable by the scientific community.
To help with the analysis, these data are supplemented with auxiliary information. These pipelines are delivered to SOC, which is responsible for mass production, archiving and distribution of the data to the scientific community. Scientists then work on interpreting the data (comparison with models, statistical analyses, etc.).
