Operational history

(Note: Adapted from Planck Collaboration 2011, A&A 536, A1)

The major operational phases and milestones are:

1. Launch and transfer to orbit: (provide date, and summary overview)

Planck was launched from the Centre Spatial Guyanais in Kourou (French Guyana) on 14 May 2009 at its nominal lift-off time of 13:12 UT, on an Ariane 5 ECA rocket of Arianespace2. ESA’s Herschel observatory was launched on the same rocket. At 13:37:55 UT, Herschel was released from the rocket at an altitude of 1200 km; Planck followed suit at 13:40:25UT. The separation attitudes of both satellites were within 0.1 deg of prediction. The Ariane rocket placed Planck with excellent accuracy (semimajor axis within 1.6% of prediction), on a trajectory towards the second Lagrangian point of the Earth-Sun system (L2). After release from the rocket, three large manoeuvres were carried out to place Planck in its intended final orbit. The first (14.35 ms−1), intended to correct for errors in the rocket injection, was executed on 15 May at 20:01:05 UT, with a slight overperformance of 0.9% and an error in direction of 1.3 deg (a touch-up manoeuvre was carried out on 16 May at 07:17:36 UT). The second and major (mid-course) manoeuvre (153.6ms−1) took place between 5 and 7 June, and a touch-up (11.8 ms−1) was executed on 17 June. The third and final manoeuvre (58.8ms−1), to inject Planck into its final orbit, was executed between 2 and 3 July. The total fuel consumption of these manoeuvres, which were carried out using Planck’s coarse (20N) thrusters, was 205 kg. Once in its final orbit, very small manoeuvres are required at approximately monthly intervals (1 ms−1 per year) to keep Planck from drifting away from its intended path around L2. The attitude manoeuvres required to follow the scanning strategy require about 2.6 ms−1 per year. Overall, the excellent performance of launch and orbit manoeuvres will lead to a large amount (∼160 kg, or ∼40% of initial tank loading) of fuel remaining on board at end of mission operations.

Planck started cooling down radiatively shortly after launch. Heaters were activated to hold the focal plane at 250 K, which was reached around 5 h after launch. The valve opening the exhaust piping of the dilution cooler was activated at 03:30 UT, and the 4He-JT cooler compressors were turned on at low stroke at 05:20 UT. After these essential operations were completed, on the second day after launch, the focal plane temperature was allowed to descend to 170 K for out-gassing and decontamination of the telescope and focal plane.

2. Commissioning & Calibration and Performance Verification: (provide date, and summary overview)

The first period of operations focussed on commissioning activities, i.e., functional check-out procedures of all sub-systems and instruments of the Planck spacecraft in preparation for running science operations related to calibration and performance verification of the payload. Planning for commissioning operations was driven by the telescope decontamination period of 2 weeks and the subsequent cryogenic cool-down of the payload and instruments. The overall duration of the cool-down was approximately 2 months, including the decontamination period. The sequence of commissioning activities covered the following areas:

– on-board commanding and data management;

– attitude measurement and control;

– manoeuvreing ability and orbit control;

– telemetry and telecommand;

– power control;

– thermal control;

– payload basic functionality, including:

– the LFI;

– the HFI;

– the cryogenic chain;

– the Standard Radiation Environment Monitor;

– the Fibre-Optic Gyro unit (FOG), a piggy-back experiment which is not used as part of the attitude control system.

The commissioning activities were executed very smoothly and all sub-systems were found to be in good health. The most significant unexpected issues that had to be addressed during these early operational phases were the following.

– The X-band transponder showed an initialisation anomaly during switch-on which was fixed by a software patch.

– Large reorientations of the spin axis were imperfectly completed and required optimisation of the on-board parameters of the attitude control system.

– The data rate required to transmit all science data to the ground was larger than planned, due to the unexpectedly high level of Galactic cosmic rays, which led to a high glitch rate on the data stream of the HFI bolometers (Planck HFI Core Team 2011a); glitches increase the dynamic range and consequently the data rate. The total data rate was controlled by increasing the compression level of a few less critical thermometers.

– The level of thermal fluctuations in the 20-K stage was higher than originally expected. Optimisation of the sorption cooler operation led to an improvement, though they still remained ∼25% higher than expected (Planck Collaboration 2011b).

– The 20-K sorption cooler turned itself off on 10 June 2009, an event which was traced to an incorrectly set safety threshold.

– A small number of sudden pressure changes were observed in the 4He-JT cooler during its first weeks of operation, and were most likely due to impurities present in the cooler gas (Planck Collaboration 2011b). The events disappeared after some weeks, as the impurities became trapped in the cooler system.

– The 4He-JT cooler suffered an anomalous switch to standby mode on 6 August 2009, following a current spike in the charge regulator unit which controls the current levels between the cooler electronics and the satellite power supply (Planck Collaboration 2011b). The cooler was restarted 20 h after the event, and the thermal stability of the 100-mK stage was recovered about 47 h later. The physical cause of this anomaly was not found, but the problem has not recurred.

– Instabilities were observed in the temperature of the 4He-JT stage, which were traced to interactions with lower temperature stages, similar in nature to instabilities observed during ground testing (Planck Collaboration 2011b). They were fixed by exploring and tuning the operating points of the multiple stages of the cryo-system.

– The length of the daily telecommunications period was increased from 180 to 195 min to improve the margin available and ensure completion of all daily activities.

The commissioning activities were formally completed at the time when the HFI bolometer stage reached its target temperature of 100 mK, on 3 July 2009 at 01:00 UT. At this time all the critical resource budgets (power, fuel, lifetime, etc.) were found to contain very significant margins with respect to the original specification

3. Nominal Mission: (provide date, and summary overview)

4. Extended Mission: (provide date, and summary overview)

5. LFI-only phase: (provide date, and summary overview)

6. End-of-life: (provide a vague estimation)

Provide a description of the product Planck Operational State History.