The Planck mission
Planck is a space telescope of the European Space Agency designed to answer key cosmological questions. Its main goal is to determine the geometry and content of the Universe, and which theories describing the birth and evolution of the Universe are correct. To achieve this ambitious objective, it observed the Cosmic Microwave Background radiation (CMBCosmic Microwave background), emitted about 14 billion years ago, around 380,000 years after the Big Bang. The CMBCosmic Microwave background permeates the Universe and is observed to have a blackbody spectrum with a temperature of 2.7 K. Small deviations from isotropy encode a wealth of information on the properties of the Universe in its infancy. The objective of Planck is to measure these properties with an unprecedented accuracy and level of detail.
As with all ESAEuropean Space Agency scientific missions, Planck was developed in a partnership with the European scientific community. Two consortia of scientific institutes, each led by a Principal Investigator, developed and delivered to ESAEuropean Space Agency two instruments designed specifically for Planck. Each of these instruments targets a specific number of wavelength bands within the range in which the CMBCosmic Microwave background is observable. Together, the two instruments are capable of collecting data of a quality adequate to measure the CMBCosmic Microwave background signal and distinguish it from other confusing sources. A large telescope collects the light from the sky and delivers it to the instruments for measurement and analysis. The reflectors of the Planck telescope were developed and delivered to ESAEuropean Space Agency by a Danish consortium of institutes. ESAEuropean Space Agency retains overall management of the project, develops and procures the spacecraft, integrates the instruments into the spacecraft, and launches and operates it. Planck was launched on May 14th 2009 on an Ariane 5 rocket together with the Herschel Space Observatory. After launch, they were both placed into orbits around the L2 Lagrange of the Sun-Earth system, located about 1.5 million km from the Earth. From that far vantage point, Planck swept the sky regularly in large swaths, and covered it fully about six 5 times for the HFI(Planck) High Frequency Instrument and 8 times for the LFI(Planck) Low Frequency Instrument. Each of the two instrument consortia operated their respective instrument and processed all the data into usable scientific products. At the end of the mission the consortia delivered the final products to ESAEuropean Space Agency, which archives them and distributes them to the community.
The last command to the Planck satellite was sent on the 23rd of October 2013, marking the end of operations.
Early operations and transfer to orbit
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 Arianespace. ESAEuropean Space Agency’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). The orbit describes a Lissajous trajectory around L2 with a ∼6 month period that avoids crossing the Earth penumbra for at least 4 years.
After release from the rocket, three large manoeuvres were carried out to place Planck in its intended final orbit. Once in its final orbit, very small manoeuvres were 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 has lead to a large amount 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.
Commissioning and initial science operations
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 commissioning activities were executed very smoothly and all sub-systems were found to be in good health. The commissioning activities were formally completed at the time when the HFI(Planck) High Frequency Instrument 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.
Calibration and performance verification
Calibration and performance verification (CPVCalibration and Performance Verification) activities started during the cool-down period and continued until the end of August 2009. On completion of all the planned activities, it was concluded that the two instruments were fully tuned and ready for routine operations. No further parameter tuning was expected to be needed, except for the sorption cooler, which requires a weekly change in operational parameters. The scientific performance parameters of both instruments was in most respects as had been measured on the ground before launch. The only significant exception was that, due to the high level of Galactic cosmic rays, the bolometers of HFI(Planck) High Frequency Instrument were detecting a higher number of glitches than expected, causing a modest (∼10%) level of systematic effects on their noise properties. The satellite did not introduce any major systematic effects into the science data. In particular, the telemetry transponder did not result in radio-frequency interference, which implies that the data acquired during visibility periods is useable for science.
The First Light Survey (FLS) was the last major activity planned before the start of routine surveying of the sky. It was conceived as a two-week period during which Planck would be fully tuned up and operated as if it was in its routine phase. This stable period could have resulted in the identification of further tuning activities required to optimise the performance of Planck in the long-duration surveys to come. The FLS was conducted between 13 and 27 August, and in fact led to the conclusion that the Planck payload was operating stably and optimally, and required no further tuning of its instruments. Therefore the period of the FLS was accepted as a valid part of the first Planck survey.
Routine operations phase
The routine operations phase of Planck is characterised by continuous and stable scanning of the sky and data acquisition by LFI(Planck) Low Frequency Instrument and HFI(Planck) High Frequency Instrument. It started with the FLS on 13 August of 2009.
The Planck satellite generates (and stores on-board) data continuously at the following typical rates: 21 kilobit s−1 (kbps) of house-keeping (HKHouse Keeping) data from all on-board sources, 44 kbps of LFI(Planck) Low Frequency Instrument science data and 72 kbps of HFI(Planck) High Frequency Instrument science data. The data are brought to ground in a daily pass of approximately 3 h duration. Besides the data downloads, the passes also acquire realtime HKHouse Keeping and a 20 min period of real-time science (used to monitor instrument performance during the pass). Planck utilises the two ESAEuropean Space Agency deep-space ground stations in New Norcia (Australia) and Cebreros (Spain). Scheduling of the daily telecommunication period is quite stable, with small perturbations due to the need to coordinate the use of the antenna with other ESAEuropean Space Agency satellites (in particular Herschel). At the ground station the telemetry is received by redundant chains of front-end/back-end equipment. The data flows to the mission operations control centre (MOC[ESA's] Mission Operation Center [Darmstadt, Germany]) located at ESOCEuropean Space Operations Centre (Darmstadt) in Darmstadt (Germany), where it is processed by redundant mission control software (MCS) installations and made available to the science ground segment. To reduce bandwidth requirements between the station and ESOCEuropean Space Operations Centre (Darmstadt) only one set of science telemetry is usually transferred. Software is run post-pass to check the completeness of the data. This software check is also used to build a catalogue of data completeness, which is used by the science ground segment to control its own data transfer process. Where gaps are detected, attempts to fill them are made as an offline activity (normally next working day), the first step being to attempt to reflow the relevant data from station. Early in the mission these gaps were more frequent, with some hundreds of packets affected per week (impact on data return of order 50 ppm) due principally to a combination of software problems with the data ingestion and distribution in the MCS, and imperfect behaviour of the software gap check. Software updates implemented during the mission have improved the situation such that gaps are much rarer, with a total impact on data return well below 1 ppm. Redump of data from the spacecraft is attempted when there have been losses in the space link. This has only been necessary on very few occasions. In each case the spacecraft redump has successfully recovered all the data.
All the data downloaded from the satellite, and processed products such as filtered attitude information, are made available each day for retrieval from the MOC[ESA's] Mission Operation Center [Darmstadt, Germany] by the LFI(Planck) Low Frequency Instrument and HFI(Planck) High Frequency Instrument Data Processing Centres (DPCs).
The scanning strategy is the following: the spin axis follows a cycloidal path on the sky by step-wise displacements of 2 arcmin approximately every 50 min. The dwell time (i.e., the duration of stable data acquisition at each pointing) has varied sinusoidally by a factor of ∼2. Planck’s scanning strategy results in significantly inhomogeneous depth of integration time across the sky; the areas near the ecliptic poles are observed with greater depth than all others.
The scanning strategy for the second year of Routine Operations was exactly the same as for the first year, except that all pointings are shifted by 1 arcmin along the cross-scanning direction, in order to provide finer sky sampling for the highest frequency detectors when combining two years of observations.
The remaining two years of operations including surveys 6 to 8 of the LFI(Planck) Low Frequency Instrument only phase made use of a change in the cycloid phase of 90 degrees in order to spread out scanning directions over the sky.
Orbit maintenance manoeuvres were carried out at approximately monthly intervals. Although the manoeuvres only required a few minutes, preparations, post-manoeuvre mass properties calibration[LFI meaning]: absolute calibration refers to the 0th order calibration for each channel, 1 single number, while the relative calibration refers to the component of the calibration that varies pointing period by pointing period., and re-entry into scientific slewing mode increased the overhead to several hours. The manoeuvres were carried out without disturbing the path of the spin axis from its nominal scanning law. The dwell times of pointings before and after the execution of the manoeuvre were reduced to allow all pre-planned pointings to be carried out.
While the Planck detectors are scanning the sky, they also naturally observe celestial calibrators. The main objects used for this purpose are the Crab Nebula, and the bright planets Mars, Jupiter and Saturn.
Major operational milestones
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 Arianespace.
On 13 August 2009, Planck started its first all sky survey after successfully concluding its commissioning phase.
On 14 January 2012, Planck’s HFI(Planck) High Frequency Instrument completed its survey of the early Universe after running out of coolant as expected. Able to work at slightly higher temperatures, the LFI(Planck) Low Frequency Instrument continued surveying the sky until 3 October 2013.
On 4 October 2013 Planck ended its routine operations, after executing its last observation of the Crab Nebula, and started its decommissioning activities.
On 19 October 2013, the sorption cooler and the two instruments (LFI(Planck) Low Frequency Instrument & HFI(Planck) High Frequency Instrument) were switched-off.
The final command to the Planck satellite was sent on 23 October 2013, marking the end of operations.
The main unplanned events included the following:
- Very minor deviations from the scanning law include occasional (on the average about once every two months) under-performance of the 1-N thrusters used for regular manoeuvres, which implied the corresponding pointings were not at the intended locations. These deviations had typical amplitudes of 30 arcsec, and have no significant impact on the coverage map.
- The thruster heaters were unintentionally turned off between 31 August and 16 September 2009 (the so-called “catbed” event).
- An operator error in the upload of the on-board command timeline led to an interruption of the normal sequence of manoeuvres and therefore to Planck pointing to the same location on the sky for a period of 29 h between 20 and 21 November 2009 (“the day Planck stood still”). Observations of the nominal scanning pattern resumed on 22 November, and on 23 November a recovery operation was applied to survey the previously missed area. During the recovery period the duration of pointing was decreased to allow the nominal law to be caught up with. As a side effect, the RF transmitter was left on for longer than 24 h, which had a significant thermal impact on the warm part of the satellite.
- As planned, the RF transmitter was initially turned on and off every day in synchrony with the daily visibility window, in order to reduce potential interference by the transmitter on the scientific data. The induced daily temperature variation had a measurable effect throughout the satellite. An important effect was on the temperature of the 4He-JT cooler compressors, which caused variations of the levels of the interference lines that they induce on the bolometer data (Planck HFI(Planck) High Frequency Instrument Core Team 2011a). Therefore the RF transmitter was left permanently on starting from 25 January 2010 (257 days after launch), which made a noticeable improvement on the daily temperature variations.
- During the coverage period, the operational star tracker switched autonomously to the redundant unit on two occasions (11 January 2010 and 26 February 2010); the nominal star tracker was restored a short period later (3.37 and 12.75 h, respectively) by manual power-cycling. Although the science data taken during this period have normal quality, they have not been used because the redundant star tracker’s performance is not fully characterised.
For more information
A complete overview of the Planck mission and its science programme can be found in the Blue Book.
A complete list of Planck publications can be found here.
- Planck early results. I. The Planck mission, Planck Collaboration I, A&A, 536, A1, (2011).