Planck Pre-Launch Status Papers - APC - Université Paris Diderot ...

J. A. Tauber et al.: Planck pre-launch status: The Planck mission2. Satellite descriptionFigures 2 and 3 show the major elements and characteristics ofthe Planck satellite. Planck was designed, built and tested aroundtwo major modules:1. a payload module (see Fig. 5) containinganoff-axis telescopewith a projected diameter of 1.5 m, focussing radiationfrom the sky onto a focal plane shared by detectors of the LFIand HFI, operating at 20 K and 0.1 K respectively; a telescopebaffle thatsimultaneouslyprovidesstray-lightshieldingand radiative cooling; and three conical “V-groove” bafflesthat provide thermal and radiative insulation between thewarm service module and the cold telescope and instruments.2. a service module (see Fig. 6) containingallthewarmelectronicsservicing instruments and satellite; and the solarpanel providing electrical power. It also contains the cryocoolers,the main on-board computer, the telecommand receiversand telemetry transmitters, and the attitude controlsystem with its sensors and actuators.The most relevant technical characteristics of the Planck spacecraftare detailed in Table 1.Fig. 1. The fully assembled Planck satellite a few days before integrationinto the Ariane 5 rocket. Herschel is visible by reflection on theprimary reflector. Photo by A. Arts.possible for detailed descriptions. In addition to a summary ofthe material presented in the accompanying papers, this one alsoincludes a description of the scientifically relevant elements ofthe satellite performance, of its planned operations, and a briefoverview of the “science ground segment”. The main accompanyingpapers, most of which are part of this special issue ofAstronomy & Astrophysics,include:– Tauber et al. (2010), describing the optical performance ofthe combined payload, i.e. telescope plus instruments;– Mandolesi et al. (2010), describing programmatic aspects ofthe LFI and its development;– Bersanelli et al. (2010), describing in detail the design of theLFI;– Mennella et al. (2010), describing the test and calibrationprogramme of the LFI at instrument and system levels priorto launch;– Villa et al. (2010), describing the test and calibration of theLFI radiometer chains;– Sandri et al. (2010), describing the design and test of the LFIoptics;– Leahy et al. (2010), describing the polarisation aspects of theLFI, and its expected performance in orbit;– Lamarre et al. (2010), describing in detail the on-ground design,manufacture, test and performance of the HFI;– Pajot et al. (2010), describing the test and calibration programmeof the HFI prior to launch;– Ade et al. (2010), describing the design, test and performanceof the cryogenic elements of the HFI focal plane;– Holmes et al. (2008), describing the design, manufacture andtest of the HFI bolometers;– Maffei et al. (2010), describing the design and test of the HFIoptics;– Rosset et al. (2010), describing the polarisation aspects ofthe HFI.2.1. PointingPlanck spins at 1 rpm around the axis of symmetry of the solarpanel 2 .Inflight,thesolarpanelcanbepointedwithinaconeof 10 ◦ around the direction to the Sun; everything else is alwaysin its shadow. The attitude control system relies principally on:– Redundant star trackers as main sensors, and solar cells forrough guidance and anomaly detection. The star trackerscontain CCDs which are read out in synchrony with thespeed of the field-of-view across the sky to keep star imagescompact.– Redundant sets of hydrazine 20 N thrusters for large manoeuversand 1 N thrusters for fine manoeuvers.An on-board computer dedicated to this task reads out the startrackers at a frequency of 4 Hz, and determines in real time theabsolute pointing of the satellite based on a catalogue of brightstars. Manoeuvers are carried out as a sequence of 3 or 4 thrustsspaced in time by integer spin periods, whose duration is calculatedon-board, with the objective to achieve the requested attitudewith minimal excitation of nutation. There is no further activedamping of nutation during periods of inertial pointing, i.e.between manoeuvers. The duration of a small manoeuver typicalof routine operations (2 arcmin) is ∼5 min.Largermanoeuversare achieved by a combination of thrusts using both 1 Nand 20 N thrusters, and their duration can be considerable (up toseveral hours for manoeuver amplitudes of several degrees). Theattitudes measured on-board are further filtered on the ground toreconstruct with high accuracy the spacecraft attitude (or ratherthe star tracker reference frame). The star trackers and the instrumentalfield-of-view were aligned on the ground independentlyto the spacecraft reference frame; the resulting alignmentaccuracy between the star trackers and the instruments was of2 In reality, Planck spins about its principal axis of inertia, whichdoes not coincide exactly with the geometrical axis; this difference willevolve slowly during the mission due to fuel expenditure. After ongroundbalancing, the difference (often called “wobble angle”) is predictedto be ∼–14 arcmins just after launch (mainly around the Y axis,see Fig. 3), and to vary between ∼–5 arcmins after the final injection manoeuverinto L2 (when most of the fuel has been expended), to ∼+5arcminat end of the nominal mission lifetime.Page 3 of 22