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Planck
 
Spacecraft Overview

Planck will travel in a Lissajous orbit around the Earth-Sun L2 point, about 1 million miles further from the Sun than we are. The two-ton Planck spacecraft will spin at about one rpm about an axis which points towards the Sun. The 1.5 m aperture telescope looks about 80° away from the spin axis and scans circles in the sky. The Planck Spacecraft is shown in the image below on a mounting fixture at the launch facility.


HFI's focal plane
The Planck spacecraft on a mounting fixture at the Kourou launch facility in February 2009. The telescope baffle is seen at the top of the spacecraft. The instrument focal plane is the gold assembly in the middle of the telescope enclosure. The three V-groove radiators are seen just below the telescope enclosure (the top of the highest V-groove shield is black and the bottom two are shiny silver colored). Below the V-groove shields is the spacecraft bus: silver on top and black on the sides. The solar panel and primary communications antenna are under the spacecraft bus and therefore cannot be seen in this picture. (Image credit: ESA)


Two continuous operation sorption coolers, developed by the Jet Propulsion Laboratory (JPL) as a NASA contribution to the European Space Agency (ESA), will be flown on the Planck mission. They will provide refrigeration at a temperature of 17 K (that's -429° F, or only 30° F above absolute zero). These coolers are part of a sophisticated cryogenic cooling chain design that combines:
  • Passive radiative cooling of the telescope enclosure to approximately 40 K and precooling of the three active coolers to ~47 K
  • Two active sorption Joule-Thomson (J-T) coolers, each of which provides approximately 1.1 W of refrigeration at ~17 K to the Low Frequency Instrument (LFI) and High Frequency Instrument (HFI)
  • A 4.5 K J-T cooler developed by the UK's Rutherford Appleton Laboratory which is precooled by the sorption J-T cooler
  • A 100 mK dilution cooler developed by France's Aerospatiale which is precooled by both the sorption J-T cooler and the 4.5 K J-T cooler
As with most fundamental physics experiments, the data to be taken by Planck are more sensitive to the specifics of the experimental apparatus and it's immediate environment than to the science of interest. Uncertainties or oscillations in pointing, supply voltages, thermal fluctuations, and stray light (much of this is thermal emission from different surfaces) all impact the data collected by the instrument. As an example, the spacecraft emits nearly 2,000 W, yet the HFI is sensitive to energy levels on the order of 10-18 W: a ratio of 1021 to 1!! As a consequence, even extraordinarily minute spacecraft thermal emission fluctuations that reach the HFI detector could be confused with sky emission.

Changes over some timescales, especially at about the 1 minute spin period, are particularly critical, as they are difficult to distinguish from the observed sky. Even worse, this does not mean that these changes necessarily have a frequency of 0.01667 Hz; but only that they have a non-zero Fourier component at this frequency. The result is that the mission configuration and design, along with the instrument designs, operational and data analysis strategies, must all be driven to minimize systematic effects on the final science data products by reducing the level of these effects and ensuring that they are well understood so that the residual effects can be confidently removed in software without adversely affecting the science results. Ensuring the successful accomplishment of the science goals is the major factor affecting the mission design, and therefore the design of the sorption J-T coolers.[1]

All warm components of the instruments are mounted on the spacecraft bus. This includes not only the instrument electronics but the warm compressors of the sorption coolers and the RAL 4.5 K mechanical J-T cooler, the gas storage tanks for the dilution refrigerator and all of their associated electronics. The optical bench and telescope are thermally isolated from the warm spacecraft bus by twelve low conductance struts and three V-groove shields. The specular, low-emissivity V-groove shields are set at angles of 5, 10 and 15 degrees relative to the top of the spacecraft bus. The radiative heat transfer between two facing surfaces of the V-groove shield with emissivity (ε) is proportional to ε2, while that between infinite parallel planes is ε/2. The extra energy radiated by the facing V-groove surfaces is radiated to space.

The V-groove shields therefore can provide extremely good radiative isolation between objects at different temperatures even with surfaces of only moderately low emissivity. In addition, they can be highly efficient at intercepting conductive thermal loads and radiating them to space. The reduction in heat transfer in this arrangement is significantly superior to that typically achieved by conventional Multi-Layer Insulation (MLI) between two parallel plates. The V-groove design concept was originally invented by Ray Garcia of the California Institute of Technology's Jet Propulsion Laboratory. Since then, this new technology has been validated in thermal vacuum and vibration tests.[2] [3] Similar "bounce-view-of-space" tricks are commonly employed in high performance radiators (e.g. the NIMS radiative cooler for the Galileo Orbiter[4]) and therefore substantial flight heritage data exists for evaluating the long-term behavior of such surfaces.

The measured nominal V-groove shield temperatures during satellite system tests were 140 K for the first thermal shield, 90 K for the second, and 46 K for the telescope enclosure. The Front End Unit (FEU) is located within the telescope enclosure. This contains the frontend radiometer of the LFI as well as the HFI bolometer array.

Two sorption coolers developed by NASA's Jet Propulsion Laboratory will be flying on the Planck mission. The second cooler will be turned off and used as a backup unit should anything happen to the primary cooler. Each cooler can provide 1.1 W of cooling at approximately 18 K with an end-of-life input power of 470 W. In addition the cooler electronics will use up to 110 W of power.


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