Coolest spacecraft ever in orbit around L2

Jul 03, 2009
Planck's orbit around L2, the second Lagrange point of the Sun-Earth system. Credits: ESA

( -- Last night, the detectors of Planck's High Frequency Instrument reached their amazingly low operational temperature of -273°C, making them the coldest known objects in space. The spacecraft has also just entered its final orbit around the second Lagrange point of the Sun-Earth system, L2.

Planck is equipped with a passive cooling system that brings its temperature down to about -230°C by radiating heat into space. Three active coolers take over from there, and bring the temperature down further to an amazing low of -273.05°C, only 0.1°C above - the coldest temperature theoretically possible in our Universe.

Such low temperatures are necessary for Planck’s detectors to study the (CMB), the first light released by the universe only 380 000 yrs after the Big Bang, by measuring its temperature across the sky.

Like measuring the heat of a rabbit on the Moon
The detectors will look for variations in the of the CMB that are about a million times smaller than one degree - this is comparable to measuring from Earth the heat produced by a rabbit sitting on the Moon. This is why the detectors must be cooled to temperatures close to absolute zero (-273.15°C, or zero Kelvin, 0K).

Details on the different stages of the cool-down process are available via the 'Planck in depth' link at right.

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Planck will scan the entire sky to build the most accurate map ever of the Cosmic Microwave Background (CMB), the relic radiation from the Big Bang. The spacecraft will spin at 1 rotation per minute around an axis offset by about 85° so that the observed sky region will trace a large circle on the sky. As the spin axis follows the Sun the circle observed by the instruments sweeps through the sky at a rate of 1° per day. Planck will take about 6 months to complete a full scan of the sky, allowing the creation of two complete sky maps during the nominal mission lifetime (about 15 months). Credits: ESA (animation by C. Carreau)

Arriving at L2
Starting at 13:15 CEST yesterday, the Planck Mission Control Team conducted a crucial orbit insertion manoeuvre designed to place the satellite into its final orbit about L2.
Once commanded, the burn was auto-controlled by Planck, with the thrusters operating for between 12 and 24 hours. The manoeuvre directed the satellite into its final operational orbit around the second Lagrange point of the Sun-Earth system, L2.

The thruster burn was planned to deliberately underperform by a small margin, necessitating a small 'touch up' manoeuvre in the coming days to bring the satellite fully onto its planned trajectory.
"While this manoeuvre itself is routine, it represents the final major step in the long voyage to L2, and everyone here is quite happy to see Planck getting into its operational orbit," said Chris Watson, Spacecraft Operations Manager, speaking in the mission's Dedicated Control Room at ESA’s European Space Operations Centre, Darmstadt, Germany.

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Located in the focal plane of the telescope, Planck’s Low Frequency Instrument (LFI), and the High Frequency Instrument (HFI), are equipped with a total of 74 detectors covering nine frequency channels. These detectors must be cooled to temperatures around or below 20 K so that their heat does not swamp the faint microwave signals they are designed to detect. The first part of this animation shows the different stages of Planck’s active cooling system. This active cooling system consists of a three-stage refrigeration chain which takes over after the passive cooling system cools the telescope to about 50 K. The first stage makes use of liquid hydrogen to reduce the temperatures to 20 K. The second stage is a mechanical cooler (a pump) that uses liquid helium (4He) to bring the temperatures down to 4 K. The third stage makes use of a mixture of Helium 3 and Helium 4 (3He and 4He) to reach an amazing low temperature of just 0.1 K. The second part of the animation shows microwave light collected by telescope reaching the instrument detectors via the conical feed horns. The bolometric detectors of the HFI, located behind the horns, absorb the light and heat up slightly. Credits: ESA (animation by AOES Medialab)

The manoeuvre was planned to change the satellite’s speed by 211.6 km/hour, ending with a final speed of 1010 Km/hour with respect to the ground. Together with Earth and the virtual point L2, Planck will then be orbiting the Sun at a speed of 106 254 km/hour (29.5 km/second).

At the start of yesterday’s manoeuvre, Planck was located 1.43 million km from Earth.

Science operations to begin soon
All commissioning activities are on schedule, and this phase of the mission is practically complete. Over the next few weeks, the operation of the instruments will be fine-tuned for best performance.

Planck will begin to survey the sky in mid-August.

Source: European Space Agency (news : web)

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User comments : 4

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4 / 5 (1) Jul 03, 2009
"...this is comparable to measuring from Earth the heat produced by a rabbit sitting on the Moon. " who makes up these comments? sports writers? (not to split hares...)
not rated yet Jul 03, 2009
no...a hare brain!..I couldn't resist..
not rated yet Jul 03, 2009
oh...split hares...i missed that, good too....anyway it is pretty darn cool!..oops...
back to reality..does this experiment help us understand the "shape" of the universe?
not rated yet Jul 07, 2009
This article would have been improved by a discussion of the CMB and why measuring it to the described accuracy is important.

For those interested, the reason that measuring the CMB is so important is because there are several alternate models of the processes that occurred in the first part of the big bang. Inflation theories explaining the uniformity of the cosmos predict minute variations in the CMB which Planck can detect. Previous measurements had several strange artifacts that were barely on the level of detectability.

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