On July 2 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.

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 absolute zero - the coldest temperature theoretically possible in our Universe.

Such low temperatures are necessary for Planck’s detectors to study the Cosmic Microwave Background (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 temperature 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.

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.

"This artist's impression shows the focal plane unit of the Planck telescope. Planck's instruments will operate at a few degrees above absolute zero. To achieve this, a series of cooling stages are required. Without any additional cooling, the spacecraft itself can passively cool to around 50 K (about -223ºC). (Credit: ESA/AOES Medialab)"

Source: European Space Agency