Planck’s cooling system - cut view

Away from sunlight it can get very cold in space, but not cold enough for the Herschel and Planck missions, which ESA and European industry have equipped with state-of-the-art refrigeration systems to make the detectors of the two spacecraft among the coldest objects in the cosmos for the duration of their missions.

Both are due to be launched in May aboard a shared Ariane 5 to begin their journey to the same area of space, both orbiting around the Lagrange Point 2 (L2), 1.5 million km away behind Earth relative to the Sun - Herschel in a 800 000 km Lissajous orbit and Planck in a 300 000 km Lissajous orbit - and both are investigating the coolest parts of the Universe.

Herschel will be observing new-born stars and galaxies, asteroids and clouds of dust and gas, while Planck will map tiny variations in the temperature of the so-called Cosmic Microwave Background – the last remnants of the Big Bang, just 2.7 degrees above absolute zero. The primary detectors of the instruments on both spacecraft must be kept as cold as possible in order to obtain high-resolution data through the background noise whilst making observations.

Herschel’s cryostat vacuum vessel

The decade-long Herschel-Planck programme enabled European industry to maintain and extend its skills in space-borne cryogenics, as originally built up during ESA's 1990s Infrared Space Observatory (ISO) programme.

Herschel carries with it the equivalent of a vacuum flask filled with liquid helium supercooled to 1.65K – that is, just 1.65°C above absolute zero. The spacecraft detectors will remain cooled to operational temperatures until the last of its around 2300 liters of superfluid helium in the cryostat vapourises. In fact Herschel houses the single largest cryostat to fly in space, the quantity of helium defining the mission lifetime of three and a half years. Two of the instruments have in addition built-in coolers to bring the operating temperatures of the detectors down to 0.3K – a third of a degree above absolute zero.

The Planck spacecraft incorporates first a passive cooling system using radiative shields facing deep space to achieve temperatures down to about 60K before innovative 'active' cooling systems more akin to a refrigerator, with no less than three different cryogenics systems nestled within each other that finally bring the detector temperatures down to operating temperatures, in one case down to 0.1K – a remarkable tenth of a degree above absolute zero.