Why do some space telescopes have to be cooled?
The Herschel Space Telescope
It is cold in space, unimaginably cold for us. The temperature that prevails there and the temperature of the cosmic background radiation (see the astronomy question from week 10: Where is the coldest point in the Universe?) is around minus 270 degrees Celsius. This is only slightly above absolute zero, which is minus 273 degrees Celsius, the lowest possible temperature. In spite of this, some space telescopes have to be cooled – but not others.
Space telescopes that make observations in the visible light spectrum (such as the Hubble space telescope) do not need any special cooling. On the contrary, they must – just like any other satellite – be kept within a certain temperature range. This is different for the various instruments on board and lies roughly between minus 20 degrees Celsius and plus 50 degrees Celsius. Thermal control systems are used to achieve the optimum temperature for each instrument and component.
For certain measurements, this is not cold enough
However, space telescopes that make their observations in the infrared range – that is, thermal radiation – must be cooled. This is because the telescope itself has a certain temperature and continually radiates heat that would interfere with the measurements taken by the heat sensor. In order to minimise this characteristic radiation, the telescope must be cooled as far as possible below the temperatures at which it will make its observations in space.
The Herschel space telescope, which is scheduled for launch on 14 May, will measure thermal radiation at very low temperatures. This radiation penetrates interstellar dust clouds, allowing us a glimpse of the furthest regions of the cosmos and letting us draw conclusions about the formation of galaxies and stars.
Parts of the Herschel telescope must be cooled to around minus 270 degrees Celsius to prevent measurements made by its sensors from being distorted by their characteristic radiation. A large, insulated shield protects the telescope from the radiation emitted by the Sun and the Earth, which would otherwise cause it to heat up. The telescope is actively cooled by the slow evaporation of over 2300 litres of liquid helium, in a similar way to the evaporative cooling mechanism employed in a refrigerator. Excess heat is then rejected into space by radiators. In this way, the sensors can be cooled to almost minus 271 degrees Celsius and the telescope reaches its highest level of measuring sensitivity.