How Webb stays cool

Infrared light: a different view of the cosmos

One of the most well-known space telescopes, Hubble, has been in operation for over three decades. It can see the same type of light that is visible to the human eye, as well as UV and near-infrared radiation. However, many celestial objects, such as planets, low-mass stars, and distant galaxies, also shine brightly in – or emit – another type of light: infrared light.

The James Webb Space Telescope will gather infrared light and help researchers study many cosmic objects and phenomena that Hubble cannot observe. But there's a catch: Earth and the Moon glow quite brightly in the infrared and can be a source of light pollution, which can interfere with Webb's observations.

The Webb Telescope must therefore be sent much farther out than Hubble, located only 550 kilometres above Earth's surface. In contrast, Webb will perform its observations in the vicinity of Lagrange 2, a point in space located about 1.5 million kilometres from Earth – nearly four times farther away than the Moon!

Shielded from the Sun

Another potential source of light pollution for an infrared telescope is its own instruments. To counteract this, Webb was designed to have two sides:

• the hot side, which faces the Sun and heats to about 85 degrees Celsius; and
• the cold side, which is protected from the Sun's energy and stays at a frigid -233 degrees Celsius to ensure that the telescope's delicate mirrors and instruments do not radiate in infrared light themselves.

Webb's five-layer, diamond-shaped sunshield is designed to protect it from the Sun's radiation. In fact, the massive, tennis court-sized sunshield blocks out 99.9999% of the Sun's rays – the equivalent of sunscreen with SPF 1 million!

Webb's open-concept design also helps the observatory keep its cool. Unlike other telescopes, like Hubble, whose mirrors are contained in a tube, Webb's large golden mirror is out in the open, which prevents heat from getting trapped inside the telescope and its components.

Packing up the sunshield for launch

With its 6.5‑metre golden mirror, the Webb Telescope is the largest space telescope ever built. Due to its impressive size, the whole observatory must be folded up before being placed inside an Ariane 5 rocket and launched into space.

This preparation is no easy task: some of Webb's most delicate components are its very thin sunshield layers. Ahead of the telescope's launch later this year, engineers at Northrop Grumman in California recently folded and packed up the sunshield – like a parachute being carefully folded into a backpack!

A refrigerator in space

For most of Webb's components, simply being protected by the sunshield is sufficient to keep them cool and prevent them from producing their own infrared light.

One exception is the Mid-Infrared Instrument (MIRI), provided by the European Space Agency and NASA, which will observe a type of infrared light with a longer wavelength than the other instruments. Its specialized observations mean that MIRI must be cooled to an even lower temperature: about -266 degrees Celsius, or 33 degrees colder.

To achieve this, MIRI is equipped with a "cryocooler" – in essence, a sophisticated refrigerator in space! Rather than using ice or other consumable coolant, the cryocooler is made up of pumps and compressors distributed throughout the instrument to cool it down and keep its temperature at optimal levels.

An international collaboration between NASA, the European Space Agency and the Canadian Space Agency, the James Webb Space Telescope is the most complex and powerful space telescope ever built. Canada contributed two key elements to Webb: the Fine Guidance Sensor (FGS) and the Near-Infrared Imager and Slitless Spectrograph (NIRISS).

Explore further

• About the James Webb Space Telescope
• What Webb will observe
• Webb: Hubble's successor