Space Telescopes

Why put a telescope in space?

Earth’s air stops most kinds of electromagnetic radiation coming from space before they reach the ground.^[1] Radio waves, visible light and part of the ultraviolet do get down to sea level, and some infrared can be caught from mountain tops; balloons and rockets can climb higher, but only for months or minutes.^[1] For long observations, the best place for a detector is on a satellite in orbit.^[1]

Blocking is one problem; blurring is the other. ESA points out that light crossing our restless atmosphere loses its fine detail, and that a telescope in space avoids this.^[6] From orbit, Hubble gets a clean view and can record wavelengths that the air would absorb or partly filter.^{[3, 6]}

Light our eyes cannot see

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays are all forms of electromagnetic radiation.^[1] Each photon (a single packet of light) carries the least energy in radio and the most in gamma rays.^{[1, 2]}

Infrared has longer waves that pass through all but the thickest dust, so it can show objects hidden inside dusty clouds.^{[4, 14]} As the universe expands, light from the most distant galaxies is stretched into the infrared, a shift called cosmological redshift, so only infrared instruments can see them.^{[14, 4]} Ultraviolet comes from the hottest, biggest and youngest stars, so it marks where galaxies are making new stars.^[4] X-rays come from matter heated by extreme gravity, strong magnetic fields or explosions.^[15]

“Infrared” itself is drawn differently in different places. Infrared astronomers treat their band as roughly 1 to 100 microns (millionths of a metre).^[1] A wider definition runs from the red edge of visible light out to about 1 mm.^[2] Webb was built for 0.6 to 28.8 microns.^[9]

The main missions

Hubble went up on 24 April 1990 aboard the space shuttle Discovery.^[3] It was the first astronomical observatory in Earth orbit able to take images from the ultraviolet to the near infrared.^{[3, 6]} ESA notes that its 2.4 m main mirror had a serious flaw: the edge was too flat by around a fiftieth of a hair’s width, enough to ruin sharp images.^[6] NASA and ESA engineers designed corrective optics, fitted on the first servicing mission in December 1993.^[6] Between 1993 and 2009 astronauts visited Hubble five times to swap worn parts such as batteries and gyroscopes.^[3]

Webb, the James Webb Space Telescope, is an infrared observatory meant to study every stage of cosmic history, from the first glow after the Big Bang to the birth of planetary systems.^{[7, 9]} Its main mirror is about 6.5 metres across at its widest point, and four instruments do the science: NIRCam, NIRSpec, MIRI and NIRISS.^[8]

Spitzer, the last of NASA’s four Great Observatories, worked in infrared with an 85 cm telescope.^{[15, 18]} Its liquid helium coolant ran out on 15 May 2009, but the spacecraft was not decommissioned until 30 January 2020.^[18]

Chandra takes X-ray images of black holes, neutron stars and cosmic explosions.^{[16, 17]}

Kepler, launched on 6 March 2009, hunted planets by watching for transits: small dips in a star’s brightness when a planet passes in front of it.^[19] In May 2013 it lost the second of its four reaction wheels, ending new data for the original mission.^[19]

Fermi launched on 11 June 2008 to collect gamma rays, with energies thousands to hundreds of billions of times those of visible light.^[21]

Gaia, Planck and Euclid are ESA missions.^{[22, 23, 24]} ESA says Gaia built the largest and most precise 3D map of our galaxy from nearly two billion objects.^[22] Planck, in ESA’s words, set out to measure the leftover glow of the Big Bang, the cosmic microwave background, more accurately than ever.^[23] Euclid is an ESA survey mission built to investigate dark matter and dark energy.^[24]

Mirrors, and staying cold

Large telescopes gather and focus light with curved mirrors, and a bigger mirror catches more light, much as a wider bucket fills faster in the rain.^{[25, 10]}

Webb’s main mirror is the largest ever flown, too wide for its rocket, so it is split into 18 six-sided segments that folded up for launch.^{[10, 9]} The segments are made of beryllium for strength and low weight, with a very thin gold coating because gold reflects infrared so well.^{[10, 9]}

Warm objects glow in infrared, including the telescope itself. If Webb’s mirror were as warm as Hubble’s, the faint light of far galaxies would vanish in that glow.^{[10, 8]} Webb therefore shelters behind a five-layer sunshield, and a cryocooler chills MIRI’s detectors to 7 K.^{[9, 11]} ESA explains that Planck’s detectors were also cooled close to absolute zero, because their own heat would otherwise spoil the readings.^[23]

Where they orbit

Hubble circles Earth about once every 95 minutes, so it passes through Earth’s shadow on each orbit.^{[3, 12]}

Webb instead orbits the Sun about 1.5 million km from Earth, near a spot called L2.^[12] L2 is one of five Sun-Earth Lagrange points, where the pulls of the Sun and Earth combine so that a small object there takes one Earth year to go round the Sun.^[13] Webb’s halo orbit around L2 keeps it out of the shadows of Earth and the Moon, which reduces temperature swings.^[9] From there the Sun, Earth and Moon all sit on one side, so a single sunshield can block them all.^[12] Unlike Hubble, Webb is too far away for a servicing visit.^[9]

How the colour pictures are made

Hubble images all start in black and white and are coloured afterwards, usually to bring out interesting features; sometimes the colours match what our eyes would see, but often not.^[5] Blending images taken in different bands gives a fuller picture: one view of the galaxy M82 shows Chandra’s X-ray data in blue and Spitzer’s infrared in red.^[5]

Some discoveries

ESA reports that, using exploding stars called Type Ia supernovae as distance markers, Hubble found that the expansion of the universe has been speeding up for several billion years.^[6] Spitzer was the first telescope to detect light from an exoplanet.^[18] Kepler left more than 2,600 exoplanet discoveries and showed that our galaxy holds more planets than stars.^[20] Fermi revealed the Fermi Bubbles, a structure in our galaxy 50,000 light-years across.^[21]

Some results are still estimates: Kepler’s data suggest that perhaps 20 to 50% of stars have small, possibly rocky, Earth-size planets in their habitable zones.^[20]

Common misconceptions

“Space telescopes beat ground telescopes because they are bigger.” Hubble’s 2.4 m mirror is smaller than those of many observatories on the ground; its edge comes from being above the air.^[3]

“Webb can point anywhere.” Warm bodies close by would swamp its faint infrared signals, so it has to keep its back to them: the Moon, our own planet, the Sun, Venus and Mercury are all off-limits.^[9]

What this page does not cover

Space radio telescopes, a list of Webb discoveries, Chandra’s orbit and instruments, telescope costs, ground-based telescope technology such as adaptive optics, Hubble’s altitude, future and planned telescopes, and whether any mission is operating today.

Going further

See Light and Waves for how electromagnetic waves behave, Orbits and Gravity for how L2 works, Satellites and Space Exploration for other spacecraft, and Stars, Galaxies and Cosmology for what these telescopes study.