How space telescopes see what ground observatories cannot

Light from stars and galaxies travels across the universe in many forms, not just the visible colours the human eye can see. Ultraviolet, infrared, X-ray and gamma-ray light carry information about hot young stars, cold dust clouds, exploded stars and black holes. Earth's atmosphere, however, absorbs or scrambles most of that light before it reaches the ground, according to NASA Science and the European Space Agency (ESA).

Why Earth's atmosphere is a barrier

ESA explains that the atmosphere creates two separate problems for infrared astronomy. First, it absorbs most infrared wavelengths outright, letting only a few narrow bands reach ground-based telescopes. Second, the atmosphere itself glows in the infrared, often emitting more radiation than the faint cosmic object astronomers are trying to study, which buries the signal in noise.

ESA also notes that the atmosphere blocks out most high-energy and ultraviolet radiation, including X-rays, gamma rays and most ultraviolet rays. NASA notes that Chandra, its X-ray observatory, orbits up to an altitude of roughly 139,000 kilometres (86,500 miles) above Earth — about 200 times higher than Hubble — specifically because X-rays never reach the surface.

Moving a telescope above the atmosphere, or far from Earth altogether, removes both problems at once: there is no air left to absorb the light, and no atmospheric glow to drown it out.

Different telescopes, different light

Each major space observatory is tuned to a specific part of the spectrum. Hubble, launched in 1990, observes mainly ultraviolet and visible light (0.1 to 2.5 microns) and orbits about 300 miles (483 km) above Earth, still within reach of occasional servicing missions, according to NASA Science. The James Webb Space Telescope, launched in December 2021, is built for infrared light and sits far more distant, near the second Lagrange point (L2) roughly one million miles from Earth, where its own heat and Earth's heat cannot interfere with the faint infrared signals it is trying to detect.

Webb's primary mirror spans 6.5 metres (21.3 feet), compared with Hubble's 2.4 metres (7.9 feet), giving it more than six times the light-collecting area. NASA Science states that Webb can detect objects up to 100 times fainter than Hubble can see. Because the expansion of the universe stretches light from the most distant, earliest galaxies into infrared wavelengths — a redshift effect — Webb's infrared sensitivity lets it look further back in cosmic time than Hubble's ultraviolet-and-visible instruments can.

Webb carries dedicated instruments for different infrared ranges. Its Near-Infrared Camera (NIRCam) covers 0.6 to 5 microns and serves as Webb's primary near-infrared imager, according to NASA Science's instrument page. The Mid-Infrared Instrument (MIRI) covers 5 to 28 microns and combines a camera, a spectrograph that splits light into a spectrum to measure brightness at each wavelength, and a coronagraph that blocks a star's glare so fainter planets nearby can be seen. MIRI needs an additional cryocooler to stay colder than the rest of the observatory so its detectors are not blinded by Webb's own residual heat.

Not every space observatory produces images. ESA's Gaia spacecraft, launched on 19 December 2013 and stationed near L2, is building an extraordinarily precise three-dimensional map of more than two billion stars across the Milky Way, tracking their motions, luminosity, temperature and composition, according to ESA's Gaia mission page. ESA says Gaia improved positional accuracy by a factor of a hundred compared with its predecessor, the Hipparcos satellite; the mission entered its post-operations phase after facing sensor and micrometeoroid issues in 2024.

At the high-energy end of the spectrum, NASA's Chandra X-ray Observatory, launched on 23 July 1999, detects X-rays from extremely hot phenomena: exploded stars, galaxy clusters and material swirling around black holes, according to NASA's astrobiology mission page.

Putting the pieces together

ESA's Herschel project scientist Göran Pilbratt described the value of infrared observation specifically for studying how stars and planets form: "By observing in the infrared we can study how things get formed, the very early steps, because formation processes very often happen in cool and dusty places," he said, according to ESA. Infrared light passes through dust clouds that block visible light, letting instruments such as Webb's NIRCam and MIRI see newborn stars and planets hidden inside nebulae.

Because no single telescope covers the whole spectrum, astronomers routinely combine data from several observatories. NASA Science describes Hubble and Webb as complementary partners rather than rivals: Webb's deeper infrared vision cuts through dust and gas, while Hubble's visible and ultraviolet vision complements Webb's infrared views, together producing broader views of the same objects.