Space Telescopes

Overview

A space telescope observes from above Earth's atmosphere, which both blurs images and absorbs most of the electromagnetic spectrum outright. Different observatories are built for different wavelength ranges, because the detector technology and optics required vary enormously across the spectrum.

• The atmosphere blocks ultraviolet, X-ray, gamma-ray and much infrared light entirely — those wavelengths are only observable from space.
• Above the atmosphere a telescope reaches its diffraction limit continuously, with no waiting for good seeing.
• JWST operates near the Sun–Earth L2 point, roughly 1.5 million kilometres out, and cannot be serviced.
• Hubble's servicing missions are the reason it remained scientifically current for decades.

Why go to orbit at all

Two independent reasons. First, atmospheric turbulence blurs images from the ground; a space telescope is limited only by its optics and reaches its diffraction limit continuously. Adaptive optics has narrowed this gap substantially for large ground telescopes, but only over restricted fields and in favourable conditions.

Second, and more fundamentally, the atmosphere is opaque across most of the spectrum. Ultraviolet, X-ray and gamma-ray astronomy are impossible from the ground at any aperture, and much of the infrared is absorbed by atmospheric water vapour. No ground-based improvement can recover wavelengths that never arrive.

Where they orbit, and why it matters

• Low Earth orbit: accessible for servicing, as Hubble was, but Earth blocks part of the sky and the thermal environment cycles rapidly with each orbit.
• Sun–Earth L2, about 1.5 million kilometres beyond Earth: Sun, Earth and Moon stay in roughly the same direction, so a single sunshield can block all three and allow passive cooling to very low temperatures. JWST, Gaia, Planck and Euclid use this region. It is far beyond any crewed servicing capability.
• Earth-trailing heliocentric orbit: used by Spitzer and Kepler, giving a stable thermal environment and unobstructed sky at the cost of a steadily increasing communication distance.
• Highly elliptical orbits: used by some X-ray observatories such as Chandra to spend most of the time outside Earth's radiation belts.

Different wavelengths, different machines

Optical and ultraviolet telescopes use conventional reflecting optics. Infrared telescopes must be cold, because a warm instrument glows in exactly the band it is trying to observe — hence JWST's five-layer sunshield and its operating temperature near 40 kelvin.

X-ray photons pass straight through ordinary mirrors at normal incidence, so X-ray telescopes use grazing-incidence optics: nested shells that deflect photons at very shallow angles, as Chandra and XMM-Newton do. Gamma rays cannot be focused at all by any mirror, so gamma-ray observatories such as Fermi use coded masks or particle-tracking detectors and reconstruct arrival directions computationally.

What they have produced

Hubble, launched in 1990 and upgraded across several shuttle servicing missions, delivered the deep field observations that constrained galaxy evolution, refined the extragalactic distance scale, and contributed to the discovery of cosmic acceleration. Its serviceability is why it stayed at the frontier for decades — an option no L2 observatory has.

Kepler and TESS turned exoplanet detection into a statistical science by monitoring vast numbers of stars photometrically. Gaia is measuring positions, parallaxes and motions for more than a billion stars, and its data underpins a large share of the stellar parameters catalogued on this platform. JWST has extended infrared sensitivity far enough to characterise exoplanet atmospheres and observe galaxies at very high redshift.

In-depth, individual pages in this category.

• Hubble Space Telescope

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  The iconic orbiting observatory of optical astronomy.

• James Webb Space Telescope

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  The great infrared observatory of the modern era.

• Spitzer Space Telescope

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  NASA's pioneering infrared observatory.

• Chandra X-ray Observatory

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  NASA's window onto the high-energy X-ray universe.

• Kepler Space Telescope

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  The telescope that revealed thousands of exoplanets.

• TESS

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  NASA's all-sky survey for nearby exoplanets.

Continue in the data

Catalogues, hubs, and reference pages that hold the underlying records for this topic.

• ObservatoriesSpace and ground observatories with instruments.
• InstrumentsCameras, spectrographs, and detectors.
• Data archivesWhere the observations are published and preserved.
• ImagesLicensed imagery from these observatories.

Frequently asked

Why is JWST so far from Earth?

Because it observes in the infrared and must stay extremely cold. At the Sun–Earth L2 region, about 1.5 million kilometres beyond Earth, the Sun, Earth and Moon all lie in roughly the same direction, so a single sunshield blocks all three and lets the telescope cool passively to around 40 kelvin. The cost of that location is that servicing is not possible.

Can Hubble be repaired again?

Not with current capability. Its five servicing missions were flown by the Space Shuttle, which was retired in 2011, and no operational vehicle is currently equipped for the task. Hubble continues to operate on its existing hardware, with its orbit gradually decaying.

Is JWST a replacement for Hubble?

No — they observe different wavelengths. JWST is optimised for infrared, extending to ranges Hubble cannot reach, which is what lets it see very distant redshifted galaxies and cool objects. Hubble covers ultraviolet and visible light, where JWST has little or no capability. They are complementary instruments, and both were designed to be.

Why can't X-ray telescopes use normal mirrors?

Because X-ray photons are energetic enough to pass through or be absorbed by a mirror surface struck head-on. They can only be reflected at very shallow grazing angles, so X-ray telescopes use nested cylindrical shells that deflect photons through small angles onto a focus — a geometry that looks nothing like a conventional telescope.