Earth's atmosphere blurs starlight and blocks X-rays completely, so telescopes above it see sharper images and light that never reaches the ground.
Look at a star on a clear night and it twinkles. That twinkle is the atmosphere bending the light as warm and cool air moves. It blurs telescope images. Worse, the air completely blocks some kinds of light. NASA notes that Earth's atmosphere blocks X-rays, so X-ray telescopes must be in space.
NASA's Hubble Space Telescope was launched from the space shuttle Discovery in 1990. It orbits about 483 km above Earth and has a 2.4 m main mirror. Because it avoids the atmosphere's distorting effects, it sees far more sharply than a telescope of the same size on the ground. It observes from ultraviolet to near-infrared light (115–2,500 nanometres).
AstroSat, launched on 28 September 2015 on PSLV-C30, is India's first dedicated astronomy satellite. It weighs 1,515 kg and orbits at about 650 km, tilted only 6° to the equator. Its five instruments observe in ultraviolet, a limited optical range, and X-rays from 0.3 to 100 keV. ISRO highlights that it can study the same object at many wavelengths at the same time from one spacecraft.
XPoSat is India's first dedicated mission to measure the polarisation of X-rays from bright cosmic sources in extreme conditions. NASA notes that such X-rays come from very hot places, like pulsars and the discs of gas around black holes. It carries POLIX (8–30 keV), which measures polarisation, and XSPECT (0.8–15 keV), which measures spectra and timing. PSLV-C58 completed its mission on 1 January 2024, and the mission is planned to last about five years. The rocket's spent upper stage was reused as the POEM-3 platform and re-entered on 21 March 2024.
Check Hubble's altitude: it orbits about 500 km up, above almost all of the air that blurs ground-based telescopes. Track the Hubble Space Telescope
X-ray astronomers describe light by photon energy instead of wavelength. The link is E = hc / λ, which in handy units is:
E (keV) ≈ 1.24 / λ (nm)
AstroSat's range: 0.3 keV → λ ≈ 1.24 / 0.3 ≈ 4.1 nm; 100 keV → λ ≈ 0.0124 nm. XPoSat's POLIX: 8 keV → 0.155 nm; 30 keV → 0.041 nm.
Compare visible light at about 500 nm: a 10 keV X-ray photon carries roughly 4,000 times more energy than a visible photon. Such photons are absorbed by air long before they reach the ground, so the only option is to fly above the atmosphere.
Why is sharpness better in space? A telescope's finest possible detail is set by diffraction:
θ ≈ 1.22 × λ / D
For Hubble, D = 2.4 m; at λ = 500 nm:
θ = 1.22 × 5 × 10⁻⁷ / 2.4 ≈ 2.5 × 10⁻⁷ rad
Convert to arcseconds (1 rad = 206,265 arcsec): 2.5 × 10⁻⁷ × 206,265 ≈ 0.05 arcsec. Above the atmosphere there is no air turbulence to blur the image, so a space telescope can work close to this limit.
Earth's atmosphere absorbs X-rays, so they never reach telescopes on the ground.
AstroSat, launched in 2015 on PSLV-C30.
The polarisation of X-rays from bright cosmic sources, using its POLIX instrument.