Why does a telescope need to be above the Earth's air to study some objects?

On 28 September 2015, a PSLV rocket lifted off from Sriharikota carrying AstroSat, India's first space observatory dedicated to astronomy. It was placed in an orbit about 650 km above the Earth.

Stars, galaxies and other objects in space give off light of many kinds, not only the light our eyes can see. Some of the most interesting objects glow brightly in ultraviolet light or in X-rays, which the Earth's atmosphere blocks. To study them properly, a telescope has to be above the air.

What makes AstroSat unusual is that it can watch the same object in several kinds of light at the same time: ultraviolet, some visible light and X-rays. Many observatories study only one kind, so scientists often have to combine observations taken at different times by different telescopes. AstroSat can gather them all at once, from one satellite.

Among its aims is to understand violent processes in pairs of stars that include neutron stars and black holes, some of the most extreme objects in the universe. The data it collects is sent down to a control centre in Bengaluru, where it is processed and shared with scientists.