High in the sky

The Indian Space Research Organisation (ISRO) recently launched Astrosat, India’s first dedicated multi-wavelength space observatory. The successful launch saw the satellite being thrust into space in a 650-km altitude orbit. Having a mass of about one-and-a-half tonnes, the satellite carried five instruments to observe the universe mainly in the ultraviolet, optical and x-ray parts of the electromagnetic spectrum. Though a space observatory in its own right, it is 10 times smaller than the Hubble Space Telescope, launched by NASA 25 years ago. While the Hubble Telescope, which is still in operation, is serviced by space shuttle astronauts, this facility would not be available to the Astrosat, which is expected to function for five years. Designing the Astrosat involved more than a decade of planning and combined efforts of the country’s science fraternity.

In the past decades, research in astronomy and astrophysics has progressed significantly and several astronomical satellites have been launched to detect radiation in the ultraviolet (UV), infrared (IR), radio, x-ray and gamma rays of the electromagnetic spectrum from a plethora of celestial bodies. Earlier launches of several satellites and high altitude balloons showed that celestial bodies emit copious amounts of x-rays and ultraviolet  rays. However, Earth’s thick atmosphere with ozone layer does not allow harmful UV radiation, x-rays and gamma rays to penetrate to the ground.

When Wilhelm Roentgen discovered x-rays at the turn of the 19th century, hardly any one could have imagined that the astronomical objects would emit x-rays with an intensity that is millions times higher than the Sun’s radiation output. There are stars with surface temperatures five to 10 times greater than the sun, emitting much of their radiation in UV form. The cosmos is awash with x-rays, ultraviolet and infrared radiation from innumerable celestial objects and since these radiations do not penetrate below the Earth’s atmosphere, there was a need for launching space telescopes and observatories.

Several satellites have been launched with instruments to detect x-rays. For instance, Chandra, launched more than 15 years ago, in a highly elliptical orbit has observed and detected several thousands of high energy x-ray sources. The International UV Explorer (IUE), and the GALEX satellite are some of the satellites dedicated to detect and monitor UV sources. There are also many satellites designed specifically to observe gamma rays like the Compton Gamma Ray Observatory (CGRO) and GLAST. The 16-tonne CGRO was brought down after it completed its mission of providing among other aspects, information about the gamma ray bursts. It indicated that celestial bodies can emit in a few seconds what the sun would emit in its entire life time of ten billion years and all in high energy gamma and x-rays. Remember that the energy emitted by the sun every second is equivalent to what mankind may be able to consume in several million years. There are also several dozens of stellar mass black hole binaries in our galaxy spewing out x-rays and other high energy radiation.

Apart from the long serving Hubble, we also have the William Herschel IR Telescope (with its 3.5 metre mirror) and the Planck Observatory (to study different spectral regions of the cosmic microwave background). The James Webb Space Telescope, which the expected to be the successor of Hubble, is to be launched in 2018 and would carry a telescope three times larger in diameter than Hubble. Thus space astronomy, or astronomical observations carried out from space satellites (flying high above the obscuring atmosphere) is now all the rage and would become increasingly important in future. Many of these space observatories observe only a narrow path of the electromagnetic spectrum with instruments sensitive to mainly UV or x-ray or infra-ray.

Unique satellite
The unique feature of Astrosat is that it would deploy five different instruments for simultaneous observations in a wide spectral range ranging from optical to energetic x-rays. This range also includes the all important UV region. The payloads of the satellite include the UV Imaging Telescope (UVIT), the Large Area X-ray Proportional Counter, the Soft X-ray Telescope, the Scanning Sky Monitor and the Cadmium-Zinc-Telluride Imager. It was considered challenging to develop the high resolution UVIT, its gold coating and materials, thermal control design of all the instrumentation and optical elements to control contamination.

More specifically, Astrosat would enable simultaneous observation in wavelengths of optical, ultra-violet, low and high energy x-rays. Scientific objectives mainly include detection of transient bright x-ray sources, study star birth regions and high energy processes in stellar systems, especially in binary systems having compact (dense) objects like neutron stars and black holes. The x-ray instruments onboard Astrosat are particularly suitable for temporal and spectral studies of neutron stars and black holes. It also records  variations in x-ray output from x-ray binaries, active galactic nuclei and does limited deep-field survey of the universe in the ultra-violet region.

Simultaneous observations at entirely different wavelengths would complement each other and could resolve several enigmas concerning bizarre celestial objects.

Space telescopes work in tandem with ground observatories and any interesting observation is pursued in detail by the latter. The satellite’s x-ray detectors can cope with very bright sources that would saturate those on satellites like Chandra and XMM-Newton and could take over the gap caused by Rossi X-ray Timing Telescope, which became defunct in 2012. Indian researchers would now have to no longer rely on other space agencies for x-ray and other astronomical data. Atrosat’s multi-wavelength capability would help us gather information from several satellites, understand the universe  and get fuller picture of cosmic processes and events.

It would enable the country to do cutting-edge research in space astronomy and supplement the vast invaluable information already available. There may not be any immediate commercial or social benefits. However, the sensitive instruments developed to detect radiation in several bands of the electromagnetic spectrum could hopefully enable efficient array of photovoltaic devices to convert solar energy (in different bands) to useful power. It is hoped that the next five years would result in invaluable scientific results from Astrosat.