India’s first astronomy satellite will be launched on September 28. ISRO has noted that while it has launched payloads capable of making astronomical observations before, this is the first time one dedicated to astronomy will be launched. Called ASTROSAT, it was first scheduled for launch in 2005, then in 2010, and finally in 2015 with delays largely due to putting the scientific payload together. ASTROSAT will be a multi-wavelength mission, observing the cosmos in X-ray, visible and UV light.

ASTROSAT is one of two scientific missions that have long been overdue – the other being the Aditya-1 mission to study the Sun. ASTROSAT comprises five scientific instruments, all of which had been delivered to the ISRO Satellite Centre by 2014. They are the UV Imaging Telescope, the Scanning Sky Monitor, the Cadmium-Zinc-Telluride Imager, the Soft X-ray Telescope and three identical Large Area Xenon Proportional Counters. The Soft X-ray Telescope reportedly took 11 years to be built.

X-ray and UV radiation fall in the short-wavelength part of the electromagnetic spectrum, and their emissions in the universe can’t be detected at ground level because the high-energy photons that constitute the radiation can’t easily penetrate Earth’s atmosphere. The opposite is true for long-wavelength radiation like radio waves. As a result, the most powerful and effective X-ray and gamma-ray satellites are in Earth-orbit whereas radio-telescopes – with their giant telltale antenna dishes – are on ground.

Transmission properties of radiation of different wavelengths. Source: Caltech
Transmission properties of radiation of different wavelengths. Source: Caltech

One of the better known examples of multi-wavelength space-borne observatories is the Hubble Space Telescope, which makes observations in the UV, visible and infrared parts of the spectrum. However, comparisons between the telescopes are unfounded because Hubble’s optical mirror is eight-times as wide as ASTROSAT’s, allowing for a deeper field of view and much better imaging. Nonetheless, ASTROSAT will be able to contribute in the study of time-variable sources of radiation by being able to observe the sources in UV and X-ray wavelengths simultaneously.

Eleven years after the project was first okayed, the satellite is slated to be launched on board a PSLV rocket on its 30th flight from the Satish Dhawan Space Centre at Sriharikota on September 28. Four smaller American, one Indonesian and one Canadian satellites will also be launched as part of the same mission. ISRO has stated that open observing time will be available on the satellite’s instruments from September 2016, from their perch in the near-Earth orbit at an altitude of 650 km. ASTROSAT cost Rs.178 crore.

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