Astrosat gives worrying moments to scientists

Indian astronomers spent 72 agonising hours earlier this month as the newly launched astronomical satellite, Astrosat, failed to execute its first task.

A week after its successful launch on September 28, the indigenous space observatory was to capture the Crab Nebula, a supernova remnant in the constellation of Taurus.
The instrument for the job was Cadmium-Zinc-Telluride Imager (CZTI), designed by scientists at the Tata Institute of Fundamental Research, Mumbai.

It was made fully operational on October 5 and was oriented towards the Crab Nebula.
A quick look at the first set of images, however, brought disappointment as the CZTI didn’t pick up the object. This was surprising as Crab Nebula is the brightest hard X-ray source in the space, visible even to a small detector.

Scientists at TIFR, Inter University Centre for Astronomy and Astrophysics, Pune and Indian Space Research Organisation (Isro) had no explanation why one of the most photographed object in the sky remained elusive to Indian telescopes.

Theories were debated, discussed and discarded. A tense session of self introspection followed, said an IUCAA scientist. Questions followed one after the other to explain the strange behaviour.

There were some doubts if the mission operation team had put the satellite in right orientation, but the question was too embarrassing even to ask as such a large mistake was not expected from the seasoned hands at Isro. The tense moments, however, ended on October 9 when the Crab Nebula was finally detected by both mission operation centre at Bengaluru and payload operation centre, IUCAA, Pune.

It was later realised that during the first orbit, the Crab could not be detected because the satellite happened to pass through the South Atlantic Anomaly (SAA) region in the space when Crab was in the field of view and the detectors were off.

SAA is a high risk zone for astronomy payloads as they are exposed to several minutes of strong cosmic radiation. The International Space Station requires extra-shielding to deal with this problem.

The Hubble Space Telescope does not take observations while passing through the SAA.
It is thought to be the reason for early failures of the Globalstar network’s satellites in Astrosat deliberately kept the SAA avoidance zone wide to protect the instruments, and detectors were switched off in this interval. When all the data were systematically analysed, astronomers could see the Crab emerging from Earth’s shadow, giving them a sigh of relief. Other Astrosat payloads would now be made operational to look at interesting stars and black holes.