Astronomy

To the stars and beyond

Print edition : October 30, 2015

ISRO Chairman A.S. Kiran Kumar (centre) with (from left) scientists K. Sivan, B. Jayakumar, K. Suryanarayana Sarma and M. Annadurai after the launch of Astrosat from Sriharikota in Andhra Pradesh on September 28. Photo: R. Senthil Kumar /PTI

Astrosat, India’s first satellite mission dedicated to observing distant celestial objects, will bring in tremendous amounts of information for major academic and research institutions.

IN just 30 days from August 27, 2015, three events took place that confirmed India’s position as a powerful space-faring nation. They were (1) the Indian Space Research Organisation’s second consecutive success with its indigenously built cryogenic engine aboard the Geo-synchronous Satellite Launch Vehicle (GSLV-D6) on August 27, which eliminated India’s dependence on foreign launchers to put its two-tonne class of communication satellites into orbit; (2) India’s spacecraft to Mars completing, on September 24, one year of orbiting around the red planet, an extraordinary achievement for a country’s debut mission to Mars; and (3) ISRO putting Astrosat, India’s first space observatory, into orbit on September 28.

As the clock struck 10 in the morning on September 28, ISRO’s Polar Satellite Launch Vehicle (PSLV-C30), equipped with powerful strap-on booster motors, lifted off from the first launch pad at Sriharikota in Andhra Pradesh and put Astrosat into orbit 23 minutes later. The PSLV-C30 also put six other satellites from abroad—one each from Canada and Indonesia and four from the United States—into orbit. All the six are maritime surveillance satellites. It was the 30th successful mission in a row for the PSLV.

Astrosat, weighing 1,513 kg, is India’s first satellite mission dedicated to observing distant celestial objects. An orbiting space observatory, it carries five scientific instruments/telescopes that will study astronomical objects such as black holes, pulsars, quasars, white dwarfs and active galactic nuclei. It will study these both in the Milky Way and in the galaxies beyond.

Astrosat’s design is novel. Its telescopes enable simultaneous multi-wavelength observations of astronomical objects from a single platform. Be it a black hole, a white dwarf or a neutron star, Astrosat can observe it [that is, each astronomical object at a time] simultaneously in the visible, optical, ultraviolet (UV), low energy X-ray and high energy X-ray regions of the electromagnetic spectrum. This eliminates the need for sending separate satellites with a telescope each to study the astronomical phenomena in visible, optical, ultraviolet or X-ray wavebands. Astrosat can investigate astronomical objects using UV light and X-rays, which cannot be done from the ground. Study of distant galaxies in the UV range can unravel mysteries about the formation of galaxies themselves and the birth of stars in the galaxies. Astrosat’s lifespan is expected to be five years.

In a press conference at Sriharikota after the PSLV-C30 put Astrosat into orbit, A.S. Kiran Kumar, ISRO Chairman, said: “One of the unique features of this satellite is the involvement of major Indian academic institutions such as the IUCAA [Inter-University Centre for Astronomy and Astrophysics], the TIFR [Tata Institute of Fundamental Research], the IIA [the Indian Institute of Astrophysics], the RRI [Raman Research Institute] and the international agencies—the Canadian Space Agency [CSA] and the University of Leicester. We have been able to inculcate in these institutions the habit of developing the basic instruments, with all the payloads having been built by these institutions with some support from ISRO. This has taken a long time, almost a decade because of the many problems we faced. But at the end, the development has taken place in these institutions and we are happy about it.”

Of the five scientific payloads on board Astrosat, the Ultraviolet Imaging Telescope (UVIT) was jointly developed by the IIA, Bengaluru, and the IUCAA at Pune in collaboration with ISRO and the CSA. This instrument can observe the sky in the visible (what can be seen with the naked eye), near UV and far UV regions of the electromagnetic spectrum.

On its website, the CSA quoted John Hutchings of the National Research Council, Canada, who is the Principal Investigator for Canada’s contribution, as saying: “These are not your run-of-the-mill stars and galaxies. They are so powerful, they affect the whole universe. We will be able to see where they form and study their brightness, distribution, life cycle and more.”

Hutchings added: “All of the hottest and most exotic objects in the universe radiate strongly in the ultraviolet range…. By exploring distant galaxies in the ultraviolet light, we can study the formation and life cycle of galaxies, as well as star formation within galaxies. That is one of the science drivers of this project.”

In collaboration with the CSA, Hutchings co-led the development of the three Canadian detectors for the UVIT, the twin UV and visible imaging telescopes for Astrosat. “This is a technology that Canada has never developed before. The detectors capture each photon of light as it arrives and records its location and time of arrival. These are then stored, and an image is created. Also, the UVIT telescopes are far more capable than those flown previously, and can observe far larger areas of the sky,” he said.

The TIFR and the RRI developed the second payload, called Large Area X-ray Proportional Counter (LAXPC). It will study the variations in the emissions of X-rays from sources such as X-ray binaries, active galactic nuclei and other cosmic sources.

The Soft X-ray Telescope (SXT) was developed by the TIFR in collaboration with the University of Leicester, United Kingdom, and ISRO. The SXT will investigate how the X-rays coming from different celestial bodies vary with time.

The Cadmium Zinc Telluride Imager (CZTI), which functions in the X-ray region, can sense X-rays of high energy. It may detect sudden bursts of gamma rays, giving a clue to the evolution of the universe. This payload was developed by the TIFR and the IUCAA in collaboration with ISRO. The Scanning Sky Monitor (SSM) will keep surveying the sky for transient phenomena such as cosmic objects in X-ray waveband, which flare up for a brief period of time.

K. Suryanarayana Sarma, Project Director, Astrosat, called the payloads “sophisticated and sensitive astronomy instruments”. He said “the unique feature” of the satellite was that it could observe an object simultaneously in multiple wavelengths such as the visible, optical, UV, soft X-ray and hard X-ray wavebands. “This brings out a lot of science information. Otherwise, we have to correlate different data from different satellites, put them on a time-scale and combine them. But here we can get a total picture [of the event].”

Since the five instruments had to be operated simultaneously, precise pointing towards the object of interest had to be done. “So there was a challenge in the design of the control system of the satellite,” Sarma said. There should be no “jitter” and the satellite “should steadily point towards the object which is under observation”, he said. (The attitude and orbit control system, or AOCS, accurately maintains Astrosat’s orientation with the help of reaction wheels, magnetic torquers and thrusters.)

After the observation of a cosmic object, which can last from 30 minutes to a few hours, is completed, the satellite can re-point to another object of interest. “While doing the re-pointing from one object to another, we should avoid the sun. There are some operational constraints in each of the telescopes and instruments [such as] there should not be [interference from the] sun and bright objects. We have to avoid them and go to the next observation point. So there are design features of the satellite which take care of these things,” said Sarma.

Another problem is the continuous change in the thermal environment when re-pointing is done from one target of interest to another. Astrosat will face sun-load, albedo load, and so on. (Albedo is the fraction of the solar energy reflected from the earth back into space.) “So the thermal design of the satellite was complex. We have achieved all this. Before the launch, we tested Astrosat in thermo-vacuum chambers. It was working fine,” Sarma said.

Sarma argued that the Hubble Telescope could not be compared to Astrosat. (While Hubble weighs 11.1 tonnes, Astrosat weighs 1.5 tonnes.) Hubble belongs to a different class of satellite. He said: “Hubble is huge. It weighs several tonnes. It carries big telescopes. It has very precise bands where you can do observations. We are unique in our class of satellite weighing 1.5 tonnes. We have different features. It is not an apple-to-apple kind of comparison…. Our science is not inferior. It is a different kind of output from our satellite.”

Kiran Kumar also emphasised that the “prime” feature of Astrosat was that it could simultaneously study an astronomical object in multiple wavelengths from the same platform. Besides, it had the SSM, which could look for transient astronomical events and when it found such an event, it could “immediately tap” it for observation.

These were features that were different from what existed in other satellites that carried astronomy payloads. “As far as the sensitivity of Astrosat’s X-ray instrument is concerned, it is better than what is planned two years down the line. So Astrosat is a good combination of what can be achieved with the resources we have,” he asserted.

Challenges involved

The ISRO Chairman spoke about the other challenges involved in building Astrosat. From the visible wavelength where the objects could be seen with the human eye, “as you go towards shorter and shorter wavelengths such as far UV and X-ray”, the kind of technology needed to make telescopes kept changing, he said. As wavelengths became smaller, the mirrors that were used in telescopes and the surface accuracies required were significantly higher. Very precise, super-polishing techniques were needed for the mirrors. Contamination could significantly affect the performance of the telescopes.

During the entire course of the telescopes’ operation, cleanliness had to be maintained. Special efforts were made to ensure that the telescopes did not get contaminated when Astrosat was transported from Bengaluru to Sriharikota, during the mating of the satellite with the PSLV or when the PSLV with the Astrosat stood in the launch pad.

“Similarly, when you go to X-ray telescopes, reflection is possible only from grazing incidence. There is a significantly different technology that is required in this telescope, and K.P. Singh’s team from the TIFR has done a wonderful job in realising this telescope,” said Kiran Kumar. Furthermore, when one went from one wavelength to another, the detection system changed, the collection system changed and the coatings required [for the mirrors] changed.

M. Annadurai, Director, ISRO Satellite Centre (ISAC), Bengaluru, where Astrosat was built, said all the five instruments would start working together two months after the satellite was put into orbit on September 28. The instruments would be switched on one after another. Soon after each instrument is made operational, the raw, first-cut data will start coming in.

The SSM was the first instrument to be switched on soon after the satellite was put into its precise, equatorial orbit. The science data gathered by the five payloads will be beamed to the ground station at the Mission Operations Complex of ISRO Telemetry, Tracking and Command Network (ISTRAC) at Bengaluru. The Indian Space Science Data Centre (ISSDC) at Byalalu village, about 40 km from Bengaluru, processes the data, archives them and distributes them, first to the scientists who built the instruments. After a lock-in period of six months to one year, when the instrument-builders alone will have access to the data, the ISSDC will disseminate the data to researchers and students. An announcement of opportunity has been made to this effect.

“The ISSDC is the repository,” said Annadurai. “It receives the data and disseminates them to the individual Payload Operation Centre (POC). Each of the five instruments has a POC, to which the respective data will go,” he added.

But before the data are given to the scientific community, they will be analysed and annotated, that is, the time of their arrival, their significance, etc., will be noted. If the data reveal that some calibration needs to be done to the instrument, the Principal Investigator of the instrument will do it.

All this activity will begin after November 28. The UVIT will be the last payload to be switched on, 59 days after Astrosat went into its orbit on September 28. After the UVIT is switched on, all the five payloads will start functioning together.

“Astrosat is a scientific mission for the country,” summed up Kiran Kumar. “It will bring in tremendous amount of information for the institutions [IUCAA, IIA, RRI and TIFR]. They can make use of the data for their research activities. Our effort is always to make more and more institutions and students start using the data. Just to give you an idea: after we hosted the data from Chandrayaan-1 in the ISSDC, many students are using it for doing their PhD.”

Srivatsan Sridhar from Chennai, doing his PhD in the Observatory of Nice, University of Nice, France, on “Statistical Analysis of Galaxy Clusters and Cosmological Constraints from Euclid Survey”, is excited about Astrosat. He said, “We will be seeing the past live” with Astrosat. (Since some of the astronomical events which happened in the past would take several light years to reach the ground, watching them with telescopes amounts to watching the past live.)

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