AstroSat, Launched on 28 September 2015 With Five Instruments, Has Worked for a Decade, Caught Extreme-Ultraviolet Light From a Galaxy 9.3 Billion Light Years Away and Measured X-ray Polarisation From the Crab Pulsar

AstroSat, which ISRO describes as the first dedicated Indian astronomy mission, has been in orbit for ten years. A PSLV-C30 rocket lifted it from Sriharikota on 28 September 2015, and its five instruments study the same objects in ultraviolet, visible light and X-rays at the same time. Its ultraviolet telescope has picked up extreme-ultraviolet light from a galaxy 9.3 billion light years away, and its X-ray camera has measured how the X-rays from the Crab pulsar are polarised through each of the pulsar’s 33-millisecond turns.

What went up on 28 September 2015

ISRO’s mission page gives AstroSat a lift-off mass of 1,515 kg and an orbit 650 km up, tilted 6 degrees to the equator. PSLV-C30 was the 31st flight of the Polar Satellite Launch Vehicle and its 30th success in a row. It also put six smaller satellites from Canada, Indonesia and the United States into orbit, seven in all. A later PSLV flew 104 satellites in one launch, which we cover in the PSLV-C37 article.

After separation, AstroSat’s two solar panels opened automatically. The first six months went on checking and calibrating the payloads, and science observations began after that, according to ISRO. The Mission Operations Complex of ISRO’s telemetry network in Bengaluru flies the satellite. The Indian Space Science Data Centre at Bylalu, near Bengaluru, processes, archives and distributes what the five payloads send down. The minimum useful life was set at five years. ISRO’s solar observatory, Aditya-L1, was launched in 2023.

ISRO lists five scientific aims: to understand high-energy processes in binary star systems that contain neutron stars and black holes, to estimate the magnetic fields of neutron stars, to study star-birth regions and high-energy processes in star systems beyond the Milky Way, to detect new, briefly bright X-ray sources, and to carry out a limited deep survey of the sky in ultraviolet.

Five instruments on one satellite

The idea behind AstroSat is simultaneity. Many objects change brightness within seconds or hours, so looking at one in ultraviolet today and in X-rays next month does not show what it was doing at any single moment. ISRO’s mission page calls this the unique feature of the mission: simultaneous observations of one object across the spectrum from a single satellite.

The Ultra Violet Imaging Telescope (UVIT) images in far ultraviolet (130 to 180 nanometres), near ultraviolet (200 to 300 nm) and visible light (320 to 550 nm), with a photon-counting detector in each channel. The telescope is 38 cm in diameter, and ISRO’s release on the AUDFs01 result says UVIT was built by teams from IIA, IUCAA and TIFR in India and the Canadian Space Agency, led by Shyam Tandon of IUCAA. Ultraviolet from space cannot be seen from the ground, because Earth’s atmosphere absorbs it.

The Soft X-ray Telescope (SXT) is a focusing X-ray telescope with a CCD camera covering 0.3 to 8 keV. A keV, or kiloelectronvolt, is the energy unit X-ray astronomers use, and a higher number means a harder, more penetrating X-ray. The Large Area X-ray Proportional Counters (LAXPC) are three identical detectors for 3 to 80 keV, with an effective area above 8,000 square centimetres between 5 and 30 keV. They do not make pictures. They record how a source’s total brightness changes within a one-degree field of view.

The Cadmium Zinc Telluride Imager (CZTI) is a hard X-ray camera for 10 to 100 keV that forms coarse images through a coded mask, a plate of open and blocked squares whose shadow pattern reveals where the X-rays came from. Its detector has 16,384 pixels. The Scanning Sky Monitor (SSM) covers 2.5 to 10 keV and looks for new X-ray transients while watching known sources. A small charged particle monitor warns the instruments when the satellite passes through regions of high-energy particles.

Who built what, and the fifteen years it took

ISRO’s tenth-anniversary note says that ISRO centres (URSC, LEOS, SAC, VSSC and PRL) and research institutes (TIFR, IIA and IUCAA) developed four of the five payloads, with the Canadian Space Agency a partner on UVIT and the University of Leicester in the United Kingdom a partner on SXT.

A. R. Rao of TIFR, who was principal investigator of CZTI, gives the split in his 2023 history of TIFR’s space astronomy. TIFR took prime responsibility for delivering LAXPC, SXT and CZTI. IIA in Bengaluru was responsible for UVIT. IUCAA in Pune worked on SSM and CZTI, RRI in Bengaluru on LAXPC, and PRL in Ahmedabad on CZTI’s polarisation side. The SSM was designed, built and delivered at ISRO Satellite Centre, and CZTI was assembled and tested at VSSC in Thiruvananthapuram. ISRO’s release on the CZTI work adds that SAC in Ahmedabad provided its analysis software and that IUCAA designed its coded mask and operates the payload operation centre.

Rao writes that the project received in-principle approval in 2000 and formal approval in 2003, with a launch targeted for 2006, and that it took fifteen years to complete. He names P. C. Agrawal of TIFR and S. N. Tandon of IUCAA as its driving forces. For how ISRO itself began, see the Vikram Sarabhai article.

A galaxy leaking extreme-ultraviolet light

The result ISRO puts first among AstroSat’s discoveries is the detection of AUDFs01, a galaxy 9.3 billion light years away, in the field that the Hubble Space Telescope imaged as its Extreme Deep Field. A team led by Kanak Saha of IUCAA, with colleagues in India, Switzerland, France, the United States, Japan and the Netherlands, reported it in Nature Astronomy on 24 August 2020.

The light in question is Lyman continuum emission, ionising radiation from young stars that can strip electrons from hydrogen atoms. Astronomers want to know which objects produced enough of it to end the cosmic dark ages, a few hundred million years after the Big Bang. Almost all such light is absorbed by gas inside the galaxy or on the way to Earth, so detecting any of it is rare. The paper reports an escape fraction above 20 per cent, meaning more than a fifth of the ionising light got out, from a low-mass galaxy at a redshift of 1.42. The authors say leakers had been found at redshifts below 0.4 and between 2.5 and 3.5, but not at this intermediate distance, near the peak of cosmic star formation.

ISRO’s release quotes Saha saying the team expected to struggle to convince others that UVIT had recorded the emission “when more powerful HST has not”, and attributes the detection to UVIT’s lower detector background than the ones on Hubble. Anne Verhamme of the Geneva Observatory is quoted saying AUDFs01 gives the first observational constraint in this extreme-ultraviolet regime.

A follow-up by Suraj Dhiwar, Saha and colleagues, posted in January 2024 and accepted by the Astrophysical Journal Letters, reports Lyman continuum from nine galaxies and one active galactic nucleus at redshifts of about 1.1 to 1.6 in another deep field, with escape fractions of roughly 10 to 55 per cent.

X-ray polarisation from the Crab pulsar and the gamma-ray bursts

Polarisation is the direction in which a light wave’s electric field oscillates, and it carries information about the magnetic fields near a source. ISRO’s account is that X-ray polarisation had been seen from the Crab Nebula in 1975 and from no other source since, because it is so hard to measure.

The CZT Imager measures it by counting X-ray photons that scatter between two neighbouring pixels. If the pairs line up in one direction more often than chance allows, the X-rays are polarised. The team, led by Santosh Vadawale of PRL Ahmedabad, tested the method on the ground before launch, observed the Crab pulsar for about eighteen months and combined data taken months apart. To keep the timing accurate to microseconds, the Giant Metrewave Radio Telescope near Pune and the Ooty radio telescope tracked the pulsar’s radio pulses and supplied a timing model. The paper appeared in Nature Astronomy in November 2017.

The finding was that polarisation changed most in the “off-pulse” part of the cycle, when no contribution from the pulsar was expected. Dipankar Bhattacharya of IUCAA is quoted in ISRO’s release saying the result tells theorists that the pulsar’s emission extends into what was thought to be the off-pulse region.

CZTI also doubles as a wide-angle monitor for gamma-ray bursts, the brief flashes of gamma rays that arrive from random directions. A team including Vadawale and Bhattacharya analysed the 11 brightest bursts it caught in its first year. Five showed polarisation at roughly 3 sigma or higher, one at about 2 sigma, and the other five yielded upper limits, in the 100 to 300 keV range. The paper in the Astrophysical Journal in 2019 says that sample is too small to separate competing models of how bursts radiate, and that more observations should help.

A butterfly, a star that regained its youth, and black-hole spin

ISRO’s picture-of-the-month archive records several UVIT results by IIA astronomers. N. Kameswara Rao and colleagues imaged the Butterfly Nebula, NGC 6302, about 3,800 light years away. In far ultraviolet, gas extends to 5.5 light years from the centre, nearly three times the extent seen in visible light. They attribute it to hydrogen molecules excited by the central star, and suggest it may point to two stars at the centre. The work appeared in Astronomy & Astrophysics in 2018.

Snehalata Sahu and colleagues used UVIT on the globular cluster NGC 5466, about 52,000 light years away. They found that a blue straggler star, a star that looks younger than its neighbours, had gained mass from a companion that became a white dwarf. ISRO’s page says it is only the second such pair known in a globular cluster.

ISRO’s tenth-anniversary note also lists the first science result, a puzzle about a red giant star unusually bright in both ultraviolet and infrared, as well as galaxy mergers and what it calls very fast spinning black holes. Rao’s history adds that LAXPC observed the black hole GRS 1915+105 several times and found a new intermediate state, that SXT measured coronal emission from stars, and that joint LAXPC and SXT data helped measure black-hole spin.

Who can use the data

ISRO opened AstroSat’s archive to the public on 26 September 2018, with data from all five payloads available through ISSDC’s AstroBrowse portal. Observing time is allocated through calls for proposals, called Announcements of Opportunity, which have been open to Indian and international scientists. ISRO and IUCAA run an AstroSat Support Cell in Pune to help with proposals and data. ISSDC’s guidelines give each proposing team a 12-month proprietary period, counted from the day the processed data reaches them, after which the archived data is open to registered users.

ISRO’s tenth-anniversary note counts close to 3,400 registered users from 57 countries, and says AstroSat has brought astrophysics research into 132 Indian universities, with about half the users Indian scientists and students.

Ten years on

ISRO marked the tenth anniversary on 28 September 2025 and said that although AstroSat has far outlived its design life, all five scientific instruments are working satisfactorily. A page dated 29 January 2026 says the U. R. Rao Satellite Centre is organising an international conference in Bengaluru to review the scientific results, discuss technical challenges and consider future directions.

In his 2023 history, Rao writes that the results so far, though significant, lack the punch of a collector’s issue of the Astrophysical Journal Letters, and that easier data access and more sophisticated analysis software could make more of the instruments’ niche capabilities. ISRO’s anniversary note says the observatory is expected to keep producing new results in the coming years.