How ISRO’s AstroSat space observatory has transformed our understanding of the universe in 11 years

On September 28, AstroSat completed eleven years, more than double its intended lifespan.

During this period, the mission has transformed our understanding of the universe's most energetic events: burning-hot stars, catastrophic stellar explosions, neutron stars (formed when a massive star runs out of fuel and collapses), galaxy collisions and gamma-ray bursts. It has also shown that India can design, build, and operate a complex multi-wavelength observatory at the cutting edge of modern astronomy.

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The idea for AstroSat began in 1996, during a meeting led by K Kasturirangan, the then chairman of the Indian Space Research Organisation (ISRO). The goal was ambitious: a compact multi-wavelength observatory. The reasoning was solid. Many of the universe's most energetic and violent processes, such as flares on distant stars and thermonuclear explosions on white dwarfs (the dense core remnant of a star that has exhausted its nuclear fuel), emit radiation across multiple wavelengths simultaneously. To fully understand these space events, astronomers need to observe the same object simultaneously in ultraviolet, X-ray, and gamma-ray light.

Think of a crime scene. A detective arrives to find evidence. Fingerprints reveal who touched a surface, but they don’t identify the weapon. CCTV footage shows movement but does not detail the time of death. A DNA swab links a suspect, but it doesn't explain what happened. The toxicology report uncovers the poison, while blood spatter analysis reconstructs the struggle. Only by putting together all these pieces of evidence, gathered at once and compared against a single timeline, does the full picture of the crime come to light.

In astrophysics, the "crime scene" could be a supernova (a violently exploding star with increased luminosity after eruption), a flaring star (a variable star which shows sudden increased brightness), or a hungry black hole (a dense celestial body with a strong gravitational pull through which light cannot pass). Visible light acts like witness testimony, showing what can be seen. But the intense, new material shines in ultraviolet (UV); the chaotic fall of superheated matter emits X-rays; and the most powerful forces burst in gamma rays. If astronomers only examine visible light, they are akin to detectives relying on a single blurred photograph while ignoring vital evidence.

Regrettably, Earth's atmosphere interferes. It absorbs important X-rays and UV rays before they can be studied from the ground. To decode the cosmic mysteries, we needed to place our "detective", AstroSat, above the atmosphere in space. This would allow it to collect a full spectrum of evidence at once and compare the clues in real time, revealing the true nature of the universe's violent events.

AstroSat was designed for this job.

It was built as a compact observatory capable of scanning celestial objects across a wide spectrum, from ultraviolet radiation at 130–300 nanometres to X-rays and gamma rays (0.3–100 kiloelectron volts), wavelengths that Earth's atmosphere mainly absorbs. Such simultaneous observations are crucial for understanding the complicated physics behind these dramatic cosmic events.

This capability placed AstroSat alongside major international observatories such as NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton. However, unlike those larger facilities, AstroSat needed to be compact and light enough to launch on an Indian vehicle while carrying multiple telescopes and detectors, most of which were developed in India. Key partners included the Tata Institute of Fundamental Research (TIFR), the Indian Institute of Astrophysics (IIA), the Inter-University Centre for Astronomy and Astrophysics (IUCAA), various ISRO centres and Indian industry collaborators.

One remarkable technological advance was India's first space-qualified soft X-ray focusing telescope, which uses grazing-incidence optics. Unlike regular light or radio waves, X-rays easily pass through most materials, making them very difficult to collect and focus with standard mirrors. To solve this problem, scientists came up with a clever solution. Imagine skipping a flat stone across a pond's surface. If you throw it straight down, it sinks. But if you toss it at a shallow angle, it skips before finally sinking. X-rays behave similarly. Mirrors angled at shallow positions can gently deflect X-rays, focusing them into a beam that hits detectors. This innovation enabled AstroSat to focus X-rays from distant cosmic objects.

The Soft X-ray Telescope (SXT) was developed at TIFR by a team led by the first author. The hard X-ray detectors included three Large Area Xenon Proportional Counters (LAXPCs) and one Cadmium-Zinc-Telluride Imager (CZTI), developed by his colleagues.

ISRO provided the satellite platform, orientation system, thermal control, communications, and mission operations support. Launched aboard PSLV-C30 in 2015, AstroSat operates successfully, with its scientific data archived and shared freely through the Indian Space Science Data Centre.

Over the past eleven years, AstroSat has observed hundreds of cosmic targets: stars, star clusters, white dwarfs, neutron stars, black holes, supernova remnants, and galaxies.

Hot cosmic objects like young, massive stars and white dwarfs shine brightly in ultraviolet light, while extreme events, from stellar flares and material falling onto compact objects to supernova shock waves, produce abundant X-rays. AstroSat's Ultraviolet Imaging Telescopes (UVIT) have discovered many hot and unusual stars, including blue stragglers, white dwarfs and other rare stellar populations. Its SXT detected significant X-ray flares from Proxima Centauri, the closest star to our Sun, which has an Earth-sized planet. This intense activity raises an important question: can any planet orbiting such a star support life?

AstroSat's ongoing monitoring of the Andromeda Galaxy has identified dozens of novae, stellar explosions triggered by rapid thermonuclear reactions on the surfaces of white dwarfs. These observations have helped astronomers understand how such bursts evolve over time.

The mission has also captured extraordinary details of novae within our Milky Way, including one of the fastest stellar outbursts recorded in recent times. By observing these events simultaneously in ultraviolet and X-rays, AstroSat has provided fresh insights into how matter accumulates onto compact stars.

Another significant target has been the Cygnus Loop, a massive supernova remnant about 10,000 years old. Supernova explosions create and scatter heavy elements, the essential building blocks of planets and life. AstroSat's SXT and UVIT instruments mapped the distribution of heated gas and tracked elements like oxygen, neon, magnesium, silicon, sulphur, and iron throughout this vast structure.

One of the brightest X-ray sources in the Milky Way are X-ray binaries, systems where material from a normal star spirals onto a neutron star or black hole. As the material falls inward, it heats to extreme temperatures, emitting intense X-ray radiation. AstroSat's LAXPCs and SXT have studied many of these systems in unprecedented detail. In neutron-star binaries, the mission detected spectral features indicating magnetic fields billions to trillions of times stronger than Earth's. In black-hole systems, it monitored rhythmic and arhythmic variations, instabilities in swirling accretion disks (disk like flow of gas, plasma and particles around a celestial body) and explosive thermonuclear activity, offering insights into gravity-dominated regimes.

The mission has significantly impacted our understanding of galaxies and cosmic evolution. Ultraviolet observations of nearby galaxies have clarified the history of star formation by identifying regions rich in young, massive stars. In one giant spiral galaxy located about 56 million light-years away, AstroSat found over a hundred active star-forming regions. When combined with data from NASA's Chandra observatory, these observations showed that ten of these regions align with bright X-ray sources, an important discovery.

AstroSat's reach extends to the distant universe. It has detected signs of a smaller galaxy being engulfed by a larger one, demonstrating the "cannibalistic" growth of galaxies over time. In another deep observation, it found a remote galaxy from a time when the universe was only about one-third its current age. The mission has also examined ultraviolet and X-ray radiation from accretion disks and jets surrounding supermassive black holes at the centres of galaxies.

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AstroSat's Cadmium-Zinc-Telluride Imager (CZTI) has tracked approximately 750 gamma-ray bursts, some of the most powerful explosions in the universe. Measurements of gamma-ray polarisation from a few exceptionally bright events are providing new insights into the physical processes behind these mysterious phenomena.

The mission's scientific contributions extend beyond individual discoveries. As of today, AstroSat data has led to more than 570 research papers in top international journals and numerous doctoral theses from Indian universities and research institutes. Many undergraduate and graduate students have gained practical experience through AstroSat observations.

Over a decade after its launch, AstroSat acts as a landmark achievement for Indian science. It has demonstrated that a home-grown observatory, built through close collaboration between research institutions, industry, and ISRO, can deliver world-class astronomy. With the satellite still functioning robustly, India's window on the energetic universe remains wide open. And AstroSat may continue exploring the cosmos for years to come.