AstroSat is India's first dedicated multi-wavelengthspace telescope. It was launched on aPSLV-XL on 28 September 2015.[1][2] With the success of this satellite,ISRO has proposed launchingAstroSat-2 as a successor forAstroSat.[3]
A number of astronomy research institutions in India, and abroad have jointly built instruments for the satellite. Important areas requiring coverage include studies ofastrophysical objects ranging from nearbySolar System objects to distant stars and objects atcosmological distances; timing studies of variables ranging from pulsations of hotwhite dwarfs to those ofactive galactic nuclei can be conducted withAstroSat as well, with time scales ranging from milliseconds to days.
The sanctioned cost of Astrosat was ₹177.85 crore (equivalent to about 20 million USD as of March 2025).[5]Astrosat was successfully launched on 28 September 2015 from theSatish Dhawan Space Centre on board aPSLV-XL vehicle at 10:00AM.
Artist's conception of a binary star system with one black hole and one main sequence starTilted view of Astrosat
AstroSat is a proposal-driven general purpose observatory, with main scientific focus on:
Simultaneous multi-wavelength monitoring of intensity variations in a broad range of cosmic sources
Monitoring the X-ray sky for new transients
Sky surveys in the hard X-ray and UV bands
Broadband spectroscopic studies of X-ray binaries,AGN,SNRs, clusters of galaxies, and stellar coronae
Studies of periodic and non-periodic variability of X-ray sources
AstroSat performs multi-wavelength observations covering spectral bands from radio, optical, IR, UV, and X-ray wavelengths. Both individual studies of specific sources of interest andsurveys are undertaken. While radio, optical, and IR observations would be coordinated through ground-based telescopes, the high energy regions, i.e., UV, X-ray and visible wavelength, would be covered by the dedicated satellite-borne instrumentation ofAstroSat.[6]
The mission would also study near simultaneous multi-wavelength data from different variable sources. In abinary system, for example, regions near the compact object emit predominantly in theX-ray, with theaccretion disc emitting most of its light in the UV/optical waveband, whereas the mass of the donating star is brightest in the optical band.
The observatory will also carry out:
Low- to moderate-resolutionspectroscopy over a wide energy band with the primary emphasis on studies of X-ray-emitting objects
Timing studies of periodic and aperiodic phenomena in X-ray binaries
TheUltra Violet Imaging Telescope (UVIT) performs imaging simultaneously in three channels: 130–180 nm, 180–300 nm, and 320–530 nm. The three detectors are vacuum image intensifiers manufactured byPhotek, UK.[9] The FUV detector consists of aCsIphotocathode with aMgF2 input optic, the NUV detector consists of CsTephotocathode with afused-silica input optic and the visible detector consists of an alkali-antimonidephotocathode with afused-silica input optic. The field of view is a circle of ~28′ diameter and the angular resolution is 1.8" for the ultraviolet channels and 2.5″ for the visible channel. In each of the three channels a spectral band can be selected through a set of filters mounted on a wheel; in addition, for the two ultraviolet channels a grating can be selected in the wheel to do slitless spectroscopy with a resolution of ~100. The primary mirror diameter of the telescope is 40 cm.[10]
TheSoft X-ray imaging Telescope (SXT) employs focusing optics and a deep depletion CCD camera at the focal plane to perform X-ray imaging in the 0.3–8.0 keV band. The optics will consist of 41 concentric shells of gold-coated conical foil mirrors in an approximate Wolter-I configuration (the effective area of 120 cm2). The focal plane CCD camera will be very similar to that flown on SWIFT XRT. The CCD will be operated at a temperature of about −80 °C by thermoelectric cooling.[10]
TheLarge Area X-ray Proportional Counter (LAXPC) covers X-ray timing and low-resolution spectral studies over a broad energy band (3–80 keV),Astrosat will use a cluster of 3 co-aligned identical Large Area X-ray Proportional Counters (LAXPCs), each with a multi-wire-multi-layer configuration and a Field of View of 1° × 1°. These detectors are designed to achieve (I) wide energy band of 3–80 keV, (II) high detection efficiency over the entire energy band, (III) narrow field of view to minimize source confusion, (IV) moderate energy resolution, (V) small internal background and (VI) long lifetime in space. The effective area of the telescope is 6000 cm2.[10]
TheCadmium Zinc Telluride Imager (CZTI) is a hard X-ray imager. It will consist of a Pixellated Cadmium-Zinc-Telluride detector array of 500 cm2 effective area and the energy range from 10 to 150 kev.[10] The detectors have a detection efficiency close to 100% up to 100 keV, and have a superior energy resolution (~2% at 60 keV) compared to scintillation and proportional counters. Their small pixel size also facilitates medium resolution imaging in hard x-rays. The CZTI will be fitted with a two dimensionalcoded mask, for imaging purposes. The sky brightness distribution will be obtained by applying a deconvolution procedure to the shadow pattern of the coded mask recorded by the detector. Apart from spectroscopic studies, CZTI would be able to do sensitive polarization measurements for bright galactic X-ray sources in 100–300 keV.[11]
TheScanning Sky Monitor (SSM) consists of three position sensitive proportional counters, each with a one-dimensional coded mask, very similar in design to the All Sky Monitor on NASA'sRXTE satellite. The gas-filled proportional counter will have resistive wires as anodes. The ratio of the output charge on either ends of the wire will provide the position of the X-ray interaction, providing an imaging plane at the detector. The coded mask, consisting of a series of slits, will cast a shadow on the detector, from which the sky brightness distribution will be derived.
TheCharged Particle Monitor (CPM) will be included as a part ofAstrosat payloads to control the operation of the LAXPC, SXT and SSM. Even though the orbital inclination of the satellite will be 8 deg or less, in about 2/3 of the orbits, the satellite will spend a considerable time (15–20 minutes) in theSouth Atlantic Anomaly (SAA) region which has high fluxes of low energy protons and electrons. The high voltage will be lowered or put off using data from CPM when the satellite enters the SAA region to prevent damage to the detectors as well as to minimize ageing effect in the Proportional Counters.
The Ground Command and Control Center forAstrosat is the ISRO Telemetry, Tracking and Command Network (ISTRAC) at Bangalore, India. Command and control of the spacecraft, and scientific data downloads is possible during every visible pass over Bangalore. 10 out of 14 orbits per day are visible to the ground station.[12] The satellite is capable of gathering 420 gigabits of data every day that can be downloaded during the 10 visible orbits by the Tracking and Data receiving center of ISRO in Bangalore. A third 11-meter antenna at theIndian Deep Space Network (IDSN) became operational in July 2009 to trackAstrosat.
ISRO has set up a support cell for AstroSat atIUCAA,Pune. AMoU was signed between ISRO and IUCAA in May 2016. The support cell has been set up to give opportunity to the scientific community in making proposals on processing and usage of AstroSat data. The support cell will provide necessary resource materials, tools, training and help to the guest observers.[13]
This image ofNGC 2336 was one of the first images takes by Astrosat-1, The Near-UV (200-300 nm) and Far-UV (130-180 nm) images were captured by Ultra-Violet Imaging Telescope(UVIT)
April 2009 : Scientists fromTata Institute of Fundamental Research (TIFR) have completed the developmental phase of complex science payloads and have begun integrating them before delivery of the 1,650 kg satelliteAstrosat. The challenges in the design of payloads andAttitude Control System have been overcome and in a recent review committee meeting, it was decided that the delivery of the payload to the ISRO Satellite Centre will begin from the middle of 2009 and continue until early 2010 to enable the launch of ASTROSAT in 2010 using ISRO workhorse PSLV-C34.[15]
May 2015 : The integration ofAstrosat is complete and final tests are under way. ISRO issued a press release stating that "The satellite is planned to be launched during the second half of 2015 by PSLV C-34 to a 650 km near equatorial orbit around the Earth."[16]
24 July 2015: Thermovac completed. Solar panels attached. Start of final vibration tests.[10]
10 Aug 2015: All tests passed. Pre-shipment review successfully completed.[10]
28 Sep 2015: ASTROSAT has been successfully launched into orbit.[17]
15 April 2016: The satellite has completed its performance verification and started its operations.[18]
29 Sep 2018: The satellite has completed 3 years since its launch on 2015. It has observed over 750 sources and resulted in close to 100 publications in peer-reviewed journals.[19]
29 Sep 2020: The satellite completed its mission life of 5 years and will continue to remain operational for many years.[20]
29 September 2025 : The Satellite completed 10 years in orbit.[21]
Agamma-ray burst was detected byAstrosat on 5 January 2017. There was a confusion whether this event was related to the gravitational wave signal detected byLIGO from the black hole merger eventGW170104 on 4 January 2017.[22]Astrosat helped in distinguishing between the two events. The gamma-ray burst from 4 January 2017 was identified as a distinct supernova explosion that would form a black hole.[22]
Astrosat also captured the rare phenomenon of a 6 billion year old small star orblue straggler feeding off and sucking out the mass and energy of a bigger companion star.[23]
In July 2018,Astrosat has captured an image of a special galaxy cluster that is more than 800 million light years away from Earth. Named abell 2256 the galaxy cluster is made of three separate cluster of galaxy that are all merging with one another to eventually form a single massive cluster in the future. The three massive cluster contain more than 500 galaxies and the cluster is almost 100 times larger and more than 1500 times massive as our own galaxy.[28]
On 26 September 2018, the archival data of AstroSat was publicly released.[29] As of 28 September 2018, data from AstroSat has been cited in around 100 publications in refereed journals. This figure is expected to rise after the public release of data from AstroSat.[30]
In 2019 AstroSat observed a very rare X-ray outburst in aBe/X-ray binary system RX J0209.6-7427. Only a couple of rare outbursts have been observed from this source hosting a neutron star. The last outburst was detected in 2019 after about 26 years. The accreting neutron star in this Be/X-ray binary system was found to be an ultraluminous X-ray Pulsar (ULXP) making it the second closest ULXP and the first ULXP in our neighbouring Galaxy in theMagellanic Clouds. This source is the first ULX pulsar discovered with the AstroSat mission and only the eight known ULX pulsar.[31][32][33]
In August 2020,AstroSat had detected extreme-UV light from a galaxy located 9.3 billion light-years away from Earth. The galaxy called AUDFs01 was discovered by a team of Astronomers led by Kanak Saha from theInter-University Centre for Astronomy and Astrophysics, Pune.[34][35]
In August 2025, researchers from ISRO,Haifa University ,URSC &IIT-Guwahati observed the origins of X-ray flickering on a black hole named GRS 1915+105 and discovered that it's X-ray brightness alternated between bright and dim phases, each lasting several hundred seconds.This discovery will provide critical insights into how black holes interact with surrounding matter and release energy.[36]
Launches are separated by dots ( • ), payloads by commas ( , ), multiple names for the same satellite by slashes ( / ). Crewed flights are underlined. Launch failures are marked with the † sign. Payloads deployed from other spacecraft are (enclosed in parentheses).
