ASTROSAT

1.1 What does ISRO call ASTROSAT?

ISRO calls AstroSat the first dedicated Indian astronomy mission. It studies cosmic sources in X-ray, optical and ultraviolet (UV) bands at the same time.

ISRO says this simultaneous view from a single satellite is one of the mission’s unique features.

1.2 What did ISRO build it to study?

ISRO lists five scientific objectives for the mission.

• Understand high-energy processes in binary star systems that contain neutron stars and black holes.
• Estimate the magnetic fields of neutron stars.
• Study star birth regions and high-energy processes in star systems beyond our galaxy.
• Detect new, briefly bright X-ray sources in the sky.
• Do a limited deep field survey of the Universe in the ultraviolet region.

1.3 Why watch in many kinds of light at once?

Different kinds of light show different things about the same object. Here is an analogy. A doctor may use a plain photo, an X-ray and an ultrasound of one patient. Together they tell more than any single picture.

ASTROSAT does something similar for a star or galaxy, using one satellite. It also has to be in space. ISRO says Earth’s atmosphere absorbs UV radiation, so it can only be observed from space. ESA says the atmosphere blocks out all X-rays.

1.4 Is ASTROSAT only an X-ray satellite?

No. This is a common mix-up. ISRO says the payloads cover the near and far UV, a limited optical range and X-rays from 0.3 keV to 100 keV.

2.1 What are the five science instruments?

ISRO’s payload page names five science instruments. It adds a Charged Particle Monitor. That monitor detects high-energy particles along the orbit and alerts the instruments.

The table covers the five science instruments only. It leaves out the Charged Particle Monitor. Bands follow ISRO’s payload page. ISRO’s invitation for observing proposals gives 15 to 100 keV for CZTI, so check which page a question follows. A keV is a unit of energy.

2.2 How do you remember the jobs?

• UVIT takes the UV and visible pictures.
• SXT focuses soft X-rays into an image.
• LAXPC does not make pictures. It tracks how a source’s brightness changes with time.
• CZTI sees hard X-rays through a coded mask.
• SSM watches for new X-ray transients. Think of it as a sky watchman. That is only an analogy.

2.3 How is UVIT built?

UVIT is a twin telescope system, says the PIB release of 4 December 2025. One telescope observes the far UV band. The other observes the near UV and visible bands. ISRO’s discovery page gives the diameter as 38 cm.

3.1 How was ASTROSAT launched?

PSLV-C30 launched ASTROSAT on 28 September 2015. It lifted off from the First Launch Pad at the Satish Dhawan Space Centre, Sriharikota. ISRO calls it the 31st flight of PSLV and the thirtieth consecutive successful one.

ISRO says six smaller satellites from Canada, Indonesia and the USA also launched with ASTROSAT. That made seven satellites on this flight.

• ISRO gives the PSLV-C30 height as 44.4 m.
• ISRO gives its lift-off mass as 320.2 tonne.

3.2 What are its mass and orbit?

• Lift-off mass: 1515 kg. ISRO splits this into a main bus of 660.2 kg and payloads of 854.8 kg.
• Orbit: 650 km, inclined 6 degrees to the equator. ISRO calls the orbit type near equatorial.
• Shape: a cuboid of 1.96 m by 1.75 m by 1.30 m.
• Power: 1626 W.

Remember 1515 kg and 650 km. The 2016 exam question used very different numbers.

3.3 How long was it meant to last, and what did it cost?

ISRO says the minimum useful life of the mission was expected to be 5 years. A minimum is a floor, not a limit. In its note of 28 September 2025, ISRO says ASTROSAT has far exceeded its design life. It adds that all five science instruments were working satisfactorily.

The Department of Space gave the cost in a Lok Sabha reply, released on 2 December 2015. The sanctioned cost was ₹177.85 crore. The cost incurred up to the end of March 2015 was ₹172.68 crore, which was provisional.

4.1 Which bodies built ASTROSAT?

ISRO’s spacecraft page lists ISRO as both manufacturer and owner. ISRO’s note of 28 September 2025 calls ASTROSAT a multi-institute mission and gives more detail.

• ISRO centres named: URSC, LEOS, SAC, VSSC and PRL.
• TIFR, IIA and IUCAA contributed to four of the five science instruments.
• UVIT also collaborated with the Canadian Space Agency (CSA).
• SXT also collaborated with the University of Leicester, UK.

4.2 Who built UVIT?

The PIB release of 4 December 2025 says the Indian Institute of Astrophysics (IIA) designed, assembled, tested and delivered UVIT. It says IIA led a national consortium. The consortium had support from IUCAA, TIFR, many ISRO centres and the Canadian Space Agency.

ISRO’s discovery page words it a little differently. It says teams from IIA, IUCAA, TIFR and CSA developed UVIT. It names Shyam Tandon of IUCAA as the leader and says ISRO gave full support.

When a question asks who built UVIT, read the options carefully. The PIB release names IIA as the lead institute.

4.3 Who flies it from the ground?

• The Mission Operations Complex (MOX) of ISTRAC, Bengaluru, manages the satellite for its entire mission life.
• The Indian Space Science Data Centre (ISSDC), at Bylalu near Bengaluru, processes, archives and distributes the science data.
• ISRO says the ground station and ISSDC sit together in the Indian Deep Space Network Complex at Bylalu.

5.1 Is ASTROSAT run like an ordinary satellite?

ISRO says the satellite is operated as an observatory. Researchers send proposals. Observing time is given on the basis of those proposals, through ISRO’s Announcements of Opportunity (AO).

An Announcement of Opportunity is an invitation to submit proposals for observations. ISRO and IUCAA, Pune, set up an AstroSat Support Cell. It helps with proposals and with using the data.

5.2 What are the steps?

ISRO’s ground segment page describes how a proposal reaches the sky.

• Step 1: the proposal enters the AstroSat Proposal Processing system.
• Step 2: the AstroSat Time Allocation Committee (ATAC) selects sources. Peer review and comments from the technical committee help it.
• Step 3: the AstroSat Technical Committee (ATC) studies whether the sources can be observed within the spacecraft’s limits.
• Step 4: the Payload Programming System makes a time-lined command sequence.
• Step 5: the command sequence is uplinked to the satellite.

5.3 Who could apply, and when?

• The first invitation sought proposals for observations from October 2016 to March 2017. It called this an exclusive opportunity for Indian principal investigators.
• ISRO says that from October 2017 the observatory has been open to the Indian and international astronomy community.
• As of 28 September 2025, ISRO reports a registered userbase close to 3400 from 57 countries. About half are Indian scientists and students.
• ISRO says ASTROSAT has brought astrophysics research into 132 Indian universities.

6.1 What was the first result?

ISRO’s note of 28 September 2025 says ASTROSAT began its science work by solving a two-decade-old puzzle. It concerned a red giant star that was unusually bright in both UV light and infrared. The same note lists other results. One is that emission from the butterfly nebula extends three times more than the earlier known size.

6.2 Which results came with a date?

The table lists only these five dated items. It leaves out later papers. A preprint is a paper posted before it is formally published.

6.3 Why was AUDFs01 notable?

AUDFs01 lies in the Hubble Extreme Deep Field. Dr Kanak Saha of IUCAA led the team. The result was reported in Nature Astronomy.

Dr Saha said UVIT could do this because the background noise in its detector is much less than on the Hubble Space Telescope. Nature says the wide-field UVIT made the observation.

7.1 What happened, and when?

The table lists only the nine dated items used in this article. It is not a full history of the mission.

7.2 What did ISRO say at ten years?

ISRO’s note of 28 September 2025 says ASTROSAT completed a decade of discoveries and service to the global scientific community. It says all five science experiments were working satisfactorily. It also says the observatory is expected to give many more results in the coming years.

Ten years is twice the five-year minimum that ISRO had expected. ISRO itself says ASTROSAT has far exceeded its design life.

8.1 What happened on 4 December 2025?

The Indian Institute of Astrophysics (IIA) held a one-day workshop on 4 December 2025. It marked ten years since the doors of UVIT opened on 30 November 2015. PIB reported it the same day.

8.2 What did the release say UVIT has done?

• As of 4 December 2025, UVIT had observed a total of 1,451 targets in the sky.
• In ten years it had led to about 300 research articles and 19 PhD theses.
• Annapurni Subramaniam, Director of IIA, said UVIT is India’s first UV space telescope. She said it is the only operational telescope capable of observing in the far-UV apart from the Hubble Space Telescope.
• The release says UVIT combines a large field of view with spatial resolution better than 1.5 arcseconds.

8.3 What may come next?

The PIB release says the workshop also aimed to plan for future space UV telescopes. The astronomers discussed how a larger facility called INSIST (Indian Spectroscopic and Imaging Space Telescope) can be made possible. The release says this would build on more than two decades of experience with UVIT.

A 2022 paper by INSIST’s proposers says the proposal answered an ISRO call for proposals. It says the proposal won seed funding and had completed two years of pre-project phase.

ISRO’s report of the 2017 science meet also mentions AstroSat-2. It says the instruments, spacecraft and pipeline have to be made “in the bigger way”. None of the sources read here gives a launch date for AstroSat-2 or INSIST.

9.1 What did UPSC ask?

UPSC asked about ASTROSAT in the Civil Services (Preliminary) Examination 2016, General Studies Paper I, as Question 87. The official paper (Series A) reads:

With reference to ‘Astrosat’, the astronomical observatory launched by India, which of the following statements is/are correct?
1. Other than USA and Russia, India is the only country to have launched a similar observatory into space.
2. Astrosat is a 2000 kg satellite placed in an orbit at 1650 km above the surface of the Earth.
Select the correct answer using the code given below.
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

The official Series A answer key lists option D for Question 87. In the official paper, option (d) reads “Neither 1 nor 2”.

See UPSC CSP 2016 GS Paper I, Q87. View this question.

9.2 How do you reach the answer step by step?

• Step 1: test statement 1. ESA’s XMM-Newton was launched on 10 December 1999. It carries 3 X-ray telescopes and an optical monitor. The official UPSC key marks statement 1 as wrong. XMM-Newton is only an example of a space observatory that ESA has flown.
• Step 2: test statement 2. ISRO gives about 1515 kg and a 650 km orbit, not 2000 kg and 1650 km. Statement 2 fails.
• Step 3: neither statement is correct. Pick “Neither 1 nor 2”, which is option D.

9.3 Quick Q&A