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1. Why is ASTROSAT a multi-wavelength observatory ?

Supposing we cannot touch a human body but are trying to understand what it is made of solely by looking at the radiation from it. The visible light will tell us about the skin, infrared about the temperature and blood vessels, ultraviolet about the tissues, and x-ray shadows about the bones inside. All these pieces of information are essential in arriving at a complete picture of the human body.

Astrosat Telescopes

An artist’s impression of the Astrosat & its High-Energy radiation detecting Telescopes

The only way in which we can understand the universe is by looking at the light emitted by them. Celestial objects emit light of all wavelengths, from radio and microwave to infrared, visible and ultraviolet, up to x-rays and gamma rays. Light of different wavelengths are emitted by different physical processes in an object and it is only by studying the emission in all of these wavebands can we hope to truly make sense of that object. In addition, the high energy sky, i.e., emission from the sky observed at x-ray and gamma-ray wavelengths, is highly variable and changes constantly. Hence it becomes crucial to be able to observe all these wavebands simultaneously. What is unique about ASTROSAT is that it can observe the universe in the near and far ultraviolet, soft x-rays and hard x-rays and it can do so at the same time !

2. Why is ASTROSAT in space ?

One of the many reasons life as we know it could survive on earth is that our atmosphere acts as a protective blanket. It is largely opaque to ultraviolet, x-rays and gamma rays (and also parts of infrared and microwave radiation), which is a good thing since these radiations can cause damage to living cells. In fact, the only wavebands our atmosphere is transparent to are the visible and the radio.

Therefore, if we want to observe ultraviolet and x-ray emission from sources in the sky, we need to put our telescopes outside the atmosphere. This is why ASTROSAT is a space observatory that ISRO had launched using its PSLV rocket.


3. How long will the mission last ?

The mission is expected to last atleast 5 years. ASTROSAT may well continue to function after five years, but that will be determined by the leakage of gas and build-up of impurities from inside the instruments, etc. However, a minimum of five years of operation is aimed at.


4. How heavy and how large is ASTROSAT ?

The satellite weighs 1515 kgs, about the weight of a car.

It is about 6 metres in height and the deployed solar panels will be 7 metres long.


5. Where will ASTROSAT get its power from ?

ASTROSAT has two solar panels on either side. These convert sunlight to electricity using the photo-electric effect and hence need to point at the Sun. These deliver a total of 1.6 kilowatt power. These solar panels will charge the lithium ion batteries and it is from these batteries that the spacecraft will be drawing power.


6. What kind of a rocket will be used to launch ASTROSAT ?

The Polar Satellite Launch Vehicle (PSLV) in its XL version will launch ASTROSAT into its orbit. In fact, this is the first time that ISRO is launching a rocket solely for astronomy. The launch vehicle has four stages and is equipped with six strap-on motors. The PSLV-C30 weighs 320 tons at lift-off and is 15 storeys high !

This will be the 31st launch of the PSLV. The launch took place at 10.00 am IST on 28th September 2015, and the rocket took only 23 minutes to place ASTROSAT into its final orbit.


7. What kind of an orbit will ASTROSAT be in ?

ASTROSAT will go around the Earth at a height of 650 km in a roughly equatorial orbit. The orbit is almost parallel to the Earth’s equator, inclined at an angle of 6 degrees. The observatory will go around the Earth once every 97 minutes, or 15 times a day.


8. How will ASTROSAT know where to point to ?

A space telescope like ASTROSAT is a precious resource and even a single second of its lifetime cannot be wasted. Based on the accepted proposals for observations from astronomers, a schedule will be made ahead of time. This schedule will take into account various constraints like the position of the sun and the earth as the satellite goes in its orbit etc. Once the schedule is made, commands will be uploaded to ASTROSAT that will tell it where to point and what to do for each instant of time.


9. How will ASTROSAT manage to point to different objects in the sky ?

A gyroscope on board the ASTROSAT will help maintain an absolute direction with respect to the stars, like in most satellites. The satellite also has a flywheel which will store rotational kinetic energy,. This flywheel will be spun up by motors to a high speed. Whenever ASTROSAT needs to turn towards another direction to point to a particular object in the sky, the stored energy in the flywheel is used to torque the satellite and point it in that direction.


10. How will we get data back from ASTROSAT ?

ASTROSAT has an antenna on board, that will be used to transmit the data towards earth. This antenna, operating in the X band (around 8 GHz), can transmit up to 105 Megabits per second. ISRO has a dedicated antenna at Byalalu, near Bengaluru. Whenever ASTROSAT passes over Bylalu, the science and engineering data that is transmitted is received by this antenna and this data is then stored at the Indian Space Science Data Centre (ISSDC). ASTROSAT will be visible from Byalalu only for a few minutes during each orbit. All the required data has to be downloaded as well as new commands uploaded within this time ! Byalalu is also the location of the Indian Deep Space Network (IDSN), used for planetary missions.

11. Is this India’s first telescope in space ?

Certainly not. High energy astronomy started in India with sounding rockets and balloon launches early on. An X-ray payload was even included in Aryabhata, India’s first satellite that was launched in 1975. The next experiment was a Gamma Ray Burst (GRB) detector on the satellite SROSS-C2 in 1994. This was followed by the Indian X-ray Astronomy Experiment (IXAE) on IRS-P3 in 1996.

These successes and the experience gained in these missions led the Indian community to propose a dedicated observatory like ASTROSAT.


12. How expensive is ASTROSAT ?

The spacecraft and payload development cost approximately 178 crores, excluding the cost of the launch itself.


13. How does ASTROSAT compare with other telescopes ?

The instruments onboard the ASTROSAT have unique capabilities compared to the various international telescopes of the past. The UVIT has a resolution of 3 keV.

An important capability of ASTROSAT is that right from ultraviolet up to hard x-rays, all the telescopes on board can detect individual photons, and not just that, but measure the energy and the time of arrival of every photon for each of the x-ray events !


14. What kind of objects will ASTROSAT look at ?

ASTROSAT can observe any object that emits ultraviolet and x-ray radiation. Since these are high energy wavebands, they are emitted by energetic processes in objects, which are very different from radio or visible emission. Hot young stars and dense compact white dwarfs are bright in the ultraviolet. Spectral lines from gas heated by these stars to about tens of 1000s of Kelvin also falls in this waveband (e.g. planetary nebulae). Million degree hot gas caught in the gravity of clusters of galaxies emit in x-rays. However, the x-ray telescopes will mainly be used to study exotic objects like neutron stars and black holes. Due to their strong gravity, some of these compact objects are able to pull in matter either from a companion star or from the interstellar gas around them. This matter, as it slowly spirals on to the compact objects, gets heated and emits in x-rays. This emission is highly variable in time and ASTROSAT will be used to understand what happens in these very strange sources.


15. What new information will ASTROSAT provide about these objects ?

The data that ASTROSAT provides about these objects will be able to answer many fundamental questions. The ultraviolet data will tell us how massive stars form and evolve in galaxies as well as the nature and distribution of interstellar dust in our own galaxy. Detection and studies of transient sources, i.e., new sources that suddenly become bright and then slowly fade away, is an important area of study. Some of these sources are Gamma Ray Bursts, but some of them are unknown in origin.

Using multi-wavelength observations of neutron stars, we can measure their mass and radii. These measurements alone can tell us what is the possible state of matter deep inside these highly dense stars, something we do not yet know. Spectral features from rapidly rotating neutron stars can provide many clues to the structure of the magnetic fields of these exotic objects, which have the highest magnetic fields in the universe. The spectra as well as the differing nature of variability in different wavebands of stellar mass black holes that undergo outbursts will answer questions about how exactly a black hole accretes matter around it, as well as produce jets. All of these will also provide tests of general relativity. Similarly, the multi-wavelength variability of AGNs will tell us what the structure of the accreting disk is.


16. Can ASTROSAT see black holes ?

In a sense, yes indeed ! Black holes by themselves of course cannot be seen; they have, by definition, too strong a gravity to let light escape from them. How then can we know where they are ? We can locate them by detecting what they do to their surrounding environment.

Matter near a black hole will be attracted by its strong gravity. However, this matter does not directly fall into the black hole. Instead, it slowly spirals into the black hole by forming a flat disk, gradually losing its angular momentum. While doing so, this gas is heated to extremely high temperatures and emits in many wavebands. This radiation is what we use to infer the presence of the black hole in the centre.

17. What are the instruments being flown on ASTROSAT ?

There are five instruments, or telescopes, on the ASTROSAT. These are the

  • UVIT : Ultra-Violet Imaging Telescope
  • LAXPC : Large Area X-ray Proportional Counter
  • SXT : Soft X-ray Telescope
  • CZTI : Cadmium-Zinc-Telluride Imager
  • SSM : Scanning Sky Monitor

Of these, the UVIT, LAXPC, SXT and CZTI are co-aligned and look at the same part of the sky at a given time. The UVIT observes in the near (200-300 nm) and far ultraviolet (130-180 nm) as well as the visible at the same time. The SXT is a focussing telescope between 0.3-8 keV, LAXPC operates from 3-80 keV, whereas the CZTI goes from 10-100 keV.

The SSM is actually three small position sensitive x-ray detectors that continuously patrol the sky, looking for sources that have suddenly become very bright, so that the entire satellite can then be rotated to point the other four major instruments at it.


18. Who built ASTROSAT ?

ASTROSAT was built by a number of institutes in India, all working together. The ISRO Satellite Centre in Bengaluru (ISAC), Tata Institute of Fundamental Research (TIFR) in Mumbai, Indian Institute of Astrophysics (IIA) and Raman Research Institute (RRI) in Bengaluru, Inter University Centre for Astronomy and Astrophysics (IUCAA) and the National Centre for Radio Astrophysics (NCRA-TIFR) in Pune and the Physical Research laboratory (PRL) in Ahmedabad were responsible for developing the various instruments and the scientific data analysis. In addition, the SXT was developed jointly with the University of Leicester in UK and the UVIT was developed jointly with the Canadian Space Agency. The satellite assembly and the rocket itself, of course, were made by ISRO.


19. What were some of the challenges in designing ASTROSAT ?

ASTROSAT is the first major space based observatory that India has built, and hence the project presented challenges. The SXT is the first focussing imager in x-rays that was made in the country. The LAXPC is one of the largest proportional counters in the world, with very high time resolution. The UVIT had to be made and housed in incredibly pure and clean atmosphere to prevent contamination. There is something in each of the instruments that Indian scientists were designing and building for the first time in the country, which is why ASTROSAT is truly a trail-brazer in Indian astronomy instrumentation.


20. Who will be able to use ASTROSAT ?

ASTROSAT is a public observatory and is therefore available for any potential researcher in India and abroad. The first 6 months after launch will be devoted to extensive tests of all the systems. The next 6 months will be for observations of the sky by the teams that built the instruments. However, a year after the launch, a certain fraction of ASTROSAT time will be available for any scientist in India who proposes an observation that passes review. Two years after the launch, international scientists can apply as well. In addition, once an observation is done by ASTROSAT and some time has passed, the entire data will be made public to anyone who is interested !


21. Can I use ASTROSAT myself ?

Yes. A year after launch, when some of ASTROSAT’s time will be available for people in India, there will be periodic calls for proposals. Anybody with a good scientific idea and a plan of observation can propose for time. These proposals will be reviewed by a panel of experts. If your proposal is found to be good, you will be given time to observe with the ASTROSAT !

However, since time of ASTROSAT is precious, only sound scientific proposals that can efficiently utilise ASTROSAT well will be accepted. Various training schools have been, and will continue to be organised for college and university students and faculty to make them potential ASTROSAT users and these schools are a good way to equip yourself scientifically for this task.


22. Where can I see the data and images taken by ASTROSAT ?

All data taken by ASTROSAT will be stored at the Indian Space Science Data Centre (ISSDC) by ISRO. All data taken by ASTROSAT will be made public to everyone after a specific time period. In addition, images taken by the instruments that are exciting and have popular appeal for everyone will be released periodically on the ISRO website.


23. Is there a connection between ASTROSAT and other Indian telescopes ?

Most certainly. Just as having multiwavelength data in the ultraviolet and x-ray will help us decode the nature of celestial objects, combining these with data in the radio, infrared and visible will be even more useful. In addition, when a transient source is detected by ASTROSAT, follow-up observations in the radio and visible bands are essential in understanding the nature of the source. For these reasons, ASTROSAT data will be combined with, for example, radio data from the Giant Metrewave Radio Telescope near Pune, and the infrared-visible telescopes in Devasthal, Mt. Abu and Hanle, as well as telescopes around the world.


24. Why should a developing country like India invest in a space telescope ?

There are many answers to this question. First, every human being, whichever country they live in, has a curiosity about the world around them, and will strive to satisfy this curiosity. Astronomy is probably the oldest science because some of the first questions humans asked were about the earth, the sun and the moon, and the stars above us. Astronomy excites people’s imagination, gives us a sense of something bigger than ourselves and broadens our world view. And every country needs to foster this.

Second, a developing country needs to develop technological and scientific capability parallel with other capabilities. India has had a strong tradition in both high energy experiments as well as satellite launches and ASTROSAT builds on these to achieve a world class experimental facility. The history of science and technology shows us that unforeseen socially and economically useful applications arise out research in apparently unrelated basic sciences. For example, astronomy research has given rise to technologies like wireless, CCDs used in cameras, GPS and many important applications in medical imaging and communication. Any country that invests in basic science invariably reaps a reward in extremely useful technology a few years down the line.

Lastly, in a country with high educational aspirations but a lack of good quality universal science education, projects like ASTROSAT have the potential to excite the imagination of students and teachers alike. It is up to us all now to harness this excitement.


25. Will ASTROSAT benefit science education in schools ?

We certainly hope so ! Any world-class experimental science facility that tries to answer fundamental questions about the world that we live in, has the potential to interest a student in science. Good examples are the moon landing, Voyager’s mission, the Large Hadron Collider, the GMRT, Chandrayaan and Mars Orbiter Mission (MOM) and so on. It is up to us to devise ways by which these projects can be used in our classrooms in an exciting way.

All of the data taken by ASTROSAT as well as the software to analyse this data will be made available to everyone free of cost, through the internet. This data is on young hot stars, neutron stars, black holes, transient objects and so forth. It is expected that there will be tremendous interest in this data. We need to devise simple tasks and procedures for using such data, in order to make it accessible to students as well as teachers. Citizen science projects can be built around these data as well and the possibilities are being actively discussed. Since ASTROSAT is an Indian satellite, we have a higher degree of access to it and it is up to us to utilise this opportunity well.




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