The Indian Space Programme

by Gurbir Singh

  Mars Exospheric Neutral Composition Analyser

  MENCA was built at the Space Physics Laboratory located within the VSSC complex. At 3.56 kg, it is the heaviest of the five instruments and based on the CHACE (fitted to the MIP) used to analyse the contents of the tenuous lunar atmosphere as the MIP descended from orbit to surface impact.

  MENCA is designed to collect data on how far the outer Martian atmosphere (known as the exosphere) extended and what it was made of. It is unique in that it does not analyse electromagnetic emissions or reflections from Mars but particles that make up the exosphere in-situ. Taking advantage of the highly elliptical orbit, MENCA samples its environment five times along each orbit and build up a radial profile of the composition of the exosphere between 400 km and 77,000 km. Over time, MENCA will build a profile of Mars’ exosphere in altitude, daily and seasonal variations. It will also be used to study the environment of Phobos during its encounters. During the development stage, MENCA was repeatedly tested and calibrated. The vacuum chamber used to simulate space-like environment has a built-in facility through which known gases can be introduced at a controlled rate. Also, a reference mass spectrometer is used to ensure that MENCA calibration is consistent with a known source. MENCA was calibrated on three separate occasions at the VSSC where it was developed. The first instance was before testing and evaluation, second after testing and evaluation and a third just before it was transported from VSSC to ISAC in Bangalore for integration into the spacecraft.

  MENCA was activated to test functionality while MOM was still in Earth orbit and later during the cruise to Mars. It was first operated in Martian orbit on 29 September 2014 just five days after arrival. MOM’s orbit was temporarily lowered to 260 km to collect data from four orbits during late December 2014. These measurements allow scientists to understand why Mars’ atmosphere is so tenuous, about 1% that of Earth’s, and how the current carbon dioxide rich constitution came about. They also allow scientists to model the mechanism and the rate at which these molecules escape from Mars’ upper atmosphere.[898] Data from MENCA is formatted to Planetary Data System (PDS) standard and archived at the ISRO Space Science Data Centre.

  Mission Status

  India’s first mission to Mars, which was developed on a very short time scale, has succeeded in every respect. It entered orbit as planned, all onboard instruments have been returning data. MOM has survived a communication blackout and a whiteout when planetary alignment forces MOM to fend for itself without any assistance from ISRO. A blackout (Earth–Sun-Mars), is a period when the Sun and Earth appear in the same direction and ISRO cannot discern the weak radio signals from Mars are drowned out by the closer Sun. A little like listening to a phone call in a noisy room. The maximum duration of the blackout in June 2015 was 17 days (between 6 – 22 June). A whiteout (Sun–Earth–Mars) is a period when the Sun and the Earth appear in the same point in the Martian sky, and MOM cannot discern the weak radio signals from Earth because of the interference from the Sun. The maximum duration of a whiteout was been around 14 days during 16 – 29 May 2016.

  Figure 15‑8 Mars full disc captured by Mars Colour Camera from an

  altitude of 66543 km in October 2014. Credit ISRO

  As the first mission to Mars, ISRO had designated MOM as a technology demonstrator with an operational period of 6 months. Once the initial six months of operations were complete, ISRO declared this primary mission a success. Since then, MOM has been operating in an open-ended extended mission phase. MOM completed its 100th orbit on 22 June 2015. With ample reserves of fuel, the data that the onboard instruments it continues to return data. By August 2016, MOM had generated six peer-reviewed publications, and more results will be published following the Announcement of Opportunity making MOM data available to researchers.[899] Data from MOM’s instruments can be used as an independent source to support scientific observations and conclusions made by other spacecraft.

  MOM remains healthy as a spacecraft, and each of the five onboard instruments continues to observe Mars and return data from orbit. When MOM was launched, its mere five instruments were not expected to make any ground-breaking scientific discoveries. After all, what could MOM achieve with its collection of meagre instruments that has eluded the American and European missions that have been scrutinising Mars for decades with higher specification instruments? A possible, although not probable, achievement would be if a detection of methane by the methane sensor were linked to a surface feature imaged by the colour or thermal camera helping to identify a specific geographic location as a methane source. Those sites could then be scrutinised by future missions for the potential Martian microbes on the surface of Mars. The mission, spacecraft and all onboard instruments have enjoyed remarkable success. However, a risky and hazardous scenario could have ended MOM mission in February 2017. In its then orbit, MOM would have entered a series (in consecutive orbits) of solar eclipses that would have lasted for up to eight hours.

  These eclipses would have coincided with MOM’s orbit at apogee, where it moves the slowest and thus would have stayed in the shadow longer. With the solar panel as the only power source, the battery would have drained completely, raising for the first time the prospect of MOM shutting down since leaving Earth, even though it still had about 30 kg of propellant, sufficient for several years of operation. On 17 January 2017, ISRO manoeuvred MOM into a new orbit that not only avoided the February 2017 eclipses altogether but also extended its life. In the words of ISRO chairman Kiran Kumar “Because of the crucial orbital change, the MOM now gets three additional years’ life. We are expecting it to transmit data till 2020.”[900]

  The new orbit will reduce the time spent in the shadow by about half for these and future eclipses. Following this manoeuvre, MOM still has 13 kg of fuel, which will allow it to continue to gather data into the next decade. In June 2017, MOM completed 1000 days in Martian orbit, well beyond the intended 3 months.

  Astrosat - Astronomy from Orbit

  Observational astronomy using telescopes has been conducted from India since the 17th century by gifted observers, such as Father Eugène Lafont and C.V. Raman. They observed from places, such as Calcutta and Trivandrum, and official observatories, such as Kodaikanal and Madras. However, some types of astronomical observations cannot be made from India or anywhere else on the surface of the Earth. The history of science illustrates that new instruments or vantage point give rise to the tantalising possibility of new scientific discovery. For ISRO, Astrosat would be a fresh new vantage point.

  Although we can see stars in the night sky, most of the electromagnetic radiation coming to Earth from space is blocked by the atmosphere. Only visible light and some ultraviolet, infrared, and short-wave radio can make it from space to the surface of the Earth. When scientists want to study other wavelengths including X-rays, they have to get their instruments into space.

  Figure 15‑9 Electromagnetic Spectrum. Credit NASA

  The first X-rays from the sun were detected by a V2 missile launched in the US in 1948.[901] This was one of about 100 V2s that were recovered from Germany by the US in the few weeks after the end of World War II. In 1962, a sounding rocket accidentally detected the first X-ray source beyond the solar system.[902] India’s first satellite Aryabhata, launched in 1975, carried a science payload that collected X-ray data from several cosmic sources, including a black hole. About a decade later, an experiment (Anuradha) to study low energy cosmic rays was designed, built and tested in India. It operated successfully between 29 April and 6 May 1985 on-board Spacelab-3 within the Space Shuttle Challenger.

  During this time, the experiment recorded 10,000 alpha particle events, a similar number of heavier ions and about 15,000 galactic cosmic rays.[903] An X-ray experiment dubbed Indian X-ray Astronomy Experiment as part of Indian Remote Sensing Satellite (IRS-P3) payload and operated between 1996 and 2004. It collected data to generate a light curve of variation in X-ray sources. It also observed binary and intense X-ray sources. On 28 September 2
015, India launched its first dedicated astronomy satellite to observe the cosmos using wavelengths that are otherwise blocked by the atmosphere. Astrosat carries six instruments looking at a range of frequencies accessible from space. Canada and the UK contributed, but Astrosat is predominantly an Indian mission with input from six Indian institutions.[904]

  With a total mass of 1,513 kg at launch with 42 kg of propellant, Astrosat is designed to operate for a minimum of five years in a 650-km equatorial orbit inclined by 6°.

  Figure 15‑10 Astrosat and its Science Payload. Credit Adapted from ISRO

  The orbit and inclination were selected to allow Astrosat to avoid an unusual phenomenon known as the South Atlantic Anomaly (SAA). It is a section of an inner Van Allen radiation belt that surrounds the Earth but dips to lower altitudes over the South Atlantic. The SAA is a high radiation hazard for all spacecraft in LEO that must travel through it. The Hubble Space Telescope observations are paused during this time amounting to a loss of up to 15% of observing time.

  The ISS crew also take precautions, and spacewalks are not scheduled during these times. High energy radiation (UV, X-rays and gamma-rays) to which Astrosat instruments are tuned are produced during some of the most energetic events in the cosmos. During its five years, Astrosat will look at individual hot, bright stars, very small but extremely dense neutron stars, rotating neutron stars, supernovae, black holes and supermassive black holes at the centres of galaxies.[905] Astrosat will investigate these phenomena with its six instruments built mostly, but not exclusively, in India.

  Instrument

  Origin

  Purpose

  Charged Particle Monitor (CPM)

  TIFR

  Not strictly a science instrument. Designed to prevent damage to Astrosat’s sensitive sensors by intense radiation, the CPM is constantly monitoring the radiation in Astrosat’s environment. When it detects excessive radiation from the Sun, Moon and Earth, it places other instruments in a safe mode.

  Scanning Sky Monitor (SSM)

  ISAC and Inter University Centre for Astronomy and Astrophysics

  Detect intergalactic X-rays (2.5–10 keV) and map their sources in the cosmos.

  Large Area X-ray Proportional Counters (LAXPC)

  TIFR

  Similar to SSM but LAXPC detects X-rays of higher energy (3–80 keV). It is located on a platform that allows it to track an object for greater detail

  Ultraviolet Imaging Telescope (UVIT)

  ISRO, TIFR, Indian Institute of Astrophysics, and Canadian Space Agency.

  This instrument consists of two telescopes, one sensitive to the visible spectrum (visible 320–550 nm) and the other Near Ultraviolet (NUV 200–300 nm).

  Soft X-ray Telescope

  TIFR and University of Leicester, the UK

  Designed to observe low energy (0.3–8 keV) X-ray sources.

  Cadmium Zinc Telluride Imager (CZTI)

  TIFR and IUACC

  A special sensor designed to detect hard (10–100 keV) X-rays and gamma-rays

  Table 15‑6 Overview of Astrosat’s Science Payloads

  Four of the instruments UVIT, CZTI, LAXPC and SX are located on the same side, so they point in the same direction. The SSM on a rotating platform scans the sky looking for sporadic X-ray sources. The role of the last instrument, the CPM, is to monitor the radiation in the environment where Astrosat operates to help ensure that Astrosat subsystems and instruments avoid damage from charged particles, predominantly in the SSA. In this sense, it is not a science instrument like the others.

  Charged Particle Monitor

  Astrosat will avoid the SAA completely in about a third of its orbits. During other orbits, it will spend about 15 to 20 minutes of its 97-minute orbit in the SSA. The CPM detects this and other radiation hazards and shuts down or suspends instruments to prevent damage. The intensity and distribution of the charged particles in the SAA are well known but can dynamically change because of solar flares or coronal mass ejection events. These changes are unpredictable, so too is the precise time Astrosat enters and leaves the SAA. The CPM is designed to detect the actual radiation levels in real-time to allow Astrosat to enter a temporary safe mode if required. The CPM was the first sensor to be switched on, a day after launch. Although its primary function is to ensure that Astrosat runs efficiently and safely, its data will help refine the SAA model, making it scientifically valuable. Unlike the other instruments, CPM was not a proposal-based instrument. It was developed by TIFR, and it runs at all times.

  Scanning Sky Monitor

  The SSM is located on a rotating platform looking for transient X-ray sources in the energy range of 2.5–10 keV. It was developed by ISAC and the Inter University Centre for Astronomy and Astrophysics and weighs 75.5 kg. The high energy of X-rays cannot be focused using lenses or mirrors, so a different detection mechanism is used. The SSM instrument consists of three almost identical separate gas-filled containers with a total area of 180 cm2. An incoming X-ray generates a voltage within the container by ionising the gas (mostly argon, some xenon and a tiny bit of methane).[906] While the detection principle is similar to that used in the LAXPC, the SSM is designed to record directional information of incoming X-rays. The voltage pulse, along with the time and position within the container, and the orientation of the rotating platform at the time of detection are used to identify in the source in the sky generating the X-rays. In addition to the scanning mode, the SSM can operate in a stare mode to examine specific objects of interest. For example, on 26 October 2015, the day it was first switched on, first it detected during the scanning mode and then ‘stared’ at the neutron star binary pulsar 4U0115+63. The SSM is highly sensitive. It has a restricted field of view and must avoid bright objects, including the Earth, Moon, Sun and zodiacal lights (a collection of dust within the ecliptic that is lit up by the Sun). With a field of view of 22° by 100° at any one time, the SSM can monitor half of the sky facing away from the Sun about four times a day.

  Large Area X-ray Proportional Counters

  The Large Area Proportional Counters (LAXPC) instrument is similar to the SSM, but instead of operating on a rotating platform and scanning the sky for transient and random X-ray events, LAXPC has a field of view of 0.9° by 0.9° and targets individual objects. It is collocated on a platform with three other instruments that can track a specific object in the sky. LAXPC detects X-rays of energies (3–80 keV) higher than SSM (2.5–10 keV) and also uses containers of gas where X-rays trigger a voltage pulse, its amplitude proportional to the energy and number of X-rays detected.[907] The three identical LAXPC containers are rectangular (120 cm x 50 cm x 70 cm) in shape. At a total area of 6,000 cm2 these are currently the largest LAXPCs operating in space. LAXPC can detect X-rays at a very short timescale, in the order of a few milliseconds.

  At 419 kg, the LAXPC is the heaviest instrument on Astrosat. Despite the large collecting area, LAXPC requires a day or two of collecting data to complete an observation of a single source. The high energy X-rays LAXPC is tuned to detect are not emitted by the Earth or the Moon, so it has only to avoid the Sun (by at least 30°) when observing. The higher energy X-rays in LAXPC’s range allow it to observe some of the most energetic events in the cosmos. It will search for black holes with mass in the order of magnitude as the Sun, as well as supermassive galaxies that are nine orders of magnitude more massive. It will also study pulsars, quasars, neutron stars and violent events deep in the nucleus of active galaxies.

  The unique feature of Astrosat is the ability for several instruments to observe a single object at the same time. Observation by one instrument can initiate a more detailed observation by another. For example, LAXPC could observe in detail bursts and flares that were initially detected by the SSM. On 19 October 2015, when it was first switched on, it observed a dark area of the sky before moving to a known stable source used for calibration, the Crab Nebula. By chance, these initial observations helped to identify a small drift of the satellite (0.25° per hour), which the e
ngineers were able to fix easily.[908] The LAXPC counters were activated on 20 October 2015 with the voltage gradually increased to the high voltage required for operation. On 24 October 2015, LAXPC observed another well-known source GRS 1915+105 which further aided calibration.

  Soft X-ray Telescope

  Unlike light, X-rays are not reflected by mirrors, so an X-ray telescope uses a different technique to construct an image. Most X-rays are either absorbed by or penetrate glass, but if the angle is shallow (a grazing angle), then X-rays are reflected but only at a shallow angle. Astrosat’s SXT is based on this technique and uses two reflections down the side of the telescope tube lined with conical reflecting surfaces (0.2 mm aluminium plated with gold) rather than a mirror. This approach only works for lower energy (0.3–8 keV) X-rays, otherwise known as soft X-rays. The SXT has a diameter of 38 cm and a length of nearly 250 cm. The reflected X-rays are captured on a sensitive CCD at the prime focus operating at a very low temperature of -80°C to minimise background noise. SXT avoids the Sun (45°) and the Earth (6° the upper atmosphere can generate X-rays as a result of the solar wind) but is not concerned with avoiding the Moon since it is not a source of X-rays. TIFR developed the telescope. The CCD detector was provided by the University of Leicester in the UK.