India’s first dedicated astronomy satellite, AstroSat, was launched into a low Earth orbit on Monday atop the country’s Polar Satellite Launch Vehicle. Along with six foreign satellites, AstroSat lifted off from the Satish Dhawan Space Centre, Sriharikota, at 10:00 local time (04:30 UTC).
India’s AstroSat spacecraft is the country’s first spacecraft dedicated to astronomical research – although India has previously conducted astronomical studies through payloads hosted on its other satellites and multi-purpose spacecraft such as its first satellite, Aryabhata, which carried an astronomical payload in addition to technology demonstration and ionospheric research objectives when it launched forty years ago.
Development of AstroSat began in 2004, with the satellite then scheduled for a 2007 launch.
A multi-spectral astronomical observatory to be operated by the Indian Space Research Organisation (ISRO), AstroSat carries a suite of five instruments to make observations at optical, near-ultraviolet and x-ray wavelengths.
Designed for a five year mission the satellite has five mission objectives: to help scientists develop an understanding of highly energetic physical processes at work in binary systems consisting of a star and either a neutron star or a black hole; to determine the properties of magnetic fields produced by neutron stars; to study regions of stellar birth in other galaxies; to detect new short-lived interstellar x-ray sources and to conduct a partial ultraviolet deep-field sky survey.
The 1,470-kilogram (3,241 lb) AstroSat spacecraft carries 43 kilograms (95 lb) of fuel for a total launch mass of 1,513 kilograms (3,336 lb).
Powered by a pair of 36 amp-hour lithium ion batteries the satellite generates electricity through two solar arrays producing a total power of up to 2.1 kilowatts. Attitude control is provided by magnetorquers, reaction wheels and eight monopropellant thrusters.
The thrusters, which each produce a force of 11 Newtons (2.5 lb), use hydrazine propellant and also provide the satellite with the ability to manoeuvre in orbit.
The five astronomical instruments aboard AstroSat are the Ultraviolet Imaging Telescope (UVIT), the Large Area X-ray (or Xenon) Proportional Counter (LAXPC), the Soft X-ray Telescope (SXT), the Cadmium Zinc Telluride Imager (CZTI) and the Scanning Sky Monitor (SSM).
The UVIT instrument consists of two telescopes which can observe targets at optical and both near and far ultraviolet wavelengths. The telescopes feed photon-counter imagers for each wavelength, allowing simultaneous observations, with different filters available to be applied to each channel. The instrument has an imaging resolution of 1.5 to 1.8 arc seconds.
Mounted on the top surface of the spacecraft, along with UVIT, the Soft X-Ray telescope was developed by the Tata Institute of Fundamental Research (TIFR). Intended to observe soft X-rays – less energetic waves than their hard x-ray counterparts – the instrument can observe particles with energies of between 300 and 8,000 electron volts. The telescope has a field of view of 40 arcminutes, with a resolution of three arcminutes.
LAXPC is a large-area, low-resolution instrument intended to detect incident x-rays with energies of 3 to 80 kiloelectron volts, allowing a study of how these emissions vary between different X-ray sources. The Cadmium Zinc Telluride Imager is intended to detect hard X-rays – a more energetic type of X-ray usually associated with deep-sky origins.
It is a higher-resolution detector than LAXPC, whose observations it is intended to compliment, and will also be used for gamma-ray burst detection.
The Scanning Sky Monitor (SSM) will be used to conduct a survey of X-ray sources in binary star systems, monitoring the systems for changes throughout the life of the mission. It will also be used to watch for bright but short-lived X-ray sources which can then be targeted for further observation using the spacecraft’s other instruments.
In addition to AstroSat, Monday’s PSLV launch carried six small satellites into orbit: LAPAN-A2 for Indonesia’s Lembaga Penerbangan dan Antariksa Nasional, or National Institute for Aeronautics and Space; exactView-9 for Canada’s exactEarth and four Lemur-2 satellites for Spire Global of the United States.
The primary objective for all six of these satellites is maritime surveillance, with all of the spacecraft carrying Automatic Identification System (AIS) equipment to collect and relay tracking data and distress signals from ships at sea.
The four Lemur-2 satellites, named Joel, Peter, Jeroen and Chris, are the first part of a much larger constellation intended to aid maritime safety by relaying AIS signals and collecting meteorological data.
Each spacecraft is a three-unit CubeSat with a mass of around four kilograms (9 lb), carrying two payloads named SENSE and STRATOS. SENSE is the spacecraft’s AIS receiver, while STRATOS monitors the occultation of signals from GPS satellites as the pass through Earth’s atmosphere in order to infer atmospheric pressures, temperatures and humidities.
Spire Global, at the time known as Nanosatisfi, launched the prototype Lemur-1 spacecraft atop a Dnepr rocket last June, with the final Lemur-2 constellation expected to reach 50-100 spacecraft.
The exactView-9 spacecraft is a 5.5-kilogram (12 lb) nanosatellite, built by the University of Toronto’s Institute for Aerospace Studies (UTIAS), which is based on the University’s Generic Nanosatellite Bus. UTIAS constructed the spacecraft and arranged launch services with ISRO under its Nanosatellite Launch Services (NLS) programme, with the launch designated NLS-14 under their programme.
In addition to its AIS payload, LAPAN-A2 carries amateur radio equipment and cameras for Earth observation, following on from the earlier LAPAN-Tubsat – later renamed LAPAN-A1 – which operated from 2007 until 2013.
AstroSat was launched atop a Polar Satellite Launch Vehicle (PSLV), flying the PSLV-XL configuration. Equipped with elongated boosters to augment the first stage, the XL variant is more powerful than the standard PSLV allowing it to carry heavier payloads into orbit.
A third configuration, the PSLV-CA (Core Alone) omits the boosters entirely and is used for smaller payloads. The PSLV which will carry AstroSat into orbit is PSLV-C30; the thirty-first PSLV to fly and India’s forty-eighth orbital launch to date overall.
Monday’s launch made use of the First Launch Pad at ISRO’s Satish Dhawan Space Centre on Sriharikota Island. Constructed in the 1990s for the PSLV when it entered service, the First Launch Pad replaced now-disused pads to the south of the modern launch complexes that were used for India’s earlier rockets – the Satellite Launch Vehicle and Augmented Satellite Launch Vehicle.
When launching from the First pad, the PSLV is assembled vertically on the launch pad using a mobile tower which is then rolled away from the vehicle. By contrast rockets launched from the nearby Second Launch Pad are assembled vertically away from the pad and rolled to the launch complex atop a mobile launch platform.
The PSLV is a four-stage rocket, with six PS0M-XL boosters strapped to the first stage to provide additional thrust. Powered by S-12 solid rocket motors, four of these strapons ignited in pairs 0.42 and 0.62 seconds after the PS1 first stage’s S-138 solid-fuelled motor, which itself fired upon the countdown reaching zero.
Following ignition of the strapon motors, the rocket lifted off and began climbing away from Sriharikota. The remaining pair of solid rocket motors ignited 25 seconds after liftoff.
The first pair of ground-lit solid rocket motors burnt out and separated 69.9 seconds into the PSLV’s flight, with the second pair falling away two tenths of a second later. The air-lit motors separated from the rocket 92 seconds into the flight.
Following the separation of the solid rocket motors, the first stage continued to burn until the one minute, 52-second mark in proceedings, at which point it burnt out and separated with the second stage firing two tenths of a second later to continue powered flight.
The second stage of the PSLV is the PS2; a liquid-fuelled stage powered by a Vikas engine that burns a mixture of unsymmetrical dimethylhydrazine (UDMH) and hydrazine hydrate, known as UH-25, with dinitrogen tetroxide used as an oxidiser.
The Vikas engine is a licence-produced derivative of France’s Viking engine, used by the Ariane family of rockets between 1978 and 2004. Around 57 seconds into the second stage’s burn, the payload fairing separated from around the satellites at the nose of the rocket.
Now at an altitude of 114 kilometres (71 miles), the rocket no longer required this component to shield the satellites as it will have climbed above the atmosphere.
Second stage flight lasted from the stage’s ignition until its separation two minutes and 30.79 seconds later, with the stage’s engine shutting down shortly before the spent stage was jettisoned. The solid-propellant third stage, or PS3, ignited its S-7 motor 1.19 seconds after staging to begin a 70-second burn.
A coast phase followed third stage flight, with the third stage remaining attached for approximately four minutes and 13 seconds after burnout. After staging the coast continued for a further seven minutes and ten seconds before fourth stage ignition at 16 minutes, 57 seconds mission elapsed time.
The liquid-fuelled fourth stage, or PS4, is powered by two L-2.5 engines which burn monomethylhydrazine (MMH) and mixed oxides of nitrogen (MON-3). It performs injection of the payloads into their target orbits, providing the final thrust to reach their desired trajectories.
During Monday’s mission with PSLV C30 it made a four-minute, 59-second burn to achieve AstroSat’s planned circular orbit at an altitude of 650 kilometres (404 miles, 351 nautical miles) and an inclination of six degrees.
Separation of the AstroSat spacecraft came 22 minutes, 32.92 seconds after liftoff; 37 seconds after the end of powered flight. Thirty seconds later, LAPAN-A2 was deployed with exactView-9 following twenty five seconds later.
The first pair of Lemur spacecraft were separated thirty five seconds after exactView; the third spacecraft deploying ten seconds later and the fourth twenty seconds after that. The fourth Lemur was the final spacecraft to separate from the PSLV, completing the launch.
Monday’s launch was the fourth of five orbital missions which India has planned for 2015, with a further PSLV launch scheduled for December to deploy the TeLEOS-1 remote sensing satellite.
The third PSLV launch of 2015, the AstroSat launch comes a month after ISRO’s successful launch of GSAT-6 using its larger Geosynchronous Satellite Launch Vehicle, which the agency now stands ready to declare operational for the second time – the rocket having experienced four consecutive launch failures and a redesign of its upper stage since its first operational flight in September 2004.
The launch of AstroSat was the fifty-fourth orbital launch of 2015 worldwide – including Proton and Falcon 9 launches which failed to achieve orbit and a Vega mission in February which achieved orbit during testing at the end of a successful suborbital mission and was subsequently not catalogued.
Before December’s PSLV launch, ISRO will be involved in another launch, with the European Ariane 5 rocket lofting India’s GSAT-15 communications satellite in early November.