India launches first solar research mission, Aditya-L1, aboard PSLV

by William Graham

India’s space program continued its busy 2023 with the launch of its first solar research mission, Aditya-L1, on Saturday. Liftoff, aboard the country’s Polar Satellite Launch Vehicle (PSLV) XL rocket, occurred at 11:50 Indian Standard Time (06:20 UTC) from the Satish Dhawan Space Centre.

Aditya-L1 will be the first mission conducted by the Indian Space Research Organisation (ISRO) that is dedicated to studying the Sun. Bound for the Earth-Sun L1 Lagrange point, it will also be the first Indian spacecraft to operate at this location. ISRO expects the mission to contribute to global research into the Sun, with a focus on space weather and the solar wind.

Key science objectives for the mission are to help increase understanding of the phenomena of coronal heating, or how the Sun’s corona – its outer atmosphere – has a far higher temperature than the surface of the Sun itself, and study how the corona accelerates the solar wind. Other major objectives include understanding how solar flares and coronal mass ejections are initiated, studying the dynamics of the Sun’s atmosphere, and monitoring the distribution and anisotropy (lack of uniformity by direction) of particles in the solar wind.

Saturday’s launch comes just ten days after ISRO celebrated another major milestone with the successful landing of its Vikram probe on the Moon as part of the Chandrayaan-3 mission. With the agency having already carried out the first successful launch of its new Small Satellite Launch Vehicle rocket earlier this year, and now gearing up for the first uncrewed test flight of its Gaganyaan crew capsule in early 2024, this is an important time for India in space.

Aditya-L1 carries a suite of seven instruments that will be used to study the Sun. The spacecraft was constructed in-house at ISRO and has a mass of 1,475 kilograms. It is expected to operate for at least five years. The main instrument is the Visible Emission Line Coronagraph, which was built by the Indian Institute of Astrophysics to study the Sun’s corona and coronal mass ejection events. It uses an internally occulted reflective coronagraph to block direct light from the Sun itself to allow its detectors – including an imager, a multi-split spectrograph, and a polarimeter, to observe the corona.

The Solar Ultraviolet Imaging Telescope was developed by the Inter-University Centre for Astronomy and Astrophysics. Its purpose is to make observations of the full disc of the Sun at near-ultraviolet wavelengths — allowing for study of the photosphere and chromosphere, as well as measuring fluctuations in the amount of ultraviolet light radiated by the sun.

ISRO’s U R Rao Satellite Centre has contributed a pair of x-ray imaging payloads to the mission: the Solar Low Energy X-ray Spectrometer (SoLEXS) and the High Energy L1 Orbiting X-ray Spectrometer (HEL1OS). These will be used to help study solar flares, with SoLEXS observing soft, or lower-energy, x-ray emissions and HEL1OS focussing on hard, or high-energy, emissions.

Aditya Solar Wind Particle Experiment (ASPEX) was built by the Physical Research Laboratory at Ahmedabad to study the solar wind in an effort to unlock a better understanding of processes within the Sun. It consists of two sensors: the Solar Wind Ion Spectrometer, which will be used to detect and measure protons and alpha particles; and the Suprathermal and Energetic Particle Spectrometer, which will detect higher-energy ions.

Aditya-L1 atop PSLV C57’s fourth stage, shortly before being enclosed in the payload fairing. (Credit: ISRO)

The Plasma Analyser Package for Aditya (PAPA), developed by the Vikram Sarabhai Space Centre’s Space Physics Laboratory, will also contribute to Aditya’s study of the solar wind. PAPA consists of a Solar Wind Electron Energy Probe and Solar Wind Ion Composition Analyser to record the flux, density, and composition of electrons and ions in the solar wind, helping to build a detailed record of how these vary over time.

By studying particles in the solar wind with ASPEX and PAPA, the Aditya mission may be able to validate theories about how wave-particle interactions might play a role in the processes of solar wind acceleration and coronal heating.

The final instrument aboard Aditya, the Magnetometer, will measure the magnetic fields in situ at the spacecraft’s location. Two sets of sensors are mounted on a six-meter deployable boom, with one set at the end of the boom and the other halfway along it, providing two sets of measurements.

ISRO used its PSLV rocket to deploy Aditya-L1 into an initial low-Earth orbit. From here, the spacecraft will, under its own propulsion, perform a series of maneuvers to place itself into a halo orbit around the Earth-Sun L1 Lagrange point. Lagrange points are locations in space where the gravitational pull of two celestial bodies are in equilibrium, allowing objects to remain at, or orbit, the point with minimal station-keeping.

Aditya’s journey to the Earth-Sun L1 point. (Credit: ISRO)

The Earth-Sun L1 point is located directly between the Earth and the Sun, at about 1.5 million kilometers from Earth. Placing solar research missions at L1 ensures their view of the Sun will not be interrupted by Earth while ensuring the spacecraft also remains relatively close to Earth for communications and data downlink.

Upon arrival at the L1 point, Aditya will join four other spacecraft currently operating at L1: the joint NASA-ESA Solar and Heliospheric Observatory, NASA’s Advanced Composition Explorer and Wind missions, and the National Oceanic and Atmospheric Administration’s Deep Space Climate Observatory. Aditya will be the first Indian spacecraft to operate at L1 and is expected to reach its final orbit around the Lagrange point about 125 days after launch.

To deploy Aditya-L1, the most powerful version of the PSLV rocket, PSLV-XL, was employed. This is a four-stage launch vehicle employing both solid and liquid-fuelled propulsion, capable of placing payloads of up to 1,750 kilograms into a 600-kilometer Sun-synchronous orbit or 1,425 kilograms to a geosynchronous transfer orbit.

The rocket that carried Aditya into orbit was designated with flight number PSLV C57, and was the fifty-ninth PSLV rocket to fly. The rocket is ISRO’s workhorse, carrying out the majority of the agency’s satellite launches as well as occasional commercial missions. It first flew in September 1993 and had – prior to Saturday’s launch – carried out 55 of its missions successfully, with two failures and one partial failure due to an off-nominal orbit. PSLV’s last 17 missions have all been successful.

PSLV C57 leaving the assembly building for the launch pad. (Credit: ISRO)

Saturday’s launch will took place from the Second Launch Pad at the Satish Dhawan Space Centre (SDSC) on Sriharikota, a barrier island in the Bay of Bengal. First used in 2005, this launch pad has hosted PSLV, Geosynchronous Satellite Launch Vehicle (GSLV), and GSLV Mk.III (also known as Launch Vehicle Mark 3) launches and is one of two pads available for PSLV missions, along with the nearby First Launch Pad. SDSC has been the site for all of India’s orbital launches to date.

PSLV’s first stage is powered by an S-138 solid rocket motor. Depending on the configuration, up to six PS0M-XL solid rocket motors can be attached as boosters to augment its thrust, with PSLV-XL using the full set of six. Once Saturday’s countdown reached zero, the first stage ignited, followed by the first two pairs of boosters at T+0.42 seconds and T+0.62 seconds. Climbing away from the SDSC, PSLV ignited the final pair of boosters around 25 seconds into flight. Separation of the ground-lit boosters occurred around the 70-second mark, while the air-lit boosters separated about 92 seconds into the mission.

About 108 seconds after liftoff, the first stage exhausted all of its propellant and burned out. The spent stage was jettisoned, and the vehicle’s second stage took over. Designated PS2, or PL40, the second stage is powered by a single Vikas engine burning hypergolic liquid propellants. Its fuel is UH25, a mixture of three parts unsymmetrical dimethylhydrazine to one part hydrazine hydrate, while its oxidizer is dinitrogen tetroxide. This propellant combination can be stored at room temperature and ignite easily without the need for complex ignition systems on the vehicle; however, they are also toxic and highly flammable. The Vikas engine itself is derived from the Viking engine used on early versions of the European Ariane series of rockets.

PSLV’s second stage burn lasts about two and a half minutes. While the second stage is firing, the rocket will enter space, and shortly afterward shed its payload fairing. The fairing, which ISRO calls a “heat shield,” helps to protect the satellite and preserve the rocket’s aerodynamic profile during its ascent through the atmosphere, but is no longer needed once PSLV reaches space and is discarded to reduce the rocket’s weight.

PSLV C57 with Aditya-L1, prior to launch. (Credit: ISRO)

The third stage, or HPS3, ignited shortly after the second stage has shut down and separated. This solid-fuelled stage consists of an S-7 motor burning for approximately 70 seconds. After burnout, the mission entered a coast phase as the rocket climbs towards its apogee – or the highest point on its trajectory. Third stage separation took place during this coast, leaving PSLV’s fourth stage to complete the insertion of Aditya into its initial orbit.

The fourth stage is capable of completing multiple burns to carry out complex satellite deployments, ensuring payloads can be delivered to precise target orbits or, on multi-satellite missions, allowing satellites with differing orbital requirements to be accommodated. This stage, known as PS4, has a pair of small engines burning monomethylhydrazine and mixed oxides of nitrogen (MON-3 – a mixture of 3% nitric oxide and 97% dinitrogen tetroxide). Once the fourth stage has completed its burn, Aditya-L1 will separate to begin its own journey to L1.

(Lead image: PSLV C57 rolling out to the Second Launch Pad. Credit: ISRO)

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