XPoSAT: How ISRO’s Black Hole Mission Will Script A ‘POEM’ In Space, And Why It Is Special

XPoSAT: The X-ray Polarimeter Satellite (XPoSAT), India’s first mission to study black holes and neutron stars, is special for several reasons. XPoSAT is India’s first dedicated polarimetry mission to study the dynamics of bright astronomical X-ray sources in extreme conditions, which means the spacecraft will analyse X-rays emitted by celestial bodies such as black holes, neutron stars, active galactic nuclei, and pulsar wind nebulae by measuring the beams of light in which the vibrations of electromagnetic waves are confined to one plane. Also, XPoSAT will script a ‘POEM’ in space. 

ISRO launched XPoSAT on January 1, 2024, atop a Polar Satellite Launch Vehicle (PSLV), from Satish Dhawan Space Centre, Sriharikota, Andhra Pradesh. The 2024 New Year’s Day launch, called PSLV-C58, is the first orbital launch attempt of 2024, PSLV’s 60th mission, and ISRO’s 92nd mission overall. 

Two scientific payloads onboard XPoSAT will study interesting celestial objects. 

The PSLV-C58 rocket has injected XPoSAT into a 650-kilometre orbit with an inclination of six degrees to Earth. This is an eastward low-inclination orbit. 

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How XPoSAT will script a ‘POEM’ in space

The fourth stage of PSLV, or PS4, has been lowered to a 350-kilometre orbit. In order to take PS4 to a lower orbit, it had to be ignited for a second time. 

PS4 is also called the PSLV Orbital Experimental Module (POEM), and will perform orbital experiments. Since this is the third time ISRO has used PSLV’s fourth stage to exhibit orbital experiments, XPoSAT’s PS4 is called POEM-3. 

ISRO’s novel idea allows the space agency to use a spent PS4 stage to conduct in-orbit scientific experiments in microgravity conditions for an extended duration of four to six months. This is a sustainable practice because otherwise, the stage would have ended up as space debris immediately after the mission was accomplished. 

PS4, the uppermost stage of PSLV, uses two liquid engines for propulsion, and is responsible for the correct injection of the rocket’s payloads into the intended orbits.

POEM, which derives power from the solar panels mounted around the PS4 tank and a Lithium-ion battery, navigates using four Sun sensors, a magnetometer, gyroscopes, and NavIC. A gyroscope is a device used for measuring orientation and angular velocity, and is built into compasses on ships and aircraft. NavIC is the acronym for Navigation with Indian Constellation, and is the operational name for Indian Regional Navigation Satellite System (IRNSS), which is an independent regional navigation satellite system providing accurate real-time positioning and timing services to users in the country as well as regions extending up to 1,500 kilometres from its boundary. 

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POEM carries dedicated control thrusters using Helium gas storage, and is enabled with a telecommand feature.

POEM-3 is equipped with 10 payloads:

• Women Engineered Satellite (WESAT)
• Radiation Shielding Experimental Module (RSEM)
• BeliefSat
• Green Impulse TrAnsmitter (GITA)
• Launching Expeditions for Aspiring Technologies Technology Demonstrator (LEAPTD)
• RUDRA 0.3 HPGP
• ARKA-200
• Dust Experiment (DEX)
• Fuel Cell Power System (FCPS)
• Si-based High-Energy Cell

The LBS Institute of Technology for Women in Thiruvananthapuram has designed WESAT, which will compare solar irradiance, the power per unit area received from the Sun, to ultraviolet index, which refers to the amount of skin-damaging ultraviolet radiation reaching the Earth’s surface at any instant of time. 

RSEM, developed by Bengaluru-based start-up TakeMe2Space, will evaluate the effectiveness of coating made of Tantalum, the 73rd element of the Periodic Table. 

Mumbai-based KU Somaiya Institute of Technology’s BeliefSat is an amateur radio satellite. 

GITA, developed by Thane-based aerospace company Inspecity Space Labs, is a green bipropellant CubeSat propulsion unit. This means that it is a unit which could be used to power a CubeSat, and uses two green propellants. 

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Hyderabad-based Dhruva Space’s LEAPTD will demonstrate a subsystem of a microsatellite. 

Bengaluru-based Bellatrix Aerospace’s RUDRA is a green monopropellant thruster, which means it uses a single, clean propellant. 

The Physical Research Laboratory’s (PRL’s) DEX will measure interplanetary dust count. 

FCPS and the Si-based High-Energy Cell, developed by the Vikram Sarabhai Space Centre (VSSC), will demonstrate the functioning of a fuel cell, and the functioning of a silicon-based high-energy cell, respectively, according to ISRO.

What makes XPoSAT special

XPoSAT is special because it is the second spacecraft in the world which will study celestial bodies using polarimetric techniques, after NASA’s Imaging X-Ray Polarimetry Explorer (IXPE). 

The planned mission duration of XPoSAT is five years. 

XPoSAT carries two payloads: Polarimeter Instrument in X-rays (POLIX), and X-ray Spectroscopy and Timing (XSPECT). 

Bengaluru-based Raman Research Institute has developed POLIX, and UR Rao Satellite Centre’s (URSC’s) Space Astronomy Group has developed XSPECT. 

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POLIX is the primary payload, and will measure polarimetry parameters such as the degree and angle of polarisation in the medium X-ray energy range. In these electromagnetic waves, photons with an energy range of eight to 30 kiloelectronvolts are present. 

POLIX is the first payload to conduct polarimetry measurements in the medium X-ray energy band. 

It will observe about 50 bright astronomical objects of different types. 

XSPECT will analyse spectroscopic information of soft X-rays, or photons of astronomical origin in the energy range of 0.8 to 15 kiloelectronvolts. This means that XSPECT will obtain information related to the absorption and emission of radiation by astronomical objects. 

XSPECT will observe X-ray pulsars, which are rotating neutron stars; black hole binaries, in which two black holes orbit each other; and magnetars, which are exotic neutron stars with extremely powerful magnetic fields. 

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Black holes, neutron stars, active galactic nuclei, and pulsar wind nebulae are created as a result of complex physical processes, and their emission mechanisms are difficult to understand. 

Several space-based observatories have obtained a multitude of data on celestial bodies by understanding spectroscopic and timing information of emissions, but have not provided much details on the exact nature of the emissions. This is where polarimetry comes into the picture. 

Polarimetric measurements will allow astronomers to have a better understanding of the nature of these emissions. 

Overall, XPoSAT will analyse about 50 different types of cosmic wonders. 

XPoSAT has a unique set of science objectives.

The satellite will study the distribution of magnetic fields emitted by astronomical objects, measure geometric anisotropies, or different values of a particular parameter in different directions, understand the structure of black hole binaries, analyse supernova remnants, and confirm the production of X-rays from neutron stars and pulsars.

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