XPoSat mission explained

"India’s first dedicated polarimetry mission to study various dynamics of bright astronomical X-ray sources in extreme conditions."

“Exploration is wired into our brains. If we can see the horizon, we want to know what's beyond.” - Buzz Aldrin

Greetings, fellow Bohron

Happy New Year! What an amazing start to 2024 for India. Indian Space Research Organization (ISRO) successfully launched its XPoSat mission on New Year's Day. You must have heard about it in the news and might be interested in diving deeper into its technical details. But you might be also thinking, why is this mission needed anyway? Let’s begin from there.

PSLV-C58 lifts off from Satish Dhawan Space Center. Sriharikota at 9:10 a.m. on January 1, 2024. Source

Why is XPoSat important?

In 2021, NASA launched Imaging X-ray Polarimetry Explorer (IXPE). Its objective was to study X-ray cosmic sources like active galactic nuclei, microquasars, X-ray binaries, supernova remnants, pulsars and magnetars. XPoSat will also study 50 such brightest known sources in the universe. So it will complement the efforts of IXPE.

Analysis of the polarized radiation reveals plenty of information about the nature of its source like the strength and configuration of the magnetic field around it as well as its geometry and the X-ray production mechanisms.

Illustration of NASA’s IXPE. Source

Now that we know the motive behind this mission, let’s explore it in detail.

What is XPoSat?

XPoSat, short for X-Ray Polarimeter Satellite, is an ISRO space observatory launched on 1 January 2024. Its main objective is to search and capture X-ray emissions from extreme celestial sources like pulsars, black holes and neutron stars to study the nature of these sources. It’s expected to be operational for five years.

Detailed diagram of XPoSat showing various components. Source

What is PSLV C-58?

Polar Satellite Launch Vehicles (PSLVs) are a family of third-generation launch vehicles developed by ISRO. PSLV C-58 was the 60th flight in ISRO’s PSLV program that carried with it XPoSat and a variety of other payloads.

PSLC C-58 mounted on the launchpad. Source

Payloads of XPoSat

XPoSat has carried with it two payloads to conduct various scientific experiments:

POLIX

The POLarimeter Instrument In X-rays is developed by the Raman Research Institute, Bengaluru. POLIX is responsible for measuring the polarisation of X-rays in the energy band 8-30keV emitted by nearly 50 cosmic sources.

POLIX payload. Source

XSPECT

X-ray SPECtroscopy and Timing payload is developed by the Space Astronomy Group of U.R. Rao Satellite Centre, Bengaluru. XSPECT will perform spectroscopic analysis of X-rays emitted by cosmic sources within the energy band 0.8-15 keV.

XSPECT Payload. Source

Besides, POLIX and XSPECT will also carry out polarisation and spectroscopic measurements of X-ray emissions from cosmic sources in the common energy band (8-15 keV).

How is a rocket launched into space?

Modern launch vehicles employ the multistage technique to inject payloads into their intended orbits. This fundamental approach to launch vehicle design was proposed by French civil engineer Louis Damblanc in 1936.

A multistage rocket is a launch vehicle that uses two or more rocket stages, each with its own engine and propellant.

A multistage rocket is basically two or more rockets stacked upon each other. When a rocket runs out of its fuel, it is detached from the rest of the system. This marks the completion of a stage. The ejection of stages helps decrease the mass of the remaining rocket, increasing the upward thrust. This way, the rocket can achieve its desired final velocity and altitude.

An illustration showing various stages of separation in a multistage rocket. Source

How does PSLV C-58 work?

PSLV C-58 is a four-stage multi-rocket. PS4, the fourth and final stage, used two liquid engines for propulsion. Why two engines? This is because the rocket also carried ten other payloads besides XPoSat. These ten payloads are part of the PSLV Orbital Experimental Module-3 (POEM-3) experiment which will be carried out at an altitude of 350 km. However, the altitude requirement of XPoSat is 650 km. So when XPoSat was ejected after reaching this altitude, the PS4 stage was restarted to reduce the orbit to POEM-3’s altitude.

XPoSat and POEM-3 accommodation inside the PS4 module of PSLV C-58. Source

I hope you have some insight into the ins and outs of this mission. I'll leave you with an analogy:

The restarting strategy of PS4 is similar to what a player does in the Flappy Bird game. To avoid hitting a pipe, we have to lift our finger from the spacebar time and time again. In a sense, we are restarting a bird’s engine!

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Sources:

1. XPoSat - Wikipedia.

2. Revised XPoSat Mission Brochure. - ISRO

3. PSLV - ISRO.

4. PSLV-C58 - Wikipedia.

5. Multistage rocket - Wikipedia.

6. IXPE - NASA.

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