International Space Agencies – Missions and Discoveries

Polar-Areas Stellar-Imaging in Polarisation High-Accuracy Experiment (PASIPHAE)


From UPSC perspective, the following things are important :

Prelims level : PASIPHAE

Mains level : Theory of expansion of the universe

The development of a vital instrument PASIPHAE, which will be used in upcoming sky surveys to study stars, is being led by an Indian astronomer.


  • PASIPHAE stands for Polar-Areas Stellar-Imaging in Polarisation High-Accuracy Experiment.
  • It is an international collaborative sky surveying project. Scientists aim to study the polarisation in the light coming from millions of stars.
  • The name is inspired by Pasiphae, the daughter of Greek Sun God Helios.
  • The survey will use two high-tech optical polarimeters to observe the northern and southern skies, simultaneously.
  • It will focus on capturing starlight polarisation of very faint stars that are so far away that polarisation signals from there have not been systematically studied.
  • By combining the data, astronomers will perform a maiden magnetic field tomography mapping of the interstellar medium of very large areas of the sky using a novel polarimeter instrument known as WALOP.

Why is PASIPHAE important?

  • Since its birth about 14 billion years ago, the universe has been constantly expanding, as evidenced by the presence of Cosmic Microwave Background (CMB) radiation which fills the universe.
  • Immediately after its birth, the universe went through a short inflationary phase during which it expanded at a very high rate before it slowed down and reached the current rate.
  • However, so far, there have only been theories and indirect evidence of expansion associated with the early universe.
  • A definitive consequence of the inflationary phase is that a tiny fraction of the CMB radiation should have its imprints in the form of a specific kind of polarisation (known scientifically as a B-mode signal).
  • All previous attempts to detect this signal met with failure mainly due to the difficulty posed by our galaxy, the Milky Way, which emits copious amounts of polarized radiation.
  • Besides, it contains a lot of dust clouds that are present in the form of clusters. When starlight passes through these dust clouds, they get scattered and polarized.

What will PASIPHAE do?

  • The PASIPHAE survey will measure starlight polarisation over large areas of the sky.
  • This data along with distances to the stars will help create a 3-Dimensional model of the distribution of the dust and magnetic field structure of the galaxy.
  • Such data can help remove the galactic polarized foreground light and enable astronomers to look for the elusive B-mode signal.

What is WALOP?

  • Wide Area Linear Optical Polarimeter (WALOP) is an instrument when mounted on two small optical telescopes, that will be used to detect polarized light signals emerging from the stars along high galactic latitudes.
  • The images will simultaneously have the finest of details of a star along with its panoramic background.
  • WALOP will operate on the principle that at any given time, the data from a portion of the sky under observation will be split into four different channels.
  • Depending on the manner in which light passes through the four channels, the polarisation value from the star is obtained.
  • That is, each star will have four corresponding images which when stitched together will help calculate the desired polarisation value of a star.

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