PRATUSH Telescope

PRATUSH, short for Probing ReionizATion of the Universe using Signal from Hydrogen, is a proposed Indian space mission designed to explore the Cosmic Dawn — the epoch when the first stars and galaxies formed, ending the Universe’s “Dark Ages.” The mission is being developed by the Raman Research Institute (RRI), Bengaluru, in collaboration with the Indian Space Research Organisation (ISRO). It represents a major step in India’s efforts to study the early Universe using advanced space-based radio astronomy.

Background and Purpose

After the Big Bang, the Universe expanded and cooled, eventually forming neutral hydrogen gas. For hundreds of millions of years, this hydrogen-filled Universe remained dark until the first luminous sources — stars and galaxies — ignited, emitting radiation that began to ionise the hydrogen around them. This transformation period is known as the Epoch of Reionisation (EoR).
The PRATUSH mission aims to detect the faint radio signals emitted by neutral hydrogen from this early era, particularly the 21-centimetre spectral line, which is a powerful tool for tracing the conditions of the Universe before and during the formation of the first cosmic structures. By measuring this signal, PRATUSH will provide new insights into when and how the first sources of light emerged.

Mission Concept and Design

PRATUSH is conceived as a space-based radio telescope designed to observe the redshifted 21-cm line of hydrogen across a wide range of radio frequencies. Because the signal from the early Universe is extremely weak and easily masked by terrestrial interference, PRATUSH will operate from lunar orbit or possibly the far side of the Moon, a region naturally shielded from Earth’s radio emissions.
Key design elements include:

• Instrument Type: A radio radiometer optimised for detecting the global (sky-averaged) signal rather than creating detailed maps.
• Frequency Range: Approximately 30–250 megahertz, corresponding to hydrogen signals from redshifts between about 6 and 40, covering the period from the Cosmic Dark Ages to the Epoch of Reionisation.
• Orbit and Platform: Planned deployment in lunar orbit or on a lunar far-side lander to ensure a radio-quiet observational environment.
• Payload and Mass: A compact and lightweight payload (less than 100 kilograms), incorporating an ultra-stable receiver, frequency-independent antenna, and onboard data processing system.
• Data Processing: Use of advanced digital correlators and a low-power, high-performance computer to process signals in situ before transmission to Earth.

Scientific Objectives

The main scientific goals of PRATUSH are to:

• Detect and characterise the global 21-cm hydrogen signal from the early Universe.
• Determine the timing and duration of the Cosmic Dawn and Epoch of Reionisation.
• Understand the formation and evolution of the first stars, galaxies, and black holes.
• Study the thermal history of the intergalactic medium and its interaction with the first radiation sources.
• Provide observational constraints for cosmological models describing early structure formation.

By doing so, PRATUSH will address some of the most fundamental questions about the origin of cosmic light and the processes that shaped the Universe’s earliest structures.

Scientific Importance

Detecting the 21-cm hydrogen line from the early Universe is one of the most challenging frontiers in modern cosmology. This faint radio signal carries vital information about:

• The first sources of light: The properties of primordial stars and galaxies.
• Heating and ionisation: How ultraviolet and X-ray radiation altered hydrogen in the early cosmos.
• Cosmic timelines: When reionisation began and how rapidly it progressed.

Ground-based observatories such as EDGES and SARAS have attempted to detect this signal but are limited by Earth’s atmosphere and radio interference. A lunar or space-based observatory like PRATUSH avoids these limitations, providing a clean environment for ultra-sensitive observations.

Engineering Innovations

PRATUSH incorporates several technological advancements:

• Compact Radiometer Design: High sensitivity within a small, efficient instrument profile.
• Thermal and Electrical Stability: Essential for maintaining calibration over long observation periods.
• Radio-Quiet Environment Utilisation: The lunar far side offers a shielded location free from Earth-originating radio noise.
• Onboard Calibration Systems: To ensure accurate separation of cosmological signals from foreground noise.
• Low-Power Architecture: A specially designed single-board computer controls the entire system to minimise energy consumption.

These features collectively enable PRATUSH to detect extremely faint signals that are billions of times weaker than local radio noise.

Challenges

The mission faces several technical and scientific challenges:

• Foreground Removal: Galactic and extragalactic radio emissions are millions of times stronger than the target signal.
• Instrument Calibration: Achieving and maintaining long-term frequency stability is crucial.
• Data Interpretation: Extracting cosmological information from complex and noisy data requires sophisticated modelling.
• Lunar Deployment Logistics: Ensuring reliable communication and power supply for instruments operating near the Moon’s far side.

These challenges make PRATUSH an ambitious and high-precision experiment, pushing the limits of both radio engineering and cosmological data analysis.

Expected Outcomes

If successful, PRATUSH will:

• Detect or place strong limits on the redshifted hydrogen 21-cm global signal.
• Establish the timeline for the emergence of the first luminous sources.
• Improve understanding of how reionisation reshaped the Universe’s structure.
• Provide valuable data to refine cosmological theories and simulations.
• Demonstrate India’s capability in conducting deep-space radio astronomy missions.

The mission’s findings will also complement global research efforts to study the early Universe, helping to integrate Indian science into international cosmological studies.

Future Prospects

PRATUSH is currently in the conceptual and developmental phase, with design work and prototype testing underway at the Raman Research Institute. Once the final payload is validated, the mission is expected to proceed toward integration and eventual launch with ISRO’s lunar or deep-space platforms.
In the long term, PRATUSH could pave the way for a series of lunar-based radio observatories, contributing to the establishment of a permanent radio astronomy presence on the Moon. It may also inspire future collaborations aimed at mapping the three-dimensional structure of the early Universe.

Significance

The PRATUSH telescope represents a pioneering effort by India to explore one of the last unobserved epochs of cosmic history. By seeking to capture the Universe’s earliest whispers of light, it combines cosmology, engineering, and space science in a mission of both national and global importance.