PRATUSH: Unveiling The Universe's First Light From The Moon's Quiet Side
Mayuri S Rao of RRI explains the challenge of the experiment as the signal they are trying to detect has never been observed before.
By Anubha Jain
Published : September 8, 2025 at 6:08 PM IST
Bengaluru: Scientists at Bengaluru’s Raman Research Institute have engineered the laboratory model receiver of PRATUSH (Probing Reionisation of the Universe using Signal from Hydrogen), a proposed cosmology experiment to study the formation of the first stars and galaxies. Mayuri S Rao, Associate Professor-I at Raman Research Institute and Principal Investigator of PRATUSH, discussed the project in detail with ETV BHARAT in an exclusive interview.
Currently at the concept stage, PRATUSH aims for a two-year mission once approved. Over this period, the data collection would be structured in a way that the scientists need about 100 hours of high-quality data from the Moon’s far side. However, from lunar orbit this time won’t be continuous — one orbit may allow only 5 minutes, another 45 minutes. The data will be buffered on board and downlinked to ground stations when the satellite moves to the near side facing Earth.
PRATUSH: Scientific significance and positioning
Rao stated that the mission aims to detect extremely faint signals emitted by neutral hydrogen from the early universe. These signals hold the key to understanding how the first stars and galaxies came into existence.
"Detecting this signal from Earth is particularly challenging. Apart from its faintness, the frequency band needed for the experiment overlaps with heavy radio noise generated on our planet," Rao explained. "For instance, FM radio stations transmit in the 88–108 MHz range, which coincides with the frequencies of interest. The far side of the Moon offers a solution."
PRATUSH would orbit the Moon and observe the sky-spectrum over 55-110 MHz radio frequencies from the Moon’s far side. "Shielded from Earth’s radio emissions, it provides a radio-quiet environment, free from ionospheric disturbances and terrestrial interference. This makes PRATUSH uniquely positioned to study these early cosmic signals from a pristine location in space," she further said.
Apart from radio-frequency interference from human activity and the ionosphere that can distort or confuse the signals reaching the antenna, the reason why the far side of the Moon was selected for orbital observation was the effect of the environment itself—whether soil, rock, or even a laboratory table—which can alter an antenna’s properties. On the far side of the Moon, all three of these issues are eliminated, making it an ideal location for our observations," Rao said.
The 21-cm hydrogen signal
Discussing the importance of the 21-cm hydrogen signal in studying the Cosmic Dawn and the Epoch of Reionisation, she explained that a hydrogen atom consists of a proton and an electron, both possessing a property called “spin.” These spins can be either aligned or anti-aligned, with the latter being the more stable configuration
"When an electron flips from the aligned state to the anti-aligned state, it emits a photon of light. This light corresponds to a frequency of about 1,420 MHz (or a wavelength of 21 cm)—known as the '21-cm line'. This 21-cm signal was produced even by the earliest, primordial hydrogen when the universe was in its infancy," she said.
She added that due to the expansion of the universe, the wavelength of the radiation had been stretched over billions of years. What initially measured 21 cm now reached Earth at wavelengths of several metres, corresponding to frequencies around 100 MHz. By studying this stretched 21-cm signal, she said, scientists could trace the fingerprint of hydrogen throughout cosmic history and gain insights into how the first stars and galaxies were formed.
Digital receiver for PRATUSH
In an engineering feat, the researchers have demonstrated that a compact digital receiver developed for the proposed lunar payload PRATUSH with all its complexities, can be controlled with a simple Raspberry Pi, a common credit–card–sized single-board computer that has demonstrated the required sensitivity for the experiment. This receiver orchestrates a complex radiometer system—managing the antenna, analogue receiver, and a powerful Field-Programmable Gate Array (FPGA) that processes streams of cosmic data.
While explaining the credit-card–sized digital receiver and its compact technology, Rao said, “A mobile phone today is a very sophisticated small computer, but it can't be sent into space because the technology for that isn’t viable yet. Our challenge was to show that a complex receiver — which has to perform highly sensitive measurements — can still be controlled by a simple, low-cost, tiny computer like the Raspberry Pi, commonly used by hobbyists. We’ve successfully demonstrated that this is possible.”
Describing the role of the Field-Programmable Gate Array (FPGA) in processing cosmic signals and reducing noise, Rao explained that an FPGA is similar to the chip inside a computer or phone, but more advanced. It receives the incoming data, processes it, and generates the final measurement spectra that scientists want to study.
"In other words, it takes raw signals, converts them into digital form, and transforms these digitised signals into spectra, which are then collected and managed by the single-board computer," she said.
PRATUSH calibration and data collection
Speaking about how PRATUSH will collect and transmit data while orbiting the Moon and how it ensures accurate calibration and avoids interference, Rao told ETV Bharat, “PRATUSH will take scientific measurements on the far side of the Moon and downlink the data when it moves to the near side within view of ground stations. The receiver will periodically switch between the antenna and a known noise source to calibrate itself accurately, a method already demonstrated in the laboratory with the required sensitivity."
"In addition, several techniques, including careful PCB design and the use of shielding materials, are planned to minimise the self-generated radio noise of the receiver. These will be validated at the required levels with space-qualified materials," she said.
Design challenges
Discussing the hurdles, Rao said, “This was a highly challenging experiment because the signal we are trying to detect has never been observed before. As with any such project, the first step is to design a very clean experiment and operate it in an equally clean environment—in our case, the far side of the Moon."
"For example, an antenna cannot be designed in isolation from the environment where it will operate. Since PRATUSH is a space-based proposal, we had to co-design the antenna along with the satellite, going through many iterations. We have now arrived at the baseline design, which will be implemented in the space mode with necessary modifications," she added.
Risk and mitigation
PRATUSH is designed as a standalone experiment with a dedicated spacecraft. Rao explained that using a dedicated platform allows far better control over electronic noise. If it will fly alongside other instruments and experiments, the team would have to ensure all of them remained radio-quiet. However, the launch itself need not be dedicated—PRATUSH can share a launch with other payloads and then be injected into its own orbit.
Talking about the technical and scientific risks foreseen for PRATUSH and how the team plans to mitigate them, Rao said that there are two primary technical challenges from an R&D perspective:
- Although such experiments can be built with high sensitivity, they must operate on the far side of the Moon without becoming a source of radio noise themselves. We need to ensure that the noise from our own electronics is suppressed to acceptable levels, and all of this must be space-qualified.
- The antennas will experience extreme temperature swings—from very hot to extremely cold—which can reach several hundred degrees Celsius and potentially alter the antenna’s shape. Managing these effects is critical for mission success.
PRATUSH’s global significance
“Everything we are demonstrating is on par with the global standard and is competing neck and neck with our counterparts worldwide. Successfully carrying out this mission would be a major boost for Indian astronomy and firmly place India on the global map in this field," Rao said.
Emphasising the next steps for the mission, Rao said, “With the lab model complete, we’ve reached a key milestone. We’re now gearing up for reviews with ISRO and plan ground-based technology demonstrations. The review process will then decide the project’s next steps.”
Discussing the theoretical predictions PRATUSH aims to test, Rao said the mission targets a broad family of cosmological models and should be able to constrain most of the standard ones, which can tell more about first stars and galaxies.
She said that two major unresolved questions remain in what is popularly called the Big Bang model: first, the period of inflation immediately after the Universe’s birth, and second, the phenomenon we are currently investigating. There is a major gap in our understanding of the Universe’s evolution. Filling this missing piece would mark a significant milestone in cosmology and astrophysics.
