From moon’s far side, a satellite to look for the first radio signal
Scientists are developing small lunar-orbiting satellites, including the proposed CosmoCube mission, to detect the faint 21-cm radio signal emitted by neutral hydrogen in the early universe.
The signal is extremely difficult to isolate because it is buried beneath foreground radio emissions (from the Milky Way and other sources) that are thousands of times stronger.
Such missions plan to use the Moon's far side as a radio-quiet shield, since it blocks interference from Earth-based FM radio, satellites, and telecommunications for part of every lunar orbit.
India's own PRATUSH (Probing ReionizATion of the Universe using Signal from Hydrogen) mission, under study with the Indian Space Research Organisation (ISRO), aims to make a similar measurement from lunar orbit.
The 21-cm Hydrogen Line and Cosmic Dawn
The 21-cm line is a spectral line produced by a hyperfine "spin-flip" transition in the ground state of the neutral hydrogen atom — when the electron's spin flips relative to the proton's spin, the atom emits radiation at a rest-frame wavelength of 21 cm (frequency ~1420 MHz). Because neutral hydrogen filled the universe for hundreds of millions of years after the Big Bang, before the first stars and galaxies formed, this line acts as a fossil radio signal from that era. As the universe has expanded, this signal has been cosmologically redshifted, so it now arrives at Earth (and lunar orbit) at much lower frequencies, in the tens to a few hundred MHz range.
Key Details
- The "Dark Ages" span roughly from recombination (~380,000 years after the Big Bang, when the universe first became transparent) to the appearance of the first stars (~100–300 million years after the Big Bang).
- The subsequent "Cosmic Dawn" and "Epoch of Reionization" (roughly redshift z ~ 6–15) is when ultraviolet radiation from the first stars and galaxies ionized the neutral hydrogen filling the universe.
- Ground-based radio telescopes cannot observe below ~45 MHz because Earth's ionosphere blocks these frequencies and human radio interference drowns out the signal — this is why space- and Moon-based platforms are being pursued.
- India's SARAS (Shaped Antenna measurement of the background Radio Spectrum) experiment, developed at the Raman Research Institute (RRI), Bengaluru, is a ground/balloon-based precursor effort at the same kind of measurement; its SARAS 3 results in 2022 statistically challenged an earlier disputed 2018 detection claim by the US-based EDGES experiment.
CosmoCube and PRATUSH both attempt to detect this same 21-cm "global signal" (the all-sky-averaged brightness rather than a spatial map) using the Moon's far side as a natural radio-interference shield — something no Earth-based or standard Earth-orbiting instrument can achieve as effectively.
PRATUSH — India's Lunar 21-cm Cosmology Mission
PRATUSH is a scientific payload concept proposed to ISRO to measure the sky-averaged (global) redshifted 21-cm signal from the Cosmic Dawn and Epoch of Reionization, by orbiting the Moon and observing the low-frequency radio sky (roughly 40–200 MHz) while shielded from Earth's radio noise on the lunar far side.
Key Details
- PRATUSH stands for "Probing ReionizATion of the Universe using Signal from Hydrogen."
- It was proposed to ISRO under an announcement of opportunity for science payloads (2018) and remains in pre-project/study phase, distinct from an approved, funded mission.
- Its scientific goal — like CosmoCube's — is to detect the faint absorption/emission dip in the radio spectrum caused by neutral hydrogen interacting with the first generation of stars, which would provide direct evidence about when and how the first luminous objects formed.
- The mission concept envisions transmitting recorded far-side data back to Earth during the near-side portion of each lunar orbit, since direct-to-Earth transmission is not possible while on the shielded far side.
PRATUSH represents India's contribution to the small but growing global effort — alongside missions like CosmoCube — to use lunar orbit as an observatory for the earliest, otherwise unobservable chapter of cosmic history.
Why the Moon's Far Side Matters for Radio Astronomy
The Moon's far side never faces Earth and is permanently shielded from Earth-originated radio frequency interference (RFI) — a natural, cost-free radio-quiet zone unavailable anywhere in Earth orbit or on Earth's surface.
Key Details
- A satellite orbiting the Moon spends part of each orbit over the far side, shielded from Earth's RFI for a portion of that time (for a low lunar orbit of a few hours, this "quiet window" is a large fraction of each pass).
- This same radio-quiet property is why the Moon's far side is also proposed as a site for a possible future lunar radio interferometer array (concepts such as FarView) for cosmology beyond just the global 21-cm signal.
- Detecting the signal further requires advanced statistical (Bayesian) techniques to separate the cosmological signal from foreground Galactic and instrumental noise that can be orders of magnitude stronger.
This RFI-free environment is the central design rationale for both CosmoCube and PRATUSH — it is what makes a Moon-based measurement scientifically superior to equivalent instruments on Earth or in near-Earth orbit.
- 21-cm line rest-frame frequency: ~1420 MHz (redshifted down to ~40–200 MHz range for Dark Ages/Cosmic Dawn observations).
- Dark Ages span: ~380,000 years to ~100–300 million years after the Big Bang.
- Epoch of Reionization: roughly redshift z ~ 6 to 15.
- Ground-based radio telescopes are effectively blind below ~45 MHz due to Earth's ionosphere.
- PRATUSH proposed to ISRO in 2018; currently in pre-project studies phase.
- SARAS 3 (Raman Research Institute) statistically challenged the 2018 EDGES cosmic dawn signal claim with results published in 2022.