Astronomers are looking forward to opening a new window on the universe by posting high-resolution telescopes on the moon, and in orbit around it. There are numerous proposals to do this from astronomers around the world — including one from India called PRATUSH.
On the earth, optical telescopes (which collect visible light at longer wavelengths) and radio telescopes (which collect radio waves with the shortest wavelengths) have to peer through layers of the planet’s atmosphere. While it is becoming increasingly difficult for optical instruments to see through the polluted skies, radio telescopes also contend with radio and TV signals adding to the cacophony of the electromagnetic ‘hiss’ from the communications channels used by radar systems, aircraft, and satellites. It also does not help that the earth’s ionosphere blocks radio waves coming from outer space.
A pristine desolation
Scientists tried to find a way out of this by launching radio telescopes into orbit around the earth. But this only made the problem worse, as orbiting telescopes started receiving radio noise from the whole planet along with signals from outer space. So astronomers are now seriously considering an idea they have toyed with since the 1950s: placing optical and radio telescopes on the far side of the moon, which always faces away from the earth.
The pristine, airless desolation of the moon provides optical telescopes crystal-clear seeing conditions throughout the long lunar night, which lasts two weeks at a time. Radio telescopes on the lunar far side will also be protected by a 3,475-km-thick wall — a.k.a. the moon (its diameter is 3,476 km) — that blots out radio transmissions from the earth and electrically charged plasma winds blowing from the Sun.
In the past, the enormous costs involved discouraged scientists from setting up lunar telescopes. But renewed interest among spacefaring nations to return to the moon promises to open up “the most radio-quiet location in the solar system”, to quote The Royal Society, to astronomers.
The oldest light in the universe
Once upon a time, cosmologists believe, everything in the cosmos was condensed into an infinitesimally small, incredibly dense blob in the void that exploded with a ‘Big Bang’. The resulting fireball cooled as it spread and its blinding light faded into a gathering darkness. At some point, the young universe resembled a formless sea of murky matter, highlighted only by traces of primordial hydrogen and helium.
This darkness persisted from some 300,000 to half a billion years after the Big Bang, which is why there is so little direct evidence today of this important period in the cosmic story. The blackness in the heavens was banished only when the first stars switched on their nuclear power-plants and the cosmos continued to expand. We see this expansion now as a faint glow called the cosmic microwave background (CMB) — the oldest light in the universe — which can be captured by radio telescopes.
Meanwhile, the universe went ‘quiet’ for tens of millions of years afterwards as gravity began to build the first stars and galaxies. This period of time between the initial scattering of the CMB radiation and the birth of the first stars is known as the Dark Ages. It is believed the neutral hydrogen pervading the cosmos during the Dark Ages absorbed some of the CMB radiation to produce an extremely small dip in the frequency of the spreading radio waves.
China may be the first, again
Terrestrial instruments can’t detect this minute frequency drop. Instead, moon-based instruments are our best bet to spot this signal from the Dark Ages, which would be essentially free from the influence of any starlight (since there were no stars then).
“We want to study the Dark Ages period because it connects how the early universe evolved into the universe we see today,” Aritogi Suzuki, who heads the Lunar Surface Electromagnetic Experiment, or LuSEE Night, a joint NASA-Berkeley Lab project, scheduled for launch in December 2025, told this author via email. “We are going to land on the far side of the moon, near the equator of the moon, and almost exactly opposite from the earth. This location is helpful because it best shields radio frequency noise coming from the earth.”
LuSEE Night will be followed by many moon-bound instruments currently in various stages of planning with space agencies like NASA and the European Space Agency (ESA). NASA’s Long-Baseline Optical Imaging Interferometer, for instance, is scheduled to be launched in parts before this decade is out. Once assembled on the moon’s far side, it will study magnetic activity on stars and the centres of active galaxies in visible and ultraviolet wavelengths.
ESA is getting ready to launch a radio telescope to the moon’s far side on board its lunar lander, ‘Argonaut’, by 2030. Other European projects on the anvil include super-sensitive detectors to hunt for the elusive ripples of gravitational waves in space-time and an infrared telescope located inside a permanently shadowed crater near the lunar south pole.
First off the block, however, could be China, with a moon-orbiting radio telescope scheduled for launch in 2026. Another of its satellites, Queqiao-2, intended as a communications relay between the earth and future missions, probably entered into orbit around the moon on March 24. Its payload includes a 4.2-m antenna that will be used as, among other things, a radio telescope.
PRATUSH radio telescope
Although the technologies for these instruments exist, it is difficult for scientists to deploy them on the moon. “An alternative approach,” Dr. Suzuki said, “would be to orbit … the moon instead of landing on the surface and study the data when the satellite is behind the moon.”
This is what Indian scientists plan to do with the radio telescope PRATUSH (Probing ReionizATion of the Universe using Signal from Hydrogen), to be sited on the moon’s far side. PRATUSH is being built by the Raman Research Institute (RRI) in Bengaluru with active collaboration from the Indian Space Research Organisation (ISRO).
Initially, ISRO will place PRATUSH into orbit around the earth. After some fine-tuning, the space agency will launch it moonwards. “Although earth orbit will have significant radio frequency interference (RFI), it will have advantages compared to ground-based experiments, such as operating in free space and lesser ionosphere impact,” Mayuri S. Rao and Saurabh Singh, principal investigators at RRI, explained in an email. “PRATUSH in lunar orbit will have the ideal observing conditions operating in free space with minimal RFI and no ionosphere to speak of.” It will carry a wideband frequency-independent antenna, a self-calibrating analog receiver and a digital correlator to catch radio noise in the all-important signal from the Dark Ages.
As astronomers open new windows from the moon to look at the far reaches of the universe, who knows what discoveries await them. One thing is certain: they are in for some exciting times as the cosmos yields clues to some of its greatest mysteries, such as dark energy (which pushes the universe in every direction at an accelerating rate), primordial black holes and, indeed, the very nature of the cosmos.
Prakash Chandra is a science writer.
