Moonshadow: China Mulls Fleet of Orbital Telescopes

A team of Chinese radio astronomers has proposed an ambitious project to put a fleet of ‘mother’ and ‘daughter’ radio telescopes into lunar orbit. Dipping in and out of the moon’s shadow, the satellites will peer into the “grand mist of the primordial universe,” just after the cosmic dawn.

In the beginning, there was only primordial hydrogen, floating in the cosmic dark. Then, just a few hundred million years after the Big Bang, the first stars flared to life. The very oldest stars formed out of primordial hydrogen and gathered into galaxies that we can still see. But very old things are also very distant. The farthest, oldest stars have light that is redshifted entirely out of the visible spectrum, down into the infrared.

Those ancient hydrogen atoms hold the key to the so-called “dark ages” of the universe, and they can teach us about the structure of the universe we live in today. But it’s impossible to detect their ultra-redshifted light from Earth because of interference from human radio activities, said project scientist Chen Xuelei, of China’s National Astronomical Observatories in Beijing. So Chen and his colleagues offered a different solution: a family of ‘mother’ and ‘daughter’ satellites, that would make their observations in the radio shadow of the moon.

‘Mother and Daughter’ Satellites

Chen and his team have dubbed their project Hongmeng, after “the grand mist of the primordial universe” in Chinese mythology. Their design involves a ‘mother satellite,’ coordinating an array of nine ‘daughter satellites’ in orbit around the moon. Each daughter satellite carries a radio receiver, with which it will observe the hydrogen atoms from the far side of the moon. Once the satellites return to the near side of the moon, the mother satellite will phone home with the observation data.

Radio noise pollution can be ten thousand times stronger than the target signals, said Chen. That noise can come from sources all over, from Earthly radio broadcasts, to signals like pulsars, from beyond the rim. Consequently, the mission would need to dedicate one of its daughter satellites to monitor the radio “room noise.” This allows mission scientists to pick out their desired signals from the cosmic din.

Hongmeng’s daughter satellites will hunt for signals with frequencies as low as 1-30 MHz. These ultra-long waves can get through when all other signals are absorbed by interstellar gas and dust.

The mother satellite would pair off signals collected by any two daughter satellites, using a technique called interferometry, which uses subtle timing differences to create a high-resolution image of a radio source. The technique is just like Earthside interferometry. In fact, earlier this year, the collective efforts of hundreds of astronomers turned a dozen radio observatories into one gigantic, Earth-sized telescope. With that resolving power, they captured the first direct images of Sagittarius A*, the behemoth black hole at the center of the Milky Way. In lunar orbit, Hongmeng would create a radio telescope the size of the moon.

‘Discover the Unknown Unknowns’

Chen and colleagues first proposed the Hongmeng mission in 2015. Now that the satellite array has completed a five-year feasibility study and a test flight with drones, it’s time to seek funding. If the project gets approval, Chen says the team could have satellites in the sky by 2026 for a three-year mission.

“With such a dynamic design, Hongmeng can complete an all-sky, high-resolution survey at unprecedented low frequencies within just one year,” said Wu Ji, former director of the National Space Science Center. Wu participated in early stages of the Hongmeng project, but is no longer directly involved.

“It is time to consider a new space mission to explore and reveal the mysteries of this wave band,” the team said in the study. “As a first explorer mission, the main science objectives of the DSL are (1) to open up a new window of observation by mapping the sky and cataloging the major sources at this wavelength, to reveal new astrophysical phenomena at this wavelength, and to discover the unknown unknowns; (2) to explore the dark ages and cosmic dawn by making high precision global spectrum measurements.”

Shadow of the Moon

Even in the radio shadow of the moon, the mission will face some major engineering challenges. One problem is the huge temperature swings between the unobstructed sun and the dark of the moon’s shadow. Like Hongmeng, the James Webb space telescope sees in the infrared. But the two take a very different approach to the shadow of the moon.

NASA put the James Webb telescope in orbit around the Earth-Sun L2 point for thermal stability. Even in deep space, Webb needs active cooling, because its own body temperature is enough to blind its infrared eyes. Orbiting L2, the telescope will stay well clear of the shadows of Earth or the moon.

But where Webb seeks thermal stability by avoiding the change from shade to sun and back again, Hongmeng dives in and out of the shadows. This thermal flux can distort data and even damage delicate wiring. How Hongmeng will cope with the stress — and clean up the signal — remains to be seen.

Nevertheless, Chen remains optimistic. “We’ve sorted out all key technologies,” he said, “and are confident about sending satellites to the lunar orbit.”

Feature image credit: Chinese Academy of Sciences, via SCMP

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