China has successfully launched a satellite to the Moon that will perform radio astronomy as well as communicate between Earth and a separate lunar lander, which is set for launch later this this year. Dubbed Queqiao, or Magpie Bridge from an ancient Chinese folklore tale, it took-off from the Xichang Satellite Launch Center on 21 May. It will now be put at Langrange Point 2 “L2” – a gravitational-balance point about 65000 km behind the Moon – where it will stay visible both to ground stations on Earth and the future lander.

Due to tidal locking between the Earth and the Moon, only one side of the Moon is visible to Earth. This “far side” remains of enormous interest to scientists as studies have indicated that there is a very different world on the far side, being geologically more ancient and dominated by highlands, unlike the planar landscape that prevails on the near side.

A radio antenna behind the moon will open up a new window on the universe Marc Klein Wolt

The far side is also of interest for the radio-astronomy community. While almost all celestial radio wave frequencies can be received on Earth, those that are below 30 MHz are blocked by the atmosphere. Yet such frequencies contain important information about the early universe and can only be measured from a special vantage point like the back of the moon, which is free from atmospheric and man-made interference. This means the far side is one of the best places to measure the 21 cm hydrogen emission line that can be used to study the mass and dynamics of galaxies and will allow scientists to peer into the “cosmological dark ages” – a period between the Big Bang and the birth of the first stars.

Queqiao will include a Dutch-built antenna – the Netherlands-China Low-Frequency Explorer (NCLE) – that is designed to measure radio waves between 1-80 MHz. “A radio antenna behind the moon will open up a new window on the universe,” says NCLE project leader Marc Klein Wolt, who is managing director of the Radboud Radio Lab at Radboud University. According to Albert-Jan Boonstra from the Netherlands Institute for Radio Astronomy in Dwingeloo, the Dutch antenna is especially designed to receive low-frequency radio waves over a larger range. “We have found ways to avoid the electromagnetic interference of the satellite itself and successfully developed a broadband receiver,” he says.

Limited observations

Also riding on Queqiao is a pair of microsatellites that will be released into an elliptical lunar orbit for similar radio astronomy experiments as the Dutch antenna. The twin microsatellites, developed by Chinese scientists, will carry out interferometry tests to demonstrate the feasibility of a future microsatellite array, which would be more sensitive than a single probe in detecting faint-radio signals from afar. However, due to the size of the microsatellites, their observation times will be limited to 10 minutes from the far side and 20 minutes of data transmission from the near side every orbit. Read more China unveils plans for next-generation X-ray observatory

As well as performing radio astronomy, another key aim of the Queqiao mission is to enable the transmission of commands and data from Earth to the Chang’e-4 lander that will launch later this year. Chang’e-4, which will land in the South Pole-Aitken Basin area, will be the first mission to land on the far side of the Moon and it will also have a low-frequency radio-spectrum analyzer, which has been developed by scientists from the Institute of Electronics, Chinese Academy of Science in Beijing.