Chinese satellite set to join ‘radio telescope’ covering area 30 times the size of Earth
- Orbiting lunar satellite will join observatories on Earth in a giant network of antennas to form the Space Very Long Baseline Interferometry
- It will be the largest facility ever created for astronomical observation, researchers say – but satellite’s hydrogen clock and frequency range pose challenges
A number of observatories on Earth will then join the lunar outpost in forming a giant network of antennas – technically equivalent to a radio telescope nearly 400,000km (250,000 miles) wide.
For comparison, the world’s largest single-dish telescope is China’s Five-hundred-metre Aperture Spherical radio Telescope (FAST).
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“It can produce images of some important celestial phenomena such as black holes and gravitational lenses with unprecedented resolution in the X-ray band,” said a project scientist who requested not to be named because he was not authorised to speak to the media.
In theory, a telescope spanning an area from the Earth to the moon could capture the action in a baseball game on Mars. The greater the distance between two observatories, the higher the image resolution.
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Combining several small telescopes to form a large one – known as very long baseline interferometry (VLBI) – was invented in the 1950s.
While studying distant galaxies, University of Cambridge astronomers Martin Ryle and Antony Hewish noticed that the same radio signal could be picked up by different observatories at different times.
If observational data from these sites was coordinated precisely and analysed together, astronomers could obtain images with resolution far higher than those achieved by a single observatory. Ryle and Hewish shared the Nobel Prize in Physics in 1974 for their discovery.
The first image of a black hole was produced in 2019 by the Event Horizon Telescope – a VLBI network that used observatories around the world to effectively create a diameter about the size of the Earth.
Russia and Japan have each launched a satellite to extend the scale of VLBI observation beyond Earth, but without reaching as far as the moon.
There are some question marks, however, over the feasibility of the Chinese project.
For it to operate like a single telescope, data collected at different locations must be timed accurately. This requires an atomic clock known as a hydrogen maser that can work for long periods with extremely high stability.
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But size and weight constraints mean the precision of a hydrogen clock on a satellite tends to fall short of that of larger machines on Earth, usually by an order of magnitude. It is not certain that the small, distant observatory in the moon’s orbit could stay in sync.
In a paper published on Monday in domestic peer-reviewed journal Acta Astronomica Sinica, a research team led by Professor Liu Qinghui of the Shanghai Astronomical Observatory said that the atomic clock would not be a problem.
Their conclusion was based on a recent experiment involving Tianwen 1, a Chinese spacecraft orbiting Mars.
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Liu and his colleagues tracked extremely weak signals from Tianwen using the hydrogen maser built for the telescope programme, and found that the small atomic clock and the bigger version on the ground differed less significantly than expected.
Further questions surround the lunar orbital station’s antenna, which is about four metres wide – small enough to reduce signal detection capability.
It picks up radio signals in a frequency range above those of many ground-based telescopes, including FAST. That affects the telescope’s overall sensitivity, because only a relatively small number of Earth observatories could join the project.
Nonetheless, it would expand the astronomical observation infrastructure into the solar system, the researchers said.
If it works as planned, China is expected to launch further satellites and establish a fixed observatory on the surface of the moon or even Mars, to create an interplanetary telescope.
