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Contacting extraterrestrials may be better done with X-rays than by radio


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They penetrate better through outer space


Opinion is divided on whether Homo sapiens should announce its presence to the universe by broadcasting messages to any putative extraterrestrials who may be listening, or should keep schtum, for fear of attracting unwanted attention. But if attempts at contact are to be made at all, then they might as well be done properly.


Past efforts, including one in the 1970s to a star cluster 25,000 light-years away and another in 2017 to a planet a mere 12 light-years away, have used radio. Hang Shuang and his colleagues at the Nanjing University of Aeronautics and Astronautics, in China, think this approach foolish. Radio waves spread out quickly, and are also absorbed and scattered by interstellar dust. On top of this there are many sources of radio in the universe, which creates a confusing background. Instead, Mr Hang proposes using x-rays.

x-rays diverge more slowly than radio waves. They are also better at penetrating dust. And there is little x-ray background to confuse them with. They would therefore be suitable in principle for interstellar communication. Their value as communication tools on Earth, however, has not been obvious, so little research has been done on using them to carry messages. But not none, for Mr Hang and his colleagues have actually built a prototype of an x-ray transceiver that has a particular, specialised purpose. This is to eliminate the communications blackout which a spacecraft experiences during re-entry into Earth’s atmosphere. The blackout is a result of the craft being surrounded by a plume of incandescent plasma generated by the heat of re-entry. Such a plasma is impenetrable by radio waves, but can be pierced by x-rays. Using their prototype, Mr Hang and his colleagues are able to encode messages into x-rays, transmit them through a vacuum, and then decode them at the other end.

A practical version of this system would not broadcast signals directly to Earth from the re-entering craft. Rather it would transmit them to a satellite that then relayed the message Earthward by more conventional means. The reason for the detour is that, though x-rays penetrate dust, they are absorbed by the sorts of gases that make up Earth’s atmosphere. The re-entry transceiver works because the period of re-entry blackout happens high in the atmosphere, where the air is thin. A signal beamed through the thick air of the lower atmosphere would, by contrast, be absorbed.

xcom, as Mr Hang dubs his putative x-ray Aldis lamp, would be a more powerful version of such a spacecraft transmitter. To avoid atmospheric absorption it would have to be put into space to operate. Ideally, it would sit on the far side of the Moon, shielded from interference from Earth.

By a lucky coincidence, the China National Space Administration, the country’s space agency, has just demonstrated, with the landing of its lunar probe Chang’e-4, that it can position equipment on that part of Earth’s natural satellite. Whether the agency’s research interests stretch as far as the hunt for extraterrestrial intelligence remains to be seen. But xcomwould certainly be a novel approach to the question.



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