Space-based submarine detection may be upon us soon
Chinese scientists are working on a space-based device that could track gravitational ripples produced by submerged submarines
By Stephen Chen
South China Morning Post
Chinese astronauts have played many roles in space, including teacher, mechanic and tourist.
But all the science classes, repair missions and spacewalk flag-waving have tended to obscure the fact that they are, first and foremost, members of the People’s Liberation Army.
China’s manned space programme has so far given its astronauts few opportunities to fulfil military roles, but that will all change when its space station is completed in the next six years.
One task on their to-do list could be detecting and tracking nuclear submarines from space, using a technological breakthrough achieved by Chinese scientists.
The two-man Shenzhou XI spaceship that blasted off from the Jiuquan Satellite Launch Centre in Inner Mongolia on Monday morning will soon dock with the Tiangong-2 space laboratory, launched last month, which is carrying the world’s first space-based cold atom clock.
The ultra-accurate timepiece
The ultra-accurate timepiece shares its core technology with cold atom interferometers, which can measure tiny changes in gravitational pull with unprecedented sensitivity, and one of the devices, to be built and put on the Chinese space station, could potentially be used to track nuclear submarines.
Nuclear submarines can be massive, with the largest measuring more than 170 metres in length and displacing 48,000 cubic metres of water, and when they cruise several hundred metres below the ocean’s surface they generate many gravitational ripples. An extremely sensitive detector could catch and analyse the invisible ripples to locate and follow the submarine.
Using cold atom interferometry to detect submarines is a controversial technology, with some scientists saying the enormous engineering challenges involved mean it will never work, especially over the long distances involved when using a space-based platform. Others think it’s worth a try.
China could be the first nation to do so, according to a researcher at the Beijing-based China Academy of Space Technology, which has initiated and designed most of China’s space projects.
The cold atom interferometer would be part of a super-cold atom laboratory in the space station’s experimental module, said the researcher, who declined to be named because he was not authorised to speak to the media about the project.
“The technology’s potential military value is not discussed in public, but it’s an open secret in the research community,” he said.
Professor Tu Liangcheng, who has studied the precise measurement of gravity at Wuhan’s Huazhong University of Science and Technology, said the Chinese government had substantially increased funding for submarine detection technology in recent years.
“There is a shift in the navy’s attitude to submarine warfare,” said Tu, who has been involved in military research projects.
In the past, China paid more attention to developing its own submarines and the technology to make them quiet, powerful and able to stay submerged for longer. It largely ignored the activities of other countries’ nuclear submarines, unless they ventured into Chinese waters.
But funding for submarine-hunting technology, including gravitational measurement, had now increased significantly, Tu said.
The change reflects China’s ambition to develop a “blue-water” navy able to protect its national interests along important maritime trade routes spanning the globe.
Tu said the Chinese navy desperately wanted to be able to track foreign nuclear submarines, but it was 30 years behind the capabilities of the United States when it came to submarine-detection technology.
“Now we have enough money, and China’s strength in this field of research in on par with the US and Europe,” Tu said. “But the pressure is high, there is high expectation of a quick breakthrough, and we are short of hands.”
There are many ways to measure gravitational variations. The US-German gravity recovery and climate experiment (Grace) mission, launched in 2002, uses a pair of satellites to measure “bumps” – fluctuations – in the earth’s gravitational field.
But the cold atom interferometer to be placed on the Chinese space station should be more precise because it will be able to measure bumps at the atomic level.
Because the movement of single atoms is difficult to observe directly, the device will split a beam of slow-moving atoms in two and then merge the beams to produce a band with interference patterns.
The up and down movements of the atoms caused by gravity would change the look of the patterns, and scientists could use the information to detect gravitational anomalies in a specific area.
Professor Zhan Mingsheng, who led a research team studying space-based cold atom interferometers at the Chinese Academy of Sciences’ Wuhan Institute of Physics and Mathematics, said Chinese scientists would be able to shrink the device, currently the size of a room, to something that could fit comfortably into the back of a car.
But he was sceptical about whether the technology would be able to detect a submarine in the foreseeable future.
“The biggest problem is noise,” he said. “The technology is very sensitive indeed, but also because of the sensitivity it can become an ear that can hear everything. The signal that you want to detect will simply be drowned out and lost in the background noise.”
The device would also suffer disturbances on a large platform such as the space station, where a lot of equipment could cause trembles. Even an astronaut’s cough could produce a false alarm, Zhan said.
The space-based detector would also have to compete with other platforms, such as planes and ships, which were closer to the target and likely to produce more precise locking, even though they could not offer the same global view as the space station, Zhan said.
Once a nuclear submarine entered a big, deep ocean such as the Pacific, it used to be believed that it would remain undetectable until it surfaced. But a space-based submarine detection platform could locate it precisely.
Besides the cold atom interferometer, China might consider other unconventional ways to detect submarines.
Nuclear submarines’ fission reactors produce neutrinos, extremely small particles can pass through materials such as water and walls without effort, potentially exposing a submarine’s location.
China has built some of the world’s largest and most advanced neutrino detectors, one at Shenzhen’s Daya Bay nuclear power plant and the other at a hydropower station in Sichuan, which at 2.4km below the earth’s surface is the world’s deepest underground laboratory.
Cao Jun, the researcher who led the Daya Bay neutrino detection project, said that detecting the elusive particles from a source 50 km away, using current technology, required a detector weighing 20,000 tonnes. But new neutrino detection technologies were emerging, which might satisfy the military’s need for a portable platform.
A future technological breakthrough might enable scientists to develop neutrino detectors that could be placed on ships or space stations, Cao added.
The US and Europe also launched cold atom interferometer projects, but they were either cancelled or delayed, mainly due to funding shortfalls, and some Western scientists have expressed interest in joining the Chinese project.
China has told the European Space Agency that there could be a place for European astronauts on the Chinese space station, and some European astronauts have started learning Chinese to prepare for the trip.
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