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The Mengtian lab module being moved to the Wenchang spaceport. The clocks and six other cabins will ride with the Mengtian to dock with the Tiangong in October. Photo: Handout

Chinese atomic clock trio bound for Tiangong space station in boost to GPS, dark matter probes

  • Devices of exceptional accuracy seen to have profound implications for GPS, national defence, deep space exploration and fundamental physics research
  • System will be about thousands of times more accurate than clocks on navigation satellites, China’s top space scientist says
China will soon send a trio of atomic clocks to its Tiangong space station, to establish a space-based timekeeping system of exceptional accuracy.

The clocks can work together to measure time with 10-17 stability – or missing one second every few billion years – the chief scientist of the Chinese crewed space programme said.

The devices have been packed inside a pair of fridge-sized cabins and are waiting to lift off from the Wenchang spaceport in southern China in October, Gu Yidong told a recent national space science assembly in Taiyuan.

The system will be about three orders of magnitude – or thousands of times – more accurate than the hydrogen maser (an acronym for microwave amplification by stimulated emission of radiation) clocks on navigation satellites.

Illustration of the interior of the Mengtian module. Photo: Handout

They will have profound implications for hyper- accurate positioning, national defence, deep space exploration, and fundamental physics research, Gu said.

Atomic clocks are the most precise timepieces in the world, using lasers to measure the vibration of atoms – whose electrons jump back and forth between energy states like tiny pendulums swinging in sync.

Such clocks are the most important component of a navigation satellite, each of which contains multiple atomic devices contributing very precise time data to the positioning signals.

The best lab-based atomic clocks are so accurate that if they had been running since the beginning of the universe, they would be off by less than half a second now.

China’s Wentian space lab docks with Tiangong station core module

However, it is a mammoth task to downsize such clocks before sending them into space. Space-based atomic clocks need to be compact in size and robust in performance, requiring more sophisticated design and technology than those in laboratories.

In 2016, China launched the world’s first space-based, microwave cold-atom clock on board its space lab Tiangong 2.

The clock used laser beams to trap, cool and probe rubidium atoms to measure time at a few millionths of a degree above absolute zero.

Developed by physicist Liu Liang and his colleagues at the Shanghai Institute of Optics and Fine Mechanics, the clock was packed inside a box that could fit into the back of a car and operated in orbit for 34 months straight.

The new clocks on the under-construction Tiangong space station will include a rubidium microwave cold-atom clock, a strontium optical cold-atom clock, and an active hydrogen maser, Gu said.

Of these, the optical clock was developed by the National Time Service Centre in Xian in northwest China, and will be the first of its kind to be sent into space.


China grows rice, other plants in space as part of Tiangong station experiment

China grows rice, other plants in space as part of Tiangong station experiment

Elements including rubidium, strontium and caesium are the preferred choice of scientists for making atomic clocks because they are relatively stable and their vibrations are easier to observe, among other reasons.

The three clocks bound for the Tiangong space station can work independently and compare readings with each other to measure time with unprecedented accuracy.

Tests went well on the ground, and the clocks’ performance may be further improved once they become operational in space, Gu said.

The devices represent the most complex and expensive research cabins on the space station, and will also be used to test fundamental physics, helping scientists make more precise measurements of constants, look for evidence of dark matter and dark energy, and probe the merger of black holes in the distant universe.

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The clocks and six other cabins will ride with the Mengtian experimental module to dock with the Tiangong next month, completing the construction of the T-shaped space station.

Meanwhile, Liu’s team in Shanghai has been working on an ultra-precise cold-atom clock based on an unconventional laser cooling technology, which can further downsize the device to the dimensions of a shoebox.

Their 28kg (62lb), 70-watt rubidium diffuse laser clock will be launched next year, and is expected to improve the positioning accuracy of the BeiDou navigation satellites if all goes well, Liu told the meeting last month in Taiyuan, in central Shanxi province.

The clock on the Tiangong 2 was 1 metre by 0.5m (3.3 feet by 1.6 feet), while the new diffuse laser clock will be 0.3m by 0.4m, a significant reduction given its highly complex internal structure.