China's Tiangong-2 space station just spent 10 days much closer to Earth than its usual orbit, a manoeuvre that seems to have baffled experts - and China's government isn't talking.

From June 13, it dipped from its usual altitude of around 390 kilometres (242 miles) down to around 295 kilometres (183 miles), according to Harvard-Smithsonian astronomer Jonathan McDowell, and orbital information  published by the US Strategic Command's Joint Force Space Component Command, reports Space News.

It has since returned to its usual orbit - leading to speculation that the space station may be about to be decommissioned in a much more controlled manner than Tiangong-1, which slipped out of control and came plummeting down to Earth in April this year, thankfully splashing harmlessly into the ocean.

Tiangong-2, which was launched in 2016, is not a permanent fixture. The 10.4-metre, 8.6-ton craft serves as a test facility for technologies to be deployed in a large, modular space station called Tianhe that China plans to launch by 2022, weighing up to 100 tonnes. These include life support, and docking and resupply capabilities.

In October 2017, Chinese astronauts arrived for a 30-day stay on the station - China's longest stay in space, since it has been banned from participating in the International Space Station - but Tiangong-2 spends most of its time unmanned.

According to the data, most likely lowered its orbit via two burns, then returned to higher orbit via another two burns.

"My best fit to the data is that TG-2  reboosted at 0117 and 0202 UTC Jun 22, for a total delta-V of 56 m/s and a propellant usage of about 144 kg," McDowell noted on Twitter.

"Possibly just testing out the spacelab's engine reliability after 2 years in orbit, as part of end-of-life tests?"

He also noted that perhaps the China Manned Space Engineering Office (CMSEO) is planning to use the same propulsion system on Tianhe, which means on-orbit test data would be extremely valuable to have.

At time of writing, the CMSEO had not responded to ScienceAlert's request for comment.