Such close encounters could allow China to practice rendezvous procedures between its future space station and other spacecraft, as well as learn about satellite formation flying, said Brian Weeden, a former U.S. Air Force orbital analyst and now technical adviser for the Secure World Foundation.
The satellite rendezvous tests could also permit close-up inspection of another satellite, not unlike demonstrations performed by the United States and other countries in the past.
According to Weeden, China's SJ-12 satellite made at least six sets of maneuvers between June 20 and Aug. 16. During those maneuvers, the satellite made passes near SJ-06F, an older Chinese satellite that launched in October 2008.
The satellite's behavior does not fit the profile for an anti-satellite test, Weeden wrote in an analysis for Space Review. But he added that the mysterious nature of the test could have an effect on perceptions of trust and safety in space activities.
"There's no evidence there was any damage to the satellite or debris, so I wouldn't characterize it as a collision," Weeden told SPACE.com. "More like a bump."
How it happened
SJ-12 launched on June 15 from Jiuquan Satellite Launch Center in China. That launch center has also served as the starting point for China's manned spaceflights on Shenzhou spacecraft in 2005 and 2008.
Weeden said the SJ-12 satellite's first moves allowed its orbital plane to eventually match that of SJ-06F over the course of 50 days. The latter mission consists of a pair of satellites, including a smaller maneuvering satellite and a larger satellite.
One of the closest passes between SJ-12 and SJ-06F created a change in SJ-06F's orbit on the night of Aug. 18, according to public data from the U.S. Air Force. SJ-12 also made many close approaches with less than 984 feet (300 meters) between the satellites.
"As far as we can tell, SJ-12 is still doing a bunch of little maneuvers close to SJ-06F," Weeden explained.
NASA previously experienced its own satellite bump when it launched the Demonstration for Autonomous Rendezvous Technology mission on April 15, 2005. That DART mission was supposed to demonstrate automated rendezvous with a defunct U.S. Navy MUBLCOM communications satellite.
A navigation error led to DART and MUBLCOM bumping at a speed of 4.9 feet per second (1.5 meters per second) — fast enough to cause a major change in the MUBLCOM satellite's orbit.
The orbital change for the MUBLCOM satellite was about 100 times greater than the change in SJ-06F's orbit, Weeden points out. That suggests the Chinese satellites most likely did not suffer any damage during their presumed bump, if that happened.
Concerns about the unknown
The SJ (Shi Jian, or "practice" in Mandarin Chinese) satellites typically conduct what the Chinese government has termed "scientific missions."
But some outside observers believe the satellites actually receive electronic signals for the Chinese military. They point out that no scientific research based on the work of the satellites has ever appeared.
China has good reason to want to experiment with close space maneuvers, given its plans to build a space station that would require continuos resupply. Still, the lack of official Chinese information about the maneuvers has certainly allowed room for speculation.
First notice of the satellite bump came from Igor Lissov, a well-respected Russian space observer.
Russia's Interfax-AVN news service quoted him in a story on Aug. 19, and China's state-run Xinhua news agency later picked up the story.
The Russian story suggested that the Chinese space maneuvers may point to the possibility of inspecting both their own satellites and perhaps foreign spacecraft.
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