Spy satellites have a new role: as well as watching us they are now spying on each other.

The Pentagon admitted last week that it is using two covert inspection satellites developed for the Defense Advanced Research Projects Agency to assess damage to a failed geostationary satellite - something no one suspected the US could do. If such satellites can get that close to a target, they could probably attack it.

The Department of Defense says its Mitex micro-satellites, which were launched in 2006, have been jetting around the geostationary ring and have now jointly inspected DSP 23, which was designed to pinpoint clandestine missile launches and nuclear tests, but which stopped working a year after its November 2007 launch. The micro-satellites are trying to nail the problem.

Theresa Hitchens, who becomes director of the UN Institute for Disarmament Research in Geneva this week, is troubled by the secrecy surrounding launch of the Mitex craft. It raises questions about their future use, including potential anti-satellite missions, she says.

"I am positive other nations, particularly China, will find this development suspicious - and the US behaviour regarding the programme as hypocritical, given that Washington is always chastising Beijing for its lack of transparency regarding its space programmes and intentions," she says.