By Paul Rincon
Science reporter, BBC News
The destruction of a rogue spy satellite by the US military has stoked concerns about a new arms race in space.
The shootdown was described as a 'cover' by Russia's defence ministry
The US has denied Thursday's destruction of the disabled spy satellite USA 193 was a response to an anti-satellite test carried out by China last year.
The prospect of hostile operations against satellites already influences how military spacecraft are built and operated.
Satellites such as Skynet 5, operated by the UK Ministry of Defence, carry technologies designed to resist any interference - attempts to disable or take control of the spacecraft - and any efforts to eavesdrop on sensitive communications.
An advanced receive antenna allows the spacecraft to selectively listen to signals and filter out attempts to "jam" it.
But the latest actions are certain to influence how countries plan for protecting their intelligence capability in space.
Applying armour to spacecraft in order to protect them against a missile is a non-starter. No amount of shielding would be any use against a missile hitting the satellite at a closing velocity of 10km/s (22,000mph).
Military space expert Dr Stuart Eves thinks that future spacecraft are likely to incorporate stealth technology in order to better avoid detection.
"The threat side of this could be implemented by techniques a bit like those used on aircraft, such as using radar absorbent materials, radar reflective materials, or painting the satellite black," Dr Eves, from Surrey Satellite Technology Limited (SSTL), told BBC News.
But the solar panels for powering the satellite would be more difficult to hide from radar. The infra-red signature of a satellite is also hard to conceal.
Skynet 5 is hardened against "jamming" attempts
Dr Eves adds: "One of the other issues is that the ability to track objects in space from the ground extends down to objects about 10cm in size - that's effectively what you can do with existing radars.
"One of the ways of being stealthy is to be quite small and hard to see."
PalmSat, being built by SSTL and the University of Surrey, is about the size of a small soft drink can and weighs only 1kg. But it is designed to have all the sub-systems associated with a normal satellite.
"Once you get to doing something tolerably useful with something that small - which is what they are planning to do - people aren't necessarily going to know what is up there and what it is doing," said Dr Eves.
There is a secondary advantage to being stealthy. If a state knows when a foreign reconnaisance satellite is due to fly overhead, it can carry out any sensitive operations it might want to hide from the other side when the spacecraft is on the other side of the Earth.
Making satellites stealthier increases the chances of catching the other side in the act.
But the need for high quality data places limits on satellite miniaturisation.
Other experts say that "redundancy" - which essentially means having a duplicate satellite ready to launch - is the best strategy for protecting intelligence capability in a situation where satellites might be threatened by enemy operations.
Meanwhile, the US Air Force (USAF) is working on an intriguing programme: a small experimental satellite concept that would orbit close to a friendly spacecraft, keeping tabs on their surrounding space environment.
The Autonomous Nanosatellite Guardian for Evaluating Local Space, or Angels, would be the size of a small suitcase, and during launch would piggyback on the satellite that it is intended to shadow.
Demons and angels
In orbit, the Angels would detach from the main satellite and fly in formation with them. If anything went wrong with the host - such as the solar panels failing to deploy properly - the Angels could move around to inspect them and diagnose the problem.
Stuart Eves has coined the term Deployable Monitoring Nano-Satellites (Demons) to describe small satellites which could, for example, sneak up on an enemy reconnaissance satellite to photograph them.
The XSS-11 navigated around other spacecraft in low-Earth orbit
But he concedes that formation flying with an enemy satellite would be much harder, as the Demon would have to second-guess its adversary's next move.
Nevertheless, the USAF's XSS-11 satellite, launched in 2005, has demonstrated the ability to navigate autonomously around other spacecraft in low-Earth orbit.
The 2007 Chinese anti-satellite test and with the American shootdown of USA 193 both provoked criticism from other powers.
But there is, ultimately, a strong deterrent for any state planning to obliterate enemy satellites: it could generate enough orbital debris to threaten its own space assets.