The US Army's XM1063 projectile

XM1063 is the code name for the US army's new secret weapon which will "suppress" people without harming them, as well as stopping vehicles in an area 100m square. But is it a violation of chemical weapons treaties, or a welcome move towards less destructive warfare using non-lethal weapons?

Exactly how it works is classified, but we have established some details. The first part of the weapon is an artillery round - or as the army puts it, "a non-lethal personal suppression projectile" - fired from a 155mm howitzer, with a range of 28km. It scatters 152 small non-explosive sub munitions over a 1-hectare area; as each parachutes down, it sprays a chemical agent. Development was overseen by the US Army's Armament Research, Development and Engineering Center (Ardec).

A presentation by the makers, General Dynamics, says the XM1063 will "suppress, disperse or engage personnel" and "deny personnel access to, use of, or movement through a particular area, point or facility"

Experts suggest three possible payloads: an existing riot-control agent, malodorants or a new chemical agent. Existing agents include CS gas and a form of pepper spray. But these seem unlikely choices, because their effects only last minutes, and could wear off before friendly forces arrive. They could also face a legal challenge: the 1997 Chemical Weapons Convention prohibits the use of riot control agents in warfare

"The matter is further complicated if pepper gas was used as the irritant since this is a plant toxin," says Steve Wright of Leeds Metropolitan University. "Such toxins are explicitly banned."

The possibilities seem to boil down to anti-traction agents (which make the whole area impossibly slippery), a malodorant or some novel chemical agents.

Anti-traction agents are possible, but seem unlikely because research in this area (such as Darpa's Black Ice program) still seems to be at an early stage. It would be unusual for an agency to still be doing basic research when another is about to field a finished product.

A malodorant is a super stinkbomb with a truly intolerable smell. The Pentagon has been working on such chemicals for years, and a recent US army briefing on future artillery concepts specifically mentions artillery-delivered malodorants

Until the 1980s, the US maintained stockpiles of a chemical incapacitant known as BZ or Agent Buzz. BZ is a psychoactive chemical causing stupor, confusion and hallucinations lasting for more than 24 hours. It has an evil reputation, but this is based largely on rumour as few facts are available. Most people have only heard of BZ in connection with the film Jacob's Ladder. This depicted soldiers exposed to a secret chemical weapon in Vietnam with terrible results, including permanent psychosis.

"We are reaping the whirlwind today because of government secrecy in the past," says Jim Ketchum, who ran the BZ testing program in the 1960s. "It has allowed critics to make unsupportable claims about agents such as BZ without rejoinder from the government research community." Although the US is known to have been active in this area since 2000, no comments are available from researchers on non-lethal chemical agents - now termed "calmatives", whatever their chemical action.

Testing of the XM1063 was completed successfully last year and it is due for low-rate production from 2009. Ardec says that the production decision is on hold awaiting further direction from the program manager. It seems the decision on whether to enter a new age of chemical warfare now rests with the military rather then civilians. Unless put under pressure, the US Army seems unlikely to give any details of what's in the surprise package until it is used. And maybe not even then.