PAROS is an acroynm for Prevention of an Arms Race in Outer Space, and can refer to a proposed treaty or any other kind of multilateral body of work intended to address or circumvent what is perceived as the militarisation of space.

It is argued that an arms race in outer space would be undesirable to international security. Countries will be locked into diverting more resources into developing space weapons. These weapons could threaten the existing balance between states, rendering existing stockpiles redundant - and their ability to serve as a deterrent. They will also take up "space" in low Earth orbit trajectories, crowding out civilian satellites]. And should a conflict break out involving satellites there is concern that outer space would be left littered with , harmful to manned and unmanned space vehicles.

Since the 1980s the US military's ability to wage swift, conclusive and relatively bloodless wars (most of the time) has been achieved through space-based surveillance systems. This is an edge the Chinese and Russians don't have, and so they have been eager to develop legal avenues that would remove the American's advantage.

Within the United Nations there is a general consensus that the militarisation of space should be prevented, and that discussion on the matter should progress. However several states - mostly Western] - are opposed to negotiating a PAROS Treaty. There is concern that in defining "militarisation" a proposed Treaty's scope could encompass non-lethal assets vital for military purposes, such as satellites used for command and control, communication, monitoring, early warning and navigation with the Global Positioning System - stuff the Americans want.

The Americans argue - not without reason - that a PAROS treaty would be difficult to verify, and with many key concepts remaining undefined they would not want their space assets and defence strategy left in the hands of a legal interpretation. The rest of the West seek a middle line, or believe that disarmament would be better served dealing with more here-and-now matters, such as the enrichment of fissile material.

And how have the Russians and Chinese tried. At the Conference on Disarmament from 1985 to 1994 there was an ad hoc committee on PAROS. At the United Nations General Assembly (UNGA)'s First Committee for Disarmament and International Security there have been repeated attempts to bring up PAROS, with the only successful resolutions agreeing only to further discussions and submissions (which the US, alone, opposes).

Recently some states have taken a new tact in UNGA by advocating the use of "Transparency and Confidence Building Measures" that countries would be required to compile, accounting for their outer space activities. However even the Chinese and Russians have acknowledged that the degree of verification needed that would make TCBMs credible remains technically impractical.

The situation remains static. China, demonstrating its impatience and displeasure on PAROS and other space- related treaties, shot down with a ground based missile one of its ageing weather satellites on January 11 2007. This led to China's international censure, and more debris circulating in low orbit, but it is doubtful any country will change their position as a result.