Last month, a Russian military show, Voennaya Priemka, revealed one of the biggest secrets of the Soviet Union's 1970s space program: the R-23M Space Cannon. A defensive weapon designed to counter the threat of American anti-satellite weapons, the R-23M was a 23-millimeter automatic cannon that could be fired in the vacuum of space. While previously known to have been deployed, this was the first time the cannon was ever shown publicly.

Using footage from the show, Anatoly Zak, of RussianSPaceWeb.com, and others were able to create a three-dimensional model of the R-23M, published yesterday by Popular Mechanics. Fired only once in its lifetime—just as the space station carrying it was preparing to be decommissioned through a fiery re-entry—the R-23M served aboard three Almaz Orbital Piloted Stations (OPS). The Almaz program, originally undertaken in the 1960s as a purpose-built military reconnaissance space station with a reusable crew return module, was folded into the Salyut program in the 1970s.

The R-23M was based on the tail gun of the Tupolev Tu-22 "Blinder" bomber. It weighed 37 pounds and had a fire rate of above 950 rounds per minute, Zak reported, "blasting 200-gram shells at a velocity of 690 meters per second (1,500 miles per hour). According to veterans of the Almaz project, the space cannon successfully pierced a metal gasoline canister from a mile away during its ground tests."

But there were a few problems with firing the cannon. First, to aim the weapon, the entire space station had to be turned into position to fire—meaning that the crew had to have plenty of early warning to use it. An onboard optical sight provided fire control. The ship also had to burn up fuel-firing thrusters to counter the recoil of the cannon. And firing the gun caused considerable shaking aboard the station.

Only three Almaz stations were ever launched into orbit. The first, OPS-1, sustained a hull breach at launch because of a faulty weld and was never manned. OPS-2, launched in 1974, had but one crew aboard (the second crew's Soyuz suffered a failure of its rendezvous system and had to return without docking). Three bursts of the OPS-2's cannon were fired remotely before its re-entry—about 20 rounds.

After the third manned Almaz mission, the cannon was scrubbed—and so were further manned spy station missions. An unmanned variant of Almaz continued to operate into the 1990s.