Two military officers died while cleaning out a propellant storage tank at the Plesetsk Cosmodrome on November 9. The Russian authorities promptly launched an investigation into the accident, which is the fifth official fatal accident at the launch base. The preliminary result of the investigation blamed the negligence of the two dead officers who were supposed to clean a railway tank containing nitrogen tetroxide (N2O4), which is used in rocket motors as an oxidant for unsymmetrical dimethyl hydrazine (UDMH). Launch pad 14P25 where the accident occurred includes two large tanks of UDMH and N2O4.

The officers were supposed to drain the tank before proceeding with further cleaning. However, as reported by, the two inhaled nitrogen vapor and lost consciousness, falling at the bottom of the tank itself. According to the investigators, four of their fellow military colleagues tried to reach them wearing ventilation masks, but they lost consciousness too under the influence of the nitrogen vapor. Another military unit participating in the cleaning process rescued the four colleagues, but nothing could be done for the two officers who laid at the bottom of the tank for 50 minutes. The survivors were taken to the hospital with acute N2O4 inhalation poisoning. N2O4, best known in Russia as Amyl, is a highly dangerous chemical compound, which, if inhaled, can cause pneumonia and edema.

The investigation portrayed one of the most common kinds of confined spaces accidents. Ventilation masks were not effective in this situation simply because there was no oxygen to filter in the enclosed space of the tank; it had been displaced by N2O4 related gases

During the years of activity of the Plesetsk Cosmodrome, there have been quite a number of fatal accidents. In 1973, a Cosmos-3M rocket exploded while it was being prepared for launch and nine people were killed. In 1980, a Vostok-2M rocket exploded during refueling, killing 48 people. In 1987, a military unit caught fire, killing five people. In 2002, a person died following the explosion of a Soyuz-U rocket carrying ESA’s Foton-M1 satellite.

The November 9th accident occurred while European Space Agency’s Swarm satellites are being prepared for liftoff aboard a Rockot launch vehicle, scheduled for November 22. ESA and Russian officials denied that the propellant tank cleaning was related to any operations for the Swarm mission; the incident  will not prevent the planned launch.

Image caption: A military officer standing at the gate of a launch pad at Plesestk Cosmodrome (Credits: