Since the end of the U.S. space shuttle program last year, Russian craft are the only means to send crew to and from the ISS.
The unmanned Phobos-Ground probe was to have gone to the Mars moon of Phobos, taken soil samples, and brought them back. But it became stuck in Earth orbit soon after its launch Nov. 9. It fell out of orbit Jan. 15, reportedly off the coast of Chile, but no fragments have been found.
The failure was a severe embarrassment to Russia, and Popovkin initially suggested it could have been due to foreign sabotage.
But on Tuesday he said in televised remarks that an investigation showed the probable cause was "localized influence of heavily radiated space particles."
Popovkin, speaking in the city of Voronezh, where the report was presented to Deputy Prime Minister Dmitry Rogozin, said two units of the Phobos-Ground probe's onboard computer system went into an energy-saving "restart" mode, apparently due to the radiation, while the craft was in its second orbital circuit.
It was not immediately clear why the units could not be brought out of that mode.
Popovkin said that some microchips used on the craft were imported and possibly of inadequate quality to resist radiation. He did not specify where the chips were made.
Yuri Koptev, a former space agency head who led the Phobos-Ground investigation, said 62 percent of the microchips used in the probe were "industrial" class, a less sophisticated level than should be used in space flight.
Popovkin said the craft's builder, Moscow-based NPO Lavochkin, should have taken into account the possibility of radiation interfering with the operation and said Lavochkin officials would face punishment for the oversight.
Popovkin later said that a March 30 planned launch of two Russian cosmonauts, Gennady Padalka and Sergei Revin, and NASA's astronaut Joseph M. Acaba to the space station would be postponed "likely until the end of April" because of problems with the capsule. He did not offer details, but the state news agency RIA Novosti cited the director of Russia's cosmonaut-training program as saying leaks had been found in the capsule's seals.
