PARIS — Russia’s Proton rocket will return to service at the end of December to launch a large commercial telecommunications satellite following a government inquiry that found the vehicle’s Dec. 5 failure was caused by overfueling of its upper stage, Russian and International Launch Services (ILS) officials said Dec. 10.

The state commission investigating the failure, in which three Russian Glonass timing and navigation satellites were destroyed, has cleared Proton’s three lower stages from any involvement in the malfunction. Commercial Proton rockets marketed by Reston, Va.-based ILS use the same lower three stages but a different upper stage, called Breeze M. The Glonass launch used a new version of the Russian Block DM upper stage.

The Russian space agency, Roscosmos, on Dec. 10 confirmed the commission’s preliminary finding that the three Proton stages need not be grounded. A final report is due as soon as the week of Dec. 13.

James M. Bonner, chief technical officer for ILS, said the new version of the Block DM stage — which is built by RSC Energia of Korolev, Russia — features larger propellant tanks.

In what appears to have been a remarkable oversight, the personnel fueling the Block DM stage for the Glonass launch did not account for the larger tanks. That led to loading between 1,000 and 2,000 kilograms more propellant on the Block DM stage than what had been planned for the Glonass mission. Like the U.S. GPS navigation satellites, the Glonass system operates in medium Earth orbit.

As a result of the excess propellant, the Proton’s third stage, suffering from the additional weight it was carrying, underperformed, placing the Block DM stage and the stack of Glonass satellites into a lower-than-planned suborbital drop-off point.

ILS is owned by Khrunichev State Research and Production Space Center of Moscow, which is prime contractor for Proton’s three lower stages as well as for the Breeze M upper stage.

In an interview, Bonner said ILS will spend the week of Dec. 13 in Moscow reviewing the state commission’s findings. After consulting with insurance underwriters and with Paris-based Eutelsat, whose Ka-Sat satellite is ILS’s next Proton passenger, ILS will move Ka-Sat’s launch from Dec. 20 to an undetermined date in late December, he said.

“We will be there to do our due diligence and to review the state commission’s findings,” Bonner said. “Our assumption now is that this will cause a delay of seven or eight or up to 10 days.”

Russian holidays the first week of January argue against planning a launch during that period.

The state commission investigating the failure was led by G.G. Raikunov, director general of Russia’s state-owned TsNIIMash space engineering services company.

In a statement of preliminary findings that Raikunov signed Dec. 10, the commission says: “[Telemetry] data analyses … show that no issues with the functioning of [Proton’s three-stage] systems and assemblies have been detected. … In view of the above, the Interdepartmental Commission deems it possible to proceed with further technical facility processing operations of [the launch vehicle] to launch … Ka-Sat per the approved schedule.”

The Dec. 5 failure will delay Russia’s goal of placing the Glonass constellation on a par with the U.S. GPS system by late 2011.

The Russian government has made restoring Glonass to full operational capacity a major investment priority in recent years. It has been aided in this effort by gradual improvement in the operating lifespan of newer Glonass-M satellites, which are designed to function for seven years in orbit, compared with three years for the first-generation Glonass spacecraft.

A fresh design, called Glonass-K, is scheduled to be tested later this month in a launch aboard a Russian Soyuz rocket, using a Fregat upper stage, from northern Russia’s Plesetsk Cosmodrome.

Glonass-K satellites have 10-year lifetimes in addition to more broadcast signals. In a statement published on the Roscosmos website following an interview with Russia’s Izvestia news agency, Roscosmos Director Anatoly N. Perminov said the Glonass-K demonstration satellite to be launched in mid-December would now be placed into operational service to help mitigate the effects of the loss of the three earlier-generation spacecraft. The satellite is in final testing at manufacturer ISS Reshetnev in Krasnoyarsk, Russia.

The Glonass system now counts 20 operational satellites and two in-orbit spares, Perminov said. While only 18 operational satellites are needed for full Russian coverage, it takes 24 to provide a global service.

With a Glonass-K satellite launching in the coming weeks, and the integration of the two in-orbit spares, the constellation will have 23 operational spacecraft.

ISS Reshetnev is completing another Glonass satellite, presumably a Glonass-K version, that Perminov said would be ready for launch in three or four months. The possibility of accelerating production of other Glonass satellites is under study, he said.

Glonass program managers had hoped that Glonass would approach the accuracy of the U.S. GPS system — both are 24-satellite constellations in medium Earth orbit — by the end of 2011.

Russian officials also are preparing to launch Glonass signal verification payloads on Russian Luch data-relay satellites in geostationary orbit starting in 2011. This geostationary overlay, similar to architectures already in operation in the United States, Europe and Japan, is called the System of Differential Correction and Monitoring (SDCM).

Three Luch satellites are expected to be equipped with SDCM payloads and launched in 2011, 2012 and 2013 into orbital slots at 16 degrees west, 95 degrees east and 167 degrees east, respectively.

Perminov said the Proton rocket, which for ILS launches uses the Breeze M upper stage and for some government launches uses the older Block DM stage, remains one of the world’s most reliable vehicles, with a 96 percent success rating. He noted that the Breeze M caused a Proton launch failure in 2008, and that the Block DM stage “has not had any problems … for almost 15 years. Proton DM has launched six Glonass satellites in 2010, and the injection accuracy was very high.”

Peter B. de Selding was the Paris bureau chief for SpaceNews.