PARIS — The U.S. company that markets Russia’s Proton heavy-lift rocket, aiming directly at a key sales advantage touted by its principal European competitor, on Feb. 11 announced it will launch a Western-built commercial telecommunications satellite alongside a Russian-built satellite on the same vehicle in 2011.

Reston, Va.-based International Launch Services (ILS) said the SES-3 satellite, built by Orbital Sciences Corp. of Dulles, Va., and owned by SES of Luxembourg, will be placed into geostationary transfer orbit on a Proton vehicle that also will be carrying the Russian-built Kazsat-2 telecommunications satellite, owned by the government of Kazakhstan.

ILS has been saying for months that a series of upgrades to the Proton and a development program run jointly with Orbital Sciences should enable the vehicle to loft two 3,000-kilogram Orbital-built commercial satellites at a time.

The design, which ILS calls Proton Duo, would not require a heavy adaptor, which would put the combined mass beyond Proton’s lift capability.

The 2011 launch will go halfway there, with a single Western spacecraft and a Russian co-passenger. Proton manufacturer Khrunichev State Research and Production Space Center of Moscow in February 2009 launched two Russian-built satellites — the Khrunichev-built MD-1 and the Express-AM44 built by ISS Reshetnev of Krasnoyarsk — on the same Proton.

Dual launches have long been a trademark of Arianespace of Evry, France, whose current workhorse vehicle — the heavy-lift Ariane 5 — was designed specifically to take two passengers at a time to geostationary transfer orbit, which is the destination of most telecommunications satellites. Arianespace uses an adaptor weighing several hundred kilograms to separate the two satellites as they experience the high vibrations that accompany a rocket’s transit through the atmosphere.

ILS is owned by Khrunichev, which is also the manufacturer of the Kazsat-2 spacecraft.

SES spokesman Yves Feltes said the launch will be a first for ILS and Proton but that the risks appear well within SES’s comfort range.

“We have been part of dual launches with Ariane for years,” Feltes said. “In this case, Khrunichev is launching us along with a Khrunichev-built satellite on a Khrunichev-built rocket. We are confident about it.”

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