PARIS — Mobile satellite services provider Inmarsat on July 2 said it had booked one firm launch and two options — two with satellites already identified — aboard the Space Exploration Technologies Corp. Falcon Heavy rocket scheduled to make its inaugural flight in 2015.

London-based Inmarsat said that while Falcon Heavy was the nominal vehicle to be used, the contract foresees the use of SpaceX’s current Falcon 9 vehicle if needed to keep to the schedule. The satellite for which Inmarsat has a firm order for a Falcon Heavy is expected to weigh 5,900 kilograms at launch, which would be an exceptionally large payload for Falcon 9, even if the satellite makes use of electric propulsion for part of its mission.

The firm contract is for the launch, scheduled perhaps aggressively for late 2016, of a satellite being built for both Inmarsat and Arabsat of Riyadh, Saudi Arabia. Arabsat will use the satellite for conventional telecommunications services for its wholly owned Hellas Sat fleet operator of Greece. The Inmarsat payload uses S-band to provide mobile communications in Europe as part of a satellite-terrestrial broadband network, which is a new business line for Inmarsat.

Inmarsat said in June that its S-band satellite would cost about $200 million including construction, launch and insurance. Industry officials said the figure pointed to a SpaceX launch. SpaceX has captured substantial commercial satellite business, despite its rocket’s relatively short track record, with prices that are substantially lower than the competition.

A second satellite likely to be launched under the Inmarsat-SpaceX contract is the fourth satellite in Inmarsat’s Global Xpress Ka-band military and commercial mobile broadband network.

Inmarsat elected to launch all three Global Xpress satellites aboard Russian Proton rockets, commercialized by International Launch Services of Reston, Virginia. Proton’s spotty reliability record of late, with the latest failure in May, has caused bottlenecks in its launch manifest. The latest failure review has not been fully digested by ILS, and Proton’s return to flight, while expected by September, is not yet certain.

Inmarsat’s order of a fourth satellite — like the first three, from Boeing Space and Intelligence Systems of El Segundo, California — was in part a hedge against a possible launch failure, and in part a way to augment the three-satellite Global Xpress system capacity. Boeing has said it would deliver this satellite in time for a launch in mid-2016.

Inmarsat said a third SpaceX launch could be for Inmarsat’s next-generation L-band satellites, Inmarsat 6. These satellites have not been contracted.

The Boeing-built Global Xpress satellites weigh about 6,100 kilograms at launch — presumably beyond the reach of the Falcon 9 rocket. The firm SpaceX contract for the S-band satellite being built by Thales Alenia Space of France and Italy is likely to include electric propulsion for its in-orbit station keeping, a way to keep its launch weight low enough to fit into a Falcon 9.

Hawthorne, California-based SpaceX has already booked two commercial launches for satellites weighing between 5,200 and 5,300 kilograms.

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