NATIONAL HARBOR, Maryland – Satellite fleet operator Inmarsat, worried that delays in the introduction of the SpaceX Falcon Heavy rocket will compromise a major new growth initiative, has booked an option to launch the Europasat/Hellas-sat 3 satellite aboard an International Launch Services (ILS) Proton rocket in 2017, industry officials said.

Reston, Virginia-based ILS, which markets Russia’s heavy-lift Proton vehicle, on March 7 announced the Inmarsat booking without naming the satellite or providing a launch date.

But industry officials said the reservation was for the Europasat/Hellas-sat 3, under construction by Thales Alenia Space of France and Italy.

London-based Inmarsat is the second Falcon Heavy commercial customer to have sought a Plan B given the continued uncertainties in the launch schedule of Falcon Heavy, whose inaugural flight has been repeatedly delayed.

Carlsbad, California-based ViaSat Inc. in February moved its ViaSat-2 consumer broadband satellite from the Falcon Heavy to Europe’s Ariane 5 rocket for an April 2017 launch, securing what may be launch-service provider Arianespace’s last 2017 slot for a heavy satellite.

The Europasat/Hellas-sat 3 is co-owned by Inmarsat and Riyadh, Saudi Arabia-based Arabsat and carries a dual mission.

For Arabsat, it’s a Ku-band payload. For Inmarsat, it’s an S-band mobile communications payload to introduce Inmarsat’s European Aviation Network, a hybrid satellite-terrestrial service to provide broadband connectivity for commercial aviation passengers in Europe.

Hawthorne, California-based SpaceX most recently has estimated that the Falcon Heavy’s first flight, of a dummy payload, would occur late this summer. Falcon Heavy customers under time pressure have been reconsidering their options as SpaceX works out the kinks in its new-version Falcon 9 Upgrade vehicle, which has now successfully launched twice.

The first launch, of 11 Orbcomm low-orbiting machine-to-machine messaging satellites, was successfully conducted in December but did not stretch the rocket’s capacity because the Orbcomm payload was both light and destined for low Earth orbit.

Sierra Nevada Corp. of Sparks, Nevada, which built the Orbcomm satellites, on March 7 announced that all 11 had passed in-orbit checkout and that SNC was handing over operations to Rochelle Park, New Jersey-based Orbcomm.

It was not until the SES-9 flight for Luxembourg-based SES that the new-version Falcon 9 was tested to its limit.

The satellite, weighing nearly 5,300 kilograms and destined for geostationary transfer orbit, required that SpaceX load the rocket with a maximum amount of super-cooled, or densified, liquid oxygen — a complicated procedure.

After three launch scrubs and one abort just after ignition, SES-9 was successfully launched on March 4.

SpaceX modified the originally planned trajectory to place SES-9 into a higher, super-synchronous transfer orbit, in effect dropping it off closer to its final destination.

To do that, SpaceX was obliged to use more fuel than it would have otherwise, leaving too little left for much hope of successfully returning the stage for recovery and eventual reuse.

SES Technical Director Martin Halliwell said here March 7 during the Satellite 2016 conference, organized by Access Intelligence, that SES-9 will be in final geostationary position this spring 45 days earlier than it would have been if SpaceX had conducted the launch to maximize the likelihood of returning the rocket’s first stage to Earth.

“They got us to a great orbit,” Halliwell said. “We’re going to burn SES-9’s chemical propellant just about to depletion now and then switch to electric propulsion to circularize the orbit” 36,000 kilometers above the equator.

Halliwell and SES have been major backers of SpaceX and apparently remain so despite the fact that SES-9 had been scheduled to fly by September 2015 at the latest.

SES has four more Falcon 9 launches scheduled, two scheduled for 2016 and two in 2017, and has apparently made no move to secure backup reservations elsewhere.

But these four satellites are part of a long line of SpaceX customers awaiting launches in the coming months and hoping that the SES-9 experience will enable SpaceX to increase its launch rhythm. SpaceX officials have talked about launching twice a month.

Fleet operator Spacecom of Israel, whose Amos 6 launch has grown only more crucial since the November in-orbit failure of its Amos 5, told investors on Feb. 29 that the Amos 6 launch had been delayed again, to between July and September rather than the previously announced date of May.

The Inmarsat decision gives another boost to ILS, which has been struggling to win back commercial customer confidence following several Proton launch failures, all linked to quality control issues. ILS in 2015 signed multi-launch reservations with the Intelsat and Eutelsat, two of the world’s top three commercial fleet operators.