WASHINGTON — After almost three years of waiting — and six successful cargo flights to the International Space Station — SpaceX’s Falcon 9 should finally win approval to loft NASA science satellites this year, NASA’s lead launch services buyer said.

SpaceX has been working toward NASA certification since it won an $82 million contract in 2012 to launch the French-U.S. Jason-3 ocean altimetry satellite aboard a Falcon 9. The rocket is getting closer to clinching NASA’s formal seal of approval, but “has not yet achieved NASA certification,” said Jim Norman, director of launch services at NASA Headquarters here.

NASA rocket certification consists of three tiers, called categories, that determine which NASA payloads a rocket is cleared to launch. The lowest is Category 1; the highest is Category 3. The higher a rocket’s rating, the more valuable the payloads NASA will entrust to it. Rockets that have never flown must normally start at Category 1 and work their way up the certification ladder with successful launches.

“For over a decade NASA has relied on using propulsion system changes as the most significant factor in declaring a unique configuration that requires certification.”

Falcon 9, Norman said, is skipping Category 1 and proceeding straight to Category 2, thanks to a combination of commercial bookings and ISS missions paid for under a 2008 Commercial Resupply Services contract that, legally speaking, certified SpaceX for cargo deliveries, not rocket launches.

SpaceX’s launch of the Deep Space Climate Observatory on Feb. 11 did not need NASA certification because the $97 million launch was paid for by the Air Force, which is in the midst of certifying Falcon 9 for high-value military launches. The roughly $200 million satellite, originally built by NASA for Earth observations, sat in a hangar for more than a decade before the National Oceanic and Atmospheric Administration adopted it and turned it into a space weather satellite.

Category 2 certification calls for at least one successful mission and possibly as many as six. The current Falcon 9 configuration, which was introduced in September 2013 following five launches powered by the since-retired Merlin 1C engine, has flown 10 times. But SpaceX still needs NASA to formally sign off on the success of those launches and the rocket’s design before certification is finalized.

None of that has happened yet, but Norman said sign-off is on track to happen before late May or early June, when the Jason-3 satellite is scheduled for launch.

SpaceX spokesman John Taylor declined to comment on SpaceX’s certification efforts at NASA.

When SpaceX got the Jason-3 launch contract nearly three years ago, NASA anticipated a December 2014 liftoff. According to Norman, Falcon 9 certification issues are only partly to blame for delaying the launch. Another contributor was the Oct. 28 Antares launch failure that destroyed a shipment of ISS cargo being handled by NASA’s other contract carrier, Orbital ATK of Dulles, Virginia. The Antares accident forced SpaceX to launch one of its ISS supply runs sooner than expected, which forced other Falcon 9 launches to take a back seat, Norman said.

Meanwhile, SpaceX’s main U.S. competitor, United Launch Alliance of Denver, has its own NASA certification journey ahead.

Besides the U.S. military launches ULA has had largely to itself since its inception in 2006, the company has a near lock on flagship NASA science missions, such as the $2.5 billion Curiosity rover that landed on Mars in 2012. Such critical NASA payloads are entrusted only to rockets with a Category 3 certification, which so far only ULA’s flagship Atlas 5 and out-of-production Delta 2 rockets have earned. In a few years, the Atlas 5 will have to earn certification again, because of ULA’s plans to change the rocket’s main propulsion system.

“For over a decade NASA has relied on using propulsion system changes as the most significant factor in declaring a unique configuration that requires certification,” Norman said.

SpaceX had to deal with the ramifications of that rule in 2013, when the company exchanged the Merlin 1C engine that powered the first five Falcon 9 launches for the Merlin 1D the rocket has used ever since. The swap meant Falcon 9 flights that might have counted toward NASA certification suddenly did not.

ULA, unlike SpaceX, is not changing engines wholly of its own volition.

In December, as part of the Defense Authorization Act of 2015, Congress banned the use of Russian-made RD-180 engines for military launches after 2019. Banning the RD-180, which has powered Atlas 5 since 2002 under an exclusive import arrangement with engine maker NPO Energomash of Khimki, Russia, prompted ULA to move out ahead of the law’s passage to secure a permanent replacement.

In September, ULA announced it had chosen the BE-4, a methane-fueled engine made by Blue Origin, Amazon.com founder Jeff Bezos’ secretive Kent, Washington-based spacecraft company.

ULA has said it will unveil details of the revamped Atlas 5 in April. Once the design is settled, the new Atlas 5 can reclaim its Category 3 NASA certification after as few as three flights or as many as 14, depending on the level of NASA scrutiny new ULA Chief Executive Tory Bruno is willing to accept.

“While the new system will require certification, we’re confident this new system will enable us to further reduce costs while continuing to provide the most affordable and reliable launch services to our customers,” ULA spokeswoman Jessica Rye wrote in an email.

In the meantime, NASA can lean on the reliable RD-180 to launch flagship science missions for at least another four years.