WASHINGTON — Orbital Sciences Corp., which is suing United Launch Alliance (ULA) and one of its main vendors for access to the Russian-made RD-180 rocket engine, could have its day in court as soon as May after a federal judge on Dec. 6 rejected ULA’s argument that the suit should be thrown out.

Court documents filed after the hearing in Alexandria, Va.,show U.S. District Judge Leonie Brinkema denied motions brought by ULA and engine reseller RD-Amross to dismiss the suit. The companies argued, among other reasons, that Orbital had no case under U.S. antitrust laws because the Dulles-based rocket- and satellite-builder has viable alternatives to the RD-180.

Brinkema then ordered Orbital, ULA and RD-Amross — a joint venture of United Technologies Corp. of Hartford, Conn., and Russian rocket-engine maker NPO Energomash of Moscow — to prepare for pretrial conferences that will be overseen by a Virginia magistrate judge. The first of these conferences will take place Jan. 8. The last is set for April 17. Unless the case is settled, it will go to trial within four to eight weeks of the final pretrial conference, according to court documents.

Orbital is eyeing a replacement for the AJ-26 engines that power its Antares medium-lift rocket and has its eye on the RD-180, which RD-Amross sells exclusively to Denver-based ULA, a joint venture of Lockheed Martin and Boeing that launches most U.S. government satellites.

The exclusivity arrangement, Orbital alleged in a lawsuit filed in June with the U.S. District Court for the Eastern District of Virginia, gives ULA a monopoly for “launch systems and services used for medium-class payload missions.”

ULA and RD-Amross wanted the suit to be thrown out, claiming Orbital has a number of alternatives to the AJ-26, and that the RD-180, in any case, cannot be sold to any company but ULA without approval of the Russian government, over which the U.S. court has no jurisdiction.

Orbital, which is seeking $500 million to $1.5 billion in damages, said in a Nov. 12 court filing that it “will have no choice but to exit the market unless it obtains access to the RD-180 given the limited number of 40-year-old, out-of-production AJ26 engines in existence.”

AJ-26 engines are Soviet-era NK33 engines developed for a defunct Moon exploration program and refurbished and rebranded by AerojetRocketdyne of Sacramento, Calif. That company has procured 43 of these engines from the former Kuznetsov Design Bureau in Samara, Russia, of which 20 have been prepared for cargo delivery-and-disposal missions Orbital is flying for NASA to the international space station through 2016.

Orbital has raised concerns that the AJ-26 engines, mothballed for more than 40 years after the cancellation of the Soviet Union’s crewed Moon program, suffer from corrosion. AerojetRockeydyne President Warren Boley said in June that the corrosion issue has been addressed, and that the company has reached an agreement with the AJ-26’s Russian manufacturer to restart production, if Orbital places an order.