WASHINGTON — Virgin Orbit performed a key test of its LauncherOne air launch system July 10, dropping an inert vehicle from its carrier aircraft in one of the last milestones before the vehicle’s first orbital launch attempt.

Virgin Orbit’s Boeing 747 aircraft took off from the Mojave Air and Space Port in California at 11:43 a.m. Eastern, carrying a full-sized version of the LauncherOne rocket, although filled with water rather that propellants. A half-hour later, the plane released the rocket at an altitude of 10,700 meters above a test range at nearby Edwards Air Force Base.

The test was designed to test the dynamics of the release of the rocket, which on an actual mission would be allowed to fall for several seconds before igniting its first-stage engine. On this test, the rocket simply fell to the ground, impacting in an isolated test area.

The company said the test was a success. “The whole flight went incredibly well. The release was extremely smooth, and the rocket fell away nicely,” Kelly Latimer, Virgin Orbit’s chief test pilot, said in a statement after the flight. The plane behaved during the release as it did in simulations, she noted. “This was the best kind of test flight sortie from a test pilot’s perspective — an uneventful one.”

The drop test wrapped up the flight test program for Virgin Orbit. That program started with a series of flights to inspect its handling with a launch vehicle adapter installed on the plane’s left wing. The company then performed several captive carry flights, with a LauncherOne attached to that adapter but not released.

“Today’s test was a monumental step forward for us,” Dan Hart, chief executive of Virgin Orbit, said in a statement. “It’s the capstone to a thorough development program not just for a rocket, but for our carrier aircraft, our ground support equipment, and all of our flight procedures.”

Parallel to the flight test program, the company has been building and testing LauncherOne’s first and second stages. Virgin Orbit says it has completed “full thrust, integrated hotfires” of both stages on the ground.

Virgin Orbit is now finalizing the assembly of the LauncherOne that will fly the first orbital mission. That vehicle will be completed later in July, the company said, and then undergo testing before its first launch. The company said in a statement only that it expects to perform the launch “later this year,” but industry sources said they expect the company to be ready to attempt a launch in late summer or early fall.

“I’m extremely proud of the team for getting us to this point, and for their spectacular performance today,” Hart said of Virgin Orbit’s employees in the statement. “I’ve told them to take a few hours now to celebrate — our first launch campaign begins in the morning.”