WASHINGTON- Billionaire Richard Branson's Virgin Orbit aims to reach space for the first time on Sunday by launching a rocket from mid-air, a key attempt after aborting the rocket's first test launch last year.

If conditions on Sunday hold, the Long Beach, California-based company's 70-foot LauncherOne rocket will be dropped from the wing of a modified Boeing Co 747 nicknamed CosmicGirl at 40,000 feet in the air.

The rocket will then ignite its engine to pierce Earth's atmosphere and attempt to deliver 10 small satellites to orbit for NASA, as Virgin looks to prove its technology.

The window for the launch test originating from the Mojave Air and Space Port in southern California begins on Sunday at 1 p.m. EST (0600 GMT).

Virgin executives say high-altitude launches allow satellites to be placed in their intended orbit more efficiently and also minimize weather-related cancellations compared to more traditional rockets launched vertically from a ground pad.

Competition is fierce between Virgin Orbit, Firefly and U.S.-New Zealand company Rocket Lab, which are designing smaller or non-traditional systems to inject smaller satellites into orbit and meet growing demand.

LauncherOne's first attempt to reach space early last year was terminated seconds after being dropped from its carrier aircraft.

The rocket ignited its NewtonThree engine for a few seconds but cut off mid-air due to a broken propellant line, Virgin Orbit's Chief Executive Dan Hart said in July.

Virgin Orbit has fallen behind Rocket Lab, which has already completed some 17 orbital launches, though Virgin Orbit says its rocket can haul about twice the weight.

Virgin Orbit's government services subsidiary VOX Space LLC is selling launches using the system to the U.S. military, with a first mission slated for October under a $35 million U.S. Space Force contract for three missions.

(Reporting by Joey Roulette in Washington Editing by Eric M. Johnson in Seattle and Cynthia Osterman) ((Eric.m.johnson@thomsonreuters.com; +1 206 707 1218; Follow me on Twitter @ByEricMJohnson;))