Seattle — Richard Branson's Virgin Orbit on Wednesday plans to release a rocket from the wing of a modified Boeing 747 jetliner in mid-air, in a key test of its high-altitude launch system for satellites, a person familiar with the matter says.

The morning “drop test” over California's Mojave Desert is the penultimate test before Virgin Orbit offers commercial satellite launch services, ideally before the end of 2019, the person said.

On Tuesday, the company said on its website that it planned the drop test “in the very near future” but did not offer specifics. A Virgin Orbit representative declined to comment.

Virgin Orbit, Firefly and US-New Zealand company Rocket Lab are among companies designing smaller or nontraditional systems to inject smaller satellites into orbit and meet growing demand.

Competition is fierce. Virgin Orbit has fallen slightly behind Rocket Lab, which has already completed six orbital launches, though Virgin Orbit says its rocket can haul about twice the weight.

Branson may also be losing a competitor in Stratolaunch, which had designed a larger aircraft-and-rocket combination system but is shutting operations, Reuters reported in May.

Virgin Orbit also has a significant potential customer. Branson's Virgin Group has invested in OneWeb's satellite constellation project, which aims to have global broadband coverage in 2021 from about 650 satellites.

Rather than blasting off from a launch pad, Virgin Orbit's LauncherOne rocket will drop at about 10,500m from underneath the company's modified 747 jumbo jet nicknamed Cosmic Girl. Virgin Orbit's subsidiary, Vox Space, is selling launches on the system to the US military, with a first mission slated for early 2020.

Virgin Orbit said in June that it plans to bring its satellite launch system to Japan in partnership with airline operator ANA Holdings, which will provide maintenance and possibly aircraft.

That launch location will join other sites including the US, Guam and UK, which Virgin Orbit says will provide satellite makers and governments more flexibility.

High-altitude launches, Branson argues, allow satellites to be placed in their intended orbit more efficiently while avoiding some cancellations due to inclement whether on the ground.

On Tuesday, Branson's separate space tourism company, Virgin Galactic, announced plans for a stock market listing by the end of 2019 giving it much-needed funds to take on a rival suborbital tourism service being developed by billionaire Amazon.com founder Jeff Bezos.