Stratolaunch Systems, which was established in 2011 by the late Microsoft co-founder Paul Allen, got the giant plane up to 90 mph (145 km/h) during "medium-speed taxi testing" at California's Mojave Air and Space Port earlier this month.
"Paul Allen's vision for #Stratolaunch continues to take form," company representatives said last week via Twitter, in a post that also shared video of the taxi test. [Stratolaunch Test Photos: The World's Largest Plane in Action]
Stratolaunch's dual-fuselage plane features a wingspan of 385 feet (117 meters) — greater than the length of a football field, including the two end zones. The vehicle is designed to haul satellite-carrying rockets up to an altitude of about 35,000 feet (10,700 m), at which point the launchers will drop away and power their payloads up to orbit.
This air-launch strategy will enable satellites to be lofted relatively cheaply and frequently, and with a great deal of flexibility, company representatives have said.
A variety of different rockets will eventually fly between the two fuselages, if all goes according to plan. For example, Stratolaunch plans to begin using the Pegasus rocket, which debuted in 1990 and has more than 40 flights under its belt, for the company's first operational missions in 2020.
Pegasus can haul about 815 lbs. (370 kilograms) to low-Earth orbit. Stratolaunch is also developing two more powerful rockets, known as the Medium Launch Vehicle (MLV) and the MLV-Heavy, which will be able to loft about 7,500 lbs. (3,400 kg) and 13,200 lbs. (6,000 kg), respectively. The MLV's first flight is targeted for 2022, whereas the MLV-Heavy is still in early development, company representatives said.
Stratolaunch is also working on a fully reusable space plane that could carry satellites or people. This vehicle is in the design-study phase.
Stratolaunch isn't the only company working to launch space missions from the air. Virgin Orbit recently mated its LauncherOne rocket and Cosmic Girl mothership for the first time (on the ground), and Virgin Galactic is performing rocket-fired test flights of its six-passenger SpaceShipTwo suborbital space liner.
SpaceShipTwo launches from the belly of a plane called WhiteKnightTwo.
Mike Wall's book about the search for alien life, "Out There," will be published on Nov. 13 by Grand Central Publishing. Follow him on Twitter @michaeldwall. Follow us @Spacedotcom or Facebook. Originally published on Space.com.