Sir Richard Branson's Virgin Orbit company has failed in its first test launch of a new rocket.
The inaugural launch off the coast of Los Angeles appeared to be going well after it was dropped from a Virgin jumbo jet called Cosmic Girl on Monday afternoon.
But it was "terminated" soon after, the company said on Twitter.
If LauncherOne had successfully hit an altitude of 50 miles, it would have marked the first time this kind of launch system has reached space.
Virgin Orbit said in its official Twitter commentary on the launch: “We’ve confirmed a clean release from the aircraft. However, the mission terminated shortly into the flight.
"Cosmic Girl and our flight crew are safe and returning to base.”
There was no immediate word on what went wrong with the rocket, which carried a test satellite.
The highly modified jumbo jet took off from Mojave Air and Space Port in the desert, north of Los Angeles, and flew out just beyond the Channel Islands, where the drop occurred.
The rocket was supposed to fall for a few seconds before the first of its two stages ignited and hurtled it down the coast towards the South Pole for insertion of its demonstration payload into a low Earth orbit.
Virgin Orbit has six additional rockets under construction in its factory and said it will "keep moving forward" following the failed launch.
“The team’s already hard at work digging into the data, and we’re eager to hop into our next big test ASAP,” the company tweeted.
“Thankfully, instead of waiting until after our 1st flight to tackle our 2nd rocket, we’ve already completed a ton of work to get us back in the air and keep moving forward.”
SpaceX founder Elon Musk tweeted to say he was "sorry" the test flight was unsuccessful. He said on Twitter: "Sorry to hear that. Orbit is hard. Took us four attempts with Falcon 1."
On Sunday afternoon Mr Branson's company delayed the historic test flight when a sensor started "acting up".
The purpose of the flight was to gather data on every step of the launch process rather than to have a useful satellite in orbit.
The attempt followed five years of development of the 70-foot-long LauncherOne rocket.
Will Pomerantz, Virgin Orbit’s vice president for special projects, commented during a preflight briefing on Saturday that about half of first rocket launches fail.
“History is not terribly kind, necessarily, to maiden flights,” he said.
Chief executive officer Dan Hart said during the briefing that there had been numerous tests, discussions and introspection to verify that the system was ready.
“In the end the questions are always, has everything been thought about and are there any gaps or seams, and those are the questions you only learn when you commit to flight,” Mr Hart said.
Virgin Orbit is targeting the market for launching satellites ranging in size from toasters to household refrigerators.
The company says it has dozens of missions on the books for customers including the US Space Force and the Royal Air Force.
Internationally, it is working on plans for launches from the UK and Japan.