(Bloomberg) -- A crucial test launch of Richard Branson’s Virgin Orbit, a two-stage, orbital rocket system, was postponed Sunday.

“We have one sensor that is acting up,” Virgin Orbit said on Twitter.

“This means we are scrubbed for today,” the company said. “We’ll provide an update on the new launch target later today.”

Branson’s system is a rival of Elon Musk’s SpaceX for satellite launches and has so far undergone 20 tests. The test was billed as “the apex of a five-year-long development program.”

On Wednesday, Musk faces his own biggest test: launching human beings into orbit.

A SpaceX Dragon capsule atop a Falcon 9 rocket will blast off from Cape Canaveral, Florida, at 4:33 p.m. Two NASA astronauts -- Bob Behnken and Doug Hurley -- will be on board, with a docking at the International Space Station scheduled for 19 hours later.

For Branson’s test, a Boeing Co. 747, named Cosmic Girl, was scheduled to take off from the Mojave Air and Space Port in California carrying beneath it Virgin Orbit’s LauncherOne rocket over the Pacific Ocean, which would have been released midair for ignition.

The company’s ultimate goal is to use its rockets to launch small satellites into space, competing with ground-based launches, such as those from Space Exploration Technologies Corp.

“We’ve got a straightforward path to address this minor sensor issue and recycle quickly,” Virgin Orbit said.

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