By Joe Skipper and Steve Gorman

CAPE CANAVERAL, Florida (Reuters) – A planned Monday night launch of the U.S. military’s secretive X-37B robot spaceplane on its seventh mission to orbit, and its first flight atop a SpaceX Falcon Heavy rocket, was postponed for at least 24 hours, SpaceX said.

The flight was called off for the night about 25 minutes before the start of a targeted launch window set to open at 8:14 p.m. EST (0114 GMT Tuesday) as the rocket stood poised for liftoff from NASA’s Kennedy Space Center in Cape Canaveral, Florida.

In a message posted on the social media platform X, Elon Musk’s California-based rocket venture said the countdown was halted “due to a ground side issue,” adding that the “vehicle and payload remain healthy.” SpaceX did not elaborate.

The company did not immediately say when it would try again. The mission’s next launch opportunity is Tuesday night, SpaceX said.

The original plan to send the spacecraft to orbit late on Sunday was scrubbed due to poor weather conditions at the Cape.

The delays came on what was to have been the seventh mission flown by the X-37B, and its first launch atop a Falcon Heavy, composed of three reusable rocket cores strapped together and capable of lofting the vehicle far higher in orbit than ever before.

Roughly the size of a small bus and resembling a miniature space shuttle craft, the X-37B is built to deploy various payloads and conduct technology experiments in long-duration orbital flights.

The X-37B has flown six previous missions since 2010, its last flight lasting well over two years before the vehicle’s return landing in November 2022.

The U.S. Defense Department discloses few details about its operation, which is carried out as part of the military’s National Security Space Launch program.