SpaceX has postponed the next launch of its Falcon Heavy rocket until at least Tuesday evening, delaying the latest mission of a mysterious space plane that remains one of the US military’s most fascinating projects as the country races to journey deeper into the cosmos.

The secretive X-37B robotic spacecraft was set to take off aboard a SpaceX Falcon Heavy rocket at 8:14 p.m. ET from NASA’s Kennedy Space Center in Florida on Monday. But the company said Monday evening that it was pushing back the launch “due to a ground side issue” — indicating a problem with the rocket’s launchpad or fueling systems, according to a social media post from SpaceX.

The company said it is now working toward lifting off at the next available launch opportunity, which is Tuesday at 8:14 pm ET.

Resembling a miniature NASA space shuttle with the windows blacked out, the reusable X-37B space plane is set to begin its seventh experimental mission.

Many of the X-37B’s mysterious tasks have been classified, but the US Space Force did provide a few details about the goals for this uncrewed mission.