SpaceX launched a top-secret military spaceplane into orbit

• Elon Musk's SpaceX launched a mysterious military spaceplane into orbit Thursday.
• The X-37B spaceplane holds the record for the longest time spent in orbit.

A mysterious military spaceplane has blasted off on the back of one of Elon Musk's rockets.

The billionaire's rocket company SpaceX launched X-37B, a top-secret spaceplane operated by the US Space Force, into orbit using one of its Falcon Heavy rockets on Thursday.

The launch, which has been delayed several times, is the second time that Elon Musk's rocket company has launched the mysterious spacecraft.

The uncrewed, shuttle-shaped vessel holds the record for the longest time spent in orbit at 908 days, and frequently spends months on end circling the Earth conducting unspecified military operations — leading some to speculate it is being used for spying.

X-37B's true purpose is a closely guarded secret, but the Boeing-built space plane is also used by NASA to conduct experiments such as examining the impact of long-term cosmic radiation on plant seeds.

A Space Force statement previously said that X-37B would be "operating in new orbital regimes" for this mission, possibly suggesting that the space plane could attempt to reach higher and spend longer times in orbit than its previous record.

Space Force, which was announced as the first new branch of the US military in over half a century by then-president Donald Trump in 2018, has become a major customer for SpaceX in recent years.

The military command, which faced ridicule over its Star Trek-esque logo when it was first unveiled, awarded Elon Musk's rocket company $1.23 billion in launch contracts in November.

It's another sign of just how dominant SpaceX has become in the rocket industry, with the US government's reliance on the California-based company furthered by the fact it is the only US provider that can currently carry astronauts into orbit.

SpaceX has conducted 96 launches this year, averaging one every few days. That's more than any of its competitors, with big-name challengers such as Jeff Bezo's Blue Origin struggling to catch up.

SpaceX did not immediately respond to a request for comment from Business Insider, made outside normal working hours.