The U.S. Air Force launched its fifth flight of the X-37B Orbital Test Vehicle on Thursday, but the long-running project is still mysterious.
We do know the X-37 has been to space four times since April 2010 and that it spends a long time in orbit every time it flies. Its last mission lasted 718 days, or almost two years.
But there's no crew aboard the autonomous ship, and its payloads are mostly classified. The Air Force usually says it's running experiments to test new satellite technology. This time, that includes a sophisticated heat sink that could help electronics run harder and longer in space.
Some experts speculate X-37 flights might be for developing surveillance and reconnaissance tools. And a heat sink like that could be useful on a high-end spy satellite.
The X-37 has hitched previous rides to space on a United Launch Alliance rocket. But for this launch, the Air Force went with SpaceX. The private commercial carrier started doing military contract work in 2016, when it won its first contract to carry a GPS satellite for the Air Force.
While we don't know how long the X-37 will stay in orbit, launching on a SpaceX rocket means the mission will eventually pass a spaceflight milestone: It'll be the first time nearly every part of a rocket that left Earth will land itself.