By Denise Chow —NBC News
The U.S. military is preparing to launch its secretive X-37B spaceplane on a seventh mission into orbit.
The uncrewed vehicle, which resembles a miniature space shuttle, will blast off Sunday from NASA's Kennedy Space Center in Florida during a 10-minute launch window that opens at 8:14 p.m. ET. For the first time, the X-37B will enter orbit atop a SpaceX Falcon Heavy rocket.
Since its debut more than a decade ago, the X-37B has been a source of intrigue within the space community, primarily due to the mysterious nature of its activities in low-Earth orbit. Despite not knowing its true purpose or location, skywatchers have occasionally spotted and photographed the spaceplane in the night sky using telescopes.
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The U.S. Space Force does not normally disclose classified aspects of X-37B missions. Therefore, little is known about the activities performed by the robotic vehicle in orbit.
For this upcoming flight, dubbed OTV-7, the X-37B will conduct several tests, including "experimenting with space domain awareness technologies and investigating the effects of radiation on NASA materials," according to the Space Force.
On the X-37B's last mission, which lasted 908 days in orbit, the spaceplane lifted off aboard a SpaceX Falcon 9 rocket. Before that, the vehicle traveled into space five times on United Launch Alliance's Atlas V rockets. The Falcon Heavy booster is significantly more powerful than both, allowing the X-37B to operate in "new orbital regimes," Space Force officials said in a statement.
The X-37B orbital test vehicle successfully concludes its sixth mission. United States Space Force
It is not known how long the X-37B will remain in space, or where, or at what altitude it will finally orbit.
The military is silent on such operations, but the Space Force reported that the X-37B missions "are key to ensuring safe and responsible operations in space for all users of the space domain."
"This seventh flight of the X-37B continues to demonstrate the innovative spirit of the U.S. Space Force," Air Force Secretary Frank Kendall said in a statement.
The mini space plane was built by Boeing, and is about 29 feet long. Like NASA's space shuttles, the X-37B is designed to return to Earth and land on a runway.
The vehicle launched on its first mission in 2010 and has since logged 3,774 days in orbit on six flights.
In previous sorties, Space Force officials said the X-37B's civilian science experiments included testing technology to harness solar energy and transmit power to Earth, as well as experiments on how organic materials behave when exposed to the space environment for long periods of time.