A mysterious Chinese spacecraft returned to Earth after an unknown 276-day mission.

Chinese state media Xinhua News reported that a "reusable experimental spacecraft" landed in Northwest China after "276 days of in-orbit operation." The mission was described as an "important breakthrough in China's research on reusable spacecraft technologies."

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However, the mission raised eyebrows elsewhere, due to the mysterious nature of the flight, and the spacecraft in general. The government hasn't released any photos of the craft, and information on it is minimal, Space News reported. The characteristics, instruments tested, and even where the craft went are all a mystery.

However, the limitations of China's Long March 2F rocket mean that the craft could not have exceeded eight tons.

The China Aerospace Science and Technology Corp. (CASC), the spacecraft's creator, stressed in its announcement commemorating the landing that it was a breakthrough for the "peaceful" use of space.

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This was the second trip taken by the spacecraft. The first trip only lasted about four days, according to Space News. United States Space Force tracking data revealed last year that the spacecraft released a small object during its orbit.

The development may raise alarm in the U.S., which is growing increasingly concerned about China's ambitions in space, including possible military ambitions. NASA Administrator Bill Nelson warned in January that the U.S. is entering another space race with China.