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[listen to sound of plane landing]

That is the sound of the United States Air Force's unmanned spacecraft landing at the Kennedy Space Center in Florida over the weekend.

The United States Department of Defense posted a video of the landing on YouTube.

The Air Force's X-37B spacecraft passed over the state of Florida early in the morning Sunday.

It made a different sound then. It created a "sonic boom." That is the noise caused by something traveling faster than the speed of sound – or 1,225 kilometers per hour.

The sonic boom served as a surprising alarm clock for some Floridians. They took to Twitter to express their shock and describe the loud noise.

One person said she thought someone had crashed into her garage.

The X-37B was first used in 2010. Each time it goes on a mission, it stays in space and orbits the Earth many times. In its first mission, it stayed in space for about six months. Its second mission was close to a year, and its third mission was close to two years.

On Sunday, the plane ended its fourth mission about two weeks short of two years.

So, what does the plane do when it is in space?

No one really knows. It is classified. That means only certain military members are permitted to know about its mission.

The Air Force can only publicly say that the plane helps them see how new devices perform while in space for a long time.

The website Space.com reported that the Air Force was happy with the mission. The long trip did not result in any problems.

The Air Force is planning to continue launching and landing the space plane from Florida.

And that's what's Trending Today.

I'm Dan Friedell.

Dan Friedell wrote this story for VOA Learning English. His report was based on information from Space.com and CNN. Ashley Thompson was the editor.