Here's why:
* Reaching Space: The X-15 reached an altitude of 108 kilometers (67 miles), surpassing the internationally recognized Karman Line, which marks the boundary between Earth's atmosphere and outer space.
* Aircraft vs. Spacecraft: While the X-15 was a rocket-powered aircraft, it differed from traditional spacecraft in that it took off and landed on a runway like a conventional airplane.
However, there's a debate about the exact definition of "aircraft" in this context:
* Technically, the X-15 was more of a hybrid: It was a plane that could fly at extremely high altitudes, but it relied on rockets for its propulsion and was not designed to fly in space for extended periods.
* Other candidates: Some consider the Soviet Union's Buran space shuttle, which also took off and landed like a plane, to be the first aircraft in space. However, the Buran was a true spacecraft designed for orbital flight, while the X-15 was primarily a research vehicle.
So, while the X-15 is widely acknowledged as the first aircraft to reach space, there's a nuance to the discussion depending on how strictly you define "aircraft".
