“For centuries, humans all around the world have dreamed of flying to space,” says Richard Branson’s Virgin Galactic. “Now, at last, space travel is open to all of us…for the first time in history, space exploration is a possibility for everyone who dares to dream of it.”

Neil Armstrong after landing the X-15 on the dry lake bed at Edwards Air Force Base (NASA Dryden Research Center)

Virgin Galactic’s SpaceShip 2, under development by Burt Rutan’s Scaled Composites, offers flights above 100 kilometers (62 miles) at speeds reaching 2,600 mph after being carried aloft and launched from its White Knight mother ship. A seat on SpaceShip 2 costs $250,000.

SpaceShip 2 aims to duplicate the feats of the X-15 rocket planes conceived and built during the 1950s. Virgin Galactic acknowledges its debt to “the iconic X-15 program that so inspired our own vehicles.”

Three X-15s were built, and were flown 199 times between 1959 and 1968 by NASA and military pilots. They were carried aloft by a B-52 bomber from Edwards Air Force Base in California’s Mojave Desert, flew out of the atmosphere under rocket power, and glided back to earth. An X-15 reached an altitude of 67 miles in 1963, setting a record that lasted until 2004.

Unlike the Space Shuttle, which was launched into orbit, the X-15s were flown to and from space by their pilots. The flights were challenging, to say the least. On one occasion an X-15 flown by NASA pilot Neil Armstrong (who later became famous for making the first landing on the moon) skipped off the atmosphere instead of re-entering, and Armstrong found himself on a ballistic trajectory heading towards the Los Angeles area. After the X-15 dropped into the atmosphere, Armstrong was able to turn back towards Edwards, but barely succeeded in making a safe landing on the dry lake bed.

Although designed to reach similar altitudes, SpaceShip 2 will not travel nearly as fast. In October, 1967 an X-15 flew at 4,520 mph (Mach 6.7), the highest speed attained by a piloted aircraft. This record is unlikely to be broken any time soon. Despite special thermal protection, at that speed the X-15 was damaged by aerodynamic heating and shock wave effects. It was repaired, but never flew again.

Today the first X-15 hangs in the Smithsonian National Air and Space Museum on the Mall in Washington D.C. The second, which flew at Mach 6.7, is on display at the National Museum of the United States Air Force in Dayton, Ohio. Scattered fragments of the third lie on the floor of the Mojave Desert, where it fell to earth after going out of control and breaking up on re-entry in November, 1967.

In October, 2014 the first prototype SpaceShip 2 also went out of control and broke up during a test flight, killing one of its pilots and seriously injuring the other. The National Transportation Safety Board found that the accident was caused by premature activation of the control system designed to ensure safe re-entry into the atmosphere. Virgin Galactic has announced that the test program will resume with a modified second prototype this year, and hopes to begin commercial flights before the end of 2016, saying, “traveling to space will never be completely free of risk— nothing truly worth doing ever is.”

The pilots who flew the X-15 would surely agree.