At 7:45 a.m. PDT, an unmanned glider was launched from Vandenberg Air Force Base in California aboard a Minotaur 4 rocket. Overseen by DARPA, the Defense Advanced Research Projects Agency, the glider, named HTV-4, is expected to reach suborbital space before reentering Earth's atmosphere at Mach 20 with the help of rocket thrusters to stay on course. If the hypersonic glider is able to reach Mach 20, or about 13,000 mph, it will become the fastest plane ever. At that speed, the HTV-2 could travel from New York City to Los Angeles in about 12 minutes.

Part of a program named the Prompt Global Strike, the HTV-2 is just one piece in DARPA's work to develop an advanced weapons system capable of reaching any point in the world in less than an hour.

"Assumptions about Mach 20 hypersonic flight were made from physics-based computational models and simulations, wind tunnel testing, and data collected from HTV-2's first test flight -- the first real data available in this flight regime at Mach 20," said Air Force Major Chris Schultz in a statement. "It's time to conduct another flight test to validate our assumptions and gain further insight into extremely high Mach regimes that we cannot fully replicate on the ground."

During that first flight, the glider overheated and crashed after just nine minutes. As engineers attempt to go from theory to practice, this is just one of the technical problems that must be overcome. At Mach 20, the HTV-2 needs to be able to withstand temperatures of nearly 2,000 Celsius, which is hotter than the melting point of steel.

Update: In the middle of the flight, DARPA tweeted that it had lost the ability to communicate with the HTV-2. The mission was off -- again. Not to worry, though. The Falcon "has an autonomous flight termination capability," DARPA said in a follow-up message; it shouldn't come crashing down through your roof.