Right now, a Learjet 45 (like the one pictured) is circling the Lambert-St. Louis International Airport, trying to troubleshoot a landing-gear problem. Airport officials say it has a "known problem" with the nose gear, and they're preparing for an emergency landing. They also say "it's a private airplane."
The U.S. Air Force's experimental X-51A Waverider set a hypersonic flight record when it flew at Mach 5 — five times the speed of sound — for more than three minutes. The previous scramjet record was a mere 12 seconds.