How fast can Superman fly?

Superman is a comic book character created by Jerry Siegel and Joe Shuster in 1938, and his speed has never been a fixed measurement — it's a storytelling device. In the earliest issues he didn't fly at all; he leapt over buildings in a single bound and ran "faster than a speeding train." The phrase "faster than a speeding bullet" arrived with the 1940 radio serial and carried over into the Fleischer animated shorts and later television. In the Silver Age comics, starting in the 1950s, he was credited with faster-than-light speed and time travel; after the 1986 reboot, writers scaled that back to something "close to" the speed of light, and ever since, the ceiling has risen and fallen depending on the story. Asking how fast he can fly is like asking how much Thor's hammer weighs: the correct answer is whatever the plot requires.

The 1940 comparison does have a real-world translation. A handgun bullet leaves the barrel at 300-400 m/s; a 5.56mm rifle round at roughly 900-1,000 m/s, that is, Mach 2.6-2.9 at sea level. No operational aircraft has ever sustained that speed at low altitude, where aerodynamic heating would destroy an aluminum structure within minutes. At altitude, the story changes. The Lockheed SR-71 Blackbird, with a titanium fuselage, cruised at Mach 3.2 and set the 1976 absolute speed record for a manned jet aircraft, 3,529.6 km/h, which still stands. The fastest crewed aircraft of all time was the North American X-15, a rocket plane launched from a B-52: on October 3, 1967, William "Pete" Knight flew it to 7,274 km/h, Mach 6.7, at about 31,000 m altitude. Only space capsules, which re-enter at over 28,000 km/h, exceed those figures — and they aren't aircraft.

Beyond the character, physics sets three hard limits on any body flying very fast within the atmosphere. The first is drag, which grows with the square of speed: at Mach 3, a surface the size of a person needs tens of kilonewtons of thrust just to avoid slowing down. The second is heat: above Mach 5 the air compressed in front of the body exceeds 1,000°C, and at orbital speeds it turns into plasma. The third is the sonic boom, which during a low-altitude flyover of a city would shatter windows for kilometers around. A Superman genuinely flying at Mach 10 over Metropolis would, for anyone below, be indistinguishable from a meteor. Still, fiction has pushed reality forward: X-15 pilots earned astronaut wings, its data helped design the Space Shuttle, and today's hypersonic programs at NASA and the US Air Force are chasing exactly the speed range, Mach 5 to 10, where the comics place their hero.