CAPE CANAVERAL, Florida, July 21 — The commercial space launch industry operates within a set of unforgiving physical boundaries that, for all the recent hype about reusable boosters and orbital tourism, still define the economic and engineering calculus for every mission that leaves the pad. Since the dawn of spaceflight, rocket propulsion—both liquid and solid propellants—has been the only means to get a vehicle off Earth. A launch can start from a land-based pad, a sea platform, or a mid-air drop from an aircraft.
But the destination is far from arbitrary. There is no sharp line where the atmosphere ends and space begins; density simply fades with altitude.
For decades, the Fédération Aéronautique Internationale has set the Kármán line at 100 kilometres (62 miles) as the working boundary between aeronautics and astronautics, based on Theodore von Kármán’s calculation that a vehicle at that altitude would need to exceed orbital velocity to gain enough aerodynamic lift. The United States, however, has its own benchmark. Until 2021, anyone traveling above 50 miles (80 km) was designated an astronaut; now wings go only to crew members whose inflight activity was essential to public safety or contributed to human spaceflight safety.
NASA’s Space Shuttle used 400,000 feet—roughly 120 kilometres—as its re-entry altitude, the point where atmospheric drag becomes noticeable. In 2009, scientists using a Supra-Thermal Ion Imager measured a boundary at 118 kilometres above Earth, marking the midpoint of a gradual transition from relatively gentle winds to violent charged-particle flows reaching speeds over 268 metres per second.
What this means for the business of launch is straightforward: altitude alone isn’t enough. The gravitational potential energy needed to reach the Kármán line is approximately 1 megajoule per kilogram. In practice, far more energy is required because of losses from air drag, propulsive inefficiency, engine-cycle limits, and gravity drag.
