where does space start?

Space does not have a single, sharp starting line, but most space agencies and treaties treat “space” as beginning roughly 80–100 km above Earth’s surface, near the so‑called Kármán line. The most commonly quoted value is 100 km (about 62 miles), but some scientific work and U.S. agencies use 80 km (about 50 miles) as the practical boundary.

What “space” usually means

From an Earth‑dweller’s point of view, space starts where the atmosphere is too thin for conventional aircraft to generate enough lift and where orbital motion, not wings, is what keeps you aloft. This is the idea behind the Kármán line, named after physicist Theodore von Kármán, who estimated where this transition in flight physics happens.

The Kármán line

• Many international and regulatory bodies treat 100 km as the Earth–space boundary for legal and record‑keeping purposes.

• This value is somewhat arbitrary but convenient, because the atmosphere is already extremely thin and orbital dynamics dominate above this height.

The 80 km definition

• Some scientists and recent analyses argue that the physical transition to “space‑like” conditions occurs closer to 80 km.

• The United States has historically awarded astronaut wings to people who fly above about 80 km (50 miles), effectively treating that altitude as the start of space for those honors.

Why there’s no exact edge

Earth’s atmosphere thins out gradually and fades into the surrounding environment rather than ending abruptly. Even hundreds or thousands of kilometers up, there are still a few atmospheric particles, so any boundary between sky and space is partly a human‑chosen definition rather than a hard physical edge.