What altitude marks the lower boundary of the space domain?

• A. 50 kilometers.
• B. 100 kilometers.
• C. 200 kilometers.
• D. 500 kilometers.

Answer: (B)

The boundary between the atmosphere and space is defined by the altitude where air becomes too thin for conventional aircraft to operate and where orbital dynamics take over. This boundary is conventionally set at about 100 kilometers above Earth's surface, known as the Kármán line. At that height, the air density is so low that generating enough lift to fly like a airplane would require speeds similar to orbital velocity, signaling the shift from atmospheric flight to spaceflight. That’s why 100 kilometers is used as the lower limit of the space domain in most definitions. Some sources reference lower or higher thresholds for specific purposes, but 100 kilometers is the standard starting point. Higher altitudes like 200 or 500 kilometers are well into space, but they’re not the boundary itself.