What Is the Maximum Altitude Someone Could Come Back From Space Without Burning at Reentry?

If we lift off a person to the limit of space or beyond, what is the maximum altitude he could come back from without being burned alive at reentry? originally appeared on Quora - the place to gain and share knowledge, empowering people to learn from others and better understand the world.

Answer by Robert Frost, Instructor and Flight Controller at NASA, on Quora:

If we lift off a person to the limit of space or beyond, what is the maximum altitude he could come back from without being burned alive at reentry?

It's not about altitude. It's about velocity.

Objects that enter Earth’s atmosphere burn not because they are falling from great height, but because they are traveling through the atmosphere at great speed. A returning spacecraft enters the atmosphere at about Mach 25.

It’s usually assumed that the mechanism of heating in re-entry is by friction (i.e. viscous drag in the atmosphere). This is the predominant mechanism only at lower altitudes, as air density increases. During the fastest and hottest part of the descent, something different happens.

A re-entering vehicle develops a very energetic pressure wave at its leading surfaces. The energy density is sufficient to cause atmospheric molecules to dissociate, and their component atoms to become ionized. The vehicle thus descends in a superheated shroud of incandescent plasma.

Plasma does not conform to the gas laws of conventional thermodynamics, although it does share one familiar property—a proportionality between pressure and temperature in a contained system. The formation of the pressure wave, therefore, also creates extreme temperatures. The plasma stream is electrostatically charged, too, and so it concentrates at acute surface contours. The resultant effect is particularly intense local heating at the airframe’s leading edges.

An approximate rule-of-thumb used by heat shield designers for estimating peak shock layer temperature is to assume the air temperature in Kelvin to be equal to the entry speed in meters per second—a mathematical coincidence. For example, a spacecraft entering the atmosphere at 7.8km/s would experience a peak shock layer temperature of 7800 K.

In 2012, Felix Baumgartner jumped from a balloon at about 120,000 ft (38 km). The balloon had negligible speed. Baumgartner’s suit was designed to protect him from the extreme cold at altitude, not the extreme heat of reentry.

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