Why does Earth’s atmosphere (and you!) not rush off into the vacuum of space?

Have you ever walked outside, looked up, and wondered why you are not flying off into space?! Sounds perhaps a bit ridiculous, but sci-fi movie buffs have likely seen plenty of examples when the space station airlock is released and those onboard are holding on for dear life! The quick explanation for why an astronaut is forcibly drawn towards the vacuum of space when the airlock barrier separating the two environments suddenly disappears is that the astronaut has been caught within an incredible change in pressure over a tiny distance. Since pressure imbalances tend to flow from high to low (same principle that results in general wind direction), the unfortunate astronaut must move along this tight pressure gradient from inside the once sealed space station (relative high pressure) out into the vacuum of space (relative low pressure).

Fortunately, as you read this, perhaps coffee in hand, you are not hurling up into the heavens above! But, there certainly still exists a vertical pressure gradient between the surface and the vacuum of space. The origin of atmospheric pressure at the surface is due to the combined weight of air molecules stacked overhead. Higher pressure in the lower atmosphere compresses the air to make it denser (more molecules forced to occupy a smaller area). Moving the other way to the edge of the atmosphere, you would progressively encounter less air molecules, diminishing overhead weight, lower pressure, and find air molecules more spread out. So, a vertical atmospheric pressure gradient force must exist and would be directed out from the Earth’s surface towards the vacuum of space.

At this point, though, we can thank the Earth’s sufficient mass and resulting gravitational strength compressing air molecules enough to have an atmosphere suitable for life. Gravity is essentially our “airlock” and really is why the air molecules comprising the atmosphere are hugging the planet like a blanket manage to stay put and you remain safely on the ground and able to enjoy a breathable atmosphere! 

We call the balance between the vertical atmospheric pressure force wanting to escape and gravity pulling back down the “hydrostatic equilibrium”. This is a foundational concept for studying the atmospheric sciences.

Photo Credit: Jonny William Malloy