The human body can withstand a lot before giving up and dying: falls from second-story windows, years of fevered substance abuse, wolf attacks, etc. We have a pretty good idea of what it canât tolerate, but some ways of dying instantly have received less attention than others, and speed is one of these. Weâve all seen pictures of people moving at top-speedâbut is there a velocity beyond which those blown-back cheeks actually fly off your face?

For this weekâs Giz Asks we reached out to experts in space travel and physiology to figure out how fast you can travel before it actually kills you. Technically, it turns out, thereâs no real limit to that number; it all depends on the conditions. Speed can kill youâbut it canât do the job alone.

### James A. Pawelczyk

Assistant Professor, Kinesiology and Physiology, PennState, who flew aboard the NASA STS-90 Space Shuttle mission as a Payload Specialist

I could be in a re-entering spacecraft, and I could be moving at 25 times the speed of sound. Clearly, that wouldnât kill meâastronauts do that on a regular basis throughout the year. But if I were to stick my head out the window, weâd have an entirely different story.

If we rephrase the question, then, to âwhat is the maximal dynamic pressure a human body can withstandââwell, I donât have a quick number for you, that would have to be calculated, but the issue becomes that you canât just turn it into a velocity, because it depends on the medium youâre traveling through, which determines the dynamic pressure.

If youâre in the air, thatâs going to dependent on what altitude youâre at; if youâre in water, itâs going to be a much slower velocity, because the dynamic pressure is going to be greater at the same speed, as itâs a thicker, more dense medium.

And then thereâs the issue of acceleration, which is also an issue of pressureâthe force against the body. Now youâre dealing with how rapidly you accelerate. And before it kills you, thatâs going to be a function of: what is the acceleration, and what is the rate at which youâre experiencing it? Weâve seen humans experience, for a very brief period of time, as much as 40 Gs acceleration, which is 40 times the force of gravityâand theyâve survived.

Colonel John Stappâs amazing self-experiments in the 1960s on rocket sleds are worth noting here. He was not only the military investigator in charge of those experiments, but the subject as well.

They were designed to test ejection seat technology, at a time when the US was developing supersonic aircraft. The question they wanted to answer was: whatâs a safe speed that a human can eject from an aircraft? It depends on altitude, but also on how quickly we accelerate away from the aircraft. And indeed we did see human survival at level as great as 40 Gs. Having said that, if I were to put an unprotected human being on a centrifuge, and to put them even as high as 5 or 6 Gs, and were I to then rotate them continuously, to the point that they were to lose consciousness, then continue to rotate them, they would die, ultimately. If I were to take a person and suspend them at one time the force of gravity, and allow them to hang there in a harness until they passed out, and continued to leave them thereâyes, you could cause death that way, but that would take a matter of many minutes or hours.

### John Linder

Professor, Physics and Astronomy, The College of Wooster

Itâs not the speed that kills you, itâs the acceleration, and speeds are relative.

Fifty years ago, Apollo astronauts reached almost 25 thousand miles per hour relative to Earth when falling back from the moon. But just standing on Earthâs equator, you travel about a thousand miles per hour relative to Earthâs poles, due to Earthâs spin, and 67 thousand miles per hour relative to the sun, due to Earthâs orbit, and so on.

You could use mid-sized black holes to gravitationally slingshot crewed spacecrafts to near light speeds, but youâd need infinite energy to accelerate a mass to light speed itself. That 670-million-miles-per-hour speed limit is invariant, and tension between the relative speeds of Newtonâs mechanics and the invariant light speed of Maxwellâs electromagnetism is famously reconciled by Einsteinâs modification of mechanics near light speed.

### Jonathan Clark

Associate Professor of Neurology and Space Medicine at Baylor College of Medicine, who worked at NASA from 1997 to 2005 and was a six-time Space Shuttle crew surgeon

Speed is just a distance-per-time-unit measure, so the constraints to speed are really dependent on other environmental factors.

The velocity isnât the issue as much as the change in velocity, which is acceleration. When you go to space, you have to get enough speed to get out of Earthâs gravity field. To get to lower orbit, astronauts have to get to 17,500 miles per hour, and to do that they have to change their velocity. They launch such that theyâre taking the gravity from the front of their chest to the back of their chestâthatâs called the G direction. Typically, the best way to tolerate it is going from front to back, which is why astronauts launch on a couch, sitting down.

The other constraint of velocity is in the atmosphere. John Paul Stapp, when he did his sled run, got up to 46, 47 Gs, and he was probably going close to 5 or 600 miles an hour. If you looked at his face, you would see that it was being blown and heavily distorted, and that his hands were actually restrained on his lap so they wouldnât flail around. Speed going through an atmosphere causes whatâs called aerodynamic flail, and that can kill you. When youâre in outer space you can go as fast as you wantâbut you need the protection of a vehicle, or a pressure suit, to keep you from the exposure to the vacuum of space.

We know that humans have gone 25,000 miles per hour going to the moonâthe speed itself is not an issue, it was mainly the acceleration to get out of the Earthâs atmosphere that they had to endure.

Once theyâre on their way and speeding up, thereâs no constraints to speed. Weâll eventually send humans to mars and theyâll be going 35,000 miles per hour.

The two programs I was involved with, the Red Bull Stratus and the space diveâthe goal was for a human without a vehicle to break the speed of sound, and that was accomplished because they wore a pressure suit. The reason they didnât have aerodynamic flail issues was that thereâs very little atmosphere above a hundred thousand feet

You can attain very high speedsâat least supersonic onesâas long as youâre protected, or (if youâre free falling from space) youâre at an atmospheric density thatâs not going to cause that flail to develop.