Can You Propel Yourself in Space by Farting?
The subreddit AskScience is an intellectual gold mine. Curious folk of all ages pose rousing scientific questions to be answered by scientists and experts. Often, the queries deal with peculiarities of how the world and universe function, or the psychological oddities of human behavior. Other times, they detour into the ridiculous. Last month, user sleepwalken asked a question that was both hilarious and thought provoking:
"If you farted hard enough in space, could you move yourself around?"
The slightly sophomoric, yet stimulating question quickly rose to the top of the forum, and received a serious response from user VeryLittle, a physicist and moderator of the Physics subreddit.
"Essentially, farts are rocket fuel," he (or she) replied. "Gas diffusing will carry a small amount of momentum backwards, so it must exert a force on the person, pushing them forward."
However, that force would be very small, he calculated. The average person produces roughly one liter of flatulence a day, composed mostly of hydrogen, carbon dioxide, and methane, along with smaller amounts of nitrogen and oxygen. "That comes out to 0.5 g of flatulence every day for a normal person," VeryLittle wrote.
Now, let's guess that a fart leaves the butthole at about 1 m/s - again, not entirely unreasonable. So putting all this together, we can find that a day's worth of farts carries backwards momentum equal to
(1 m/s)(0.5 grams) = 0.0005 kg m/s
So for momentum to be conserved, the astronaut will now be traveling 7.7x10-6 m/s forward, which is only about 1000x faster than hair grows. If an astronaut in space farted every day, it would take 10,000 years for him to get up to a normal highway speed.
VeryLittle's analysis does seem to dash one's hopes for a "natural" anal jetpack. However, as a great many match-wielding high school boys have discovered, there's a simple way to get more energy out of one's flatulence. VeryLittle continues:
"The gases I listed above are combustible - specifically methane. Just spewing the gas backwards to get a push forward would be like putting your SUV in neutral and trying to propel it forward with a supersoaker that sprays gasoline backwards. Instead of throwing it backwards, you can explode it backwards to generate thrust, like a real rocket."
"If we take the methane to be about 1% of our flatulence, and the energy of combustion to be 890 kJ/mole, then we find that the chemical potential energy of the gas is about 100 million times greater than the kinetic energy backwards. If we had one of those fancy gas backpacks that they put on cows to harvest the methane from their farts and a jetpack to burn it, then this gas would be enough to get a particularly flatulent astronaut up to highway speed in a day."
That's on paper, at least. But how would it work in practice? Astrophysicist Ethan Siegel notes that the combustion would require some oxygen to oxidize the reactants. Moreover, most of the energy produced would turn into heat, not momentum, he says. So the acceleration would be slower than predicted.
Incidentally, NASA has been well aware of the explosive power of flatus for some time now. Back in 1969, researchers at the University of California - Berkeley sought ways to minimize the fire hazard presented by farting onboard spacecraft by feeding subjects diets designed to minimize the output of bodily gases.