Fact check: Rocket propulsion functions in space because of universal physical laws, no air required

Kate S. Petersen, USA TODAY
·4 min read
48

The claim: Propulsion can't work in a vacuum, so space travel isn't real

Some proponents of flat earth theory – the erroneous idea that Earth is flat instead of a globe – also claim that space isn't real, sometimes based on errant assumptions about vacuum chambers and launch sites.

Another iteration of this claim is spreading in an Instagram post that shows two stacked images – a photo of a fighter jet and an artist's rendering of a spacecraft.

"A jet propels off air. A spaceship propels off CGI," reads the meme. The reference to "CGI" or "computer-generated imagery" apparently suggests that photos of spacecraft traveling in space have been digitally generated.

The meme's caption in the Nov. 11 Instagram post makes the point more directly.

"There must be an external medium to have an opposite reaction - Propulsion in a vacuum is insanely absurd," it reads.

The post garnered more than 1,000 likes in less than three weeks.

But it is wrong. Propulsion functions in both Earth's atmosphere and in space because of the physical laws governing motion in the universe. Propulsion occurs when an engine produces a force called "thrust," which pushes a rocket or fighter jet in the opposite direction. Though many jet engines do make use of air in their propulsion system, air is not necessary for propulsion.

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USA TODAY reached out to the Instagram user who shared the claim for comment.

Rockets don't need air to 'propel off'

Contrary to the claim made in the post, an "external medium" is not necessary for propulsion to occur, reports All About Space Magazine.

"Rockets generate a force in one direction, called thrust, by the principle of action and reaction," reads the article. "Exhaust fumes released by explosive chemicals are pushed out of the back of the rocket at high speed, and as a result, the rocket is pushed in the other direction, regardless of any surrounding medium."

This works because of physical laws that govern motion in the universe, according to an online NASA primer on rocket function. Issac Newton's third law of motion is particularly relevant to understanding the function of rocket engines.

This law states that "for every action there is an equal and opposite reaction," reads the primer. "In the case of rockets, the action is the force produced by the expulsion of gas, smoke, and flames from the nozzle end of a rocket engine. The reaction force propels the rocket in the opposite direction."

Many jet engines use air to facilitate the explosive reactions that create propulsion, according to NASA. To create thrust in the relative vacuum of space, rocket engines use a slightly different strategy.

When rockets travel into space, they bring everything they need to function with them, reports Discover Magazine.

"On board any rocket – a chemical rocket, that is – is a fuel tank, an oxidizer tank, and something to spark an explosive reaction in the combustion chamber," reads the article.

Other rockets use hypergolic fluids, which react with each other to create the required explosive force, according to the magazine.

Fact check: Astronauts train in vacuum chambers to prepare for life in space

Evidence that space travel is real includes the development of a space tourism industry, physical samples retrieved from Earth's moon and photos of spacecraft and people in space.

Our rating: False

Based on our research, we rate FALSE the claim that propulsion can't work in a vacuum, so space travel isn't real. Rocket engines function by creating thrust, which moves the spacecraft in the opposite direction. This process works in a vacuum. Air or another "external medium" is not required.

Our fact-check sources:

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This article originally appeared on USA TODAY: Fact check: Yes, rocket propulsion works in space despite lack of air