April 2022
The Backyard Astronomer
Adam England

Atmospheric Pressure and the ISS

Imagine holding a full, unopened can of soda. The thin aluminum barrier separating you from the liquid within can flex slightly if you push on it, but it is relatively strong, enough to hold in the pressure. We can demonstrate this pressure difference by giving the can a shake and then opening the can, at which point a spray of carbonated sugar water shoots from the new opening until the outside pressure and inside pressure have balanced.

Courtesy NASA

The pressure of our regular atmosphere is usually referred to as 1 standard atmosphere, or about 14.7 pounds per square inch at sea level. The pressure in the can of soda, depending on temperature and other variables, can easily be two or three times that. This difference in pressure is what makes the soda shoot out of the can, or what makes your ears pop as you dive to the bottom of a pool, lake or the ocean, where the equivalent three atmospheres is about 30 meters or 100 feet deep, and the deeper you go, the stronger the pressure. At the deepest levels of the Mariana Trench in the Pacific Ocean, the pressure can be more than 16,000psi, or 1070atm.

But what if we go up instead of down? At 20 kilometers (about 12.5 miles) altitude, the air pressure is just 1/20th of an atmosphere. This is because the air and moisture held close to our planet’s surface by gravity prevents our atmosphere from drifting away into space, but weakens and thins over just a few miles. By the time we reach the orbit of the International Space Station at about 227mi (420km), there is essentially zero atmospheric pressure. And while this provides benefits in orbital mechanics due to negligible atmospheric drag, it presents a whole other slew of problems for keeping humans alive in space.

Spacecraft like the ISS are carefully regulated and monitored to maintain an air pressure like that of Earth at 14.7psi. As spacecraft deliver humans and supplies, they must have the same air pressure as the docking station, or opening the hatch between craft could create an event similar to the shaken soda can, but with disastrous consequences, the pressure straining docking mechanisms and potentially pushing the objects apart. The same care must be taken when an astronaut performs an extravehicular activity (EVA), also known as a spacewalk. Their suit acts as a self-contained spacecraft that must also balance pressure to maintain life support systems and not damage the suit, the station, or any other object they may be working on.

Docking and EVAs aren’t the only time that atmospheric pressure is of a concern, though.

Micrometeoroids the size of grains of sand can and do collide with spacecraft. NASA estimates that the force of a paint fleck hitting the ISS in orbit would be the comparable to that of a 550lb object traveling at 60mph on Earth, and something just a few inches across could be equivalent to a 7kg blast of TNT.

On August 30, 2018, this scenario unfolded on the International Space Station. A 2mm hole was discovered in a Soyuz capsule that had delivered astronauts to the ISS two months prior, and sensors reported a drop in pressure and an oxygen leak. Luckily the repair could be done from inside the station, and no additional damage was found. The suspected culprit was a micrometeoroid. The picture shows a similar impact on the robotic Canadarm that also chipped a window in the Russian Zvezda ISS module.

Despite the risk, brave scientists have inhabited this orbiting laboratory continuously since November 2000, moving about five miles per second and orbiting the Earth every 90 minutes, or 16 times per day. Covering a total area of nearly a football field, its highly reflective solar panels make it easy to find in the evenings as the sun sets. Amateur astronomers with binoculars can make out its elongated shape as it quickly soars overhead. To find out when it will be passing over you, visit SpotTheStation.NASA.gov to type in your zip code and receive text alerts giving times and bearings of local sighting opportunities.

If you would like to learn more about the sky, telescopes, or socialize with other amateur astronomers, visit us at prescottastronomyclub.org or Facebook @PrescottAstronomyClub to find the next star party, Star Talk, or event.

Adam England is the owner of Manzanita Financial and moonlights as an amateur astronomer, writer, and interplanetary conquest consultant. Follow his rants and exploits on Twitter @AZSalesman or at Facebook.com/insuredbyadam.