# Why would spacecraft docking to the ISS have a different atmospheric pressure?

In the SpaceX Demo 2 Crew Dragon livestream, they had to equalize pressure between the ISS and the Dragon spacecraft. The voice on the stream said that they were bringing it to levels that match Earth's. What is the pressure in the Dragon if not exactly that of Earth's? And why would that be the case? Is it intentional or something that just happens.

• were they talking about the spacecraft, or the vestibule formed between the Dragon and the ISS?
– user20636
Jun 1, 2020 at 8:46
• What is the exact pressure of Earth's atmosphere? (Hint: Why do we have barometers?) Jun 1, 2020 at 14:03
• Because it's hard to maintain the same pressure of the two precisely without connecting them. Jun 1, 2020 at 16:07
• Something that just happens. Try to buy exactly 1 pound of meat at a deli with a 3 digit display and note the looks it gets you. Jun 1, 2020 at 20:38

The pressures of both the Dragon spacecraft and the ISS match the atmospheric sea level pressure of Earth, about 1 bar. But there are small inevitable tolerances of about some millibar or less. So there is a non zero pressure difference between the spacecraft and the spacestation.

When air temperature in the closed spacecraft changes by only 0.3 K, the pressure would change by 1 mbar.

If we assume a hatch with an area of one square meter and a pressure difference of 1 mbar = 1 hPa = 100 Pa, the resulting force is 100 N. On Earth you would need about 100 N to lift a weight of 10 kg.

To remove those small pressure differences a valve is opened to align pressures on both sides before opening the hatch.

• is this first paragraph factual or a guess?
– user20636
Jun 1, 2020 at 8:44
• @JCRMt 1 atm is the design pressure in ISS. see, e.g, WIkipedia. It's hard to imagine a reason not to set Crew Dragon to 1 atm as well. Jun 1, 2020 at 11:25
• @CarlWitthoft You could drain some air to make Crew Dragon slightly lighter during launch. But then again the Crew Dragon volume is just 10 m^3, so you'd only make the ship lighter by 10 kg even at full vacuum (and the astronauts need to breathe anyways). Jun 1, 2020 at 12:35
• Side question: Why 1 bar? Couldn’t they use a lower pressure with higher (relative) oxygen content in the ISS and visiting craft? I can see a few benefits like less mass, less outgassing, lower structural stress … Jun 2, 2020 at 8:16
• @Michael something like this was used in Apollo 1: blogs.scientificamerican.com/guest-blog/… It's quite dangerous, to have more oxygen rich atmosphere. Jun 2, 2020 at 10:29