Live Updates: SpaceX Polaris Dawn Spacewalk Begins During Historic Mission

This historic spacewalk may be in full swing right now, but Polaris Dawn's four-person crew has been preparing for this moment practically since their launch two days ago.

Almost immediately after reaching orbit, Isaacman, Gillis, Menon and Poteet began a “pre-breathing process” to prepare for the spacewalk.

The goal of the pre-breathing process is to avoid decompression sickness – the same dangerous and potentially fatal illness known as decompression sickness that can occur in recreational divers when they try to ascend too quickly.

During pre-breathing, nitrogen is removed from the astronauts' blood to prevent bubbles from forming in their bloodstream when the Dragon capsule is depressurized and exposed to the vacuum of space.

In fact, the breathing protocol to which the Polaris Dawn crew will undergo is completely different from the protocol used on the International Space Station.

The space station has special airlocks where astronauts can go through a quick pre-breathing process before beginning their spacewalks. This only takes a few hours.

However, according to Gillis' earlier comments, the Polaris Dawn crew's pre-flight preparation routine lasted about 45 hours. During that time, the oxygen level in the Crew Dragon spacecraft's cabin slowly increased while the pressure decreased.

“The really cool thing about this (pre-breathing) profile is that in many ways it's much less risky than the standard on the space station,” Gillis said. “It's like opening a can of soda – and you want to open the can (and not have bubbles) because the pressure outside the can is equal to the pressure inside.”

By reducing the pressure inside the Crew Dragon, Gillis added, and donning their spacesuits precisely when the ambient pressure matches the suit pressure, crew members can better reduce the risk of unwanted bubbles.