A Billionaire in Orbit and His Crew Prepare to Walk in Space

Early Thursday morning, four private astronauts circling far above Earth will try something daring — let all the air out of their spacecraft.

That will be a prelude for two of the astronauts, who will then venture outside for the first commercial spacewalk ever, an event that is to be the highlight of the Polaris Dawn mission led by Jared Isaacman, a billionaire entrepreneur.

When is the spacewalk and how can I watch it?

The spacewalk is scheduled to begin at 5:58 a.m. Eastern time, a delay from an earlier announced start time of 2:23 a.m. SpaceX has started to broadcast live coverage of the event which you can watch in the video player embedded above.

If needed, a backup opportunity is available on Friday at the same time.

What will happen during the spacewalk?

Because there is no airlock in the SpaceX Crew Dragon spacecraft used for the flight, the only way to perform a spacewalk is to let all the air out of the spacecraft, and then open one of the hatches. That requires all four crew members to wear spacesuits.

NASA and Soviet astronauts conducted spacewalks in a similar manner in the 1960s.

As the Crew Dragon swings around Earth in an elliptical orbit that swings as close as 120 miles to the surface and as high as 460 miles, Mr. Isaacman and Sarah Gillis, a SpaceX engineer, will exit the capsule for about 15 to 20 minutes each.

They will pass through the hatch at the top of the Crew Dragon with the help of a handrail that SpaceX has named Skywalker, moving around carefully and deliberately. The two will not be outside the spacecraft at the same time.

The other two crew members, Scott Poteet, a retired Air Force pilot and the mission’s pilot, and Anna Menon, another SpaceX engineer, will remain in the capsule to manage the umbilical cords that will feed the spacewalkers oxygen and power, and monitor the readings to make sure everything is proceeding properly.

After Mr. Isaacman and Ms. Gillis return inside and close the hatch, the inside of the capsule will be repressurized with oxygen and nitrogen.

The entire spacewalk, from letting out all the air to refilling a breathable atmosphere, is expected to take about two hours.

The crew actually began preparing for the spacewalk, albeit imperceptibly, almost as soon as its members reached orbit on Tuesday. The air pressure within the Crew Dragon capsule was gradually lowered by almost half. That helped remove nitrogen from the astronauts’ blood and reduce the possibility of decompression sickness, or “the bends.”

When pressure decreases too rapidly — which can happen to deep-sea divers who return to the surface too quickly — nitrogen dissolved in the blood coalesces into gas bubbles, and those bubbles can damage organs and inflict excruciating pain.

While wearing the spacesuits, the Polaris Dawn astronauts breathe pure oxygen, which helps push out any residual nitrogen in their blood.

Isn’t this dangerous?

Space is an inherently inhospitable and dangerous environment, and during spacewalks astronauts are enclosed in a small bubble of air — their spacesuits — that keep them from suffocating in the vacuum of space.

But spacewalks are not the most dangerous part of spaceflight. No astronauts have ever died or suffered serious injury during a spacewalk. And spacewalks are not infrequent: There have been more than 270 spacewalks conducted at the International Space Station since December 1998 largely without incident. (Astronauts at the I.S.S. enter and exit the space station through airlocks, minimizing the amount of air that is released into space.)

Fatalities in space have occurred only during launches, as was the case with the loss of the space shuttle Challenger and its crew after the combustion of thousands of gallons of explosive propellants, or during landings, like when the space shuttle Columbia burned up in the searing heat of re-entry through Earth’s atmosphere.

Spacewalks nonetheless require meticulous planning, and if anything goes awry, they can be called off.

In June, for example, two NASA astronauts never left an airlock at the International Space Station after water started squirting from one of the spacesuits. Air was pumped back into the airlock, and the astronauts were back inside the space station 45 minutes later. NASA said they were never in any danger.

Have there been any spacewalk near misses?

During the very first spacewalk in March 1965, the Soviet astronaut Alexei Leonov had trouble getting back into his spacecraft. The air pressure inside the spacesuit caused it to balloon up, and he had to let much of the air out before he could fit back through his vehicle’s hatch.

More recently, in 2013, Luca Parmitano, an Italian astronaut, nearly drowned during a spacewalk at the International Space Station. A clogged filter in his spacesuit caused up to a quart of water to leak into his helmet. He had trouble breathing as water covered his eyes, nose and ears.

Why are the astronauts taking this risk?

The main goal of the spacewalk is to test new spacesuits developed by SpaceX for this mission.

“The idea is to learn as much as we possibly can about this suit and get it back to the engineers to inform future suit design evolutions,” Mr. Isaacman said.

For previous Crew Dragon missions, astronauts wore spacesuits, provided as a “just in case” precaution, during the launches and the returns to Earth.

The inside of the spacecraft is pressurized, and the astronauts do not need the spacesuits for breathing — unless something goes wrong.

In 1971, three Soviet astronauts — Georgi T. Dobrovolsky, Viktor I. Patsayev and Vladislav N. Volkov — died as they were returning to Earth from Salyut 1, the world’s first space station. A pressure valve in their Soyuz spacecraft was inadvertently jarred open, the air leaked out, and within minutes the three men, who were unsuited, were dead.

The new SpaceX spacesuits are based on the company’s earlier design, but required new capabilities — joints for easier movement, umbilical cords to provide oxygen and power, protection from micrometeoroids and controls to keep the astronauts comfortable amid the temperature extremes of space.

What has happened so far during Polaris Dawn?

After a couple of weeks of unsettled weather around Florida, the Polaris Dawn mission finally launched on a SpaceX Falcon 9 rocket early Tuesday morning.

Soon the Crew Dragon capsule and the four astronauts began an elliptical orbit that, at its highest point, was 755 miles above Earth’s surface.

That was the farthest off the planet that anyone has gone since NASA’s Apollo moon missions in the 1970s.

At that altitude, they passed through the South Atlantic Anomaly, a weak point in Earth’s magnetic field that allows high-energy charged particles from regions known as the Van Allen belts to come closer to Earth’s surface. Much of the radiation dosage from this spaceflight — equivalent to several months at the space station — occurred during these first few orbits.

Late in the day, the vehicle adjusted its orbit to a high point of about 870 miles. That surpassed the 853-mile altitude that two NASA astronauts, Pete Conrad and Richard Gordon, reached during the Gemini XI mission in 1966, which had been the record distance for astronauts on a spaceflight not headed to the moon.

After circling Earth six times in the high orbit, the Crew Dragon returned to a lower orbit, where there is less danger from radiation and micrometeoroids.

During their spaceflight, the four crew members are conducting about 40 experiments, mostly investigating how weightlessness and radiation affect the human body. They have also tested laser communications between the Crew Dragon and SpaceX’s constellation of Starlink internet satellites.