Billionaire Jared Isaacman and SpaceX engineer Sarah Gillis completed the first privately funded spacewalk Thursday — a critical aspect of a risky, somewhat troubled mission designed to test new SpaceX technology, gather valuable data about high-altitude space travel, and generally push the limits of space exploration.

The mission, called Polaris Dawn, was supposed to launch in late August, but was grounded due to safety concerns. But on Tuesday, Isaacman and Gillis — along with two other crew members, former Air Force pilot Scott Poteet and SpaceX engineer Anna Menon — finally took off.

Since then, the astronauts have gone about 870 miles from Earth, which makes Polaris Dawn the farthest crewed space mission since the Apollo 17 mission to the moon in 1972. They came closer to Earth — away from the high radiation levels they faced at the furthest point of their journey — for Thursday’s spacewalk.

“Back at home, we all have a lot of work to do, but from here, Earth sure looks like a perfect world,” Isaacman said, observing the planet from outside his SpaceX Crew Dragon spacecraft Thursday.

Though the spacewalk is complete, the astronauts aren’t done. Their mission is set to last five days — much shorter than typical NASA missions which can last weeks or months — but still includes dozens of experiments, and, critically, safely reentering the Earth’s atmosphere.

SpaceX’s current mission is supposed to be the first of three Polaris missions, all funded by Isaacman and SpaceX, Elon Musk’s aerospace company.

The goal of the Polaris mission series is to help SpaceX develop the skills and technology needed to achieve its longer term goals of sending humans to Mars. The company hoped this first mission would be a useful test of the spacecraft and its instruments’ ability to survive the extreme conditions of space, particularly as it passes through the Van Allen radiation belts, regions in space that encircle the planet and are highly radioactive. They were detected on the first US space mission in 1958, and their highly charged, energetic particles can damage spacecraft instruments.

The crew traveled through the Van Allen belts on their way to the furthest point in their journey, and, so far, the tech aboard the capsule — and the astronauts — seem to have withstood that radiation.

The trip was also a test of SpaceX’s latest spacesuit designs, which were designed in just two and a half years — an astonishingly quick turnaround by space exploration standards — and are meant to be upgrades to the bulky suits NASA has made famous. Rather than having life support systems integrated into the suit, like in NASA’s, Space X’s new suits connect to a spacecraft via a tube — all of the Polaris Dawn astronauts used that functionality to breathe while Isaacman and Gillis performed their spacewalk.

That’s because though only Isaacman and Gillis exited the craft, all four astronauts were exposed to space (and that means, technically, all four astronauts completed a spacewalk). To complete the spacewalk, Isaacman and Gillis had to open the Crew Dragon’s hatch to exit, exposing everyone inside to vacuum. Though that was once the preferred way for astronauts to leave their spacecraft, modern astronauts usually exit via an airlock mechanism, which has two doors separated by a chamber so that the vacuum of space doesn’t enter into the spacecraft.

Exiting through the hatch appeared to be a success, and could be something future missions emulate in order to avoid the need for airlocks. During the walk — which saw Isaacman and Gillis holding onto a specialized ladder leading out of the crew capsule rather than floating freely through space — the astronauts completed a series of tests meant to check the maneuverability and functionality of the suits.

The suits were a particular area of focus for this mission not just because they are new, but because SpaceX sees getting an inexpensive, reliable spacesuit design down pat as being key to its long-term mission of supporting mass human colonization of space.

“Building a base on the Moon and a city on Mars will require millions of spacesuits; the development of this suit and the execution of the EVA will be important steps toward a scalable design for spacesuits on future long-duration missions,” the company wrote on its website.

In their final days in space, the astronauts will do other experiments meant to support that long-term goal, including some related to motion sickness and some focused on communications tests involving SpaceX’s Starlink satellites. On Earth, those are used to provide internet service, particularly to remote locations; the astronauts will do some initial tests to check the satellites’ usefulness in communicating from space.

Once back, the astronauts will have completed a historic mission full of firsts: the first private spacewalk, the most astronauts spacewalking at once, and Menon and Gillis will be the only women ever to travel such a distance from Earth.