It’s official: The Artemis II astronauts have arrived in the moon’s cosmic neighborhood.

The crew’s Orion capsule entered what’s known as the lunar sphere of influence at around 12:41 a.m. ET Monday, crossing into the region of space where the moon’s gravitational pull is stronger than the pull of Earth’s.

“That’s a significant milestone on our mission,” NASA flight director Rick Henfling said Sunday in a news briefing.

The lunar sphere of influence isn’t a physical or tangible border. Rather, it’s a mathematical boundary that signifies that the astronauts are in the moon’s vicinity.

Crossing the threshold is a major achievement for NASA. It’s the first time that astronauts have entered the lunar sphere of influence in more than half a century, since the Apollo 17 mission in 1972.

On Sunday, the astronauts beamed back a photo showing “one last look at Earth before we reach the Moon." The picture shows the planet as a distant crescent framed by the Orion spacecraft’s window.

The crew of Artemis II — NASA astronauts Reid Wiseman, Christina Koch and Victor Glover and Canadian astronaut Jeremy Hansen — started their day Sunday with a wake-up message from Apollo 16 astronaut Charlie Duke.

“John Young and I landed on the moon in 1972 in a lunar module we named Orion,” Duke said in the recorded message. “I’m glad to see a different kind of Orion helping return humans to the moon as America charts the course to the lunar surface.”

Later, Wiseman, Koch, Glover and Hansen spent time testing their spacesuits, which were newly designed for this flight.

The astronauts wear the orange spacesuits during launch and re-entry, but the suits can also be worn in emergencies to provide crew members with a breathable atmosphere for up to six days if the Orion capsule loses pressurization, according to NASA.

The Orion spacecraft also executed a 14-second-long engine burn Sunday to keep the capsule on the correct path around the moon. Although other correction burns like that had been planned for other days, Henfling said this was the first time one was actually necessary since the astronauts left Earth's orbit.

“We found that Orion was on such a pinpoint trajectory that we didn’t need to do the first two correction maneuvers,” he said.

The astronauts are set to swing around the moon later Monday, reaching an estimated distance of 252,760 miles from Earth — the farthest any humans have ventured from our home planet. They are expected to break the Apollo 13 crew’s distance record of 248,655 miles.

During Monday’s lunar flyby, Wiseman, Koch, Glover and Hansen will conduct observations of the moon and capture photos for about seven hours, starting at 2:45 p.m. ET. The observations will include parts of the moon’s surface that haven’t been seen before by human eyes.

NASA will offer live coverage of the flyby starting at 1 p.m. ET.

At the Orion spacecraft's closest approach to the moon around 7 p.m. ET, NASA estimates, it will be 4,070 miles from the moon’s surface.

The astronauts plan to snap photos with two Nikon D5 cameras and a Nikon Z9 camera, NASA officials said.

Among the 30 science targets set out for the mission, the astronauts will focus on the Orientale basin, a 3.8 billion-year-old crater that formed when a large object smashed into the moon’s surface. The nearly 600-mile-wide basin, which stretches across the moon’s near and far sides, still has distinct geological features from the ancient collision, according to NASA.

The crew will also study the Hertzsprung basin on the moon’s far side, northwest of Orientale. Unlike the more pristine Orientale basin, features in this 400-mile-wide crater have been degraded by subsequent lunar impacts, NASA said. Observing both targets will give the crew and scientists on Earth a chance to compare how the moon’s topography changes over time.

A software tool will guide the crew's observations of the science targets.

,Kelsey Young, the Artemis II lunar science lead, said the schedule is “jam-packed.” Still, there is flexibility to improvise, she said: “They are the field scientists, and they are encouraged to go off-book if what they’re seeing in front of them really compels them.”

Near the end of their lunar observation period, the astronauts will experience a roughly hourlong solar eclipse from space. The sun will start to pass behind the moon at 8:35 p.m. ET, blocking its light from the perspective of the Orion capsule.

During that time, the moon will appear mostly dark, which will give the astronauts a chance to observe the sun’s corona and look for flashes of light from rocky objects smacking into the moon.

The astronauts will also have a chance to take pictures of other planets that could be visible during the eclipse, including Mercury, Venus, Mars and Saturn, Young said.

She emphasized that the crew has a unique opportunity as the first humans to see the moon from those vantage points.

“This is exploration,” Young said. “And while we have imagery, amazing data from orbiting spacecraft, it’s these nuanced observations that we’re lacking. And so this is discovery, right? And we’re asking questions that we don’t always know the answer to.”

This article was originally published on NBCNews.com