The four astronauts aboard NASA’s Artemis II mission have entered the moon’s “sphere of influence,” where lunar gravity exerts a stronger pull on the spacecraft than Earth.

The milestone came four days, six hours, and two minutes into the mission, when the crew was roughly 39,000 miles from the moon and 232,000 miles from Earth.

The next key phase will see the astronauts travel to the far side of the moon later on Monday, venturing deeper into space than any humans have before, according to a report in the Guardian.

“We’re all extremely excited for tomorrow,” said Lori Glaze, NASA’s deputy associate administrator for Exploration Systems Development Mission, on Sunday.

“Our flight operations and science teams are ready for the first lunar flyby in more than 50 years.”

The Artemis II astronauts are the first humans bound for the moon in over half a century, continuing the legacy of the Apollo programme, which last sent humans there in 1972.

While Apollo missions flew roughly 70 miles above the lunar surface, Artemis II will approach just over 4,000 miles, providing a complete view of the moon, including its polar regions.

During the approximately six-hour flyby, the crew will observe the moon with both the naked eye and onboard cameras, capturing views of the far side that were previously difficult or impossible to see.

When Orion passes behind the moon, the spacecraft will enter a planned communications blackout of about 40 minutes, as the lunar surface blocks radio signals to Earth’s Deep Space Network.

“I think it’s important to remember that we don’t always know exactly what they’re going to see,” said Kelsey Young, lead scientist for Artemis II, during a Sunday press conference.

If all goes according to plan, Americans Christina Koch, Reid Wiseman, Victor Glover, and Canadian Jeremy Hansen could set a new record for the farthest humans from Earth.

The crew has already captured unprecedented lunar imagery. Early on Sunday, NASA shared a photo taken by Artemis II showing the moon’s Orientale basin, sometimes called the “Grand Canyon of the Moon.”

“This mission marks the first time the entire basin has been seen with human eyes,” NASA said, noting that previous images were taken only from orbiting spacecraft.

On day five of the 10-day mission, Orion was nearly 215,000 miles from Earth and 65,000 miles from the moon, according to NASA’s online dashboard.

Former astronaut Charlie Duke, who walked on the moon during Apollo 16 in 1972, gave the ceremonial wake-up call, sending encouragement to the crew and praising the continuation of the Apollo legacy.

The astronauts have conducted a manual piloting demonstration, reviewed their lunar flyby plan, and studied surface features to photograph and analyse.

Nasa chief Jared Isaacman emphasised that the mission is also focused on testing the spacecraft’s life-support systems, noting, “This is the first time astronauts have ever flown on this spacecraft. That’s what we’re most interested in getting data from.”

The crew has also tested their bright orange “survival” suits, designed for takeoff, re-entry, and emergency scenarios such as cabin depressurisation.

While the astronauts will not land on the lunar surface, they are expected to set a record for the farthest distance from Earth during their orbit.

“Over the next day, they will be on the far side of the moon, they will eclipse that record, and we’re going to learn an awful lot about the spacecraft,” Isaacman said.

The data collected will be vital in preparing for future missions, including Artemis III in 2027 and the planned lunar landing of Artemis IV in 2028.