SpaceX’s surprise announcement yesterday that it would send two private citizens around the Moon next year may mark a huge milestone: the private space industry’s version of the Apollo 8. That mission was a precursor to more advanced lunar exploration for NASA, and paved the way for the first lunar landing. The lessons of Apollo 8 may give us hints of what’s next for SpaceX’s Moon ambitions.

The Apollo 8 mission helped NASA prepare for landing on the Moon. SpaceX will face some similar challenges on its lunar mission. Like Apollo 8, Elon Musk’s customers won’t be able to return to Earth in the event that something goes wrong; they’ll be on their own for the mission, which is expected to last a week. That includes illness — one of the Apollo 8 astronauts got sick during the mission, for instance. And the mission will be a test of SpaceX’s craft: it must perform precise maneuvers in order to reach the Moon and return. This is far more complicated than the trips to the ISS SpaceX routinely takes. To understand a little bit more about the SpaceX mission, it might be helpful to know your Apollo 8 history.

Apollo 8 lifted off from Florida’s Kennedy Space Center on December 21st, 1968, with a deceptively simple mission: carry three astronauts, Mission Commander Frank Borman, Command Module Pilot James Lovell, and Lunar Module Pilot William Anders to the Moon and back. No one had done it before, so this mission was a critical first step for NASA’s eventual Moon landing.

Borman, the mission leader, was summoned to Houston by Deke Slayton, director of Flight Crew Operations. There, Borman was told that the Russian space program was looking to send a mission past the Moon before the end of of the year. “We want to change Apollo 8 from an Earth orbital to a lunar orbital flight,” Slayton said, according to Frank Poole in his book Earthrise: How Man first saw the Earth. “I know that doesn’t give us much time, so I have to ask you: do you want to do it or not?”

No craft had ever gone beyond Earth orbit before. That meant new challenges. For instance, if something went wrong, the astronauts wouldn’t be able to return home right away. In addition, the spacecraft would also fly behind the Moon, which would interrupt communication with Earth.

Borman quickly said yes.

Borman was an experienced astronaut who had joined NASA in 1962 as part of the second Astronaut Group, and was the Command Pilot for the Gemini 7 mission, flying alongside Jim Lovell in 1965. His crew would reunite him with Lovell, who would fly the Command Module, and bring on William Anders, who had been training to fly the Lunar Module. The crew threw themselves into their work preparing for the mission. By December, they were ready.

Apollo 8 lifted off on the morning of December 21st. The Saturn V “was powerful and noisy and vibrated,” recalled Borman in a NASA oral history. As the crew flew away from the Earth, they became the first humans to see our home in its entirety.

After 55 hours of flight, the crew prepared to adjust the flight so they could enter an orbit on the far side of the Moon, which they did on December 24th, 68 hours into the mission. The burn went off correctly, and the spacecraft began the first of 10 orbits around the Moon, at about 60 nautical miles above the lunar surface.

Famously, as the astronauts came around the far side of the Moon, they caught a glimpse of the Earth rising over the lunar horizon. Anders and Borman quickly grabbed their cameras and began taking pictures — resulting on one of the most iconic images ever taken, Earthrise.

The moment struck the astronauts profoundly. [T]o me the significance of this [is that the Moon is] about the size of your fist held at arm’s length,” Anders recalled in an Oral History interview, “… you can imagine … [that at a hundred arms’ lengths the Earth is] down to [the size of] a dust mode. [A]nd, a hundred lunar distances in space are really nothing. You haven’t gone anywhere not even to the next planet. So here was this orb looking like a Christmas tree ornament, very fragile, not [an infinite] expanse [of] granite … [and seemingly of] a physical insignificance and yet it was our home…”

Earthrise has been called one of the most influential environmental images ever captured. Like Carl Sagan’s famous remarks about the Earth being a pale blue dot in space, the image was a stark reminder of the fragility of our home planet. It would be an iconic symbol for the environmental movement in the coming years.

The crew also gave a tour of the spacecraft on Christmas Eve, on a live television broadcast seen by nearly a billion people. Then, they exited lunar orbit and headed back to Earth. They reentered the Earth’s atmosphere on December 27th at the speed of 24,5000 miles per hour.

While SpaceX’s mission — two tourists are going, and obviously, NASA has already pioneered the journey. But the maneuvers made by Apollo 8 help demonstrate the technical complexity of such a mission, even as this new mission will likely be carried out autonomously, simply carrying the passengers along for the ride. The agency was able to test out several important milestones in 1968: conducting critical burns and orbital injections to bring the spacecraft into a new orbit and out again, while also producing images that would inspire the world over. The mission in 2018 could beam back its own version of Earthrise — right as President Donald Trump’s government attempts to dismantle environmental protections. A half century after that flight to the Moon, SpaceX could very well create its own mark in the history books, maybe in ways we don’t yet foresee.