The Tale of the Only Robot Humans Ever Visited on Another World

From the Huygens Titan lander to the Soviet Venus probes, the robots we send to explore other worlds are normally abandoned in their adopted homelands, never to see humans again, a circumstance that inspired this heartrending xkcd comic about the Mars Spirit rover.

Only one interplanetary explorer has escaped this lonely fate. Surveyor 3, a NASA probe that landed on the Moon on April 20, 1967—50 years ago this Thursday—was visited by Apollo 12 astronauts Pete Conrad and Alan Bean during on the pair's moonwalk on November 20, 1969.

How did Surveyor 3 earn the honor of being the sole spacecraft to be reunited with humans on an alien world? The lander had a few things going for it, especially its plush location in the Ocean of Storms. This massive lunar mare is thought to have been created by a colossal impact event early in the Moon's history (though other theories about its origins have rivalled the impact hypothesis).

In any case, the rich basaltic rock in the Ocean of Storms, which is fallout of ancient molten lava, made it a popular destination for exploration during the space race. By the time Surveyor 3 landed there, the vast basin was already home to its progenitor, Surveyor 1, along with three Soviet Luna landers (5, 9, and 13), and the NASA Ranger 7 impactor.

Read More: The Other First Moon Landing

To acknowledge these exploratory efforts, the International Astronomical Union officially named the region where Apollo 12 touched down the Mare Cognitum, or "The Sea That Has Become Known," though Conrad preferred to call the landing target, "Pete's Parking Lot."

In addition to its interesting geological material, the site was selected because Apollo 12 was tasked with executing a precision landing. Surveyor 3 was an ideal bullseye as it was outfitted with expensive equipment, including a remote-controlled television camera, that NASA wanted removed and returned to Earth for study.

About two-and-a-half years after Surveyor 3 touched down, Conrad and Bean successfully landed within about 180 meters (600 feet) of it, so close that the robotic probe was sandblasted by the exhaust of Intrepid, the Apollo 12 lunar landing module, as it descended. The astronauts hit up the probe on their second moonwalk, taking photographs with the robot and scrapping it for parts.

Video footage of Surveyor 3 visit. Video: VideoFromSpace/NASA/YouTube

"Hey, we got a nice brown Surveyor here, Houston," Bean said when they arrived, according to NASA's flight journal, referring to the layer of lunar dust that had collected on the probe. "The glass is still on the top. Not a bit of it is fractured."

The astronauts' excitement about visiting the lander is palpable throughout the transcript of their EVA. Surveyor 3's perspective on the Moon was already familiar to them from the 6,315 pictures it had taken during its short weeks of lifetime (the lander did not survive the cold of its first lunar night, and was last heard from in May 1967).

Some features of the scene were new, such as the erosion over the trenches dug with the lander's soil-sampling scoop.

"Okay. We'll walk real slow," Bean said, while approaching the dead robot. "Hey, you can see…Look at there where it dug those scoops. You can still see the…Boy, that's going to make some beautiful pictures on the way that's weathered since…Doesn't look like the pictures we saw of this a long time ago. That's going to be good."

The pair were also notably pumped to retrieve the scoop from the spacecraft.

But given the limited hours scheduled for lunar EVAs, the men soon had to move on to the next errand. It was a bittersweet parting: stripped of its choicest parts, Surveyor 3 was promptly abandoned again.

If and when humans return to the Moon, or even venture to Mars, rendezvous with our robotic pioneers may become more common. That said, many space scientists argue that landing sites should remain wholly untouched to prevent destruction of data and contamination.

Indeed, Surveyor 3's camera caused a minor scandal when it was returned to Earth, after thriving bacterial communities were found on its surface, suggesting that microbial life may have survived on the Moon for two years. It has since been ruled that these lifeforms colonized the camera due to shoddy clean room practices after the instrument was returned to Earth, which led to stricter laboratory protocols at NASA.

In light of these concerns, perhaps Surveyor 3 will remain the only spacecraft ever visited by humans on another planet, while the others could be left to be buried or eroded into obscurity, over billions of years, alone.

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