The Apollo 10 lunar module Snoopy flying above the Moon's surface, taken from the main Apollo 10 Command Service Module (CSM) Charlie Brown. Apollo 10 was the full dress rehearsal of the Moon landing that took place two months later in July of 1969. May 1969. | Location: Moon.

This week marks half a century since Apollo 10, the warmup mission to the lunar landing of Apollo 11. And for one brief moment, the mission looked like it was on the verge of disaster. 

The Apollo 10′s lunar module (LM) was nicknamed Snoopy since it would be “snooping” around the moon in May 1969 and sniffing out a landing spot for Apollo 11, which would put the first humans on the surface just two months later. 

The command module was nicknamed Charlie Brown, because “as in the [Peanuts] comic, the CM Charlie Brown would be the guardian of the LM Snoopy,” according to NASA. 

The characters even decorated Mission Control in Houston: 

“Peanuts” creator Charles M. Schultz called NASA’s use of his characters’ names the “proudest moment” of his career, Craig Schulz, the artist’s youngest son, who was a writer and producer of 2015′s “Peanuts Movie,” recalled last year.  

Snoopy’s job was to go through the motions of almost landing on the moon, stopping just 47,000 feet short of the surface. Astronauts Thomas Stafford and Gene Cernan manned Snoopy while John Young remained in lunar orbit aboard Charlie Brown. 

Snoopy didn’t have enough fuel to carry out a landing, and some believe it’s because NASA officials worried that if the astronauts knew they could do it, they might go rogue and touch down anyway. According to the 2009 book Rocket Men by Craig Nelson, Cernan said: 

“A lot of people thought about the kind of people we were: ‘Don’t give those guys an opportunity to land, cause they might!’ So the ascent module, the part we lifted off the lunar surface, was short-fueled. The fuel tanks weren’t full. So had we literally tried to land on the Moon, we couldn’t have lifted off.”

While the Apollo 10 astronauts didn’t land, they did get the first up-close look at the moon, close enough to admire all the boulders: 

But something happened as the astronauts prepared to bring Snoopy back to Charlie Brown.  

“We’re in trouble!” Stafford said as the LM began tumbling.

“I saw the lunar horizon go by about seven or eight times in 10 seconds,” Cernan said later, per the Apollo Lunar Surface Journal. “That’s a hair-raising experience. That’s when I said ‘Son-of-a-bitch, what the hell happened?’” 

Since much of the mission was carried live, TV viewers and radio listeners heard the astronauts cursing as they tried to deal with an LM spinning out of control. 

“Some religious groups complained of bad language on the radio,” recalled Michael Neufeld, senior curator of the space history department at the National Air and Space Museum, according to Smithsonian. 

Cernan said he believes he flipped a switch, then Stafford flipped the same switch, unaware that Cernan had already done so, essentially canceling it out. 

“In effect, we created the problem,” he said. 

Fortunately, they were able to regain control of the tumbling LM. Once it was stabilized, they fired the ascent engine and safely returned to the command module. When Snoopy and Charlie Brown finally docked, Stafford declared: “Snoopy and Charlie Brown are hugging each other.” 

Despite the brief scare, Charlie Brown and Snoopy did the job. 

“But the thing is that this was, all in all, except for this one incident, a totally successful mission,” Neufeld said. “The overall lesson of Apollo 10 was: Everything’s ready to go.”

Charlie Brown splashed down in the Pacific on May 26, 1969. It’s currently on display at the Science Museum in London.

Before returning to Earth, Snoopy was released into its own orbit around the sun.

“I feel sort of bad about that because he’s a pretty nice guy,” Cernan said, according to NASA transcripts. “He treated us pretty well today.”

Snoopy’s current location is unknown, an enduring mystery of the space program. Amateur astronomers have attempted to locate Snoopy, but so far no one has found the historic little LM.

Since the 1960s, Peanuts has remained linked to the space program. NASA gives out Silver Snoopy awards to astronauts, employees and contractors for “outstanding achievements related to human flight safety or mission success.” The space agency has also partnered with Peanuts Worldwide on educational materials for elementary schools, including lesson plans for teachers. And “Peanuts In Space: Secrets of Apollo 10,” starring Ron Howard and Jeff Goldblum, was released this month on AppleTV.