It’s no simple feat to send a rover to space, land it on a celestial body, and get the wheels rolling. NASA has used all kinds of techniques: The Pathfinder rover landed on Mars in 1997 inside a cluster of airbags, then rolled down its landing vehicle’s “petals,” which bloomed open like a flower, to the dusty surface. Cables attached to a rocket-powered “sky crane” spacecraft dropped the Perseverance Mars rover to the Red Planet’s surface in 2021. On the moon, Apollo 15, 16, and 17 astronauts pulled mylar cables to unfold and lower their buggies from the vehicles’ compact stowage compartments on lunar landers. 

But NASA’s first-ever rover mission to the lunar south pole will use a more familiar method of getting moving on Earth’s satellite: a pair of ramps. VIPER, which stands for Volatiles Investigating Polar Exploration Rover, will roll down an offramp to touch the lunar soil, or regolith, when it lands on the moon in late 2024. 

This is familiar technology in an unforgiving location. “We all know how to work with ramps, and we just need to optimize it for the environment we’re going to be in,” says NASA’s VIPER program manager Daniel Andrews.

A VIPER test vehicle recently descended down a pair of metal ramps at NASA’s Ames Research Center in California, as seen in the agency’s recently published photos, with one beam for each set of the rover’s wheels. Because the terrain where VIPER will land—the edge of the massive Nobile Crater—is expected to be rough, the engineering team has been testing VIPER’s ability to descend the ramps at extreme angles. They have altered the steepness, as measured from the lander VIPER will descend from, and differences in elevation between the ramp for each wheel. 

”We have two ramps, not just for the left and right wheels, but a ramp set that goes out the back too,” Andrews says. “So we actually get our pick of the litter, which one looks most safe and best to navigate as we’re at that moment where we have to roll off the lander.” 

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VIPER is a scientific successor to NASA’s Lunar Crater Observation and Sensing Satellite, or LCROSS mission, which in 2009 confirmed the presence of water ice on the lunar south pole. 

“It completely rewrote the books on the moon with respect to water,” says Andrews, who also worked on the LCROSS mission. “That really started the moon rush, commercially, and by state actors like NASA and other space agencies.”

The ice, if abundant, could be mined to create rocket propellant. It could also provide water for other purposes at long-term lunar habitats, which NASA plans to construct in the late 2020s as part of the Artemis moon program

But LCROSS only confirmed that ice was definitely present in a single crater at the moon’s south pole. VIPER, a mobile rover, will probe the distribution of water ice in greater detail. Drilling beneath the lunar surface is one task. Another is to move into steep, permanently shadowed regions—entering craters that, due to their sharp geometry, and the low angle of the sun at the lunar poles, have not seen sunlight in billions of years. 

The tests demonstrate the rover can navigate a 15-degree slope with ease—enough to explore these hidden dark spots, avoiding the need to make a machine designed for trickier descents. “We think there’s plenty of scientifically relevant opportunities, without having to make a superheroic rover that can do crazy things,” Andrews says.

Developed by NASA Ames and Pittsburgh-based company Astrobotic, VIPER is a square golf-cart-sized vehicle about 5 feet long and wide, and about 8 feet high. Unlike all of NASA’s Mars rovers, VIPER has four wheels, not six. 

”A problem with six wheels is it creates kind of the equivalent of a track, and so you’re forced to drive in a certain way,” Andrews says. VIPER’s four wheels are entirely independent from each other. Not only can they roll in any direction, they can be turned out, using the rover’s shoulder-like joints to crawl out of the soft regolith of the kind scientists believe exists in permanently shadowed moon craters. The wheels themselves are very similar to those on the Mars rovers, but with more paddle-like treads, known as grousers, to carry the robot through fluffy regolith.

“The metaphor I like to use is we have the ability to dip a toe into the [permanently shadowed region],” Andrews says. ”If we find we’re surprised or don’t like what we’re finding, we have the ability to lift that toe out, roll away on three wheels, and then put it back down.”

But VIPER won’t travel very far at all if it can’t get down the ramp from its lander, which is why Andrews and his team have been spending a lot of time testing that procedure. At first, the wheels would skid, just momentarily, as the VIPER test vehicle moved down the ramps. 

”We also found we could drive up and over the walls of the rampway,” Andrews says. “That’s probably not desirable.”

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Together with Astrobotic, Andrews and his team have altered the ramps, and they now include specialized etchings down their lengths. The rover can detect this pattern along the rampway, using cameras in its wheel wells. “By just looking down there,” the robot knows where it is, he says. “That’s a new touch.”

Andrews is sure VIPER will be ready for deployment in 2024, however many tweaks are necessary. After all, this method is less complicated than a sky crane, he notes: “Ramps are pretty tried and true.”