Why it is so difficult to land upright on the moon

When the robotic lander Odysseus landed on the moon last month, the first American-built spacecraft in more than 50 years, it fell over at an angle. That limited the amount of research it could do on the moon’s surface, because its antennas and solar panels were not pointed in the right directions.

Just a month earlier, another spacecraft, the Smart Lander for Investigating Moon, or SLIM, sent by the Japanese space agency, had also toppled and landed upside down during landing.

Why is there a sudden epidemic of spacecraft rolling across the moon like Olympic gymnasts doing floor routines? Is it really that hard to land upright there?

On the Internet and elsewhere, people pointed to the height of the Odysseus lander — 15 feet from the bottom of the landing pads to the solar panels at the top — as a contributing factor to the anomalous landing.

Did Intuitive Machines, the maker of Odysseus, make an obvious mistake by building the spacecraft that way?

Company officials give a technical reason for the long, skinny design, but those Internet commentators have a point.

Something large falls over more easily than an object that is short and stocky. And on the moon, where gravity is only one-sixth as strong as on Earth, the tendency to fall over is even greater.

This is not a new realization. Half a century ago, Apollo astronauts had firsthand experience as they hopped around on the moon and sometimes fell to the ground.

Last week, Philip Metzger, a former NASA engineer and now planetary scientist at the University of Central Florida, explained on the social media site mathematics and physics why it is harder to stay on the moon.

“I’ve done calculations and it’s really scary,” said Dr. Metzger. “The lateral movement that could tip a lander of that size is only a few meters per second in lunar gravity.” (One meter per second is, in everyday American units, just over two miles per hour.)

This question of stability has two parts.

The first is static stability. If something is at a large angle, it will fall over if the center of gravity is on the outside of the landing legs.

This shows that the maximum inclination angle on Earth is the same as on the moon. It would be the same on any world, large or small, because gravity rules it out completely.

However, the answer changes if the spacecraft is still moving. Odysseus was supposed to land vertically with no horizontal speed, but due to problems with the navigation system he was still moving sideways when he hit the ground.

“Earth-based intuition is now a problem,” said Dr. Metzger.

He gave the example of trying to push over the refrigerator in your kitchen. “It’s so heavy that a little push won’t push it over,” said Dr. Metzger.

But you replace it with a piece of Styrofoam in the shape of a refrigerator, which mimics the weight of a real refrigerator in the moon’s gravity, “then a very light push will push it over,” said Dr. Metzger.

Assuming the spacecraft remains intact, it would rotate at the contact point where the landing foot hits the ground.

Dr.’s calculations Metzger suggested that for a spacecraft like Odysseus, the landing legs on the moon would have to be spread about two and a half times wider than on Earth to counteract the same amount of sideways movement.

For example, if a width of 6 feet were sufficient to land on Earth at maximum horizontal speed, then the legs would have to be 15 feet apart so as not to topple onto the moon at the same sideways speed.

Due to the simplicity of the design, Odysseus’ landing legs did not fold, and the diameter of the SpaceX Falcon 9 rocket that lifted him into space limited how wide the landing legs could spread.

“So on the moon you have to design to keep the lateral velocities upon landing very low, much lower than if you were to land the vehicle in Earth’s gravity,” wrote Dr. Metzger on X.

I also wondered about the shape of the lander when I visited Intuitive Machines’ headquarters and factory in Houston last February.

“Why so long?” I have asked.

Steve Altemus, the CEO of Intuitive Machines, responded that it had to do with the tanks that hold the spacecraft’s liquid methane and liquid oxygen propellants.

The methane weighs twice as much as the oxygen, so if the methane tank had been placed next to the oxygen tank, the lander would have been unbalanced. Instead, the two tanks were stacked on top of each other.

“That created the height,” Mr. Altemus said.

Scott Manley, commenting on rockets X And YouTubenoted that Mr. Altemus had led development of a shorter, stubby lander a decade ago while at NASA.

That test lander, called Morpheus, also used methane and oxygen propellants, but the tanks were configured in pairs to balance weight. I never intended to fly to space.

In an interview, Mr. Manley said the design would also have worked for the Intuitive Machines lander, but would have made the spacecraft heavier and more complex.

If the spacecraft required two methane tanks and two oxygen tanks, the spacecraft structure would have had to be larger and heavier. The tanks would also have been heavier.

“You have more surface area, so that’s more surface area to insulate,” Mr. Manley said. He added that it would also have required “more pipes and more valves, so more things could go wrong.”

For the landing site in the Antarctic, Odysseus’ height offered another advantage. At the bottom of the moon, sunlight shines at low angles, creating long shadows. If Odysseus had remained upright, the solar panels on the top of the spacecraft would have stayed out of the shadows longer, generating more power for the mission.

During the visit to Intuitive Machines, Tim Crain, the company’s Chief Technology Officer, said the spacecraft was designed to remain upright upon landing, even on an incline of 10 degrees or more. The navigation software was programmed to look for a place where the slope was five degrees or less.

Because the laser instruments on Odysseus for measuring altitude during descent did not work, the spacecraft landed faster than planned on a 12-degree slope. That exceeded design limits. Odysseus slid across the surface, broke one of his six legs and fell to the side.

If the laser instruments had worked, “we would have accomplished the landing,” Mr. Altemus said at a news conference last week.

The same concerns will apply to SpaceX’s giant spaceship, which will carry two NASA astronauts to the moon’s surface in 2026.

A spaceship as tall as a 16-story building will have to descend perfectly vertically and avoid significant inclines. But these should be solvable technical challenges, said Dr. Metzger.

“It takes away some of the margin of error in your dynamic stability, but not all of the margin of error,” said Dr. Metzger about a large lander. “The amount of margin you have left is manageable as long as your other systems on the spacecraft are functioning.”