When the robotic Odysseus spacecraft last month became the first American-made spacecraft to land on the moon in more than 50 years, it tipped over at an angle. This limited the amount of science it could do on the lunar surface because its antennas and solar panels were not pointing 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 flipped during landing, ending up on its head.
Why is there a sudden epidemic of spaceships rolling on the moon like Olympic gymnasts performing floor routines? Is it really that hard to land upright there?
On the Internet and elsewhere, people have pointed to the height of Odysseus’ landing gear — 14 feet from the bottom of the landing legs to the solar arrays on top — as a contributing factor to its smooth landing.
Had Intuitive Machines, the manufacturer of the Odysseus, made an obvious mistake in building the spacecraft this way?
Company officials provide a technical rationale for the tall, skinny design, but these Internet commenters have a point.
Something tall falls more easily than an object that is short and squat. And on the moon, where the pull of gravity is only one-sixth as strong as on Earth, the tendency to topple is even greater.
This is not a new realization. Half a century ago, Apollo astronauts had first-hand experience as they jumped to the moon and sometimes crashed to the ground.
On social media site X last week, Philip Metzger, a former NASA engineer who is now a planetary scientist at the University of Central Florida, explained mathematics and physics because it’s harder to stay upright on the moon.
“I’ve actually done the math and it’s really scary,” said Dr. Metzger. “The lateral motion that can overturn a lander of this size is only a few meters per second in lunar gravity.” (One meter per second is, in everyday American units, a little more than two miles per hour.)
There are two parts to this stability issue.
The first is static stability. If something is standing at a steep angle, it will fall if the center of gravity is on the outside of the landing legs.
Here, it turns out that the maximum tilt angle is the same on Earth as on the moon. It would be the same on any world, big or small, because gravity cancels the equation.
However, the answer changes if the spacecraft is still moving. Ulysses was supposed to land vertically at zero horizontal velocity, but due to problems with the navigation system, it was still moving sideways when it hit the ground.
“Earth-based intuition is now a liability,” said Dr. Metzger.
He gave the example of trying to push over the refrigerator in your kitchen. “It’s so heavy that a light push isn’t going to push it,” said Dr. Metzger.
But you replace it with a refrigerator-shaped piece of Styrofoam, which mimics the weight of a real refrigerator in lunar gravity, “then a very light push will push it,” Dr. Metzger said.
Assuming the spacecraft stays in one piece, it will rotate at the point of contact where the landing leg touches the ground.
The calculations of Dr. Metzger suggested that for a spacecraft like Ulysses, the landing legs would need to be about two and a half times as wide on the moon as on Earth to counteract the same amount of lateral motion.
If, for example, it was six feet wide enough to land on Earth at maximum horizontal speed, then the legs would have to be 15 feet apart to keep from tilting at the moon at the same lateral speed.
For design simplicity, Ulysses’ landing legs did not fold, and the diameter of the SpaceX Falcon 9 rocket that launched it into space limited how wide the landing legs could be extended.
“Well, on the moon, you have to design to keep the lateral velocities very low during landing, much lower than you would if you landed the vehicle in Earth’s gravity,” wrote Dr. Metzger in X.
I, too, wondered about the landing pattern when I visited Intuitive Machines’ headquarters and factory in Houston last February.
“Why so tall?” I asked.
Steve Altemus, the CEO of Intuitive Machines, replied that it had to do with the tanks that hold the spacecraft’s liquid methane and liquid oxygen propellants.
Oxygen weighs twice as much as methane, so if the oxygen tank were placed next to the methane tank, the lander would be unbalanced. Instead, the two tanks were stacked on top of each other.
“That created the height,” Mr. Altemus said.
Scott Manley, who comments on missiles X and YouTubenoted that Mr. Altemus led the development of a shorter, squatter lander when he was at NASA a decade ago.
This test aircraft, called Morpheus, also used methane and oxygen propellants, but the tanks were configured in pairs to keep the weight balanced. It was never intended to fly into space.
In an interview, Mr. Manley said the design would have worked for Intuitive Machines, but it would have made the spacecraft heavier and more complex.
If the spacecraft needed two methane tanks and two oxygen tanks, the spacecraft structure would have to be larger and heavier. Tanks would also be heavier.
“You have more surface area, so it’s more surface area for insulation,” Mr Manley said. He added that it would also require “more plumbing and more valves, more things to go wrong.”
For the landing site in the south pole region, Odysseus’ height offered another advantage. At the bottom of the moon, sunlight shines at low angles, producing long shadows. If Ulysses had remained upright, the solar arrays on top of the spacecraft would have remained out of shadow longer, producing 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 stay upright when landing even on an incline of 10 degrees or more. The navigation software was programmed to look for a point where the slope was five degrees or less.
Because the laser instruments on Ulysses to measure altitude did not work during descent, the spacecraft landed faster than planned at a 12-degree tilt. This exceeded its design limits. Odysseus slipped along the surface, broke one of his six legs, and tipped to the side.
If the laser instruments had worked, “we would have nailed the landing,” Mr. Altemus said during a news conference last week.
The same concerns will apply to SpaceX’s massive Starship, which will carry two NASA astronauts to the surface of the Moon as soon as 2026.
The spacecraft, as tall as a 16-story building, must descend perfectly vertically and avoid significant slopes. But these should be solvable engineering challenges, said Dr. Metzger.
“It removes some of the margin for error in your dynamic stability, but it doesn’t remove all of the margin for error,” said Dr. Metzger for a high landed. “The amount of leeway you have left is manageable as long as your other systems on the starship are working.”