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Why it’s so hard to land upright on the Moon

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When the robotic lander Odysseus last month became the first American-built spacecraft to land on the Moon. in more than 50 years, it has tipped sideways. This limited the amount of scientific research it could perform on the lunar surface, because its antennas and solar panels were not pointed in the right directions.

A month earlier, another spacecraft, the Smart Lander for Investigating Moon, or SLIM, sent by the Japanese space agency, also tilted during its landing, end on the head.

Why is there a sudden outbreak of spaceships rolling around the moon like Olympic gymnasts performing floor routines? Is it really that hard to land there?

On the Internet and elsewhere, people have pointed to the height of the Odysseus lander — 14 feet from the bottom of the landing legs to the solar panels at the top — as a contributing factor to its staggered landing.

Had Intuitive Machines, the creator of Ulysses, made an obvious mistake in building the spaceship this way?

Company officials provide a technical rationale for the tall, slim design, but these Internet commenters have a point.

A tall object falls more easily than a short, stocky object. And on the Moon, where the gravitational pull is six times weaker than on Earth, the propensity for overturning is even greater.

This is not a new awareness. Half a century ago, Apollo astronauts had first-hand experience hopping on the Moon and sometimes falling to the ground.

On the 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 why it’s harder to stand on the moon.

“I did some math and it’s really scary,” Dr. Metzger said. “The lateral movement that can tip a lander of this size is only a few meters per second in lunar gravity.” (One meter per second is, in daily U.S. units, equivalent to just over two miles per hour.)

This question of stability has two parts.

The first is static stability. If something is very tilted, it will fall if the center of gravity is outside 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 out.

However, the answer changes if the spacecraft is still moving. Ulysses was supposed to land vertically with zero horizontal velocity, but due to problems with the navigation system, he was still moving sideways when he hit the ground.

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

He gave the example of trying to overturn the refrigerator in your kitchen. “It’s so heavy that a light push won’t be enough to bring it down,” Dr. Metzger said.

But you replace it with a refrigerator-shaped piece of polystyrene, mimicking the weight of a real refrigerator in lunar gravity, “and then a very slight push will make it fall,” Dr. Metzger said.

Assuming the spacecraft remains in one piece, it would rotate at the contact point where the landing foot touches the ground.

Dr. Metzger’s calculations suggest that for a spacecraft like Odysseus, the landing legs need to be about two and a half times wider on the Moon than on Earth to counteract the same amount of lateral movement.

If, for example, six feet wide were enough to land on Earth at maximum horizontal speed, then the legs would have to be spaced 15 feet apart so as not to tip over to the moon at the same lateral speed.

For design simplicity, Odysseus’ landing legs did not fold, and the diameter of the SpaceX Falcon 9 rocket that lifted it into space limited the width of the landing legs.

“So on the Moon you have to design to keep the lateral velocities very low at the time of landing, much lower than you would if the vehicle landed in Earth’s gravity,” Dr. Metzger wrote on .

I, too, wondered about the shape of the lander when I visited the Intuitive Machines headquarters and factory in Houston in February of last year.

“Why so big?” I asked.

Steve Altemus, chief executive of Intuitive Machines, responded that it was related to the tanks that hold the spacecraft’s liquid methane and liquid oxygen propellants.

Methane weighs twice as much as 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.

“It created height,” Mr. Altemus said.

Scott Manley, who comments on rockets on X And Youtubenoted that Mr. Altemus led the development of a shorter, squatter lander when he was at NASA a decade ago.

This test lander, named Morpheus, also used methane and oxygen propellants, but the tanks were configured in pairs to keep the weight balanced. It was never planned to fly into 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 needed 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 more surface area to insulate,” Mr. Manley said. He added that it would also have needed “more plumbing and more valves, more problems.”

For the landing site in the South Pole region, Ulysses’ 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 panels atop the spacecraft would have stayed out of shadow longer, generating more power for the mission.

During Intuitive Machines’ visit, Tim Crain, the company’s chief technology officer, said the spacecraft was designed to stay upright during 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 Odysseus’s laser instruments for measuring altitude did not work during descent, the spacecraft landed faster than expected on a 12-degree slope. This exceeded its design limits. Odysseus skidded on the surface, broke one of his six legs and rolled over onto his side.

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

The same concerns will apply to SpaceX’s gigantic Starship, which will take two NASA astronauts to the surface of the Moon as early as 2026.

Starship, as tall as a 16-story building, will have to descend perfectly vertically and avoid significant slopes. But these challenges should be solved in engineering, Dr. Metzger said.

“It removes some of the margin of error in your dynamic stability, but it doesn’t remove all of the margin of error,” Dr. Metzger said of a large lander. “The margin you have left is manageable as long as your other systems on the spaceship are functioning.”



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