Why landing on the Moon standing up is so difficult – 03/06/2024 – Science

When the Odysseus lander last month became the first U.S.-built spacecraft to land on the Moon in more than 50 years, it tipped over. This limited the amount of experiments that could be done on the lunar surface, as its antennas and solar panels were not pointed in the correct directions.

Just a month earlier, another spacecraft, the Smart Lander for Investigating Moon, or Slim, sent by the Japanese space agency, also tipped over and ended up upside down.

Why did these cases happen? Is it really that difficult to land upright there?

On the internet and elsewhere, people have pointed to the Odysseus module’s height — more than 13 feet from the bottom of its landing feet to the solar panels on top — as a contributing factor to its unbalanced landing.

Did Intuitive Machines, the maker of the Odysseus, make a mistake by building the spacecraft this way?

Company representatives provide an engineering justification for the tall, slender design, but those commenting online have a point.

Something tall tips over more easily than a short, sturdy object. And on the Moon, where the force of gravity is just one-sixth that of Earth, the propensity to tip over is even greater.

This is not a new discovery. Half a century ago, astronauts in the Apollo program got firsthand experience as they bounced around the Moon and sometimes fell to the ground.

Former NASA engineer Philip Metzger, now a planetary scientist at the University of Central Florida, explained the math and physics of why it’s harder to stay upright on the Moon.

“I did the math and it’s really scary,” Metzger said last week on X, formerly Twitter. “The lateral movement that can tip a lander of this size is only a few meters per second in lunar gravity.”

There are two parts to this stability issue.

The first is static stability. If something is tilted at a considerable angle, it will tip over if the center of gravity is outside the landing legs.

Here, it was discovered that the maximum tilt angle is the same on the Earth and the Moon. It would be the same on any world, large or small, because gravity cancels out in the equation.

However, the answer changes if the spacecraft is still moving. The Odysseus was supposed to land vertically with zero horizontal speed, 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,” Metzger said.

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

But if you replace it with a refrigerator-shaped piece of Styrofoam, mimicking the weight of a real refrigerator in lunar gravity, “then a very light push will knock it over,” Metzger said.

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

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

If, for example, 1.8 meters wide was enough to land on the Earth at maximum horizontal speed, then the legs would have to be 4.5 m apart to not land on the Moon at the same lateral speed.

To simplify the design, Odysseus’s landing legs did not fold, and the diameter of the SpaceX Falcon 9 rocket that carried it into space limited how far the landing legs could spread.

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

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

“Why so loud?” I asked.

Steve Altemus, CEO of Intuitive Machines, responded that it had to do with the tanks that store 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,” said Altemus.

Scott Manley, who commentates on rockets on X and YouTube, noted that Altemus led the development of a shorter, wider lander when he was at NASA a decade ago.

That test module, 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, Manley said the design would have worked for the Intuitive Machines lander as well, 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 needed to be larger and heavier. Tanks would also have been heavier.

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

For the landing site in the south polar region, Odysseus’ height offered another advantage. At the bottom of the Moon, sunlight falls at low angles, producing long shadows. If Odysseus had remained upright, the solar panels on top of the spacecraft would have remained out of the shadows longer, generating more energy 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 when landing even on a slope of ten degrees or more. The navigation software was programmed to look for a location where the slope was five degrees or less.

Because the laser instruments on Odysseus to measure altitude were not working during the descent, the spacecraft touched down faster than planned on a 12-degree slope. This exceeded its design limits. The module slid along the surface, broke one of its six legs and toppled to the side.

If the laser instruments had been operating, “we would have nailed the landing,” Altemus said during a news conference last week.

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

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

“It removes some of the margin for error in its dynamic stability, but it doesn’t remove all of the margin for error,” Metzger said of a high lander. “The amount of margin you have left is manageable as long as your other systems on the spacecraft are working.”