Last month, when the robotic lander Odysseus became the first American-built spacecraft to land on the moon in more than 50 years, it fell at an angle. Antennas and solar panels were not pointed in the right direction, limiting the amount of science that could be done on the lunar surface.
Just a month ago, the Japan Space Agency's Smart Lander for Lunar Exploration (SLIM) also tilted and hit its head during landing.
Why are spaceships on the moon suddenly trendy, like an Olympic gymnast performing a floor routine? Is it really that difficult to land straight up there?
People on the Internet and elsewhere have pointed out that the height of the Odysseus lander (14 feet from the bottom of its landing feet to the solar panels at the top) was the cause of the unstable landing.
Did Intuitive Machines, the creators of Odysseus, make an obvious mistake in building their spaceship that way?
Company officials provide an engineering rationale for the tall, slim design, but Internet commenters have a point.
Tall objects fall more easily than short, stocky objects. And on the moon, where the force of gravity is only one-sixth that of Earth, the tendency to flip is much greater.
This is not a new realization. Half a century ago, Apollo astronauts had the experience of jumping over the moon and sometimes falling to the ground.
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“I actually did the math and it’s really scary,” Dr. Metzger said. “The lateral movement that would tilt a lander of that size is only a few meters per second in lunar gravity.” (One meter per second is the everyday American equivalent of a little over two miles per hour.)
There are two parts to this stability issue.
The first is static stability. If something is standing at too much of an angle and your center of gravity is on the outside of your landing leg, you will fall.
Here it turns out that the maximum tilt angle is the same on Earth as on the Moon. Since gravity cancels out in the equation, it will be the same in all worlds, big or small.
But if the spacecraft is still moving, the answer will be different. Odysseus was scheduled to land vertically with zero horizontal speed, but problems with its navigation system meant it was still moving sideways when it hit the ground.
“Earth-based intuition is now a problem,” said Dr. Metzger.
He gave the example of trying to push the refrigerator in the kitchen. “It’s so heavy that if you push it even a little bit, it won’t fall over,” Dr. Metzger said.
But if you replace it with a refrigerator-shaped piece of Styrofoam that mimics the weight of a real refrigerator under the moon's gravity, “if you push it very lightly, the refrigerator will tip over,” Dr. Metzger said.
Assuming the spacecraft remains in one piece, it will rotate at the point of contact where the landing foot touches the ground.
Dr. Metzger's calculations show that for a spacecraft like Odysseus, the landing legs would need to be about 2.5 times wider on the Moon than on Earth to accommodate the same amount of lateral movement.
For example, if 6 feet wide is enough to land on the Earth at maximum horizontal velocity, your legs would need to be 15 feet apart to avoid tipping onto the Moon at the same lateral velocity.
To simplify the design, Odysseus' landing legs were not retractable, and the diameter of the SpaceX Falcon 9 rocket that lifted it into space limited the width over which the landing legs could be deployed.
“Therefore, on the Moon, the lateral velocity on touchdown must be designed to remain very low, much lower than when the vehicle lands in Earth's gravity,” Dr. Metzger wrote in X.
I, too, was curious about the shape of the lander when I visited the Intuitive Machines headquarters and factory in Houston in February of last year.
“Why are you so tall?” I asked.
Steve Altemus, CEO of Intuitive Machines, responded that it had to do with the tanks holding 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, there were two tanks on top of each other.
“It created height,” Mr. Altemus said.
Scott Manley provides commentary on the Rockets. X YouTube notes that Altemus led the development of a shorter unmanned lander when he was at NASA 10 years ago.
A test lander named Morpheus also used methane and oxygen propellants, but the tanks were paired in pairs to balance weight. It was never intended to fly into space.
In an interview, Mr. Manley said the design would 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 tank would also have been heavier.
“You have more surface area, so you have more surfaces to insulate,” Mr. Manley said. He also added that it would have required “more pipe, more valves, more problems.”
For the Antarctic landing site, Odysseus's height gave him another advantage. On the bottom of the moon, sunlight shines at a low angle, creating long shadows. If Odysseus had stayed upright, the solar panels atop the spacecraft would have been in shadow for longer, producing more power for the mission.
During a visit to Intuitive Machines, Tim Crain, the company's chief technology officer, said the spacecraft is designed to remain upright when landing, even at inclines of more than 10 degrees. The navigation software was programmed to find points where the slope was less than 5 degrees.
During descent, Odysseus' altitude-measuring laser equipment failed, forcing the spacecraft to land sooner than planned on a 12-degree slope. This exceeded the design limits. Odysseus slid along the surface, breaking one of his six legs and leaning sideways.
If the laser equipment had been working, “we would have landed successfully,” Altemus said at a news conference last week.
The same concerns will apply to SpaceX's massive Starship, which will take two NASA astronauts to the lunar surface in 2026.
The spacecraft, which is as tall as a 16-story building, must land perfectly vertically while avoiding significant inclines. But this should be a solvable engineering challenge, Dr. Metzger said.
“It eliminates some of the error margins in dynamic stability, but it doesn’t eliminate all of the error margins,” Dr. Metzger said of the large landers. “As long as the spacecraft’s other systems are functioning, the remaining margins can be managed.”