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When the robotic lander Odysseus last month became the first American-built spacecraft to touch down on the moon in more than 50 years, it toppled over at an angle. That limited the amount of science it could do at the lunar surface, because its antennas and solar panels were not pointed in the correct directions.<\/p>\n
Just a month earlier, another spacecraft, the Smart Lander for Investigating Moon, or SLIM, sent by the Japanese space agency, had also tipped during landing, ending up on its head.<\/p>\n
Why is there a sudden epidemic of spacecraft rolling on the moon like Olympic gymnasts performing floor routines? Is it really that difficult to land upright there?<\/p>\n
On the internet and elsewhere, people pointed to the height of the Odysseus lander \u2014 14 feet from the bottom of the landing feet to the solar arrays at the top \u2014 as a contributing factor for its off-kilter touchdown.<\/p>\n<\/div>\n<\/div>\n
Had Intuitive Machines, the maker of Odysseus, made an obvious error in building the spacecraft that way?<\/p>\n
The company\u2019s officials provide an engineering rationale for the tall, skinny design, but those internet commenters do have a point.<\/p>\n
Something tall falls over more easily than an object that is short and squat. And on the moon, where the pull of gravity is just one-sixth as strong as on Earth, the propensity to tip over is even greater.<\/p>\n
This is not a new realization. A half-century ago, Apollo astronauts had firsthand experience as they hopped around on the moon, and sometimes tumbled to the ground.<\/p>\n<\/div>\n<\/div>\n
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 the math and the physics of why it is more difficult to remain standing on the moon.<\/p>\n<\/div>\n<\/div>\n
\u201cI\u2019ve actually gone through calculations, and it\u2019s really scary,\u201d Dr. Metzger said. \u201cThe side motion that can tip a lander of that size is only a few meters per second in lunar gravity.\u201d (One meter per second is, in everyday American units, a bit more than two miles per hour.)<\/p>\n
There are two parts to this question of stability.<\/p>\n
The first is static stability. If something is standing at much of an angle, it will fall over if the center of gravity is to the outside of the landing legs.<\/p>\n
Here, it turns out the maximum angle of leaning is the same on Earth as it is on the moon. It would be the same on any world, large or small, because gravity cancels out of the equation.<\/p>\n
However, the answer changes if the spacecraft is still moving. Odysseus was supposed to land vertically with zero horizontal velocity, but because of problems with the navigation system, it was still moving sideways when it hit the ground.<\/p>\n<\/div>\n<\/div>\n
\u201cIntuition that\u2019s based on Earth is now a liability,\u201d Dr. Metzger said.<\/p>\n
He gave the example of trying to push over the refrigerator in your kitchen. \u201cIt\u2019s so heavy that a slight push is not going to push it over,\u201d Dr. Metzger said.<\/p>\n
But you replace it with a piece of Styrofoam in the shape of a refrigerator, mimicking the weight of a real refrigerator in lunar gravity, \u201cthen a very light push will push it over,\u201d Dr. Metzger said.<\/p>\n
Assuming the spacecraft remains in one piece, it would rotate at the point of contact where the landing foot touches the ground.<\/p>\n<\/div>\n<\/div>\n
Dr. Metzger\u2019s calculations suggested that for a spacecraft like Odysseus, the landing legs need to be splayed about two and a half times as wide on the moon as on the Earth to counteract the same amount of sideways motion.<\/p>\n<\/div>\n<\/div>\n
If, for example, six feet wide were enough for landing on Earth at the maximum horizontal speed, then the legs would have to be 15 feet apart in order not to tip on the moon at the same sideways speed.<\/p>\n
For simplicity of design, the landing legs of Odysseus did not fold up, and the diameter of the SpaceX Falcon 9 rocket that lifted it to space limited how wide the landing legs could spread out.<\/p>\n
\u201cSo, on the moon, you have to design to keep the sideways velocities very low at touchdown, much lower than you would if landing the vehicle in Earth\u2019s gravity,\u201d Dr. Metzger wrote on X.<\/p>\n
I too wondered about the shape of the lander when I visited the Intuitive Machines headquarters and factory in Houston in February last year.<\/p>\n
\u201cWhy so tall?\u201d I asked.<\/p>\n
Steve Altemus, the chief executive of Intuitive Machines, replied that it had to do with the tanks that hold the spacecraft\u2019s liquid methane and liquid oxygen propellants.<\/p>\n<\/div>\n<\/div>\n
The methane weighs twice as much as the oxygen, so if the methane tank were placed next to the oxygen tank, the lander would have been unbalanced. Instead, the two tanks were stacked on top of each other.<\/p>\n
\u201cThat created the height,\u201d Mr. Altemus said.<\/p>\n