The moon has a weaker gravitational pull, making it easier for spacecraft to tipple. Designers face a trade-off between weight distribution and landing stability, a challenge that future landing missions will need to overcome.
This year, two spacecraft landed at an angle on the lunar surface. Under the gravitational pull of the Moon, which is weaker than Earth's, spacecraft are more likely to tipple.
Last month, the Odysseus, a U.S.-made robotic lander, became the first U.S. spacecraft to land on the moon in more than 50 years, but it landed at an angle. Since the antenna and solar panels are not pointing in the right direction, this limits its ability to carry out scientific missions on the surface of the moon. Just a month ago, the Japan Aerospace Research Agency (JAXA)'s "Lunar Survey Intelligent Lander" (SLIM) also toppled during landing, eventually falling upside down to the ground.
Why are spacecraft tumbling on the moon so frequently recently? Is it really hard to land upright on the moon? On the Internet, the total height of the "Odysseus" lander of 14 feet (from the bottom of the landing feet to the top solar panels) was cited as one of the factors for the inclined landing.
So, did the manufacturer Intuitive Machines make a glaring mistake when designing? The company offers an engineering justification for this tall, thin design, but there is some truth in the online reviewers.
Tall objects are more likely to tip over than chunky objects. On the Moon, where the gravitational pull is only one-sixth that of the Earth, the likelihood of a captive is even greater. This is not a new discovery. Half a century ago, Apollo astronauts experienced it first-hand while jumping and walking on the moon, sometimes falling to the ground.
Last week, Philip Metzger, a former NASA engineer and now a planetary scientist at the University of Florida, explained the mathematical and physical principles of why it is more difficult to stay standing on the moon on the social platform X. "I did the calculations and the results were terrible," Dr. Metzger said, "to be able to tip a lander of this size in lateral motion of just a few meters of the moon's gravity." ”
This question involves two aspects of stability.
The first is static stability. If the object is tilted at a large angle and the center of gravity is outside the landing feet, it will tipple. The results show that the maximum tilt angle is the same on the Moon and on Earth. This is the same on any planet because gravity is canceled out in this formula.
However, if the spacecraft is still moving, the answer changes. "Odysseus" was supposed to land vertically, without horizontal speed, but due to problems with the navigation system, it still moved to the side when it hit the ground.
Earth-based intuition has now become a flaw," Dr. Metzger said. For example, he said, try to knock down the refrigerator in the kitchen. "It's too heavy for a slight thrust to push it down," Dr. Metzger said.
But if you replace it with a piece of styrofoam that is the same shape as the refrigerator, simulating the weight of a real refrigerator under the gravitational pull of the moon," says Dr. Metzger, "then even a very light thrust will push it down." ”
Assuming that the spacecraft remains intact, 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, its landing feet need to be about two and a half times wider on the Moon than on Earth in order to counteract the same lateral motion. For example, if landing on Earth at maximum horizontal speed, 6 feet wide legs would be sufficient, then landing on the Moon at the same lateral speed, the feet would need to be 15 feet apart to not tip over.
To simplify the design, the landing feet of the "Odysseus" were not folded, and the diameter of the SpaceX Falcon 9 rocket that launched it into space limited the width of the landing legs unfolded. "So on the Moon, you have to design the lander to maintain a very low lateral velocity when landing, much lower than a spacecraft that lands under Earth's gravity," Dr. Metzger wrote on X, "which makes landing more difficult and requires more precise navigation and control." ”
For future lunar missions, engineers need to weigh the relationship between weight distribution and landing stability. A taller lander can provide more space for scientific instruments, but it is harder to stay upright. SpaceX's giant Starship will send two NASA astronauts to the surface of the moon in 2026. The starship is 120 meters tall, which is equivalent to the height of a 16-story building. It must descend perfectly vertically, avoiding significant slopes. "This reduces the margin for high lander dynamic stability, but it doesn't completely eliminate it," Dr. Metzger said, "and the remaining margin is manageable as long as the other systems on the spacecraft are working properly." ”