NASA canceled its lunar rover mission, VIPER, scheduled for July 2024. Originally, it was supposed to launch in November that year aboard the Astrobotic Griffin lander to search for ice in permanently shadowed areas on the lunar south pole, but it was not carried out due to budget issues.
The cancellation decision may have been a fortunate stroke of luck. American scientists revealed that, after reanalyzing VIPER's ground test results, the rover could have become stuck in the sand if it had gone to the moon. Although the rover's weight was reduced to simulate the moon's gravity in ground tests, the sand did not accurately mimic the lunar environment.
◇Moon gravity conditions, sand was overlooked
Dan Negrut, a professor at the University of Wisconsin's Department of Mechanical Engineering, and his research team noted, "Through computer simulations, we discovered flaws in how rovers for space exploration are tested on Earth," in a publication on the 25th (local time) in the international journal Journal of Field Robotics. The researchers predicted that if VIPER had gone to the moon as planned, it would have struggled to move in the sand.
NASA tests rovers in conditions that simulate the moon or Mars before launching them. The moon's gravity is only one-sixth, or 17%, of Earth's. Mars' gravity is about 38% of Earth's. NASA scientists tested prototypes that had their weight reduced to reflect this difference in gravity. They predicted how a lightweight rover, operated under reduced weight in the desert, would perform on the moon or Mars.
The University of Wisconsin research team revealed that the standard testing methods used until now overlooked the minor detail of Earth's gravity acting on desert sand. The rover was adjusted for lunar or Martian gravity, but the underlying soil was still influenced by Earth's gravity.
The research team conducted tests using their self-developed physics simulation engine, Project Chrono. This engine was developed for simulating the VIPER rover. The researchers stated, "Earth soil is solid and has strong support, making it less likely for the rover's wheels to move separately beneath it," adding, "In contrast, lunar regolith is influenced by the weak gravity and moves easily, reducing the rover's traction."
Walking on a hard sidewalk presents no issues, but it would be challenging to get moving on a slippery board; similarly, VIPER would have likely found it difficult to move on the moon. If the rover's wheels sank into the sand, its traction would weaken, preventing it from getting out. Indeed, NASA's Opportunity rover sent to Mars became stuck in the sand in 2006 and could not move for five weeks. In 2009, its twin, Spirit, also got stuck in the sand and ended its mission.
◇Lunar vacuum and temperature conditions must also be considered
There are indoor facilities in Korea to test lunar exploration rovers. The Korea Institute of Civil Engineering and Building Technology (KICT) has built a 'ground heat vacuum chamber.' The massive chamber, measuring 4.7 meters in height and diameter, and weighing 100 tons, is filled with soil that mimics lunar regolith and tests lunar exploration equipment in near-vacuum conditions.
The moon is not only different in gravity from Earth. The moon's weak gravity cannot hold gases. This means it is close to a vacuum. Temperature changes can reach up to 200 degrees Celsius in a day. Dr. Shin Hyu-seong from the Korea Institute of Civil Engineering and Building Technology noted, "While experimenting with lunar regolith simulants in the chamber, we confirmed that along with gravity, vacuum and temperature conditions cause mechanical differences between Earth's and lunar soil."
For example, there are many voids between lunar regolith particles. While air would fill these voids on Earth, the vacuum of the moon means that the movement between soil particles can be significantly influenced. Lunar regolith particles carry strong static electricity because high-energy particles from the sun rain down without interference on the moon. Thus, lunar soil particles repel or attract each other due to their strong static electricity.
Dr. Shin stated that a hole that may collapse during drilling on Earth can remain intact on the moon because the particles pull on each other, maintaining structure. He noted, "Merely adjusting the weight of the rover according to gravity without considering vacuum, temperature, and static electricity will be insufficient for equipment verification for long-term exploration missions."
Equipment protection must also be given special attention. While Earth's soil is rounded due to atmospheric friction, lunar soil particles are sharp in all directions due to the lack of atmosphere. Additionally, they carry static electricity, which can cling to exploration equipment or spacesuits and cause damage. It's as if tiny razor blades are sticking to them. Astronauts from Apollo 17, who went to the moon in 1972, repaired the broken wheel guards of their rover with maps and duct tape to prevent lunar sand from sticking and causing the rover to malfunction.
The United States has resumed human lunar exploration, which had been halted since Apollo 17, through the Artemis program after half a century. Plans are in place to send astronauts near the lunar south pole in 2027. Researchers at Washington State University developed key technologies to ensure the success of the 2023 Artemis lunar exploration, specifically technologies to remove sharp sand particles that could damage spacesuits and equipment, known as lunar dust.
The Washington State University researchers stated in the international journal Acta Astronautica that "In experiments conducted in a vacuum environment like that of the moon, the newly developed liquid nitrogen spray removed 98% of the lunar regolith simulants that had clung to the spacesuits." This indicates that the rover must also be equipped accordingly.
References
Journal of Field Robotics (2025), DOI: https://doi.org/10.1002/rob.22597
Acta Astronautica (2023), DOI: https://doi.org/10.1016/j.actaastro.2023.02.016