The Moon's surface before and after Israel's Beresheet Moon lander crash landed, as spotted by NASA's Lunar Reconnaissance Orbiter.Credit: NASA/GSFC/Arizona State University Commercial companies and national space agencies alike are racing to land on the Moon. Japan's SLIM Moon lander, the most recent craft to land on the lunar surface, is now in sleep mode. But this does not mark the end of Moon missions for the year. Next week, Intuitive Machines in Houston, Texas, plans send a lander to the Moon. And later this year, China and the private companies Firefly Aerospace and ispace all aim to launch robotic lunar landers. Although lunar ambitions might have risen around the world, achieving a successful touchdown with a robotic lander remains a daunting challenge. Four out of the eight lunar landing attempts made in the past five years have failed — Israel's Beresheet, India's Chandrayaan-2, Japan's Hakuto-R and Russia's Luna 25. This highlights the fact that although researchers can test for some eventualities before sending a lander to the Moon, many uncertainties remain. Nature takes a look at some key tests and challenges involved in preparing a lunar lander for its mission.
Enduring the load
Like every space-bound craft, lunar landers are subject to the intense, sustained vibrations and roar of a rocket launch. To avoid mechanical damage, the lander is tested in acoustic chambers, which have large stereo-speaker-like noise horns to simulate launch sounds, and on shaker tables that produce launch-like vibrations. Scientists also test lunar landers under the kinds of load that could be imparted during touch down. For example, the Indian Space Research Organisation (ISRO) dropped the legs of its successful Chandrayaan-3 lander, Vikram, on test beds made of simulated lunar soil to ensure that they could tolerate a high vertical velocity of three metres per second. Firefly Aerospace, based in Cedar Park, Texas, has conducted more than 100 drop tests on lunar soil simulants and sand to test its lander's legs. Firefly aims to carry ten payloads to the Moon for NASA in late 2024 as part of the space agency's Commercial Lunar Payload Services (CLPS) programme. "We even tested leg drops on concrete because it's harder than anything we'll land on," says William Coogan, Firefly's chief lunar lander engineer.
Preparing for space
In space, landers are subject to near-vacuum conditions, fast-moving orbits and harsh sunlight unfiltered by Earth's atmosphere. These can lead landers to experience swift and huge temperature changes and can cause radiation damage to electronics. To ensure their structural integrity, every lander spends days — or even weeks or months — in 'thermovac' chambers. These achieve a vacuum similar to that experienced in space and on the Moon, simulate the possible temperature swings and even replicate unfiltered sunlight using powerful xenon lamps and mirrors. Landers often host computers and avionic electronics systems made of 'radiation-hardened' components, each of which is tested to not only endure the high mechanical stresses of spaceflight, but also work despite being irradiated at dosage levels expected in each mission. Protecting lunar landers from the harsh space environment is only part of the story, however. Engineers also need to ensure that the hardware and software function together as expected. The roughly three-second delay in two-way communications between Earth and the Moon makes it impossible for engineers on Earth to reliably guide lunar landings. This means that robotic landers must function autonomously during their lunar descent. The engineer who helped India to reach the Moon Kalpana Kalahasti, associate project director of Chandrayaan-3, says her team spent the bulk of the mission's development time coming up with and overseeing tests of the lander's programs. These included fitting a helicopter with the lander's sensors so that the team could mimic different descent phases. The sensors used for the earlier, unsuccessful Chandrayaan-2 lander were tested using aeroplanes. "Since testing sensors on aircraft doesn't simulate hover or low-altitude phases of a lunar landing, we switched to using helicopters for Chandrayaan-3 to better mimic varying altitudes and velocities," says Kalahasti. The Chandrayaan-3 team also examined whether the engines achieved the required dynamic throttling during descent, and assessed the navigation system's ability to hover and avoid hazards before touchdown using crane-based set-ups on Moon-like terrain. Other tests can include antenna testing for communications equipment and optical testing for cameras. For NASA's upcoming VIPER rover mission, which is intended to traverse rocky terrain at the Moon's south pole, scouting for water ice, the agency drove a model of its rover in simulated terrain with varying slopes and rock distributions to test wheel slips, sinkages and traction, and to determine how it performed and what needed improvement.
Simulated Moon landings
When hardware can't be tested, simulations fill the gap. To get a better idea of how a lander might behave on the Moon, engineers characterize hardware sensors and put them into simulations, says Coogan. Mission teams simulate key milestones, such as reaching lunar orbit, to identify what types of problem a lander can handle by itself, and what needs to be addressed by mission control on Earth. "Some real-time data from an ongoing mission is ingested into simulations to test critical commands before sending it to a lander," says Laura Crabtree, co-founder of Epsilon3, a web-based spacecraft testing and operations platform used by several companies that are building lunar landers. This helps to give engineers a more reliable idea of how the lander will behave and respond in real-world situations. Simulations are also a great way to discover the ways a landing system might fail. "We formed a dedicated simulation group to characterize the [Chandrayaan-3] lander's ability to recover from off-track trajectories during descent," says Kalahasti. The group's members also simulated alternative paths the lander could take if something didn't work as expected. And they tested various extreme landing scenarios until the system failed. Once they knew the lander's limits, they were able to modify it as needed.
Known unknowns
However, some aspects of space travel — such as the performance of a lander's propulsion system — cannot be tested on Earth. "You can't simulate weightlessness," says Crabtree. "Until you fire a thruster, you will not definitively know the precise force it imparts." She says the solution is to make a system that compares expected versus actual thrust to understand by how much the lander's performance has deviated. Reserves of propellant are built in to make up for such differences. Russian Moon lander crash — what happened, and what's next? For example, Russia's Luna 25 lander crashed on the Moon as it tried to reduce its orbit size on 19 August 2023. The Russian space agency's investigation found that this was due to an engine firing for 50% longer than necessary. The fault probably stemmed from the software not being designed to prioritize data from the accelerometer, which would have registered that Luna 25 had achieved its desired velocity change. It's also hard to predetermine the safest patch for a lander to touch down on. "During the final landing phase, a lander will see new features not present in onboard orbital imagery, including any hazards," says Coogan. Earth-based tests of the features a lander can identify only represent some aspects of Moon-like terrain. This is why engineers tested SLIM's ability to identify features from lunar orbit before beginning its descent.
Private moonshot challenges
Private companies such as Japan's ispace and those involved in NASA's CLPS programme face extra challenges. They typically cannot invest as much money or time into lander testing as a government space agency. This was highlighted on 25 April 2023 with the crash of ispace's first lunar lander. During a media briefing, ispace's chief technology officer Ryo Ujiie said that the company changed the landing site shortly before launch, and the simulations previously used to test the lander's descent didn't use terrain representative of the conditions the lander ultimately faced. These challenges are likely to increase, because 2024 will see companies competing to be the first private enterprise to successfully land on the Moon. For these organizations, there is a trade-off between development costs and customer revenue, but a mission failure would be worse. "Unsuccessful missions can be very expensive to a company," says Coogan. doi: https://doi.org/10.1038/d41586-024-00352-w Disclosure: Laura Crabtree, who is quoted in this story, is a co-sponsor of the author Jatan Mehta's newsletter Moon Monday. Reprints and permissions
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The Moon's surface before and after Israel's Beresheet Moon lander crash landed, as spotted by NASA's Lunar Reconnaissance Orbiter.Credit: NASA/GSFC/Arizona State University 
The engineer who helped India to reach the Moon Kalpana Kalahasti, associate project director of Chandrayaan-3, says her team spent the bulk of the mission's development time coming up with and overseeing tests of the lander's programs. These included fitting a helicopter with the lander's sensors so that the team could mimic different descent phases. The sensors used for the earlier, unsuccessful Chandrayaan-2 lander were tested using aeroplanes. "Since testing sensors on aircraft doesn't simulate hover or low-altitude phases of a lunar landing, we switched to using helicopters for Chandrayaan-3 to better mimic varying altitudes and velocities," says Kalahasti. The Chandrayaan-3 team also examined whether the engines achieved the required dynamic throttling during descent, and assessed the navigation system's ability to hover and avoid hazards before touchdown using crane-based set-ups on Moon-like terrain. Other tests can include antenna testing for communications equipment and optical testing for cameras. For NASA's upcoming VIPER rover mission, which is intended to traverse rocky terrain at the Moon's south pole, scouting for water ice, the agency drove a model of its rover in simulated terrain with varying slopes and rock distributions to test wheel slips, sinkages and traction, and to determine how it performed and what needed improvement. 
Russian Moon lander crash — what happened, and what's next? For example, Russia's Luna 25 lander crashed on the Moon as it tried to reduce its orbit size on 19 August 2023. The Russian space agency's investigation found that this was due to an engine firing for 50% longer than necessary. The fault probably stemmed from the software not being designed to prioritize data from the accelerometer, which would have registered that Luna 25 had achieved its desired velocity change. It's also hard to predetermine the safest patch for a lander to touch down on. "During the final landing phase, a lander will see new features not present in onboard orbital imagery, including any hazards," says Coogan. Earth-based tests of the features a lander can identify only represent some aspects of Moon-like terrain. This is why engineers tested SLIM's ability to identify features from lunar orbit before beginning its descent. 
The new car batteries that could power the electric vehicle revolution News Feature 07 FEB 24 
Elon Musk's Neuralink brain chip: what scientists think of first human trial News Explainer 02 FEB 24 
Flexible fibres take fabrics into the information age News & Views 31 JAN 24 
