Even if asteroids that may hit the earth are successfully intercepted and destroyed on the way, humans may not be able to rest easy. A new study warns that if these fragments are diverted and hit the moon, they may still bring long-term disasters to earth civilization. Earth defense seems simple and crude in science fiction movies: the spacecraft approaches the asteroid, launches missiles, blows the target into pieces, and everything is solved. But the reality is far from optimistic.
Unless the defense system can completely "turn the incoming celestial body into dust" or completely change its orbit, the shattered asteroid debris may still cause widespread casualties and destruction on the surface through strong storms, shock waves and high temperatures. This is why countries are actively developing early warning technology in the hope of discovering "killer" asteroids long before an impact occurs.
At present, humans already have certain early detection and defense capabilities. NASA has launched free software to help the astronomy community conduct early detection and risk assessment of asteroids. Japan's Hayabusa 2 probe once blasted the asteroid Ryugu. The main purpose was to collect samples rather than to conduct actual planetary defense exercises. Since the "Star Wars" program of the last century, scientific researchers have continued to explore the use of space laser weapons to destroy or deflect asteroids, and even proposed the idea of using a "shotgun" style steel rod array to tear the target into pieces hours before impact, causing it to be burned away in the atmosphere. These solutions are all based on the premise that the debris will either be completely vaporized or move away from the Earth before entering the atmosphere.
However, the problem may become more difficult if the interception operation does not cause the debris to fall towards the earth, but "accidentally" hits the moon. Aaron Rosengren, a researcher at the Department of Mechanical and Aerospace Engineering at the University of California, San Diego, and his team pointed out that if the final fragments of a destroyed asteroid hit the moon, the consequences would not only be a danger to the safety of future lunar bases, but may also pose a long-term threat to the Earth-moon system itself. Research shows that a strong impact will eject a large amount of lunar soil into space, part of which will form a co-orbital debris group similar to the near-Earth asteroid "Kamo'oalewa" and the celestial body "2024 PT5". These debris form a dangerous "debris belt" near the orbit of the earth and the moon, and its density is high enough to make any spacecraft passing through it "tremble with fear."
What's more serious is that this debris cloud may induce a chain reaction similar to "Kessler syndrome". Once some of the debris enters low Earth orbit and collides with existing satellites, the resulting debris will hit more satellites, and so on, until low Earth orbit becomes dangerous and unusable for an extended period of time. For modern society, this almost means "technical disconnection": global navigation systems, cellular communications, satellite phones, search and rescue networks, weather forecasts, disaster warnings, military operations and even the Internet services we rely on daily will be severely impacted.
In this context, the importance of early detection is once again magnified, and it must be "very early" early. Rosengren noted that a "realistic and ideal" goal for the most worrying types of NEOs, which are hundreds of meters in diameter, would be to issue warnings five to 10 years in advance. It sounds like a long time, but from an engineering perspective it is almost enough: from first discovering the target, to confirming the probability of impact, to designing and approving a defense mission, developing and launching a spacecraft, and finally leaving enough time for the spacecraft to reach the asteroid and implement a small "orbital push" to avoid the earth after multiple revolutions.
Compared with discovering kilometer-scale "planet killers", catching small celestial bodies measuring tens to hundreds of meters is more difficult, but they also have the energy to destroy big cities. To address this challenge, Rosengren, Thomas Bewley and Ben Hansen of the University of California, San Diego, and researchers at the University of Arizona are studying extremely low-probability but potentially devastating impact scenarios to provide decision-makers with a "tipping point" basis for when a deflection mission must be implemented. They combined observation data from the Pan-STARRS array in South Africa, Chile, and Hawaii and other places, and summarized the results to the Minor Planet Center under the Harvard-Smithsonian Center for Astrophysics. The Solar System Dynamics team of the Jet Propulsion Laboratory in the United States used infrared telescopes such as NEOWISE to analyze celestial bodies approaching the Earth and estimate their sizes, especially those dark asteroids that are difficult to observe in the visible light band.
These efforts have achieved remarkable results: scientists estimate that humans have now discovered about 95% of near-Earth objects with a diameter of more than 1 kilometer, which are enough to cause global disasters. In 2022, similar detection and analysis work promoted NASA's "Double Asteroid Redirect Test" (DART) mission. The probe crashed into the asteroid Demovos and successfully changed its orbital parameters around the larger companion Didymos and the two orbiting the sun. This is regarded as an important milestone in planetary defense technology.
However, what really makes people sleepless and sleepless is still the large number of small celestial bodies that are only a few tens of meters wide. The celestial body that exploded over Chelyabinsk, Russia in 2013 was only 20 to 30 meters in diameter, causing extensive shock wave damage and casualties. Asteroids of the same level or larger are not uncommon in near-Earth space. Bewley warned that there are currently a large number of known near-Earth small objects, and astronomers continue to discover new ones every year. If any one of them appears in the wrong place at the wrong time, it will bring devastating blows to large cities.
As this latest study reminds us, mankind has gained another level of understanding of the threat from asteroids: the danger comes not only from "whether it hits the earth", but also from "how we respond", and whether this response will plant new hidden dangers for the earth-moon system. In other words, planetary defense requires a more nuanced, long-term perspective—both to avoid direct impacts and to guard against making the moon a scapegoat for creating a co-orbital debris field. For ordinary people on the ground, this may mean: rather than sleeping peacefully, it is better to continue to look up at the stars, because the risks from the sky are still very real.