Suppose the next big lunar show isn’t a manned landing, but a cosmic crash that can be seen from your garden. That’s now the question hanging over asteroid 2024 YR4, a 200-foot-diameter space rock whose path has recently been under intense examination by astronomers and planetary defense scientists alike.
Discovered on December 27, 2024, by the ATLAS survey in Chile, 2024 YR4 soon hit the headlines for all the wrong reasons. Initial estimates pointed to a more than 1% possibility of an impact on Earth in 2032 the largest ever recorded for a large asteroid. With increasing data from telescopes in Chile, Hawaii, and importantly, NASA’s James Webb Space Telescope (JWST), those Earth odds dropped to close to zero. But in a surprise, the sophisticated observations pushed the chance of a lunar impact higher, now at 4.3% on December 22, 2032 a number NASA has termed “a very small chance,” but still sufficient to warrant the world’s planetary defense community’s attention as information arrives, it is to be expected that the impact probability will change.
The asteroid itself, about the size of a 10-story building, is a near-Earth object belonging to the Apollo group, with an estimated diameter between 174 and 220 feet. The name, “city killer,” is no exaggeration; if it had hit the Earth, the impact would have blown in windows and weakened structures over a city, although not the scale of mass extinction. Instead, the Moon could be its surprise target. Dr. Paul Wiegert, physics and astronomy professor at Western University, encapsulated the uniqueness of the moment: “If YR4 hits the moon, it will be the largest asteroid to have hit the moon in about 5,000 years. It’s quite a rare event.”
If the collision were to happen, the dynamics are both violent and rich scientifically. The asteroid would hit at tens of thousands of miles per hour, creating a crater whose size and shape would vary depending on the actual impact angle and speed. The Moon’s battle-scarred surface already carries the marks of about 1.3 million craters larger than 1 km in diameter, each a reminder of cosmic history impact falling is one of the most basic processes on lunar surface. The creation of a new, large crater would not only change the face of the Moon, but also emit high-speed ejecta pieces of moon rock and regolith far beyond the impact site.
Here, the danger reaches farther than the Moon. While astronomer Pawan Kumar promises that, a collision with the moon ‘won’t be a cause for concern’ because any moon debris blasted into space from the impact ‘blow up in Earth’s atmosphere if any of it makes it to near-Earth space,’ things are not that simple for assets in lunar orbit or on the lunar surface. Satellites that offer GPS, communications, and weather forecasting may be threatened by ejecta, as may astronauts who are deployed on or near the Moon. Recent simulations of artificial and natural impacts on the Moon indicate that ejecta plumes can reach kilometers above the surface, with fragments traveling considerable distances before coming to rest. Research with the iSALE2D shock physics code has demonstrated that even impacts from spacecraft traveling at orbital speeds result in craters several meters deep and in diameter, with ejecta having the potential to destroy nearby structures the ejecta can travel several kilometers from the point of impact and hit with enough kinetic energy to do damage or injury to protected property. For an asteroid of the size of 2024 YR4, its safety exclusion radius could reach 20 kilometers, something that is increasingly important as space exploration speeds up.
For the public and amateur astronomers, the event would be nothing less than spectacular. Estimates are that the collision would most likely take place on the near side of the Moon, which is visible from Earth. “People at home will be able to see the explosion with small telescopes or even binoculars,” Wiegert says. The aftermath might also include a meteor shower as expelled lunar material enters the Earth’s atmosphere a rare, observable effect of an extraterrestrial collision.
Seeing the asteroid itself is a different issue. At its present distance, 2024 YR4 is too faint for all but the most capable professional telescopes, being at an apparent magnitude of 23. When it comes back to Earth’s neighborhood in 2028, NASA and other space agencies will take advantage to study its orbit and physical properties more closely using ground-based and space telescopes such as JWST and radar surveys NASA plans to make additional observations as the asteroid’s orbit around the Sun brings it back to Earth’s neighborhood in 2028.
Aside from the theatrics of a potential impact, the event would give planetary defense measures a unique, in-situ test. Kumar summarized it this way: “2024 YR4 is a tailor-made asteroid for planetary defense efforts. It has everything it takes to get our attention.” For scientists, the produced crater and ejecta would provide new insights into the subsurface layers and geological history of the Moon, much like past impacts have informed our understanding of lunar evolution and regolith formation.
For the moment, the world waits–eyes focused on the sky, telescopes at the ready, and planetary defense systems awaiting whatever is in store.
