Asteroid Won’t Hit Earth, But Might Hit Moon — a Potential Science Bonanza – Sky & Telescope

“We are all rooting for the Moon!” Richard Binzel (MIT) is referring to the asteroid 2024 YR₄, which for a few weeks had remained at the second-highest-rated probability of potential Earth impact of any asteroid discovered. Now, although its impact probability has fallen to virtually zero for Earth, it still has a slight chance of impacting the Moon on December 22, 2032.

An Earth impact by an object of this size — estimated at anywhere from 40 to 90 meters across — could have been serious for a local region, if populated, and astronomers around the world have scrambled for weeks to obtain observations in order to refine the object’s orbit, using the Canada France Hawaii telescope in Hawai‘i, the Magdalena Ridge Observatory in New Mexico, and even the European Southern Observatory’s Very Large Telescope in Chile..

The effort paid off, as its odds of hitting us shrank from a high of 3.1% to a less than 1 in a million chance. At the same time, the odds of it hitting the Moon have risen from a small fraction of a percent to somewhere between 1% and 2% .

Unlike an Earth impact, a hit on the Moon would not only be not a threat but could actually be an opportunity for science. A lunar impact could reveal details of the asteroid’s composition and structure — as it’s pulverized by the impact and spewed back out in a rain of debris — as well as the lunar subsurface, as viewed in ejected material. Understanding the dynamics of the impact could also help aid future asteroid redirection.

Binzel emphasizes that the probability of a lunar impact is still very low. “In all likelihood, we’ll get the all-clear for the Moon as well,” he says, “but in the meantime, we would love for that scientific bonanza to befall us.”

While observers and satellites have seen many impacts on the Moon by small asteroids over the years, they’re sporadic, producing an occasional flash. “But this would be a predictable impact of some size,” Binzel tells Sky & Telescope. “We could have all our capabilities trained on the very spot of impact and watch this natural experiment unfold before our eyes.”

Mark Boslough (University of New Mexico) is likewise intrigued by the possibilities. “We could send all kinds of observational missions to the Moon: seismometers, debris trackers, and imagers, and all sorts of instruments.”

If the asteroid did hit the moon, Boslough says, based on the uncertainty in its size, it could leave a crater anywhere from about 400 meters across to as much as 1.8 km.  across — larger than Arizona’s Meteor Crater.

Test Case

Even as the all-clear has been sounded for Earth, this asteroid provided a useful test case for how such discoveries get reported and how people will react, says Binzel, who invented the now widely used Torino Scale for communicating the risk of a future asteroid impact. This event was only the second time any object has ever reached a rating higher than 2 on the 10-point scale — hitting a peak of 3 for few weeks — it has now reverted to a safe level of zero.

But as new telescopes like the Vera Rubin Observatory and Near-Earth Object Surveyor Telescope start up, “we’re going to have a lot of forthcoming cases of new discoveries of small objects for which the followup is going to be difficult,” Binzel notes, “and we’ll have cases of uncertainties for future impact probabilities that aren’t strictly zero.” As these discoveries become commonplace, people will pay less attention unless something rises to a much higher risk level on that scale, he says.

At the same time, 2024 YR₄ serves as an example of widespread cooperative action between scientists, professional and amateur, to provide a definitive answer when a question has reached a level of global interest. Dozens of observatories contributed hundreds of observations in order to pin down the orbital uncertainties and declare Earth safe.

“The unsung heroes of the YR₄ story,” Binzel says, “are the ones who pasted themselves to the telescope to get some very difficult images and dug into archives for precovery [pre-discovery] images.”