Webb Telescope to Weigh in on Impact Odds for Asteroid 2024 YR4 – Sky & Telescope

Astronomers around the world have been working to pin down the orbit of the potentially impacting asteroid 2024 YR₄, either ruling out the chance for impact with Earth in 2032 or showing that such an impact would be likely. So far, the best hope for refining the orbit at this point rests on observations using the James Webb Space Telescope, set for this May.

The asteroid’s chance for impact currently stands at 2.1% — up slightly from late January’s estimate of 1.2% — based on continued observations of the object. So far, at least 50 different observatories around the world have contributed more than 350 observations of this asteroid, as they strive to improve the accuracy of the orbital calculations, astronomer Paul Chodas (NASA’s Jet Propulsion Laboratory) tells Sky & Telescope. The predicted impact date — if it occurs — is December 22, 2032.

But for the next few days, the asteroid will be obscured by the full moon, and as it gets farther and farther away, it is becoming much fainter and harder to observe. It’s already more than 65 million kilometers (0.4 astronomical unit) away, and at magnitude 24, it’s fading fast. Webb’s observations thus have the potential to significantly improve the precision of the known orbit, narrowing down where the asteroid in 2032 might hit Earth.

“The telescopes that can still detect this are the large aperture scopes, maybe two-meter, four-meter telescopes are required, and eventually eight-meter telescopes will be required to detect this,” says Chodas. He directs the Center for Near Earth Object Studies, which runs the Sentry page that lists the latest impact probabilities for all known potentially hazardous asteroids. (“Potentially hazardous” indicates that the asteroids are large and that their orbits intersect Earths’.)

If the Arecibo radio telescope were still operational, its radar system “would have been able to observe [2024 YR4] in December 2024, likely dramatically improving the orbit,” according to planetary scientist Michael Nolan (University of Arizona), who spent 20 years on Arecibo’s planetary radar team.

At present, 2024 YR₄’s impact probability corresponds to a 3 on the Torino scale, which estimates the likelihood and seriousness of an object’s collision — the second-highest ranking ever. The only object that briefly reached a higher Torino scale rank was the asteroid 99942 Apophis, which briefly reached a level of 4 on that scale just after its discovery in 2004. However, that larger asteroid was reclassified as having zero impact probability within a few weeks.

“Unlike Apophis, this one is not going to go away quickly,” Chodas says, “partly because it’s a smaller asteroid. Apophis was larger, and pre-recoveries were available, and radar was available, so it went away a little faster than this, but it was a few hectic days back then.”

Searching the Archives

One approach to following up on the asteroid is to look to the past. The asteroid last came close enough to Earth for observations in 2016. While it wasn’t known then, searching past observations could help pin down its orbit. “Fitting those observations [from 2016] to the orbit would dramatically improve the orbit accuracy, and we would be able to make accurate predictions,” Chodas says. But while “several teams have been searching through the archives for precovery observations,” he adds, “ to my knowledge, there’s no luck. They have not found anything.”

David Rankin (University of Arizona), who has been taking part in these searches, tells Sky & Telescope that immediate follow-up isn’t as helpful. “It’s better to wait a little bit for the uncertainties to grow, and then to shoot it,” he says. That’s why he’s looking to archival data collected by the Catalina Sky Survey.

“We have images at Catalina from 2016 that could have possibly had this object in them,” he notes. “We’ve scanned these images thoroughly and haven’t been able to find it.” But he adds that not finding the asteroid (also called a “negative observation”) doesn’t mean much regarding its impact probability. “All we know is that we could have had it, and we haven’t been able to find it.”

“At this point it’s looking like we’re probably going to be stuck with the current uncertainties, or somewhat close to these current uncertainties, until the next apparition [in 2028],” Rankin says.

Chodas likewise cautions that “relying on a negative observation is fraught with uncertainties.” While the Minor Planet Center does have a process to verify such observations, and they can be useful, “they must be used with great caution.”

For now, the best hope rests with Webb, which will observe the object in March and May, long after it becomes too faint for other telescopes. Andy Rivkin (Johns Hopkins University) says his group has secured four hours of Webb’s observing time, with initial observations to better estimate the object’s size and later images that will help refine its orbit.