‘City killer’ on the moon jili game app The threatening hunk of rock appears as just a speck of light through even the strongest astronomical tools. In reality, YR4 is likely about 60 meters (about 200 feet) in diameter, according to observations in March by the James Webb Space Telescope, the most powerful space-based observatory in operation.

“Size equals energy,” said Julien de Wit, associate professor of planetary sciences at the Massachusetts Institute of Technology, who observed YR4 with Webb. “Knowing YR4’s size helped us understand how big of an explosion it could be.”

The Webb telescope captured images of YR4 in March using its NIRCam and MIRI instruments. The Webb telescope captured images of YR4 in March using its NIRCam and MIRI instruments. A Rivkin/Webb/STScI/CSA/NASA/ESA Astronomers believe they have found most of the near-Earth asteroids the field would classify as “planet killers” — space rocks that are 1 kilometer (0.6 mile) across or larger and could be civilization-ending, said Dr. Andy Rivkin, planetary astronomer from the Johns Hopkins University’s Applied Physics Laboratory in Maryland. The planet killer that slammed into Earth 66 million years ago and led to the extinction of dinosaurs was estimated to be roughly 6 miles (about 10 kilometers) in diameter.

Smaller asteroids such as YR4, which was colloquially dubbed a “city killer” after its discovery, could cause regional devastation if they collide with our planet. About 40% of near-Earth space rocks larger than 140 meters (460 feet) but smaller than a kilometer — capable of more widespread destruction — have been identified, according to NASA.

But astronomers have never really had a chance to watch a collision of that size occur on the moon in real time, Wiegert said. The latest glimpses of YR4 on June 3 before it passed out of view revealed a 4.3% chance of a YR4 lunar impact — small but decent enough odds for scientists to consider how such a scenario might play out.

A striking meteor shower — and a risk Initial calculations suggest the impact has the largest chance of occurring on the near side of the moon — the side we can see from Earth.

“YR4 is so faint and small we were able to measure its position with JWST longer than we were able to do it from the ground,” said Rivkin, who has been leading the Webb study of YR4. “And that lets us calculate a much more precise orbit for it, so we now have a much better idea of where it will be and won’t be.”

The collision could create a bright flash that would be visible with the naked eye for several seconds, according to Wiegert, lead author of a recent paper submitted to the American Astronomical Society journals analyzing the potential lunar impact.