Free Astronomy Magazine July-August 2025

28 JULY-AUGUST 2025 ASTRO PUBLISHING have surfaces covered in loose ma- terial and higher thermal inertia. By studying how these small objects respond to sunlight, scientists ob- tain indirect evidence that they may be solid chunks of rock, possibly chipped off from bigger rubble-pile asteroids. Using data from Keck Observatory’s Multi-Object Spectrograph for In- frared Exploration (MOSFIRE), Bolin and team were able to observe YR 4 in the infrared, seeing properties of the asteroid that would otherwise be impossible to observe. The study uses additional data from the Aster- oid Terrestrial-impact Last Alert Sys- tem or ATLAS, developed by the Uni- versity of Hawai ‘ i and funded by NASA, as well as the Gemini South telescope in Chile, one half of the International Gemini Observatory, partly funded by the U.S. Na- tional Science Founda- tion and operated by NSF NOIRLab. The in- strumental window Bolin and his team had for observing this object was only 4 arc- seconds wide, pro- jecting to a very small patch of the sky, re- A n artist’s animation of 2024 YR 4 , first discovered in December 2024. YR 4 is a solid, stony type that likely originated from an asteroid family in the central Main Belt between Mars and Jupiter, a region not pre- viously known to produce Earth-crossing asteroids. [W.M. Keck Observatory/Adam Makarenko] T he first discovery images of 2024 YR 4 from 2024 December 27 05:42:49 UTC. The asteroid moved at a rate of 22.8 arcminutes per hour (9.11 degrees per day) in the northwest direction. [B.T. Bolin et al., 2025] ! quiring precision measurement only Keck Observatory could provide. “This object’s orbit was so well de- termined we knew its position to within less than an arcsecond. It was moving less than 10 arcseconds per minute, if we were off target the background static stars would have been trailed, but we got it on our first try,” said Bolin. “It was a serendipitous set of circumstances that allowed us to do these obser- vations.” Serendipitous because Bolin’s orig- inal science case was imaging for trans-Neptunian objects, but due to technical difficulties, his team was able to pivot at the last minute to image the object, obtaining data that may one day play a crucial role in saving our planet from impact. “The data from our study will be used to assess the physical proper- ties and shapes of potentially im- pacting asteroids, providing a great test case on the kind of rapid re- sponse observations that are neces- sary to characterize a potential threat like this object. The physical information about an asteroid’s physical property (rubble pile vs solid rock) is crucial for planning mitigation efforts if necessary.”