Asteroid that once threatened Earth may target the Moon

An asteroid named 2024 YR4 stirred up concerns when it was found heading near Earth in December 2024. Early projections pushed scientists to keep a close watch, especially after the International Asteroid Warning Network issued a notification on January 29, 2025.

Those first calculations suggested an impact probability of 1.3% on December 22, 2032. Postdoctoral researcher Zuri Gray from the University of Helsinki was among the early observers who noticed that this object measured somewhere between 40 and 90 meters in diameter.

Scientists track the asteroid’s path

“We have been focusing on the asteroid’s precise position and motion, and to analyze its rotation, size and shape,” said Gray. A flurry of observations began in January 2025, and efforts continued through mid-February.

“Our recent observations continue to demonstrate the vital role that Nordic Optical Telescope (NOT) plays in planetary defense efforts on a global level,” added postdoctoral researcher Grigori Fedorets.

Astronomers used the 2.5-meter NOT on La Palma for a sharper view. This telescope has proven its worth in detecting objects traveling near Earth.

Asteroid may target the Moon

Estimates briefly hit 3% probability of an Earth impact, the highest ever recorded for an asteroid of this range. Observers encountered a snag with a bright full moon that briefly overwhelmed the asteroid’s glow.

Once new data became available, predictions shifted dramatically and the chance of an Earth impact decreased below 0.001%. The latest calculations, however, revealed a 4% possibility of the asteroid striking the Moon, bringing new worries for future lunar missions.

“Should the asteroid impact the Moon, the Earth-Moon system could be clouded with particles detached from the Moon and the asteroid, which would potentially threaten the human space infrastructure and operations,” explained Professor Karri Muinonen.

Determining the actual size

On March 26, the James Webb Space Telescope offered detailed infrared observations. This step gave researchers a more precise handle on the asteroid’s diameter, since infrared measurements directly mirror an object’s size.

“The initial diameter estimate of the asteroid was limited to visible light measurements, which depend on both the size and reflectivity of the surface. However, thermal emission at infrared wavelengths is a direct indicator of the size,” noted postdoctoral researcher Eric MacLennan.

“Our first attempt to observe the thermal emission on March 8 was not successful, so it was a relief when the images were transmitted back from the telescope,” said MacLennan. The team’s efforts led to a size prediction of between about 46 and 74 meters (150 and 240 feet), with a 95% level of confidence.

This update helps scientists evaluate potential outcomes if 2024 YR4 meets the lunar surface. 

Telescopes refine asteroid risk

The University of Helsinki’s fast response at the Nordic Optical Telescope in February stood out. The research offered critical measurements that narrowed the potential risks, compared to information obtained from other similar facilities.

“We were able to very quickly observe the asteroid and refine the estimates for it. The flexible, high-quality, medium-sized astronomical facilities like the NOT are crucial for rapid follow-up of these type of potentially hazardous asteroids,” said Professor Mikael Granvik.

Continuous tracking of the asteroid targeting the moon will last until about April or May 2025. After that, 2024 YR4 will slip beyond Earth-based sensors and remain undetected until its next close pass in 2028.

Preparing for broader concerns

Finland has positioned its National Land Survey as the authority for assessing near-Earth object risks. A future Space Situational Awareness Center is also on the horizon to address threats from space debris, solar activity, and collisions.

“The comprehensive expertise of astronomers at the University of Helsinki in asteroid research is a critical part of the expertise needed at the governmental level to assess the possible impact risks caused by the near-Earth objects for Finland similar to other countries,” said academy research fellow Anne Virkki.

Any object measuring several dozen meters can cause serious regional damage if it collides with Earth’s surface. Though this asteroid’s impact chances have dropped for our planet, the Moon’s 4% possibility highlights a unique scenario.

Studies using the James Webb Space Telescope confirm that 2024 YR4’s diameter is near 60 ± 7 meters (197 ± 23 feet) . Objects of this size range can produce noticeable disruptions if they collide with a celestial body.

From a policy standpoint, the research demonstrates why repeated observations, collaborative monitoring, and robust modeling are vital.

Asteroid near Earth tracked into 2025

Scientists remain ready to gather more information during the asteroid’s final observable window in spring 2025. If its path indicates the asteroid will target the moon, efforts to predict any consequences will intensify.

Past events suggest that being able to fine-tune predictions depends on timely data. If 2024 YR4 passes near enough in 2026 for the James Webb Space Telescope to lock on again, that could either calm or reignite concerns about any final trajectory shifts.

The study is published in the journal RNAAS Research Notes of The AAS.

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