Discovered in December 2024, 2024 YR4 prompted the International Asteroid Warning Network (IAWN) to issue its first alert on January 29, 2025. Early estimates suggested a 1.3 percent chance of Earth impact on December 22, 2032, and a size between 40 and 90 meters. This led to a Torino scale rating of level 3, indicating a real but still limited concern.
In response, astronomers worldwide began an intensive observation campaign. Scientists from the University of Helsinki, including postdoctoral researchers Zuri Gray and Grigori Fedorets, were among the first to take up the challenge. Using the 2.5-meter Nordic Optical Telescope (NOT) in La Palma, Spain, they tracked the asteroid's motion, rotation, and physical properties.
"We have been focusing on the asteroid's precise position and motion, and to analyse its rotation, size and shape. This information is crucial for better predicting the asteroid's future trajectory, and ultimately refining its impact probability," said Gray.
"Our recent observations continue to demonstrate the vital role that NOT plays in planetary defence efforts on a global level," added Fedorets.
In mid-February, 2024 YR4 briefly reached a 3 percent chance of Earth impact, marking the highest ever recorded risk for an asteroid of this size category. Observation efforts paused temporarily due to interference from the full moon, but Finnish astronomers resumed tracking immediately after, leading to a steep decline in risk.
Yet while the danger to Earth has diminished, calculations now indicate a 4 percent probability of a lunar impact.
"Should the asteroid impact the Moon, the Earth-Moon system could be clouded with particles detached from the Moon and the asteroid, potentially threatening the human space infrastructure and operations," explained Academy Professor Karri Muinonen.
Further refinement came from space-based instruments. On March 26, NASA's James Webb Space Telescope successfully gathered infrared data on 2024 YR4. Postdoctoral researcher Eric MacLennan, a key analyst of the infrared measurements, noted the importance of this effort.
"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. 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," explained MacLennan.
Based on the Webb data, scientists now estimate the asteroid's diameter to be between 46 and 74 meters with 95 percent confidence, improving assessments of its potential consequences should an impact occur.
Although the immediate risk has eased, Earth-based and orbital observatories will continue monitoring 2024 YR4 through May 2025. After that, it will no longer be observable until its next close pass in 2028.
NOT's rapid response and precision during February established it as a crucial asset for planetary defense, a result of years of effort by Nordic astronomers and technicians.
"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 Associate Professor Mikael Granvik.
"We have unique scientific know-how at the University of Helsinki for contributing to planetary defence, encompassing astronomical observations, laboratory measurements, theoretical and numerical research, and characterising near-Earth object orbits, rotational states, shapes, and surface structure and composition," summarised Muinonen.
In Finland, the governmental role in evaluating asteroid threats rests with the National Land Survey. Soon, the new Space Situational Awareness Center will take on that responsibility. The Center will also monitor risks from space debris, space weather, and satellite traffic.
"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," summarizes Senior Research Scientist and Academy Research Fellow Anne Virkki.
Research Report:JWST Observations of Potentially Hazardous Asteroid 2024 YR4
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