The Highest 'Collision Probability' in History Shakes the Astronomy World... The Changing Risk of Asteroid 2024 YR4

by Lim Juhyeong

Published 03 Mar.2025 07:30(KST)

Updated 03 Mar.2025 09:59(KST)

After First Discovery Late Last Year, Collision Probability Fluctuated
Impact Risk Changed from 1% to 3% to 0% in Just One Month
Uncertainty Increased Due to Excessively Bright Skies
Will Move Away in May, Returning Again in 2028

The asteroid '2024 YR4', which once attracted astronomical attention by surpassing a 3% probability of Earth impact?the highest in observational history. Recently, the revised impact probability dropped to the 0% range, ending the 'asteroid impact risk' as a brief episode. Celestial orbit calculations inherently involve significant uncertainty, but the problem with this asteroid was that the full moon's light obstructed the observation network at the time of its first detection, interfering with orbit calculations.

Impact Risk Changes from 1% → 3% → 0% in One Month

Asteroid. The photo is a sample image and is not related to any specific content in the article. Pixabay

2024 YR4 was first detected on December 27, last year (local time) by the European Southern Observatory (ESO) telescope located in Chile. International space agencies such as NASA and the European Space Agency (ESA) estimated the asteroid's diameter to be between 40 and 90 meters.

At the time of discovery, the probability of 2024 YR4 impacting Earth in 2032 was confirmed at 1.3%. The issue was that over the following month, the impact probability suddenly surged. On the 1st of last month, it rose to 1.7%, and by mid-month it peaked at 3.1%, marking it as the 'most dangerous asteroid in observational history.' However, it later halved to 1.5%, then dropped to 0.3% on the 21st, and further down to 0.0017% on the 25th.

Full Moon Obstructed Telescope, Increasing Uncertainty

Telescope for celestial observation in Chile affiliated with the European Southern Observatory (ESO). Screenshot from the ESO website

The international community, including space agencies like NASA and ESA, has established an 'International Asteroid Monitoring Network' to observe celestial bodies. Asteroids with potential Earth impact risk are monitored using ground-based telescopes or satellite-mounted telescopes in Earth's orbit, and their trajectories are estimated through 'automated orbital dynamics software.'

Generally, since asteroids come from extremely distant locations, estimating their orbits is a very challenging task. Above all, 2024 YR4 faced a major obstacle at its initial discovery: the full moon. The sunlight reflected by the full moon narrowed the telescope's field of view, preventing proper detection of the asteroid. Fortunately, as the moon waned, the telescope's field of view gradually recovered.

Asteroids are celestial bodies with very faint light points, so they can be clearly captured when the night sky is at its darkest. ESO homepage

On the 19th of last month, NASA explained in a statement, "Because the asteroid emits very faint light, ground-based telescopes require very dark skies to observe it," adding, "When the full moon rose at the end of last year, the sky was too bright for observatories to accurately pinpoint the asteroid's light spot."

As new observational data accumulated, the risk level of 2024 YR4 plummeted, but uncertainty remains. According to NASA, 2024 YR4 is expected to exit Earth's observation zone between April and May. Before that, this month, the state-of-the-art James Webb Space Telescope (JWST) will be activated to analyze the asteroid's detailed characteristics.

How Destructive Would It Be If It Actually Collided with Earth?

Since there is no specific information about the materials or mass composing 2024 YR4, the exact impact severity cannot be determined. However, by applying NASA's estimated mass and density values based on the asteroid's expected diameter, it is projected that the asteroid would release energy equivalent to 7.8 megatons (MT) of TNT. This is 520 times the power of the 'Little Boy' atomic bomb dropped on Hiroshima during World War II.

In fact, because the celestial body is quite small, 2024 YR4 is more likely to burn up in the atmosphere, causing a 'meteor airburst.' Such airbursts do not release significant energy on the ground itself, but fragments breaking off from the original body can scatter, potentially increasing the damage area.

An estimated image of the 1908 Tunguska asteroid impact, which released energy equivalent to 7.8 megatons (MT) of TNT. Screenshot from Smithsonian YouTube.

A representative case of a meteor airburst is the 'Tunguska asteroid' that fell in Siberia, Russia, in 1908. Although there were no human casualties because the area was uninhabited, the asteroid's fragments burned forests over an area of 2,150 km² (3.5 times the size of Seoul), and the shockwave is estimated to have spread over 2,600 km².

If 2024 YR4 were to fall to Earth, the expected impact zone ranges from Central and South America to Central Africa and near India. If it lands in uninhabited oceans or plains, there would be no damage, but if it crashes near a major city, the scale of human and property damage would be difficult to estimate.

Clarity Will Come When It Returns in 2028

Observatories tracking 2024 YR4's orbit plan to continue research until the asteroid's risk is completely eliminated. The asteroid, which will disappear from Earth's view around May, is expected to be observable again around 2028.