The Odds of Asteroid YR4 Hitting Earth: Science and Uncertainty

On December 27, 2024, a small, fast-moving asteroid was spotted in the vicinity of Earth, designated 2024 YR4. This asteroid, about 50 meters in diameter and weighing approximately 200,000 tons, made its closest approach to Earth just two days earlier on December 25, 2024. Since its discovery, numerous observatories have been monitoring its movement, allowing for a relatively accurate reconstruction of its orbit. The asteroid orbits the Sun every four years, reaching a maximum distance of about 4.18 astronomical units, just outside the asteroid belt. Its next close approach to Earth, set for December 17, 2028, will miss us, but a second encounter is projected for December 22, 2032, with a closer approach.

Current data suggests that the asteroid will miss Earth in 2032 by about 160,000 kilometers, or less than half the distance to the Moon. However, the trajectory’s uncertainty is significant, with a 99.7% confidence interval spanning 1.28 million kilometers, including Earth itself, which is about 12,750 kilometers in diameter. The probability of impact has risen from 1.2% after its discovery to 3.1% as of February 18, 2025.

Does this mean an impact is imminent? Not necessarily. As more data is gathered, the probability will likely first increase and then plummet to zero. This is due to a combination of improved precision in tracking and confirmation that the trajectory safely misses Earth. Let’s explore the science behind why this scenario is expected and what humanity can do if the impact risk remains.

The Torino Scale Explained

The Torino scale categorizes the potential risks from asteroids and other space objects. Level 0 indicates no hazard, while higher levels represent increasing risk. An object that will definitely miss Earth is a Level 0. Smaller objects, under 20 meters in diameter, are also Level 0, as even if they were to strike, the damage would be minimal. Larger objects, between 20 and 100 meters, are classified as “city-killers,” while those between 100 meters and 1 kilometer are “regional devastation” threats. Anything larger poses an “extinction-level” risk.

Understanding Asteroid YR4’s Rating

Asteroid YR4 currently holds a Torino scale rating of 3, meaning a close encounter with a 1% or greater chance of localized destruction. According to NASA, objects at this level should attract attention from astronomers, but it is most likely that new observations will reassign them to Level 0, especially if the encounter is within a decade.

How Orbit Trajectories Are Tracked

Determining the trajectory of an asteroid involves several key steps:

• Identifying the asteroid’s current position and time of observation.
• Tracking its motion over time to gather more data points.
• Using gravitational models to predict past and future paths.
• Incorporating measurement uncertainties to project potential future trajectories.
• Factoring in the gravitational influence of major planets.

The Importance of Precision

Precision in the data is crucial. With more observations, particularly during the asteroid’s closest approach to the Sun, the uncertainties in the trajectory prediction decrease. However, because YR4 was discovered after its closest approach, its orbital trajectory has more significant uncertainties.

This animation was made as a composite of a series of observations from the ESO’s Very Large Telescope in January of 2025. Asteroid 2024 YR4 is currently the most potentially hazardous object ever discovered in the 21st century.

The Natural Evolution of Impact Probabilities

Typically, as more observations refine our understanding of an asteroid’s orbit, the impact probability either drops significantly or increases if a collision becomes more likely. Initially, the probability of an impact appears relatively high due to significant uncertainties. Over time, if data confirms the trajectory as safe, the probability will rapidly decrease.

Uncertainty in YR4’s Trajectory

The best estimates predict that YR4 will miss Earth by approximately 160,000 kilometers in 2032, but the uncertainty in its path is ±213,811 kilometers. This uncertainty represents a 3-sigma confidence interval. Statistically, this means there is a 99.7% chance the true value lies within this range. As observations improve, the uncertainty will decrease, and the probability of impact will either rise or fall based on the trajectory’s precision.

This diagram shows the relative sizes of a Boeing 747, the Chelyabinsk impactor, asteroid 2009 Sulawesi, and the estimated sizes (black, minimum; grey, maximum) of the Tunguska impactor and asteroid 2024 YR4. The devastation that would be caused by an impact from 2024 YR4 is presently wildly uncertain.

The Statistical Significance of Uncertainty

When dealing with uncertainties, it’s vital to understand their statistical context. A 1-sigma uncertainty suggests a 68% chance that the true value lies within the interval, 2-sigma indicates a 95% chance, and 3-sigma corresponds to a 99.7% chance. For YR4, the trajectory uncertainty is ±213,811 kilometers, but this is a 3-sigma interval. As the uncertainty narrows, the probability of impact will initially increase as less likely “miss” scenarios are ruled out, but it will plummet as precision improves and the “hit” scenarios become less probable.

The Critical Role of Precision in Observations

The current trajectory uncertainty of YR4 is ±641,435 kilometers, reflecting a 3-sigma uncertainty. To confidently rule out the possibility of an impact, the uncertainty must be reduced to below ±213,811 kilometers. Achieving higher precision in observations will lead to a more accurate prediction of the asteroid’s path, enabling us to confidently state whether an impact will occur.

This diagram shows the possible paths (red dots) of asteroid 2024 YR4 relative to Earth and the Moon’s orbit, as illustrated by the density of the dots. The most likely scenario is that the asteroid misses Earth and the Moon, with (at present) a 3.1% chance of it striking the Earth and an 0.3% chance of it striking the Moon. The most likely event is no collision at all.

Historical Context: Case Study of Apophis

The asteroid that previously held the record for highest potential terrestrial impact was 99942 Apophis, which had a 2.7% chance of hitting Earth in 2029 when it was first discovered in 2004. However, subsequent observations improved the trajectory determination, and its current risk is much lower than 1 in a million.

Potential Consequences of an Impact

The variability in 2024 YR4’s size, mass, and trajectory introduces significant uncertainty in the potential impact consequences. The asteroid could range in size from 40 to 90 meters and possess kinetic energy ranging from a few hundred kilotons to 15 megatons of TNT. A smaller impact, similar to the Chelyabinsk meteor of 2013, could cause localized damage with minimal casualties, while a larger impact could result in catastrophic destruction akin to the Barringer Crater in Arizona.

This map of Earth is overlaid with the risk corridor of impact of asteroid 2024 YR4. Several major cities lie along this risk corridor, including South American cities like Bogotá, African cities like Lagos, and Asian cities like Mumbai.

Several major cities lie within the potential impact corridor of 2024 YR4, including Bogotá, Colombia; Lagos, Nigeria; Sana’a, Yemen; Mumbai, India; and Dhaka, Bangladesh. An oceanic impact could also lead to significant coastal flooding and tsunamis, affecting billions of dollars worth of property.

Deflecting an Asteroid: Lessons from DART

If observations confirm that 2024 YR4 poses a significant threat, humanity will need to consider redirection strategies similar to those used in the DART mission. By impacting the asteroid with a spacecraft to alter its trajectory, deflection can be achieved. The optimal time for redirection is as early as possible, ideally years in advance of the potential impact. A small change in the asteroid’s speed, even just a few centimeters per second, can result in a significant shift in its path over time.