It is estimated that ~25 million meteoroids, micrometeoroids, and other space debris enter Earth’s atmosphere each day, and this 0material is mostly vaporised in the atmosphere, creating a bright trail called a ‘shooting star’. It is a well-known theory that a large 10-15 km-diameter asteroid, now called the ‘Chicxulub impactor’, struck Earth in prehistoric Mexico, at a speed of 20 kmps, causing the extinction of non-avian dinosaurs. Hence, the probability of a large celestial body colliding with our planet in future, causes significant fascination and hysteria among humans. A considerable amount of space research, therefore, goes into mitigation efforts to resolve such scenarios.
In recent times, asteroid 2024 YR4 has gained infamy. In the vast expanse of our Solar System, YR4 is a relatively small, but captivating object with a unique trajectory and mysterious nature. A major reason drawing astronomers and global space agencies to keep a constant watch over asteroid YR4 is its portending collision with Earth in 2032.
Discovery and Characteristics
YR4 was first identified on December 27, 2000, by the Catalina Sky Survey, a project focused on identifying Near-Earth Objects (NEOs). While its discovery was not widely publicised at the time, YR4 has since become an object of interest among astronomers studying NEOs and the asteroid belt.
YR4 is classified as a Near-Earth Asteroid (NEA), meaning its orbit brings it into proximity with Earth’s orbit. Specifically, its semi-major axis — the average distance between the asteroid and the Sun — places it in the category of Apollo asteroids, which are a subgroup of NEAs that cross Earth’s orbit. Asteroid YR4 is relatively small, compared to some of the larger objects in the asteroid belt, measuring only 40-90 meters in diameter, putting it in the category of small asteroids.
Unlike many asteroids that follow relatively predictable, circular orbits, YR4 is erratic. It moves in an elliptical orbit that takes it from the inner reaches of the Solar System to the outer region of the asteroid belt, located between Mars and Jupiter. This orbital eccentricity means that YR4 spends a significant amount of time far from Earth, but occasionally swings closer to our planet, making it an object of interest for future monitoring.
Astronomers continue to track YR4’s orbit to better understand the forces that shape its path through the Solar System. Its orbit is influenced not only by the Sun’s gravitational pull, but also by the gravitational effects of nearby planets, particularly Jupiter, whose massive size exerts a considerable force on objects that venture near its orbit. As a result, the future trajectory of YR4 is difficult to predict with absolute certainty, making it a subject of continuous observation. YR4 is believed to be composed of a mixture of rock and metal. However, precise data on its chemical makeup is sparse, as its relatively small size and distance from Earth make it challenging to study in detail with current observational equipment.
Many asteroids are thought to be remnants from the early Solar System, leftover material from the formation of planets and other celestial bodies. This gives asteroids like YR4 the potential to reveal important insights into the conditions that existed in the early Solar System and the processes that led to the formation of Earth and the other planets.
The impact question
Does YR4 pose a potential threat to our planet? The likelihood of an impact with YR4 in the near future was earlier pegged at 3.1%, which was reconfigured to as low as 1.5% a week ago, with scientists now stating that there is an effective 0% chance of it striking Earth. “The probability of asteroid 2024 YR4 hitting Earth was reduced to zero thanks to new data collected on February 23,” reports say.
Discovered on December 25, 2024, by the Chilean station of the Asteroid Terrestrial-impact Last Alert System (ATLAS) at Rio Hurtado, YR4 quickly climbed to the top of NASA’s Sentry Risk table, at one point having a 1 in 32 chance of hitting Earth. This elevated it to Level 3 on the Torino Scale, a system used since 1999 to categorise potential Earth impact events (see table). Level 3, which falls within the yellow band of the Torino Scale, is described as: “A close encounter, meriting attention by astronomers. Current calculations give a 1% or greater chance of collision capable of localised destruction.” When additional observations suggested it had an impact probability of over 1%, it triggered the first step in planetary-defence responses, prompting major telescopes to gather data about the asteroid and led space agencies to begin planning asteroid threat mitigation.
“The damage caused by an impacting asteroid depends greatly on its exact size and composition. The exact size of 2024 YR4 is still uncertain, but an airburst is a likely scenario for its size range. If the asteroid were to enter the atmosphere over the ocean, models indicate that airbursting objects of this size would be unlikely to cause significant tsunami, either from the middle of the ocean or even nearer shore,” NASA says.
The asteroid made a close approach to Earth at a distance of 8,28,800 km on December 25, 2024, two days before its discovery, and is now moving away. Its next close approach will take place on December 17, 2028. From early April 2025 to June 2028, YR4 is expected to be too distant for ground-based telescopes to observe. However, space-based infrared telescopes could continue monitoring it during this period; the James Webb Space Telescope is scheduled to observe it between March and May, this year.
“If the asteroid entered the atmosphere over a populated region, an airburst of an object on the smaller side of the size range, about 130-200 feet could shatter windows or cause minor structural damage across a city. An asteroid about 300 feet in size, which is much less likely, could cause more severe damage, potentially collapsing residential structures across a city and shattering windows across larger regions,” NASA further explains.
However, as of February 24, the impact probability of YR4 with Earth is 0.0017%, which puts our planet in the ‘No Hazard’ zone. “... as more observations of the asteroid were collected, and as that data was added to its orbit calculations, 2024 YR4 was deemed to have no significant chance of Earth impact in 2032 and beyond,” NASA reiterates.
“The NASA JPL Center for Near-Earth Object Studies (CNEOS) now lists the 2024 YR4 impact probability as 0.00005 (0.005%) or 1-in-20,000 for its passage by Earth in 2032,” Richard Binzel, Professor of Planetary Science at the Massachusetts Institute of Technology (MIT) and creator of the Torino scale was quoted as telling Space.com.
The fears of asteroid YR4 impacting the Earth made scientists, especially at NASA, ESA and ROSCOSMOS, scramble for solutions, an approach followed for years. The field of planetary defence has gained significant attention in recent years, especially after events such as the 2013 Chelyabinsk meteor explosion in Russia, which was caused by a relatively small asteroid entering the Earth’s atmosphere. While YR4 is not considered to be a significant threat at present, its orbit could change over time due to gravitational interactions with other bodies in the Solar System. Small perturbations in its path could, in theory, lead to a more direct trajectory toward Earth.
To mitigate the potential risks posed by near-Earth asteroids, space agencies like NASA and ESA have developed programmes to track and catalog these objects. For eg., NASA’s Planetary Defense Coordination Office (PDCO) is responsible for identifying and tracking NEOs, assessing their impact risks, and developing strategies for deflecting or mitigating the effects of potential impacts. As part of this effort, asteroid YR4, along with thousands of other objects, is regularly observed and monitored to assess any potential future risks. By studying the orbits and characteristics of these asteroids, scientists can develop better models for predicting their future movements and more accurately assess the likelihood of an impact (see table).
Monitoring NEOs also provides valuable data for future space missions. Understanding the composition, size, and behaviour of asteroids can help inform decisions about how to approach these objects with robotic spacecraft or, in the future, crewed missions.
Lastly, these experiences just show how vulnerable our planet is on an astronomical scale, bringing to mind the depth of the words of Carl Sagan’s Pale Blue Dot speech, describing the never-ending depths of the Universe: “The Earth is a very small stage in a vast cosmic arena.”