In December last year, an asteroid captured by a telescope in Chile has thrown the international community into confusion. Discovered in space about 48 million kilometers from Earth, this asteroid is estimated to be about the size of a small apartment, with a diameter ranging from 40 to 90 meters. It travels through space at a speed of 17 kilometers per second and is currently moving away from Earth.
The United States National Aeronautics and Space Administration (NASA) Center for Near Earth Object Studies (CNEOS), which serves as a frontline for Earth's defense by monitoring asteroids flying from distant space, announced on the 19th (local time) that the asteroid named '2024 YR4' has a 1.5% chance of colliding with Earth on December 22, 2032. Although this is generally a low figure, it is unusual for a wandering asteroid in space to have a collision probability exceeding 1% with Earth. This center has been updating the collision probability of asteroid 2024 YR4 almost daily since late last month. Just a day ago, the probability of a collision was reported to be 3.1%. This is the highest rate observed since astronomers began estimating the collision probabilities of asteroids near Earth, surpassing the chance (2.7%) for the asteroid Apophis, which once had the highest collision probability.
When asteroid 2024 YR4 was first disclosed on January 29, the collision probability was just 1.3%. However, in the past three weeks, the collision probability for this asteroid has fluctuated dramatically. Moon Hong-kyu, a leading researcher at the Korea Astronomy and Space Science Institute, noted, "There has never been a case where the probability of asteroid collision surged to such high levels in such a short time. It's not at a level to worry about like a disaster movie, but many space scientists worldwide are paying attention to this situation."
◇Sudden warning of collision in '2032'
Our solar system contains over 1.5 million small asteroids and comets made of rock, metal, and ice. Scientists are closely tracking about 34,600 of these that have a significant probability of colliding with Earth. However, most are wandering around like ghosts without revealing their identities. Asteroid 2024 YR4 had never been detected by human eyes until it was captured by the Asteroid Terrestrial-impact Last Alert System (ATLAS), a telescope operated in Chile with NASA's funding on December 27 of last year.
In preparation for potential collisions with large asteroids referred to as 'city killers,' the International Asteroid Warning Network (IAWN), one of the advisory groups designated by the United Nations, conducted a detailed analysis for a month and warned on the 29th of last month that "this asteroid has emerged as a potential impactor with a collision probability of 1.3% on December 22, 2032." Since this group was officially recognized as an advisory group at the United Nations General Assembly in 2013, this is the first time it has warned UN member states through the United Nations Office for Outer Space Affairs (UNOOSA) of an asteroid with a collision probability exceeding 1%. The situation has also been shared with the Korea Aerospace Administration and the Ministry of Foreign Affairs.
The international response to asteroid 2024 YR4 includes participation from the Space Mission Planning Advisory Group (SMPAG), which recently launched as a UN advisory group. This group is comprised of representatives from countries involved in space exploration, led by the European Space Agency (ESA). From April 4 to 6, a meeting of the IAWN and SMPAG steering committee was held in Vienna, Austria, attended by experts from the U.S., Europe, and other countries. In Korea, Moon, the leading researcher, represented IAWN, while Cho Sung-ki, the director of the Korea Astronomy and Space Science Institute's Space Hazard Monitoring Center, participated as a representative of SMPAG. The experts agreed to hold a meeting as soon as possible, in late April or May, to discuss the collision risk and to accurately observe the asteroid's orbit using various observational means.
◇ To know the accurate orbit is to know the accurate collision probability
For most asteroids, the probability of collision converges to either 0 or 100 over time. In other words, either nothing happens, or a collision occurs. However, to estimate collision probabilities for disaster preparedness, the asteroid's precise orbital path needs to be determined.
Jung An-young, an orbital expert at the Korea Astronomy and Space Science Institute, explained, "The phenomenon of the asteroid collision probabilities reported daily by NASA and ESA fluctuating is due to the fact that the orbit of this asteroid is still being updated in real time every day." Both Earth and the asteroid revolve around the sun in their respective elliptical orbits. Scientists say that once the precise orbit of the asteroid is known, they can use the equation proposed by 17th-century German astronomer Johannes Kepler to determine its location at any given point in time. However, little is known about the orbit of asteroid 2024 YR4, except that it has a highly elongated elliptical orbit and revolves around the sun every four years, with a day lasting only 19 minutes.
According to Dr. Jung An, tracking the orbit of an asteroid in space is similar to predicting where a person walking from a distance is going by observing them through a telescope. For example, if you observe a person walking several kilometers away only once, you won't know where they are headed. However, if you continuously observe them, you can estimate their direction. Observing for an hour is much easier than just watching for a minute to predict their future position. Newly discovered asteroids in space can also have their orbits accurately determined through long-term observations.
After discovering asteroid 2024 YR4, scientists dug into observational data from 2016 and 2020, considering the asteroid's four-year orbital period. They particularly anticipated that there might be records from 2016 when the observational conditions were optimal, expecting to soon secure an accurate orbit. However, they found no traces of this asteroid in any previous observational data. Ultimately, scientists decided to directly observe the asteroid from the beginning.
Asteroids revolve in elliptical orbits around the sun, repeatedly moving away from and then closer to Earth. To determine the orbit of an asteroid, observations must occur when it approaches Earth most closely. When it is far from Earth in its elliptical orbit, a small and dim asteroid disappears from view. In the case of asteroid 2024 YR4, although it is moving farther away, it is in an optimal observation period as it will come relatively close to Earth until March this year.
Scientists have gathered approximate orbital information on this asteroid through recent focused observations. However, Dr. Jung stated, "So far, we have only outlined the approximate shape of the asteroid’s orbit; to obtain accurate collision probabilities and orbital information, observations must occur when the asteroid approaches the same location at least twice."
With short-term collected information, it is difficult to ascertain the exact location of the asteroid. As the number of observational locations increases, inaccuracies accumulate, and the calculated orbit becomes ambiguous, leading to discrepancies between the actual orbit the asteroid is following and the predicted orbit. This is why scientists argue that both observation time and the actual interval of observations need to be extended to narrow this gap.
In a situation where the precise orbit is unknown, both Earth and asteroid 2024 YR4 are oscillating between intersecting points for a potential collision in 2032. A group of scientists is currently testing the possibility of a collision between Earth and the asteroid using a method called Monte Carlo simulation. This method involves generating thousands of hypothetical asteroids based on the currently predicted orbits and checking if the asteroid's path intersects with Earth's actual orbit on December 22, 2032. Dr. Jung remarked, "As of January, the anticipated path of the asteroid was expected to pass through a much wider range than the distance between Earth and the moon, but now it appears that Earth lies in the middle of the range through which the asteroid passes. This indicates that the probability of collision has increased."
Some experts have also suggested the possibility that asteroid 2024 YR4 could collide with the moon. Recently, ESA predicted that the asteroid is likely to pass closer to the moon than to Earth at the time of collision.
◇James Webb Space Telescope also deployed, results expected by mid-year
NASA and ESA have mobilized over 50 large telescopes, including the Very Large Telescope of the European Southern Observatory and the Magdalena Ridge Observatory in New Mexico, USA, to conduct over 350 observations since January. Initially, telescopes with apertures of 4 meters and 6 meters were deployed, but as the asteroid drifts farther from Earth, telescopes with 10-meter apertures are being utilized. Currently, asteroid 2024 YR4 is moving steadily away from Earth. NASA plans to urgently deploy the James Webb Space Telescope from March until May 20 for observations of the asteroid 2024 YR4's orbit and size. Researchers believe that as the asteroid distantly moves away, infrared telescopes that capture the faint heat reflected by the asteroid will be better suited for tracking.
IAWN and SMPAG plan to generate accurate information regarding the asteroid's orbit and collision probabilities by mid-year. Leading researcher Moon noted, "By mid-year, it is expected that there will be some clarity on whether asteroid 2024 YR4 will collide with Earth or whether the concern will end as a farce." He also mentioned that, like other asteroids, the collision probability is likely to converge to zero over time. In fact, the Apophis asteroid, discovered in 2004, was initially estimated to have a 2.7% chance of colliding with Earth in 2029, but subsequent observations concluded that there is no longer any risk of collision.
Meanwhile, there are views that the controversy surrounding collision hypotheses will continue for some time. If IAWN and SMPAG do not secure sufficient observational data on the asteroid's orbit by mid-year, the opportunity to adjust the collision probability will move to the next time the asteroid approaches Earth in 2028. Dr. Jung also stated, "Given this year's observations, it will be difficult to achieve sufficient orbital observations, so it will be when the asteroid approaches next time that clear orbital information and collision probabilities can be secured." Until then, asteroid 2024 YR4 will remain a potential collision object for Earth.
◇Warning that 100 million people could be exposed to danger in case of collision
Scientists, while being cautious against premature apocalyptic predictions, are not completely ruling out the possibility of collision. In reality, even after sufficient orbital information has been gathered for asteroid 2024 YR4, the probability of collision could remain below 1% or even increase. If such a situation arises, the matter will be reported to the United Nations Security Council.
James O'Donoghue, a professor at the University of Reading in the UK, also stated in a recent letter sent to the UK Science Media Center, "NASA's mobilization of the James Webb Space Telescope and its high-level response indicate that this issue requires serious attention."
According to the Torino scale, a measure of the potential impact of an asteroid collision, asteroid 2024 YR4 has maintained a risk grade of 3 so far. This is the risk level assigned to asteroids with a collision probability exceeding 1% within a decade. The Torino scale ranges from 0, indicating no possibility of collision, to 10, which indicates a certain collision that could endanger the entire Earth. A Torino grade of 3 is the second-highest level after the Apophis asteroid, discovered in 2004.
Scientists consider the minimum size of an asteroid that can cause damage to Earth to be 10 meters. In February 2013, an asteroid with a diameter of 20 meters entered over Chelyabinsk, Russia, causing shockwaves that injured 1,500 people. In 1908, an asteroid measuring 50 meters in diameter fell in Tunguska, Russia, destroying an area of 2,000 square kilometers.
Scientists believe that asteroid 2024 YR4 could cause enough damage to obliterate an entire city. In fact, according to the collision monitoring report released by NASA last January, asteroid 2024 YR4 is expected to enter the Earth's atmosphere at a speed of 17 kilometers per second, falling into a region covering the eastern Pacific, northern South America, the Atlantic, Africa, the Arabian Sea, and southern Asia. Areas within a 50-kilometer radius of the asteroid's landing site are expected to be destroyed. This is equivalent to the explosive power of a 7.8-megaton nuclear bomb.
Currently, Korea is not included in the crash impact zone. However, among the 100 most populous cities in the world, eight are within the impact zone, including Bogotá, Colombia; Abidjan, Côte d'Ivoire; Lagos, Nigeria; Khartoum, Sudan; Mumbai, India; Kolkata, India; and Dhaka, Bangladesh. The combined population of these cities exceeds 110 million. Leading researcher Moon stated that accurate information on the asteroid's orbit, as well as its size and composition, must be secured to prepare for any potential damage.
◇Push for orbit alteration if the collision probability is high
Currently, if a collision situation were to occur, there are options for response. One option humanity has is to change its direction by crashing a spacecraft into it. NASA successfully conducted the DART (Double Asteroid Redirection Test) experiment in 2022, where a small spacecraft was crashed into the asteroid Dimorphos, over 1 million kilometers away, to alter its orbit. Leading researcher Moon stated, "While humanity has indeed secured a means to change the direction of an asteroid, sufficient time is necessary to actually use this method. Even if this is attempted, it could lead to unfavorable outcomes for Earth." He added, "If the worst-case scenario occurs, the next approach of the asteroid in December 2028 could be humanity's last chance to save Earth."
While Western societies are mobilizing around scientists for planetary defense, China is gearing up at a national level. The China National Space Administration (CNSA) has recently posted job advertisements online, seeking personnel to work in three fields: aerospace engineering, international cooperation, and asteroid detection. China is preparing to conduct a directional change test with the asteroid 2015 XF261 in 2027.
NASA and ESA have been continuously adjusting the probability of asteroid collisions since early February. Fortunately, there has been no social turmoil so far. However, as scientists strive to provide comprehensive and accurate information regarding asteroid collisions, the ongoing debate over "asteroid collision hypotheses" is expected to create tension in the international community for the time being.
References
International Asteroid Warning Network, https://iawn.net/obscamp/2024 YR4/index.shtml
Space Mission Planning Advisory Group, https://www.cosmos.esa.int/web/smpag
NASA CNEOS, https://cneos.jpl.nasa.gov/sentry/details.html#?des=2024%20YR4