Asteroid or collision with Earth: a false alarm or a serious threat?
Asteroids are small celestial bodies that orbit the Sun in the solar system but are much smaller in size and mass than planets. With the improvement of observational capabilities, asteroids that were once hidden from astronomers due to their small size and low brightness have gradually come into view. These small bodies have become the "darlings" of astronomical research because they carry clues that can answer key questions about the origin, evolution, and the origin of life in the solar system. At the same time, they pose a threat to us because they might collide with Earth, so astronomers need to be constantly vigilant against them.
On December 27, 2024, telescopes of the "Asteroid Terrestrial-impact Last Alert System" (ATLAS) deployed by the University of Hawaii at Chile's Rio Hurtado, while conducting a near-Earth asteroid search mission for NASA, discovered a new asteroid, temporarily designated as 2024 YR4. Subsequently, observatories of the International Asteroid Warning Network (IAWN) conducted follow-up observations of this asteroid and also found traces of it in archived images from December 25 and 26.
Astronomers discovered asteroid 2024 YR4 in this image. (Image source: NASA)
As observations continued, astronomers gained more understanding of the size and orbit of this asteroid. They estimated that the diameter of 2024 YR4 is between 40 meters and 90 meters. Institutions such as NASA's Jet Propulsion Laboratory Near-Earth Object Research Center (CNEOS) and the European Space Agency Near-Earth Object Coordination Center (NEOCC) independently calculated the probability of this asteroid colliding with Earth and reached a consensus: on January 27, their calculations showed that the probability of 2024 YR4 colliding with Earth on December 22, 2032, exceeds 1%. This result has sparked high international astronomical community concern.
Continued vigilance
The passage of an asteroid through a corridor, where any part of the corridor overlaps with the Earth, is considered a threat. Within a month of its discovery, the increase in observational data allowed astronomers to make more precise orbital calculations. They predicted that the area 2024 YR4 might pass through in 2032 has been greatly reduced, but the Earth is still within this area, so the probability of it colliding with Earth has shown a gradual increase over the past month. After exceeding a threshold of 1% set by astronomers, the collision probability is still dynamically changing: on January 29th it was 1.3%, on February 5th it was 1.6%, and on February 7th it was 2.3%. According to current calculations, if a collision were to occur, the impact point could be somewhere in a belt-shaped area stretching across the eastern Pacific, northern South America, the Atlantic, Africa, the Arabian Sea, and southern Asia. An asteroid of this size would cause severe damage to the local area if it hit Earth.
The International Asteroid Warning Network notified the United Nations Office for Outer Space Affairs (UNOOSA) and the Space Mission Planning Advisory Group (SMPAG) on January 29th about the situation, and both organizations have initiated response mechanisms. According to the response standards agreed upon by the two organizations in 2017, if the estimated size of the discovered small body is greater than 10 meters and the predicted collision probability is greater than 1%, the International Asteroid Warning Network must issue a warning; and if the predicted collision time is within 50 years, the predicted collision probability is greater than 1%, and the estimated size of the body is greater than 50 meters, the Space Mission Planning Advisory Group should take further action. Based on current observational results, both the size and collision probability of 2024 YR4 meet the standards for initiating response mechanisms.
On January 31st, after an emergency meeting, the Space Mission Planning Advisory Group issued a statement indicating that they are aware of the known facts about the asteroid but believe it is premature to take immediate action. However, they will continue to closely monitor changes in collision probability and observational information about the asteroid's size, and plan to convene the next meeting by late April or early May 2025. In a statement on February 5th, the organization indicated that they have discussed the matter and believe that there is currently no need to develop specific space mission proposals.
On January 31st, 2025, 2024 YR4 was 48 million kilometers from Earth, equivalent to 160 times the distance between the Earth and the Moon. Now, the asteroid continues to orbit the Sun and is moving away from Earth until early April 2025, when astronomers will use several ground-based telescopes to track the asteroid. After that, it will become too faint to be seen by ground-based telescopes as it gets too far away. The orbital period of this asteroid is about 4 years, and when it approaches Earth again in June 2028 and can be observed, its closest distance to Earth will be about 20 times the distance between the Earth and the Moon, and it will not collide with Earth during this process.
In addition to continuing to observe 2024 YR4, astronomers are also trying to search through previous observation databases to see if images of this asteroid have ever been recorded. Although such searches are not easy, if more data can be found in the archives, it will make the calculations of 2024 YR4's orbit more accurate, and thus the collision probability will also change significantly.
Mostly a false alarm
The International Astronomical Union (IAU) has been using the Torino Scale since 1999 to classify potential Earth-impact events, including an integer scale from 0 to 10, with corresponding color coding. This scale shows the likelihood and consequences of potential events, with higher numbers indicating more unusual events that require special attention.
The black arrow shows that the rating of 2024 YR4 on the Torino Scale increased from level 1 on December 29th, 2024, to level 3 on January 27th, 2025. (Image source: NASA)
The current rating of 2024 YR4 on the Torino Scale is 3, which is very rare because this only happens for asteroids greater than 20 meters in size and with a collision probability of 1% or higher, and such asteroids are very rare. Smaller asteroids have a higher collision probability, even up to 100%, but because they are small, they usually burn up in the Earth's atmosphere without causing any harm, so such asteroids are always rated 0 on the Torino Scale. In recent years, several asteroids have reached level 1 on the scale, meaning there is a low probability of impact, but after further calculations, they all returned to level 0.
Generally, an increase in collision probability over a period of time for an asteroid like 2024 YR4 is a common phenomenon and usually ends up decreasing to 0. This is because as observations continue, the risk corridor continues to narrow and moves away from Earth, so the impact risk suddenly drops. Even if there is still some uncertainty in the calculations of the asteroid's orbit at this time, astronomers will rule out the risk of impact.
The most representative example is the asteroid Apophis, discovered in 2004. On June 19th, 2004, astronomers at the Kitt Peak National Observatory in Arizona, USA, discovered a new asteroid, designated 2004 MN4, which was later officially numbered 99942 and named after the ancient Egyptian god of destruction "Apophis," also known in Chinese as "Apep." The probability of this asteroid colliding with Earth once reached 2.7%, and its rating on the Torino Scale reached 4, making it the "most dangerous object in history" (since then, no object has broken through level 2 until the latest discovery of this asteroid). However, subsequent observations confirmed that it would safely pass by Earth in 2029, with a collision probability of 0, and it will not collide with Earth within the next 100 years.
Threat still exists
Astronomers estimate that there are more than 1 million asteroids of various sizes in the solar system, the vast majority of which are invisible small bodies in space running on their own orbits and will not affect humans. Astronomers believe that only two types of asteroids may pose a threat to Earth: one is near-Earth asteroids, which are asteroids with a perihelion distance of less than 1.3 AU (1 AU = 1.496 × 10^8 kilometers); the other is potentially hazardous asteroids, which are asteroids with a diameter greater than 150 meters and a minimum distance from Earth's average orbit of less than 0.05 AU (about 20 times the Earth-Moon distance).
The image shows the Earth and the Moon orbiting the Earth, with the yellow dots indicating the possible area of encounter between 2024 YR4 and Earth on December 22, 2032, predicted based on data up to January 31st, 2025. This area only has a 1.6% chance of intersecting with Earth. (Image source: NASA)
Although asteroids are not large, they can still have significant destructive power. Several famous events have also made the public nervous when hearing about the threat of asteroids. A mainstream view is that 66 million years ago, an asteroid impact on Earth triggered an ecological disaster that ultimately ended the era of dinosaurs ruling the Earth. Although there have been many new theories in recent years about the cause of the extinction of the dinosaurs, there is no doubt that asteroid impacts have had a tremendous impact on the evolution of life on Earth.
In modern history, the Tunguska explosion is a representative event of an asteroid impact on Earth. In 1908, an asteroid that entered the atmosphere exploded over the Tunguska region of Siberia, Russia. The explosion was equivalent to 20 million tons of TNT, causing more than 80,000 trees to be burned down in an area of over 2,150 square kilometers. In the 21st century, there have also been asteroid impact events that have attracted global attention. On February 15th, 2013, a small celestial body with a diameter of about 18 meters entered the atmosphere, becoming a super fireball that exploded at an altitude of about 30 kilometers above Chelyabinsk Oblast, Russia, causing about a thousand people to be injured and thousands of buildings to be damaged.
In 2003, a research group at NASA pointed out that an asteroid impact with a diameter greater than 1 kilometer would cause a global disaster, and an asteroid impact with a diameter greater than 140 meters would cause an intercontinental disaster. Therefore, when observing, astronomers use 140 meters and 1 kilometer as the dividing lines for the size of asteroids. In addition, asteroids with an equivalent diameter of about 50 meters and 10 meters will also cause varying degrees of harm, which are also the two size standards set by the International Asteroid Warning Network and the Space Mission Planning Advisory Group for initiating response mechanisms. Smaller asteroids will cause fireball events and generally do not cause significant harm to humans.
Although asteroids like Apophis and 2024 YR4 have attracted high attention from the astronomical community and the public, the vast majority of discovered asteroids have a negligible probability of colliding with Earth, and there is no possibility of an impact within a time scale of hundreds of years. However, what worries astronomers is that due to the small size and dimness of asteroids and our limited observational capabilities, a large number of near-Earth asteroids may not have entered the field of observation yet. It is estimated that the near-Earth asteroids we have discovered only account for 2% of the total, while 70% of asteroids with a diameter greater than 140 meters, 97% of asteroids with a diameter between 50-140 meters, and 99% of asteroids with a diameter between 10-50 meters have not yet been discovered by astronomers.
Advancing defense steadily
Although the probability of Earth being hit by an asteroid is extremely small, once it happens, it will bring regional or global impacts and even threaten the survival of humanity, so asteroid defense has become a common task for humanity. The 2013 Chelyabinsk event prompted countries around the world to pay more attention to the threat of asteroids. In 2014, the International Asteroid Warning Network was established under the framework of the United Nations Committee on the Peaceful Uses of Outer Space, and China joined this network as a full member in February 2018.
In the field of asteroid defense, there are currently two main approaches: one is to destroy the asteroid, but this is currently difficult to implement and carries significant risks; the other is to change the asteroid's orbit through impacts or other means, thereby avoiding a collision with Earth, and research guided by this idea is ongoing.
The "Asteroid Impact and Deflection Assessment" (AIDA) program, jointly carried out by NASA and the European Space Agency (ESA), is representative of the latter approach. On November 24th, 2021, NASA successfully launched the "Double Asteroid Redirection Test" (DART), the first-ever asteroid defense test mission in human history, with the goal of changing the asteroid's orbit through kinetic impact to verify asteroid defense technology. DART's target is a binary asteroid system, with the larger of the two asteroids having a diameter of about 780 meters, called "Didymos," and the smaller one having a diameter of about 160 meters, called "Dimorphos," also known as the twin big star and twin small star.
On September 12th, 2022, the probe released a companion small satellite for collecting impact data. On September 27th, 2022, DART collided with the twin star as planned, with a mass of 550 kilograms and a relative velocity of 6.6 kilometers per second. According to expectations, the impact will cause small velocity changes in the twin star, thereby affecting its period of orbit around the twin star. Based on data collected from ground observation equipment and small satellites, the impact achieved its expected goals and was a complete success.
China has also developed a similar kinetic energy impact testing plan. On China Aerospace Day in 2022, the National Space Administration has provided a clear asteroid defense plan: China will begin to establish a near Earth asteroid defense system to jointly address the threat of near Earth asteroid impacts and contribute to the protection of Earth and human security. On September 5, 2024, at the Second International Conference on Deep Space Exploration (Tiandu), the China Deep Space Exploration Laboratory released China's first near Earth asteroid defense mission plan and international cooperation concept: China's first near Earth asteroid defense mission is planned to be carried out around 2030, using the "companion flight+impact+companion flight" mode, that is, when the impactor performs kinetic energy impact on the target asteroid, the detector observes the entire impact process and continues to carry out impact effect evaluation and scientific exploration after the impact.