One rock, as big as a football field, careening through space on an elliptical orbit that lasts four years, suddenly found itself one of the most observed bodies in our solar system, according to NASA’s Planetary Defense Coordinator, Lindley Johnson, who quoted Paul Mason, a Planetary Society astronomer, at a press conference on Friday. It was first observed on the evening of December 27, 2024, at the El Sauce Observatory in Chile, and shortly thereafter caught the attention of NASA’s planetary defense team because of its estimated 3.1% likelihood of colliding with Earth in 2032, which was the highest ever recorded for an object of its size—though later refined to .004%.
Though the danger is insignificant, this close call illustrated the devastating effects of what is commonly referred to as a “Exterminator” asteroid. With a diameter of anywhere between 130 and 300 feet, the threat posed by asteroid 2024 YR4 has a diameter comparable to the Tunguska asteroid in Siberia, which occurred in the year 1908. NASA’s Andrew Rivkin estimates the resulting explosion in the Earth’s atmosphere to consist of eight megatons of TNT if asteroid 2024 YR4 enters Earth’s atmosphere and blows up in a populated zone. The Hiroshima bomb detonated at a mere 0.016 megatons of TNT, a stark contrast to the devastating eight megaton explosion produced by asteroid YR4 if it were to strike a populated zone upon Earth’s surface.
While extremely calamitous on a localized scale, this asteroid will not cause any climatic effects on a global scale, as was the case in the period at the end of the reign of the dinosaurs, caused by a gigantic asteroid measuring approximately six miles in diameter and entering Earth’s surface with a vicious impact.
The physics of such an air burst is quite well understood. It begins as the asteroid hits the atmosphere at tens of thousands of miles per hour. This builds up a pressure against the surface of the asteroid until it vaporizes, giving off a shock wave and a thermal wave. The altimeter of the detonation determines the blast radius of the damage. If this occurs over an ocean, the primary danger would be a tsunami if it occurs near a coastline. Seismic detectors would be capable of picking up such an eruption even from hundreds of miles away, said Jones.
The trajectory of the asteroid 2024 YR4 was determined by the NASA Jet Propulsion Laboratory via a combination of optical observing and radar ranging, long-developed techniques to follow near-Earth asteroids. The Torino Scale, designed to provide a way to communicate the danger of asteroids to the general public, assigns the asteroid a level of 0, which means “essentially zero” chance of collision. Several authors have noted, however, that after frequent tracking, the level of danger is likely to decline to zero after a suitable number of observations have been made. This has been noted by Richard Binzel of MIT, the inventor of the Torino Scale.
However, this incident is nevertheless a reminder of why planetary defense concepts are no longer just theoretical. In 2022, NASA’s Double Asteroid Redirection Test (DART) space mission was successful in using a kinetic impactor, intentionally impacting an asteroid, to change its orbit. This experiment was conducted on Dimorphos, which is a moonlet of the asteroid Didymos, and was successful in changing its orbit period by 33 minutes. Autonomou s navigation of DART meant it was capable of intercepting its target without any Earth input, which becomes essential in situations where signal delay from Earth takes up to 40 minutes. The transfer of momentum was achieved through both impact and ejecta, where ejecta increased the transfer of momentum by up to 4.9 times.
Other theories of deflection are still being explored. A gravity tractor would employ the gravitational pull of a spacecraft itself to gently nudge an asteroid off course over an extended period of time. Alternatively, laser ablation with either on-board laser beams or concentrated sunlight reflected using mirrors would vaporize surface material, thereby propelling the asteroid. Another method called ion beam shepherd would employ a continuous flow of charged particles on the asteroid, providing a constant but gentle push. In some situations, a standoff nuclear detonation may be necessary, but the fragmentation of an asteroid into multiple pieces poses too much risk.
The trick to all of these strategies is time. Even the most effective deflection strategies need years to accomplish the task effectively. Missions such as NASA’s soon-to-be-launched NEO Surveyor are intended to detect 90% of near-Earth objects that are at least 140 meters in diameter over the course of a decade, which is the earliest form of warning that calls for direct action to prevent an impending catastrophe like the 2024 YR4 asteroid that was discovered barely three months before it was due to fly by the Earth. The International Asteroid Warning Network (IAWN) does the vital work of monitoring the detection of an asteroid like 2024 YR4, which was detected only.
For now, 2024 YR4 will continue on its way, flying by Mars and then out to Jupiter before coming back to Earth’s orbit in 2028, when scientists will pick up observations once again. As Kelly Fast of NASA pointed out: We can find these things, make these predictions and have the ability to plan. This brief time in the spotlight is over for the asteroid, although the tools developed in its wake will be instrumental in the ability to turn disaster into a near miss.
