In late 2024, astronomers detected asteroid 2024 YR4 on a potentially hazardous trajectory towards Earth, prompting a concerted observational effort to ascertain its size—estimated to be as large as a football field—and trajectory. Fortunately, it was determined that a collision with Earth is unlikely. Nonetheless, an impact on the Moon cannot be entirely discounted.

In January of this year, another colossal asteroid, potentially a million times more massive, passed largely unnoticed.

Asteroid 2024 YR4, possessing a diameter of approximately 40 to 90 meters and labeled a “city-killer“, could inflict significant regional devastation and climatic disruption. In contrast, the more substantial asteroid, 887 Alinda, extends over four kilometers in diameter and possesses the capacity to precipitate a global extinction event.

Alinda orbits just beyond Earth’s path, while 2024 YR4 intersects it, presenting a potential but distant threat of collision.

Asteroid Orbits

Both 887 Alinda and 2024 YR4 complete three orbits around the Sun for every single orbit of Jupiter, which takes about 12 years. As a result, they are set to return to similar paths in 2028, a periodicity that makes them particularly concerning due to their predictable appearances.

Alinda was discovered in 1918 and has undergone several near passes at four-year intervals, while 2024 YR4 has been periodically passing close to Earth since 1948, though it has only recently gained attention.

Since the 1970s, intensified focus has emerged regarding asteroids exhibiting a three-to-one relationship with Jupiter’s orbit, a phenomenon first identified as noteworthy by American astronomer Daniel Kirkwood in the late 19th century.

Using sparse data from that era, he observed that none orbited the Sun in simple ratios like two-to-one or three-to-one, nor in more complex configurations.

These so-called Kirkwood gaps are not readily apparent and are primarily discerned in plots illustrating the average distances of asteroids from the Sun. For approximately a century, they remained an intriguing yet obscure aspect of our solar system.

Advancements in computational technology since the 1970s have elucidated the resonance effects acting upon such celestial bodies. This resonance occurs when asteroids exhibit motion that aligns with or multiples of Jupiter’s orbit speed.

The Kirkwood gaps arise due to asteroids interacting with Jupiter, resulting in their removal from the asteroid belt, even as their mean distances from the Sun remain stable. By encroaching closer to the inner solar system, some of these asteroids are eliminated from the gaps by impacting inner planets like Mars, Venus, or Earth.

Interestingly, the gaps are not entirely devoid of asteroids. Alinda, for example, resides within the three-to-one gap. Subsequent discoveries have identified numerous similar asteroids collectively categorized as “Alindas,” named after the initial find whose etymology remains somewhat enigmatic.

The Return of the Asteroids

While the existence of Kirkwood gaps suggests potential hazards from asteroids impacting terrestrial bodies, the implications for Alinda-class asteroids are even more alarming. These asteroids’ predictable orbits allow for the alignment conducive to potential impacts on Earth every four years.

Typically, encounters with these asteroids occur over extended intervals; however, once aligned, they return periodically with just four years between approaches. The orbital tilt plays a crucial role; significant tilts reduce the likelihood of crossing paths with Earth.

Unfortunately, both Alinda and 2024 YR4 exhibit minimal orbital tilt, positioning them as more probable candidates for an impact.

The resonant “pumping” mechanism has facilitated 2024 YR4’s intersection with Earth’s orbit, while the greater concern lies with Alinda, which may be primed for an impact in about 1,000 years due to ongoing orbital changes.

On a positive note, 2024 YR4 is set to miss Earth in 2032. However, this near-approach will alter its orbit, reducing the frequency of its return to similar paths.

Despite this alteration, its trajectory will continue to intersect with Earth’s orbit, albeit less frequently. Current calculations indicate a somewhat closeness (beyond the Moon’s distance) in 2052, with subsequent predictions being less certain.

Monitoring Other Asteroids

Despite the vastness of the solar system, Earth remains prone to impacts.

If 2024 YR4 eluded detection in 2024, what of other asteroids? Notably, the last significant impact occurred undetected on February 15, 2013, over Chelyabinsk, Russia, resulting in numerous injuries due to shattered glass from the explosive shockwave.

A more substantial event transpired in 1908 over Tunguska, Siberia, causing extensive damage to forested areas while resulting in relatively few injuries.

Staying Vigilant

While astronomers meticulously survey the night skies from terrestrial platforms, space-based initiatives, such as the forthcoming Near-Earth Object (NEO) surveyor, promise greater efficiency in detecting asteroids. These observatories can identify asteroids via their heat (infrared) radiation and, being situated in space, can also survey the daytime sky.

As asserted by Amy Mainzer, the NEO surveyor’s lead scientist, we currently catalog only approximately 40 percent of the asteroids sizable enough to inflict considerable regional harm and that closely approach Earth’s trajectory.

Set to launch in late 2027, the NEO surveyor will “find, track, and characterize the most hazardous asteroids and comets,” with the objective of achieving the U.S. Congress-mandated target of identifying 90 percent of all such entities.

Particular attention must be devoted to resonant asteroids, like 2024 YR4, as they are likely to return.