White dots of stars move together across a black background, while a fainter white dot in the middle - asteroid 2024 YR4 - remains still.
The faint dot fixed in the center of this starfield is asteroid 2024 YR4. This is a sequence of observations from ESO’s Very Large Telescope in January 2025, shortly after the asteroid was discovered to have a greater than 3% chance of hitting Earth in 2032. Further observations have revealed that this asteroid won’t hit us in the foreseeable future. But, owing to a specific orbital resonance, it – and others – will keep coming back into our vicinity. Via Wikimedia Commons/ ESO/ O. Hainaut et al.
Astronomers have determined that asteroid 2024 YR4 will not hit Earth in 2032, as had been feared.
But asteroids like this one will keep coming back regularly, thanks to a strange orbital resonance.
These dangerous space rocks sit in an orbital “Kirkwood gap,” populated by asteroids that are pushed and pulled by Jupiter’s gravity until they either leave their orbit or hit a planet like Earth.
By Martin Connors, Athabasca University. Edits by EarthSky.
Asteroids like 2024 YR4 will keep coming back
In late 2024, astronomers spotted asteroid 2024 YR4 on a trajectory that could potentially threaten Earth. This observation triggered a fervid series of observations to determine that the object, which is large enough to cause city-level damage, will not hit.
Then in January of this year, the near approach of asteroid 887 Alinda – perhaps a million times more massive than 2024 YR4 – went almost unnoticed. This asteroid is large enough to cause a global extinction event.
Alinda remains just outside Earth’s orbit, while 2024 YR4 continues to cross our orbit and still could impact Earth, although not in the foreseeable future. But, thanks to a strange orbital resonance, these asteroids are of a variety that will come back worryingly regularly. That is, until they’re ejected from their orbit … or until they collide with a planet like Earth.
On a noisy grey background, the same blocky grey asteroid appears in each of the 4 corners.
A radar image of asteroid 887 Alinda taken in January 2024. The rectangular region at the top of the asteroid is about 3 kilometers (2 miles) a side. Image via NASA/ JPL.
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Asteroids 2024 YR4 and 887 Alinda have dangerous orbits
Both 887 Alinda and 2024 YR4 orbit the sun three times for every time the massive planet Jupiter goes around once. Since Jupiter’s orbit takes 12 years, the asteroids will take four years to be back on similar paths in 2028. This special kind of asteroids is dangerous, since they come back regularly.
Alinda was discovered in 1918 and has made several sequences of near passes at four-year intervals. 2024 YR4 has made what NASA considers close passes every four years since 1948, but was only recently noticed.
Not since the 1970s has so much attention been paid to asteroids with a three-to-one orbital relation to Jupiter. Such relationships had already been noted as a curiosity by American astronomer Daniel Kirkwood in the late 1800s. Working with very sparse data, since few asteroids were known at the time, he noted that none went around the sun twice for each Jupiter orbit, nor three times, nor in more complex ratios like seven-to-three or five-to-two.
These statistical voids are known as Kirkwood gaps. And they’re not obvious, as they only show up when you plot the average distance of asteroids from the sun. The gaps remained a mere curiosity of the solar system for about 100 years.
A graph mapping asteroid quantities on the y axis and distance from the sun on the x axis. Clear gaps are present at several marked orbital ratios: 3:1, 5:2, 7:3, and 2:1.
This chart shows asteroid quantities by averaged distance from the sun, showing the Kirkwood gaps. The gap labelled 3:1 harbors both 887 Alinda and 2024 YR4, located at an average distance 2.5 times Earth’s distance from the sun. Chart via NASA/ JPL.
Understanding the Kirkwood gaps
The use of new computer technologies to calculate orbits revealed the effects of resonance to scientists in the 1970s. Resonance occurs when asteroids appear to move at the same speed that an external object orbits, or a multiple of the speed. In this case, that external object is the gravitationally dominant gas giant Jupiter.
The Kirkwood gaps are explained by asteroids interacting with Jupiter to leave the asteroid belt, even while their average distance from the sun does not change. That is, their orbits become more elliptical or oval-shaped, so they dip farther in and then farther out of the asteroid belt during each single orbit of the sun.
By dipping into the inner solar system, these asteroids are often removed by hitting an inner planet like Mars, Venus or Earth. And that’s one reason there are very few asteroids remaining with these particular orbits. Alternatively, if they don’t hit an inner planet, their orbits can become so elongated that they escape the solar system. Or Jupiter’s gravity can simply move them to a different, more stable orbit.
However, scientists have found that these Kirkwood gaps are not completely empty. They discovered 887 Alinda, for example, in the three-to-one gap. Many more such asteroids have been found, and they are generically named “Alindas” after that first discovery.
Asteroids 2024 YR4 and 887 Alinda will keep returning
So the bad news is that Kirkwood gaps are partly due to asteroids hitting inner planets, including Earth. Can it get much worse? For Alinda-class asteroids, it does. Alindas follow their pumped-up, elongated orbit every four years. So certain Alindas get a chance to hit Earth about that often.
Near passes of these asteroids tend to happen spaced by gradually longer intervals. But when perfectly aligned, they come back several times with four-year spacing. A limiting factor is how tilted their orbits are. If they are quite tilted, they are not often at a “height” matching Earth’s, so are less likely to hit.
The bad news about that is that both 887 Alinda and 2024 YR4 are very nearly in the plane of Earth’s orbit, and are not tilted much, so are more likely to hit.
The resonant “pumping” that stretches the orbit both inward and outward from the asteroid belt has already made 2024 YR4 cross Earth’s orbit, giving it a chance to impact. The much more dangerous Alinda is still being pumped; in about 1,000 years, it may be poised to hit Earth.
One piece of good news is that 2024 YR4 will not only miss in 2032, but it will come close enough that our planet’s gravity will kick it out of its Alinda orbit. It will no longer come back every four years.
However, its orbit will still cross ours, just not as often. The current orbit shows a somewhat close approach (farther than the moon) in 2052. Beyond that calculations are not very accurate.
Earth impacts
Although Earth is a small target in a big solar system, it does get hit.
Are dangerous asteroids out there likely to surprise us? The last damaging one to do so appeared undetected on February 15, 2013, over Chelyabinsk, Russia, injuring many people when its shock wave shattered glass in buildings.
In 1908, a larger explosion took place over Tunguska, Russian Siberia, a remote region where huge areas of forest were devastated but few people injured.
Black and white photo of blasted forest with mostly fallen, some standing tree trunks stripped of limbs.
Fallen trees from the 1908 Tunguska explosion in Siberia. Photo via the Soviet Academy of Science/ NASA/ Leonid Kulik/ Yevgeny Krinov.
Keeping watch
While astronomers work diligently to survey the night sky from Earth’s surface, space-based surveys like the upcoming Near-Earth Object (NEO) surveyor can be very efficient in detecting asteroids. The NEO surveyor will do so by watching for the heat (infrared) radiation of asteroids. And, being in space, the satellite can also study the daytime sky.
According to Amy Mainzer, lead on the NEO surveyor:
We know of only roughly 40% of the asteroids that are both large enough to cause severe regional damage and closely approach Earth’s orbit.
Once launched in late 2027, NEO will “find, track and characterize the most hazardous asteroids and comets,” eventually meeting the U.S. Congress-mandated goal of knowing of 90% of them.
Among asteroids, we must pay special attention to resonant ones like 2024 YR4, because – eventually – they’ll be back.
Martin Connors, Professor of Astronomy, Mathematics, and Physics, Athabasca University.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Bottom line: Asteroids like 2024 YR4 – which is set to pass very close to Earth in 2032 – will keep coming back into our planet’s vicinity, thanks to a strange orbital resonance.