Behind the scenes of the hunt for the mysterious "twilight" asteroids

Behind the scenes of the hunt for the mysterious “twilight” asteroids

According to him, the ZTF has so far identified a handful of Atiras asteroids within Earth’s orbit. It also regularly flushes out several near-Earth asteroids; about one a week. Some are closer to us than the Moon, but none are big enough or close enough to be really disturbing.

He explains that most are intermediate in size, between the Chelyabinsk meteor (about 18 meters) which shattered windows and damaged buildings when it exploded over this Russian city in 2013 and the much larger object. which razed 2,150 square kilometers of forest when it exploded over Tunguska, Russia, in 1908.

“That’s the good news,” rejoices George Helou about the objects discovered by the ZTF. “Tunguska is worrying, but most of the [objets] that we discover are smaller than that. »

But the star of this twilight search, by far, is ꞌ Ayló ꞌ chaxnim, the first Vatira asteroid to have ever been discovered.


Identified in early 2020, ꞌ Ayló ꞌ chaxnim is barely 1.5 kilometers in diameter; big enough to deal a good blow to a possible planet on which it would crash. Which astronomers say is likely to happen eventually.

“There is a very good chance that it will crash into Venus one day,” says Sarah Greenstreet of the University of Washington, who is behind a model of the future of ꞌ Ayló ꞌ chaxnim made in as part of his studies of the origins and fate of these confined asteroids.

According to his models, which agree with those of other researchers, the most likely scenario is for ꞌ Ayló ꞌ chaxnim to become entangled in Venus’ nets at some point over the next few million years. In its mad dash around the Sun, the asteroid is buffeted by Mercury’s gravity and by sunlight itself. This disrupts its orbit, deflecting it outward and setting it on a collision course with Earth’s infernal sister.

A small asteroid named 2021 PH27 could also collide with Venus. Also measuring about 1.5 kilometers in diameter, 2021 PH27 is one of three twilight asteroids spotted by Scott Sheppard and his colleagues. It lies closer to the Sun than any other known asteroid and fuses partly inside Mercury’s orbit. But its orbit is so elongated that it also cavorts beyond the orbit of Venus, making it an Atiras-class asteroid.

Like ꞌ Ayló ꞌ chaxnim, 2021 PH27 is shaken by its gravitational interactions with the planets of the inner Solar System but also by its absorption and its reflection of light during its rotation. Scott Sheppard’s models predict a brush with Venus in about a thousand years. However, it is impossible to know how this interaction will modify the orbit of the asteroid.

“In this part of the Solar System, the asteroids actually have quite chaotic lives,” explains Sarah Greenstreet. They are quite often roughed up and then disseminated. »

This complexity is one of the reasons why scientists believe it is important to study these small bodies. But it’s equally important to understand how they ended up near the Sun in the first place.


Most scientists suspect these Sun-grazing objects come from the Main Asteroid Belt, that ring of rubble scattered between Mars and Jupiter. From there, however, it is not easy for a cosmic rock to make its way to the periphery of the Sun.

“You have to have a lot of chance interactions happening to get to this part of the solar system, it’s really hard to accomplish,” says Sarah Greenstreet. It’s a long trip. »

Gravitational interactions with Jupiter can shift these objects inward or outward. Those who get pushed inward eventually find themselves in the vicinity of Mars and this can start them on a hellish trajectory towards the Sun, but that fate would be relatively rare.

“With Mars, the most likely interaction is to be thrown out and then interact with Jupiter and basically be ejected from the Solar System or collide with one of the planets, explains Scott Sheppard. So getting ejected outward is a likely outcome, and once there’s been interaction with Jupiter, that’s it, the ejection is really brutal. »

So unless they came from an unseen group of Vulcanoids, these twilight asteroids have all carved out an unlikely gravitational path. Understanding how many of these objects survive this trip is crucial to quantifying the risk they pose to Earth.

At the moment, researchers believe there are fewer than two dozen Near-Earth Twilight Asteroids measuring at least a kilometer in diameter (that’s enough to devastate an entire continent). 2021 PH7 is one of them and, according to Scott Sheppard, we know of about half a dozen others. An even smaller handful of objects of similar sizes may be moving within Venus’ orbit, although only ꞌ Ayló ꞌ chaxnim has been discovered there so far. In addition, there are likely to be a multitude of other smaller cosmic rocks that will be more difficult to flush out but pose no existential threat on a planetary scale.

According to Sarah Greenstreet, it’s no surprise that scholars stumbled upon ꞌ Ayló ꞌ chaxnim first, because it’s so imposing. “But, as we discovered relatively quickly when telescopes started scanning this part of the sky, these objects could actually be more numerous than we thought,” she notes.

The researchers will continue to scan the twilight with the ZTF and with the Chilean telescope in search of flickering lights that betray the presence of these asteroids. Scott Sheppard and his team also have another telescope in their pocket, which they use to characterize these objects and learn about their composition. Finally, Sarah Greenstreet and her colleagues will be able, they hope, to count on the Vera-C.-Rubin observatory, currently under construction in Chile, to reveal more to them.

NASA also has in its projects a space telescope specifically designed to flush out near-Earth objects: NEO Surveyor. This instrument, which may be launched before the end of the 2020s, will be able to contemplate the cosmos in the vicinity of our sun and identify new asteroids confined in our orbit. It will keep an eye on the heavens even more vigilant than our terrestrial telescopes and will ensure that nothing slips through the meshes of sunlight without our knowledge.

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