Twilight asteroids: what are they and why do they threaten the Earth?

So far, according to Helou, the ZTF has detected a handful of Atira asteroids in Earth orbit. He also discovered several asteroids on trajectories crossing the Earth, approximately one per week. Some come closer to us than the Moon, but none are big enough or close enough to be of concern.

Most are intermediate in size, Helou describes, between the roughly 60-foot-wide space rock that shattered windows and damaged buildings when it exploded over Chelyabinsk, Russia, in 2013, and the rock much larger that leveled 800 square miles of forest when it exploded over Tunguska, Russia in 1908.

“That’s the good news,” says the specialist in reference to the objects found by the ZTF. “Tunguska is concerning, but most of our findings are smaller than that.”

But by far the star of ZTF’s Twilight Quest to date has been ꞌAylo’chaxnim: the first known Vatira.

space rock disk

Seen at the beginning of 2020, Aylo’chaxnim it’s just under a mile wide, large enough to deal a painful blow if it were to collide with a planet. Which, according to astronomers, probably will be.

“Most likely in the future it will collide with Venus,” says Sarah Greenstreet of the University of Washington, who developed a futuristic model of ‘Ayló’chaxnim as part of their studies of the origins and fate of these inner asteroids.

Based on Greenstreet’s models, as well as others, the most likely scenario is that the orbit of ‘Ayló’chaxnim intertwine with that of Venus over the next million years. As it orbits the sun, the rocky body is buffeted by Mercury’s gravity and by sunlight itself, both of which can disrupt its orbit, gently pushing it toward a collision course with the hellish sister world. of the earth.

A small rock known as 2020 PH27 could also be destined to collide with Venus. About 800 meters in diameter, 2020 PH27 is one of three twilight asteroids detected by Sheppard and his colleagues.. It flies closer to the sun than any other known asteroid, coming closer to Mercury’s orbit. But its orbit is so elongated that it also oscillates in front of Venus, making it a member of the Atira class of asteroids.

As ‘Ayló’chaxnim, 2020 PH27 is driven by gravitational interactions with the inner planets, and by the absorption and emission of sunlight as it rotates. Sheppard’s models predict a close encounter with Venus in about 1,000 years, though she can’t say how the asteroid’s orbit will be altered by this interaction.

“The asteroids in this part of the solar system have quite a chaotic life,” says Greenstreet. “They get jostled and scattered quite often.”

This complexity is one of the reasons why scientists believe it is important to study these small bodies. But so does understanding how they originally ended up near the sun.

thread the needle

Most scientists suspect that these sun-grazing objects come from the Main Asteroid Belt, the ring of debris that stretches between Mars and Jupiter. However, it is not easy for a rock to break out of the belt and end up so close to the Sun.

“A lot of very fortuitous interactions have to happen for it to reach that part of the solar system, it’s really complicated,” Greenstreet said, adding, “It’s a very long journey.”

Gravitational interactions with Jupiter can push these objects in or out. Those pushed inward encounter Mars, which can lead asteroids on a spiral path towards the Sun, although this outcome is considered quite rare.

“The most likely interaction with Mars is that they get kicked out, and then they probably interact with Jupiter, ending up outside the solar system or colliding with one of the planets,” Sheppard explains, adding, ” So being kicked out is a likely outcome, and once they interact with Jupiter, it’s game over: they’re being thrown really hard.”

But unless these twilight asteroids came from an unseen population of vulcanoids, they’ve all threaded that unlikely gravitational needle. Understanding how many of these objects survive this trip is crucial to quantifying the risk they could pose to the Earth.

For now, scientists suspect there are less than two dozen twilight asteroids at least a kilometer in diameter (big enough to devastate an entire continent) in orbits traversing Earth. The 2020 PH7 is one of them, and Sheppard says about half a dozen more are known.

A much smaller handful of similarly sized objects may orbit inside Venus, although ‘Ayló’chaxnim It is the only one that has been discovered. And there will likely be many more smaller space rocks that are harder to find, but not existentially dangerous on a planetary scale.

It’s no surprise that scientists found ꞌAylóꞌchaxnim first because he’s so tall, says Greenstreet. “But the fact that it was discovered relatively quickly when telescope surveys started observing this part of the sky is also an indication that there may in fact be more of these objects than previously thought,” he says.

Scientists will continue to observe twilight with the ZTF and the Chilean Telescope, looking for faint flashes that reveal the presence of an asteroid. Sheppard and his team also use another telescope to characterize these objects and learn more about their composition. And Greenstreet and his colleagues hope the Vera Rubin Observatory, now under construction in Chile, will provide more data.

NASA is also planning a space telescope specially designed to search for near-Earth objects., called NEO Surveyor. This instrument, which could be launched in the late 2020s, will be able to peer into space near our sun and detect more interior asteroids, surveying the sky better than our ground-based telescopes and ensuring that nothing sneaks in the glare. and surprise us.

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