Scientists propose new class for asteroids within Venus's orbit - study

Scientists are proposing a new class of asteroid in the solar system following the discovery and further study of 'Ayló'chaxnim, the first asteroid whose orbit is entirely between the Sun and Venus, according to a new study.

This study, while currently in pre-print, was accepted for publication in the peer-reviewed academic journal Monthly Notices of the Royal Astronomical Society.

The discovery and further study of 'Ayló'chaxnim have surpassed the expectations of scientists, and it could add an even rarer type of asteroid class to our solar system.

Background: How many asteroids are there? How many asteroids are close to Earth?

Scientists are certain that there are over 1 million asteroids in the solar system, which have their own orbits throughout space.

Of these asteroids, the vast majority of them are located in the Asteroid Belt, located between Mars and Jupiter. 

However, a small fraction is closer to Earth and are known as near-Earth objects (NEOs) or near-Earth asteroids (NEAs), even occasionally crossing Earth's own orbit as they go around the Sun.

Most of these asteroids are what are known as Apollo class asteroids. This means that the asteroid usually orbits the Sun outside Earth's own orbit, occasionally crossing over as its own orbit overlaps with Earth's orbit.

An example of an Apollo class asteroid is asteroid 7482 (1994 PC1), a large asteroid over a kilometer wide that passed by Earth earlier in 2022.

There is another, much rarer class called Aten class asteroids. These asteroids also have an orbit around the Sun that on occasion overlaps with Earth's orbit. The key difference is that Aten class asteroids are usually located between the Earth and the Sun.

An example of an Aten class asteroid is the massive Apophis asteroid, which is set to transition into an Apollo class after a close flyby with Earth in 2029.

And then there is an even rarer type of asteroid, the Atira class asteroids. These are by far the rarest kind - we only know of under 30 of them. These asteroids are located entirely between the Sun and the Earth's orbit, and never cross over - unless something happens to its orbit thanks to interference from Mercury or Venus.

A good example of an Atira class asteroid is the asteroid 2021 PH7, which crosses both Mercury's and Venus's orbits around the Sun but has the distinction of flying closer to the Sun than anything and is the fastest asteroid in the solar system - in fact, it's second only to Mercury in terms of orbital speed.

However, scientists have long theorized about the existence of another type of asteroid, sometimes called Vatira class asteroids. 

These asteroids wouldn't just be between the Earth and the Sun - they would be entirely between the Sun and Venus.

These were only ever theorized to exist until one was finally discovered.

'Ayló'chaxnim: The asteroid within Venus's orbit

On January 4, 2020, scientists at Caltech's Zwicky Transient Facility managed to discover a massive asteroid within Venus's orbit. It had the provisional designation of 2020 AV2 before being formally named 594913 ꞌAylóꞌchaxnim. The name ꞌAylóꞌchaxnim is derived from Luiseño language used by the Pauma indigenous peoples, who were given the right to name the asteroid since the observatory where it was discovered was on their ancestral land, and translates to "Venus girl."

'Ayló'chaxnim's discovery was already significant for how difficult finding something like this should have been.

As an asteroid located between the Earth and the Sun, spotting it is very difficult because the Sun's glare obscures nearly everything.

Only during the brief hour of twilight are objects between the Earth and the Sun usually visible.

But its discovery is also monumental as it is the first asteroid ever found with an orbit entirely between the Sun and Venus. As such, the scientists behind this study are suggesting naming this class of asteroid after it: The 'Ayló'chaxnim class asteroids.

But there's a problem: It's too big.

'Ayló'chaxnim is estimated to be around 1.7 kilometers wide, which is over twice the size of the Burj Khalifa, the tallest manmade structure in the world.

The current models we have suggest that any asteroid between the Sun and Venus would be much smaller.

So how did this happen?

The idea is that 'Ayló'chaxnim originated from the main Asteroid Belt. Then, sometime within the last million years or so, it likely was knocked out of its orbit and drifted inward, before getting stuck in its current orbit.

That makes sense, but the real question is: Can 'Ayló'chaxnim manage to stay there for long? And unfortunately for the Venus girl asteroid, its prospects don't look too good.

Asteroids: The 'Ayló'chaxnim clone saga

The scientists in the study performed several simulations on asteroids like 'Ayló'chaxnim, which they referred to as 'Ayló'chaxnim clones. This helped them test what would happen to it in the future.

According to these simulations, almost all (90%) of these 'Ayló'chaxnim asteroid clones were destroyed within just 30 million years, either by an asteroid impact with a planet or by flying into the Sun.

Of the remaining 10%, most met their tragic demise within 50 million years, with most colliding with Venus but others crashing into the Sun, Mercury, Earth or even Mars.

Out of that original 10%, just 4% of the 'Ayló'chaxnim asteroid clones have different fates. Either they survive, or they wind up flying out of the solar system itself. 

When will an asteroid hit Earth?

Now, there are two conclusions we can draw from this.

The first is that, if all the asteroids between Venus and the Sun tend to be destroyed, it may be why 'Ayló'chaxnim is the first we've found.

The other conclusion is that even though the chance is very small, specifically just 0.16% chance, the 'Ayló'chaxnim asteroid could still end up hitting the Earth within the next 50 million years.

Now, the odds of this happening, as said earlier, are very unlikely. But the fact remains that the Earth has had close calls with asteroids coming from the direction of the Sun before.

Back in the fall of 2021, asteroid 2021 SG flew past the Earth and scientists didn't see it coming until after it had already happened. This happened again a month later when asteroid 2021 UA1 skimmed past the Earth at a distance of just 3,000 kilometers away from the planet - far closer than the Moon or communication satellites. And yet, because they both came from the direction of the Sun, no one saw them coming. They were only ever discovered after it had already passed.

These asteroids essentially come in from a "blind spot" of sorts and it can be very hard to see them.

Incidents like this, as well as the discovery and follow-up research on the 'Ayló'chaxnim asteroid, show the need for further research on these asteroids and of the near-Sun sky in general.