Dangerous asteroids appear every week. Should we be afraid of them?

The news that a dangerous asteroid will pass near Earth in a few days or hours seems to come every week, if not every month, whenever it passes us. Is this surprising, and how much should we fear all this? Let’s find out together!

Dangerous asteroid

Dangerous asteroids

Have you heard that a dangerous asteroid is approaching the Earth? We are not talking about 2024 YR4, which has very little chance of colliding with the Earth in 2025, or Apophis, which was scared of humanity many years before. It is about the asteroid that will fly close to the Earth in a few hours or days.

Such news is quite common. It is hard to say for sure, because asteroids are discovered near the Earth much more often than they make headlines. And this is a good thing, because then we would simply go crazy from the constant expectation of a threat.

After all, among the many cases in recent years when the media have reported on the approach of a dangerous asteroid, there has never been one that caused harm to people. After all, the only case when people were massively affected by the fall of an asteroid, the Chelyabinsk Bolide, occurred when the approach of the space body was not noticed in advance.

Chelyabinsk meteorite. Source: www.universetoday.com

So, how real is the asteroid threat if we are talking about something that could happen not in the next million years, not in the next century, but right now? To do this, we need to understand which asteroids should be considered dangerous.

Large stones

The first thing that comes to mind when we think of an asteroid that threatens the Earth is a multi-kilometer-long block hurtling toward us from the outskirts of the Solar System. It is believed that something like this once led to the extinction of the dinosaurs, and theoretically, such an object could threaten the Earth today.

But in practice, celestial rocks larger than 1 km inside Neptune’s orbit have not been discovered very often recently, and most of them move in orbits that are usually incredibly far from Earth’s orbit. If something large comes to us from the Kuiper belt, it will most likely be a comet. However, even for them, the chance of at least passing close to the Earth is incredibly small, let alone crashing into it.

From the outskirts of the Solar System, it is more likely to expect a comet than an asteroid.
Source: phys.org

Scientists have long calculated, based on the number of craters on Earth and the orbits of known asteroids, that bodies with a diameter of more than a kilometer crash into the Earth on average once every few tens or even hundreds of millions of years. The vast majority of known dinosaur species became extinct long before the asteroid hit for purely terrestrial reasons.

Dangerous asteroids

That is why, for the most part, when scientists talk about a dangerous asteroid, they are talking about those that are considered near-Earth. These are two different concepts. All objects whose orbit lies between 0.983 and 1.3 AU are considered near-Earth.

Some enormous celestial bodies are among them, but they are monitored quite regularly. So the orbits of such objects are already calculated for hundreds or even thousands of years in advance, and it is known that they do not pose a threat to the Earth. Instead, many asteroids with diameters of 100-150 meters can cause serious damage even if they fall into the ocean, as they can cause tsunamis.

1036 Ganymed is the largest near-Earth asteroid. Source: phys.org

The total number of known near-Earth asteroids is now 38,600. However, most of them are not considered dangerous because they do not approach us at a distance that, even with very strong paranoia, can be considered dangerous.

As for potentially hazardous sites, the WISE probe estimates their number to be about 4,700±1,500. Many of them have already been discovered, but a significant number remain unknown. In addition, many of those that have already been discovered are observed only from time to time, so their exact orbits are unknown, and the risk of collision is calculated very roughly.

Any asteroid that has a diameter of more than 100-150 m and approaches the Earth at a distance of less than 0.05 AU is considered dangerous. According to geological data, a collision with such an object occurs on average once every 10 thousand years. So, during the existence of mankind, we should have experienced this several times.

Orbits of potentially dangerous asteroids. Source: www.astronomy.com

However, in practice, observers consider even those asteroids that have a diameter of more than 35 m and approach us closer than 0.05 AU to be dangerous. Although they cannot cause a global catastrophe on a planet-wide scale, they are quite capable of destroying a large city. Therefore, this approach is fully justified.

And there are tens of thousands of asteroids with a diameter of several tens of meters near the Earth, and no one has counted them yet. It is these asteroids that are mostly the heroes of the news about a dangerous space rock approaching us.

What distance is considered dangerous?

Another important thing to understand is the distance. 0.05 AU, which is used in the definition of a hazardous asteroid, is 5% of the Earth’s orbit. If you look at a schematic representation of the latter, you can agree that it looks like a close flyby.

Discovery of near-Earth asteroids. Source: Wikipedia

However, in reality, these 0.05 AU are 7.5 million kilometers, or approximately 19.5 distance from the Earth to the Moon. This distance was chosen for the reason that the asteroid will rotate in its orbit over and over again, but may be deflected by the gravitational influence of other bodies, which could theoretically put it on a course that would lead to a collision with our planet.

But in fact, this is an incredible distance. Even the Apollo crews took three days to get to the Moon, and here it is almost 20 times further, and it is still considered potentially dangerous. If we compare the Earth to a human head, it would mean that all the pebbles flying at a distance of 180 meters from it would have to be considered dangerous.

Asteroids do not fly very close to our planet very often. For example, the three closest asteroid passes to us are as follows: 2020 VT4 (A10sHcN) with a diameter of 5-10 meters – 383 km from the Earth’s surface, 2020 QG with a diameter of 2-14 m – 2946 km from the surface, 2011 CQ1 with a diameter of about 2 m – 11,850 km from the surface. In other words, asteroids rarely come closer to the Earth than 10 thousand kilometers. For most of those space rocks that approach our planet shortly after their discovery, it is known for certain that a collision with them is impossible.

Orbits of man-made satellites. Source: phys.org

There is still a danger

So, all the asteroid dangers we see in the news are made up? That is not entirely true. If the news says that an asteroid is approaching, you can stay calm – almost certainly nothing will happen.

It is worth paying attention to the specific distance at which the celestial body is approaching us. Anything that passes beyond the minimum distance to the Moon (384 thousand kilometers) can be safely rejected – such a rapprochement will have no consequences. However, objects flying between the Moon’s orbit and the geostationary orbit (35,786 km) could theoretically collide with the Moon. But if it is not reported immediately, it means that the object will pass by, although the next time it will not be so lucky.

An asteroid passing at a distance of 500 to 35786 km may have some consequences for vehicles in high orbits. This is where the situation should be monitored, but even in this case, the threat remains very illusory. Finally, there are asteroid flybys at a distance of less than 500 km. There is indeed a certain risk of serious consequences here, but even then, there is no need to panic – the main danger is primarily for satellites and space stations.

Asteroids approaching us from the Sun are particularly dangerous. Source: www.reddit.com

It is not those cases when astronomers have detected an asteroid approaching the Earth that we should be afraid of, but those when they have not. Calculations of the trajectory of known objects are now very accurate and reliable. So if scientists say that an asteroid will pass at a certain distance, it means that there will be no collision.

But there are still thousands of objects that have not yet been detected. If one of them, which we have not yet seen, is on its way to approaching the Earth, we will know about it only a few days or even hours before the catastrophe, especially if it is moving from the Sun. The chance of such a scenario is small, but it is still there, and the worst part is that there is a small possibility that it will go unnoticed almost until the moment of impact. Even in this case, there is a chance that it will fall into the ocean or a deserted area, but if the rock is flying into densely populated areas, then evacuation may simply not be announced in time.

So, you need to understand that the chance of hearing a message that an asteroid will collide with your area in a few days is quite real, although unlikely. Scientists are working to find all dangerous space rocks.

One of the main tasks of the Vera Rubin Observatory, which saw its first light this week, is to detect all these “small things” that may unexpectedly approach the Earth. So we can expect that soon we will at least know in advance if any of these asteroids will hit the Earth, and not just come within a “dangerous” distance.

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