An asteroid comes close to Earth by surprise

On July 13 at 6:11 a.m. Montreal time, asteroid 2023NT1 passed a hundred thousand kilometers from Earth before continuing its course. A rather common event, since dozens of celestial objects are thus spotted each month around the blue planet. Except that it approached very close to her, only a quarter of the Earth-Moon distance. And that it was detected only 48 hours after its passage.

In the large family of celestial objects, 2023NT1 is quite common. Of a size of the order of 30 meters in diameter, according to data from theEuropean Space Agencythe equivalent of a 10-storey building, it joins the 1.3 million asteroids listed to date by NASA. Among them, there are 30,000 near-Earth objects – these objects likely to approach the Earth, and whose size can vary from a few meters to about forty kilometers.

Small, hard-to-find objects

Auriane Egal, astrophysicist and scientific advisor to the Montreal Planetarium, points out that if 2023NT1 was not spotted in time, it is because of its “small” size. “These objects with a size of less than 100 meters do not reflect much light. It’s very hard to see them when they’re far from Earth, unless you have incredible instruments, like a space telescope that is just walking around,” she explains. “So usually you only see them when they’re close to Earth. And unfortunately, sometimes they can be quite close to falling on us. But fortunately, they are also the ones that present the least risk to us. »

2023NT1 also has a small peculiarity, and not the least. “What is interesting is that it came from the direction of the Sun”, which corresponds to a blind spot for telescopes, notes the astrophysicist. “He approached us during the day. So telescopes capable of seeing these small objects could not look at that region of the sky at that time. That’s why we only saw him once he had already passed, ”she adds.

A significant threat

If it pales in the face of mastodons several kilometers in diameter, an asteroid of such a size could nevertheless do damage if it collided with the Earth. In fact, its size is not the only parameter to take into account. “Everything also depends on its speed, the angle at which it enters the atmosphere…” enumerates the researcher. “If it’s an asteroid that’s very iron-rich and very dense, it’s not going to do the same damage as if it’s a big, dirty snowball like a comet — which is very porous and less dense, and therefore a less dangerous projectile. »

The scientific adviser believes that if such an event occurred, “what would probably happen is that the object would explode in the atmosphere, fragment, and create a shock wave”. In the worst case, “it can still make a crater of the order of 100 or 200 m in diameter. And if it is very rich in iron, we can increase the size of the crater by a factor of 2,” she adds. “So we are not going to have damage on a continental scale, but rather on the scale of a city or a small region. »

For example, in 1908 and 2013, small celestial objects – 50 and 15 meters respectively – disintegrated in the sky in Siberia, releasing during their explosion an energy of the order of “1000 and 30 Hiroshima bombs”, underlines the astrophysicist. And although they disintegrated several kilometers above sea level, the damage on the ground was notable: the shock wave notably blew thousands of windows and caused the roof of a factory to collapse in 2013 in Chelyabinsk, causing a thousand wounded. For the 1908 event, “the shock knocked down all the trees within a radius of 100 km”, specifies Auriane Egal.

Low probabilities

Although it is difficult to anticipate, the collision with a small asteroid is however quite rare: the specialist estimates that an object of about twenty meters in diameter falls on Earth on average once or twice a century. And according to her, when it does, there’s a good chance it will fall into the ocean or some desert area, causing little damage.

As for large-diameter objects, the researcher is reassuring. Not only do they have an even lower probability of impacting Earth, since there are very few of them in the solar system, but they are also closely watched by space agencies. “The asteroid that was probably partly responsible for the extinction of the dinosaurs was 10 km in diameter,” she estimates. For an object of this size, “we expect an impact every 150 million years,” she adds. “And we know 95% of objects of 1 kilometer or more. We track them, we know where they are, we know where they’re going — and so far, none of them are planning to impact the Earth. »

Monitor, act

For the moment, according to the specialist, humanity is not yet equipped to avoid collision with a small celestial object, of a size similar to that of 2023NT1. But she is working on it. There very first attempt, successful, was carried out by NASA in the fall of 2022. A probe then intentionally crashed into an asteroid, deflecting it from its trajectory.

Other methods are also being considered by scientists. “We can imagine a bomb exploding next to the object, so that the shock wave pushes it further”, explains Auriane Egal. “We also have more subtle methods, where we could send a probe which would orbit around the asteroid by gradually shifting it from its orbit. »

According to her, such operations would however take “several months, even several years” to be planned. So if small objects are only detected a few hours before impact, it would be too late to act. “The protocol would then be to alert the populations and organize an evacuation if necessary,” she underlines.

In order to reduce the threat of objects coming from the direction of the Sun, such as 2023NT1, the European Space Agency also plans to launch a space probe, at thehorizon 2030which would be located between the Earth and the Sun and which would make it possible to better anticipate their arrival.

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