Why didn’t astronomers warn about the “Kyiv bolide”?

The bright flashes that took place in the sky over Kyiv on the evening of April 19, fortunately, were not caused by an enemy air raid. After a few hours it became clear that their “culprit” was a cosmic body that entered the earth’s atmosphere, warmed up when braking in it to high temperatures and exploded. The size and direction of the flight of this body are still being specified, but with a high probability it belongs to the Lyrid meteor shower, the maximum of which is observed annually on April 21-22.

At this point, the question may arise: if this flow is annual, does it mean that astronomers have known about it for a long time and could warn about an unexpected “alien”? There are exactly two answers to this question: yes, they know — and no, they could not. Let’s explain why.

The fact is that bright meteors, or bolides, are not so rare. According to one definition, they include meteor phenomena that exceed the brightness of the full Moon at the maximum brightness (-12ᵐ). No matter how strange it may seem, they happen almost every day — you can just see them only from a limited area, which may well be sparsely populated or even fall into the ocean. Some of them take place during the day. And this time, the Kyiv region was “lucky”, over which the bolide appeared in the dark, and even, as if on purpose, during active hostilities.

Bolide on April 19, 2023, registered by CCTV cameras in Kyiv and the surrounding area

Actually there is a fresh example: many of our readers remember that just a few days ago the same “space alien” appeared over Western Europe, which terrestrial astronomers were able to notice a few hours before the collision. Is this possible in the case of the “Kyiv bolide”? It is very doubtful, and there are two reasons for this. Firstly, it was significantly smaller: if the “sky show” over Europe was arranged by a fragment measuring more than a meter, then a body weighing no more than 50 kg exploded over Kyiv, so its size did not exceed 30 cm (this is assuming an average density of about 3.5 g/cm³; in fact, it should be 3-4 times less). It is almost impossible to notice such a “crumb” at a sufficiently large distance. And secondly, if it really belonged to a swarm of Lyrids, it was approaching the Earth at a speed of about 49 km/s, which also made it very difficult to detect in advance.

It is thanks to such a high speed that a relatively small body entering the earth’s atmosphere creates a bright meteor phenomenon, sometimes accompanied by an explosion with a shock wave. Recall that the kinetic energy increases in proportion to the square of the speed: a 9-gram bullet moving at a speed of 800 m/s has about the same energy as a 16-kilogram kettlebell “accelerated” to 20 m/s (i.e. up to the speed of an average car). Most of the “falling stars” that we admire on cloudless nights are actually grains of sand weighing a few milligrams — but very fast. The thermal and light energy released during their atmospheric braking is enough for us to easily see them at altitudes of 60-80 km.

A bright bolide observed over Austria on November 19, 2020. Source: AMERICAN METEOR SOCIETY

Of course, among the meteor particles there are also larger “rocks”, the sizes of which reach tens of centimeters. They create phenomena similar to those discussed in this article. But astronomers cannot know their distribution in space, so no one is able to assume that any of them will enter the Earth’s atmosphere at such a moment. The maximum that can be found in astronomical reference books is that, according to long-term observations, a certain meteor shower is characterized by a large number of bolides. By the way, the Lyrids have not been “noticed” in this so far.

There have already been assumptions about possible remnants of the bolide and even calculations of the place where they need to be searched. We should warn you immediately: attempts to find some stone fragments can most likely end in nothing. Meteor showers are matter ejected by comets (in particular, Lyrids represent the “dust trail” of comet Thatcher, observed in 1861). Not to mention the fact that these celestial bodies mostly consist of water ice by 90%, let’s remember that non-volatile cometary matter also has peculiar properties. It is very fragile and when heated to high temperatures turns into fine dust, which requires much more effort to detect than to search for “rocks from the sky”. Although this is also possible with special equipment, the main problem is to distinguish the substance of this particular bolide from the “comet dust” that has been falling on the Earth for all the long years of its existence…

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