Between heaven & earth

Christian Mähr: Asteroid approaching!

Another asteroid is threatening the Earth! It's called "2024 YR4" and was only discovered with a telescope in Chile at the end of September. A deadly danger? Probably not.

First Apophis, then Bennu, now the new one! The diameter figures vary between 50 and 100 meters. Its density is said to be similar to that of the Chelyabinsk asteroid from 2013, which had a density of 3300 kilos per cubic meter, but was only around 20 meters in size. That's 13,000 tons for a spherical shape. It hit the Earth at a flat angle at around 19 kilometers per second, which is 23 times faster than a bullet. The thing apparently disintegrated in the air, which is why the shock wave from the impact was able to spread downwards at a nice angle. The asteroid didn't hit anything in particular - but 1500 people were injured by shattered windows. As we all know, windows are closed in the height of winter, so there would certainly not have been so many broken panes in August.

2024 YR4 is five times larger and therefore 125 times heavier. That is bad. The impact velocity, on the other hand, would only be around 8 kilometers per second - that's good, because with kinetic energy, the mass only has a direct impact, while the velocity increases with its square. However, in the case of 2024 YR4, both together would still provide twenty-two times the kinetic energy of the Russian asteroid. The potential impact date, December 22, 2032, is also no consolation - as we know, the windows tend to be closed two days before Christmas.

Better a collision from behind than a "head-on"
I hasten to explain the positives: Why is the chunk so much slower than its Chelyabinsk counterpart? Quite simply, it hits the earth "from behind" - not something a driver would want, but far better than a "head-on". YR4 is part of our solar system and moves around the sun in an elliptical orbit. Like all other regular asteroids and planets, it orbits to the left, i.e. anti-clockwise if you look down on the solar system from the north. The furthest point of the orbit is almost 400 million kilometers from the sun, but the closest is only 84, significantly less than the Earth's distance from the sun - which is 150 million kilometers. It therefore cuts into the "inner orbit" every time it orbits the sun; these bodies are known as "Earth orbit crossers". If YR4 were to hit the earth in 2024, it would release around 13 million tons of TNT explosives. The largest nuclear bomb ever detonated, the Russian "Tsar Bomb", had an explosive force of 50 million tons of TNT.

When the thing hits a city, there's no need for further reflection. It is then gone, vaporized. But the probability of such a hit is vanishingly small. 70 percent of the earth's surface is covered by water, so the most likely impact is in an ocean. Unfortunately, this is no reason to rejoice, on the contrary: the result would be tsunamis that would spread in all directions around the impact site and hit all neighboring coasts.

If it hits land, it would create a crater like the famous Barringer Crater in Arizona. That asteroid 50,000 years ago was about half the size of 2024 YR4, but about three times as fast. Its density was also higher because it was made of iron. The impact created a ten-kilometer fireball, and the pressure wave had the force of a hurricane even at a distance of 40 kilometers. The crater in the ground is 1500 meters in size and 180 meters deep.

With a bit of arithmetic, 2024 YR4 has about 60 percent of the energy of the Barringer asteroid. By extrapolation, the extent of the destruction zones would have to be reduced to 84 percent. However, this only applies if the body penetrates to the ground. If it is less solid (no iron, only debris), then it probably explodes at an altitude of 30 kilometers, the area affected by the shock wave multiplies, but the destruction on the ground is much less - in the famous Tunguska impact in Siberia in 1908, for example, there was no crater at all, only the forest was knocked down over an area the size of Vorarlberg. Today, the diameter is estimated at 30 to 80 meters, the energy at a maximum of five megatons of TNT.

Probability of an impact is low
The impact in 2032 should be a little larger. The crucial question remains: will it even happen? According to current calculations, the probability is just over two percent. Possible impact locations? A strip across South America, across the Atlantic, through North Africa, across the Indian Ocean and over India - no guarantees! We won't know anything more precise until 2028, when YR4 will return to the inner reaches of the solar system and new orbit calculations will be possible.

Be that as it may, an impact would not be a catastrophe for humanity, but it would be a disaster. Advantage: it happens quickly. There is no need to worry about lockdowns, vaccinations and so on. No mask requirement either.