Collision course

TEXT Florian Aigner

Celestial disasters: distances in space are enormous, and the universe is largely a void. Nevertheless, collisions occur – again and again.

A collision is inevitable: the Andromeda Galaxy is racing towards us at a speed of about 110 kilometers per second. While it is still about two and a half million light years away, in four or five billion years it will reach our Milky Way and merge into a new, larger galaxy.

That’s less dramatic than it might sound. The stars that occupy the centers of their galaxies today will then be forced into completely new orbits around the center of the giant new galaxy. Some will probably even be jettisoned into the vastness of intergalactic space. And nobody knows what will happen to our sun in the process. But the chances of two stars or planets crashing into each other somewhere during this massive meeting of galaxies are extremely slim. Galaxies consist mainly of empty space. The distances between the stars are so unfathomable that galaxies can merge without any of their celestial bodies ever coming into contact.

On a smaller scale, however, collisions occur constantly. That can be seen, for example, on the surface of the moon, which is dotted with impact craters of various sizes. The Earth is exposed to exactly the same bombardment as the moon – only you don’t see the damage to its surface. We have an atmosphere that incinerates smaller objects as they enter. We have weather, wind and vegetation that disguise impact craters. And we have plate tectonics that melt the Earth’s substructure over hundreds of millions of years and create new plates.

Brighter than the sun

On February 15, 2013, people near the Russian city of Chelyabinsk learned how frightening it can be when a celestial object literally comes down to Earth. A meteor with a presumed diameter of some 20 meters passed through the atmosphere there at a speed of approx. 70,000 kilometers per hour. Because it entered at a fairly shallow angle, it exploded in the air before it could hit the ground. The flash this generated was brighter than the sun – clearly visible from 100 kilometers away. A huge shockwave damaged thousands of buildings, with some 1,500 residents being injured – many of them by shattered window glass. So that’s a good lesson to learn: if you are observing an explosion in the sky, keep away from the windows! Just as thunder often follows a flash of lightning, the shockwave from an explosion often reaches us after the radiance has waned.

The Chelyabinsk meteor was, however, harmless compared to the mysterious phenomenon in Tunguska, central Siberia, in 1908. What happened there is still not entirely clear because the region along the Tunguska River is remote and sparsely populated. There is no doubt that there was a huge explosion. Its flare was visible a full 500 kilometers away. The shockwave was registered and measured around the world, and trees were uprooted within a radius of 25 kilometers. An actual impact crater was never found, but it is believed that an asteroid measuring between 30 and 80 meters across exploded a few kilometers above the Earth’s surface. It is difficult to imagine the carnage that would have been unleashed had this occurred over a densely populated city.

A catastrophe of completely different proportions was sparked by the asteroid that wiped out the planet’s dinosaurs 66 million years ago. Measuring approximately 10 to 15 kilometers across, it created an impact crater with a diameter of 180 kilometers. The consequences were devastating: rock was propelled into the higher layers of the atmosphere, with much of it falling back to Earth as a hail of rubble. Huge wildfires broke out. Compared to the earthquakes that this asteroid likely triggered, even the worst seismic events in human history would have seemed trivial. Huge tsunamis devastated the coastal areas, dust darkened the sky and caused a global cold snap. The planet’s ecosystems veered out of kilter, resulting in a mass extinction of its species.

A sea of stars and a bowl of cherries

Meteoroid hits on Earth are therefore nothing unusual. But why is it, then, that stars so rarely

collide with each other while galaxies do? How common are cosmic collisions really? Or to put it another way: how empty is the universe? The answer is an issue of scale.

Galaxies are quite tightly packed: relative to their diameters, the distances between them are not particularly large. The distance between the Andromeda Galaxy and the Milky Way is approximately twenty-five times the diameter of the latter. In terms of size, this is roughly equivalent to the ratio between tree diameters and tree spacing in dense forests.

The situation changes radically when you compare the distances between stars with their diameters: stars are typically a few light years apart – and one light year equals almost ten trillion kilometers. The distance between the Sun and its nearest neighbor – Proxima Centauri – corresponds to about 30 million solar diameters. So if we think of our sun as a cherry, the closest star would be hundreds of miles away. Putting a cherry in every EU capital would simulate a density comparable to the stars in the Milky Way. The cherries would be tiny compared to the vast fruit-free space between.

Asteroid 2009JF1: Like the world’s worst soccer player

And how can we best imagine the relative levels of asteroid impacts on Earth? In 2009 scientists discovered 2009JF1, which comes particularly close to our planet. NASA estimates the probability of an impact at about 1 in 3,800. That sounds like a pretty big risk – at least compared to really rare chance events like winning the lottery.

However, the Earth’s diameter is roughly 12,700 km and, based on existing data, the asteroid can be expected to pass by some 15 million kilometers away – give or take 60 million kilometers. That would be tantamount to a striker trying to score in soccer, but missing the goal by about 8.5 kilometers! With a differential of 34 kilometers either way! That’s unlikely to worry the poorest of goalkeepers.

Even if it hits Earth, an asteroid like 2009JF1 represents no threat. Its diameter is only about 13 meters. But, of course, there are larger objects out there in space that we should actually keep tabs on. NASA and ESA take this task very seriously: over 27,000 near-Earth objects have now been catalogued, approximately half of them more than 140 meters in size. None of these have a significant prospect of hitting our planet during the next century. That said, according to NASA, fewer than half of the near-Earth celestial bodies of this magnitude have been discovered to date. So the search continues.

How do we protect ourselves from killer asteroids?

The probability of a major celestial body becoming a real threat to us soon is extremely slim. Yet one day, one will doubtless darken our horizons. Maybe in a hundred years, maybe in a thousand or a million. How could we respond if we calculated the orbit of a newly discovered asteroid and concluded it was on a collision course with Earth?

There are many theories on this. We could launch a spacecraft to deflect it from its orbit with a nuclear explosion. Alternatively, we could simply ram the asteroid with the spacecraft. Or we could try to deflect it off course by flying a huge spacecraft alongside it, thereby exerting a small gravitational pull on it for a protracted period.

More exotic approaches could also prove successful. For example, we might try to bombard the asteroid with small particles. We could also have a large mirror focus sunlight on it, as a result of which the asteroid would partly evaporate. Or we could even try painting it white. This would reflect sunlight more strongly, so the sun’s radiation would divert the asteroid slightly – possibly enough to prevent a collision.

The most realistic method will depend on how soon the celestial body is discovered, how large it is, and what technological advances humanity has made up to that point. After all, our strategies for coping with this threat are still in their infancy. Nobody knows which options will be available in a century or two! Either way, protecting our planet from devastating collisions does not seem implausible. Not every collision is inevitable. There is more than enough space in the universe for us all.

Florian Aigner is a physicist and science journalist who lives in Vienna. His book Die Schwerkraft ist kein Bauchgefühl (Gravity is Not a Gut Feeling) was published in the fall of 2020. In it he tells numerous stories that illustrate why we can rely on scientists.

180 km

Vor 66 Millionen Jahren schlug ein etwa 10 bis 15 km großer Asteroid in der Nähe der heutigen Stadt Chicxulub in Mexiko ein und erzeugte einen Einschlagkrater mit 180 km Durchmesser. Er verursachte eine globale Finsternis und eine ausgeprägte Abkühlung. Sixty-six million years ago, an asteroid about 10 – 15 kilometers in size struck near the present-day city of Chicxulub in Mexico, creating a crater 180 kilometers wide. It caused a global eclipse and a significant cooling of the atmosphere.

20 m

Der Asteroid, der am 15. Februar 2013 über der russischen Stadt Tscheljabinsk mit einer Geschwindigkeit von rund 70.000 km/h in die Erdatmosphäre eingetreten ist, hatte einen Durchmesser von etwa 20 Metern. 1.500 Menschen wurden damals durch den Einschlag verletzt.

Möglicherweise ist am 30. Juni 1908 in der Nähe des Flusses Steinige Tunguska in Sibirien ein noch größerer Asteroid einige Kilometer über der Erde explodiert. Dessen Durchmesser schätzt man auf 30 bis 80 Meter. Bei dem Ereignis wurden Bäume im Umkreis von etwa 30 Kilometer Entfernung entwurzelt und Fenster und Türen in der 65 Kilometer entfernten Siedlung Wanawara eingedrückt. The asteroid that entered the Earth’s atmosphere at a speed of around 70,000 kilometers per hour on February 15, 2013, had a diameter of about 20 meters. Some 1,500 people in and near the Russian city of Chelyabinsk were injured by the impact.

30 – 80 m

An even larger asteroid may have exploded a few kilometers above the Stony Tunguska River in Siberia on June 30, 1908. Its diameter was estimated at 30 to 80 meters. During the impact, trees were uprooted within a radius of some 30 kilometers, while windows and doors 65 kilometers away in the town of Vanavara were blown in.

13 m

Das Objekt 2009JF1 kommt unserer Erde vergleichsweise nah, besonders nah am 6. Mai 2022 um 9.13 Uhr (MEZ). Es hat einen Durchmesser von 13 Metern. Nach bisherigen Daten ist davon auszugehen, dass der Asteroid in einer Distanz in der Größenordnung von mindestens 15 Millionen km an der Erde vorbeiziehen wird. The 13-meter object 2009JF1 comes comparatively close to our Earth, and it will do so most notably at 9:13 a.m. (CET) on May 6, 2022. According to current projections, it can be assumed that the asteroid will pass Earth at a distance of at least 15 million kilometers.