Are asteroids kidding or not

Asteroids heading for Earth : How researchers want to prevent Armageddon

To look at the sky is to see black. Blacker than black. Because empty space has no color. And that is above all space: “A pretty empty place”, as astrophysicist Holger Sierks says, a small man with fine features who speaks softly, almost whispers, so that he cannot be heard a few meters further on. But its sphere of activity is enormous. A space probe, which he helped to develop, will travel 16 million kilometers in the huge nothingness of infinite depth, in which matter orbits each other, because drawing circles is what particles do in the gravity carousel of the galaxies.

So far so good. But then:


A loud bang, as it startled the residents of the Russian metropolis of Chelyabinsk in 2013. A meteorite 19 meters in diameter had burned its way through the atmosphere, bursting at a height of 30 kilometers with 40 times the force of the Hiroshima bomb, so that the pressure wave shattered windows and roofs collapsed 30 kilometers away. 1500 people were injured. The material damage amounted to $ 30 million. The glistening light of the fireball was recorded by dozens of cameras in the morning hours of February 15, so that the track could be reconstructed very well. A fragment of the asteroid was later lifted out of a frozen lake. It weighed 600 kilograms.

The Chelyabinsk meteor fall reminded the world of an ancient, forgotten danger. And he raised questions about what surrounds us. What happened if? And of course: Can something like that be stopped?

The group of scientists who can answer that is quite small. It shouldn't contain more than a hundred people, estimates one insider. Because ballistic experiments in XXL format are rarely at the top of the ranking of scientific priorities. This has made the list of unfinanced “Planetary Defense” missions particularly long.

270 million euros in tax money for the image of a crater

Holger Sierks from the Max Planck Institute for Solar System Research is one of the few. At a meeting in Göttingen at the end of November, he eagerly awaits a decision from the Council of Ministers of the European space agency Esa. He hopes that money will finally be approved for a planetary defense mission that was rejected in 2016. The plan is to send a probe called Hera to the double asteroid Didymos in 2024. The smaller of the two bodies, called Didymoon, is said to have been shot at by NASA beforehand. Hera should then measure the impact point. In other words: The Esa is supposed to make 270 million euros for the photo of a crater 16 million kilometers away?

The question of whether all the effort is worth it should not be asked of Holger Sierks. Next to the door to his office at the Max Planck Institute hangs a blurry, large-format close-up of a comet, gray rock and gray dust that was sent to Earth by the Rosetta probe, 20 meters above the ground, just before it crashed on it itself . The photo marks the last moment of the Rosetta mission, during which it was possible for the first time to approach a small body 700 million kilometers away and to set a landing module on it. So far, says Sierks, the focus has been on the nature of comets and asteroids. The next thing to do is to take care of their behavior.

For this reason, Sierks traveled to Berlin in mid-November. One of the cameras that his institute assembles and tests in dust-free laboratories is stored shockproof in his luggage. His destination: the Berlin Natural History Museum, where he presented the highly sensitive eye of a small group of specialist journalists. While school coffers bustled through the dinosaur hall and watched animated films about the creation of the universe, one floor above was a sense of doom and fate in the air. In a video message that was projected onto a screen at the end of the room, the English rock star and astrophysicist Brian May said the beautiful sentence that humans are the first species that can prevent their extinction - unlike the dinosaurs.

In 2018, 73 objects came closer to the earth than the moon. Five burned up in the atmosphere with the energy of at least one kiloton, which is one tenth of the Hiroshima bomb. As a rule, the atmosphere is only overcome by objects with a diameter of more than 100 meters, which statistically happens every 5000 years. Asteroids longer than 500 meters strike once every 300,000 years. And then there are the killers.

Bad luck for the dinosaurs, luck for us

One such hit the earth 66 million years ago on the northern tip of Mexico's Yucatán Peninsula and left a hole 180 kilometers in diameter, the Chicxulub crater, which is now half covered by water. The energy of the impact and the amounts of dust, ash and CO2 were so great that all living things larger than hamster size died in the short period of frost that followed. The clock of evolution has been turned back. Bad luck for the dinosaurs. Lucky for us.

Humans also differ from dinosaurs in that they live in the knowledge that they will die - but not all at once. Which leads the writer T. C. Boyle to ask: "If you bring Chicxulub into play, the next Chicxulub, the Chicxulub that could thunder down to obliterate everything and everyone while your eyes are still reading these lines - where does that leave us?"


Ms. Nageswaran also heard a loud bang on January 2nd, 2007, when the elderly lady was standing at the back door of the house that her son Srini Nageswaran lives in. It was half past four in the afternoon on that winter day in Freehold, New Jersey, and Mrs. Nageswaran thought the sound must be some stray fireworks from a neighborhood birthday party. So she later told the writer Ian Frazier for his "New Yorker" report "On Impact". Her son, an IT consultant, was working in the basement of the house at the same time. He hadn't heard the bang.

After dinner, which the family had together without Mrs. Nageswaran having found the bang worth mentioning, Srini went up to the bathroom. When he opened the door, a strange picture presented itself to him. There was a fist-sized hole in the ceiling above the sink through which he could see the night sky. Splinters of shingles and shreds of insulation lay around everywhere. And on the floor was a black shimmering lump of metal that had smashed another tile and looked unusually heavy considering its size of ten centimeters.

In his lectures, Holger Sierks likes to show a map of the world; he now calls it up on his computer. It shows the earth hits between 1992 and 2013, and he asks with a smile, “Where would you want to live?” As if being able to choose makes a difference. The scattering pattern is too even for there to be one place safer than another. The crater landscape characteristic of the moon would also emerge on earth if its atmospheric shield did not let everything continue to dissolve.

So far, Sierks and his colleagues are familiar with around 21,000 Near Earth Objects, so-called NEO’s. The discovery rate has increased rapidly over the past 20 years. Sierks says that today you know about half of what is really buzzing around out there, and of the large bodies you think you know well over 90 percent. His screensaver keeps reminding him. Then the earth rotates through a "soup", according to Sierks, made of tiny particles that are reminiscent of a swarm of mosquitoes in the summer heat. “Wherever there are many bodies, they collide,” he says.

Sierks initially paid little heed to this physical law. But one day a colleague showed him an earth orbit cruiser through the telescope. Sierks was deeply impressed to see the Brocken march through the field of vision at high speed. It was “a startling experience” for the nuclear physicist. He let him imagine what would happen if something like that hit the earth. "The stuff is there."

A risk of five percent

Sierks ’first grandson was born a few weeks ago. Of course, that pleases him. But he says that according to NASA calculations, the child will have a five percent chance of being hit by a hundred-meter asteroid in the course of his life, and he asks: "Would you take a car trip if the chance of an accident was five percent is? "


If you have any questions about what happens then, Kai Wünnemann is the right person to talk to. Because he knows about craters. The fact that his wife occasionally says of him, "My husband does the same thing as Bruce Willis" is a joke, he says, laughing as he walks through the shady hallways and back stairs of the Natural History Museum. Wünnemann, 50 years old, Duisburg, round face with a three-day beard, has his study here, where he deals with the consequences of meteorite strikes and is now making every effort not to sound like an action hero.

Which is not that easy. Wünnemann speaks of nuclear missiles, pressure waves and tsunamis as naturally as he does about a differential equation. He calculated and simulated all the scenarios on the computer to which Hollywood films such as “Armageddon” or “Deep Impact” refer. But even Wünnemann does not know what it really takes to stop a killer comet. He has a rough idea. For example, that a ram would be better suited than an explosion. That distracting is more effective than splitting up.

Would you like to play space billiards, Mr. Wünnemann?

“We don't need the Hera experiment,” he says, “to find out that we can throw a body off track. That's what I'm expecting. We would just like to know how he does exactly what we want. "

For almost his entire career, the geophysicist has been looking forward to the possibility of studying an artificial crater in space. Once he was close. He had obtained his PhD a few years earlier when he was on the Deep Impact Mission in the United States in 2005. Back then, material was supposed to be broken out of the interior of a comet by placing an obstacle in its path. The particles released on impact provided information about the primordial matter that rushed through space in these dust collectors from the early days of the solar system. The fact that one could also influence the trajectory with such a method did not play a role at the time.

A last desperate act

Nor could it have been found out at all. Wünnemann had previously worked on a simulation of the process and calculated the possible crater size. But at the moment of the collision with the impactor, the probe raced past the comet so quickly that the crater was never seen. It was not until years later that an image was taken from a great distance, so that the crater was the size of an image pixel.

When looking at a crater, a simple question arises: How big was the thing that made it? "Sounds trivial," says Wünnemann, "but it's not easy to answer."

Craters have long been a mystery to people. Until the late 19th century, naturalists could not imagine that anything outside the Earth could create such formations. The crater landscape of the moon was therefore attributed to burst bubbles, tides or volcanic activity.

The existence of meteorites was well known. In the same year in which Columbus discovered a new continent, the residents of Ensisheim discovered a stone in a wheat field that did not belong there. It had passed through the sky with a loud roar, which gave it the name "Thunderstone".

He was put in chains and dragged "dreyg zentner schwär" to the church of the place where the evil that emanated from him would be banished. For a long time it was impossible to explain the origin of the stone. It was said that the stone formed in a cloud and must have rained down. Then it was said that such stones were spat out by volcanoes. Also very popular: birds charred by lightning strikes.

Alfred Wegener took a different path. The meteorologist, geologist and polar researcher, who was to become famous above all for his theory of plate tectonics, narrowly escaped the bombing of the Flanders front during the First World War. Seriously wounded, he was transferred to the Army Weather Service. When he heard of the fall of a meteorite near Schwalmstädt in northern Hesse, he went there to interview witnesses. Based on their statements, he calculated the possible trajectory. In fact, a little later the meteorite was found not far from the predicted target area. Wegener's predictions about weight and height turned out to be largely correct.

Shock waves in the rock

Fascinated by the find, Wegener then interfered in the lunar debate and advocated the "crash hypothesis". He made one of the first experiments on this by piling cement powder on a spoon and throwing it energetically on a cement powder surface. Perhaps only someone could recognize the key who had witnessed the bombardment of the western front: The resulting craters were exactly like the faults that could be seen through the telescope on the moon. Wegener's article on the morphological properties of the lunar craters was an important building block for the meteorite theory.

Today Wünnemann no longer heaped cement powder on a spoon. Because meteorite craters, that much is known, are also the product of shock waves such as those created when the sound barrier is broken. Such powers cannot be established with earthly means. The speed of the approaching objects, brought up to speed in the large flywheel of celestial mechanics, is so high that rock is compacted faster than it can escape. The result: it evaporates. Above all, the meteorite itself goes up in a glowing cloud. For example, the iron meteorite that created Barringer Crater in present-day Arizona 50,000 years ago blasted a hole 27 times its own size out of the ground.

“The same experiment that left a crater of half a meter on Earth can overturn 100 to 200 times the area on an asteroid. In order to carry the momentum into the depths of an asteroid, more effort has to be made. In addition, asteroids are sometimes so porous that shooting into them would only compress cavities and cause the impact energy to evaporate. Wanting to deflect such a 'sponge' with a shock pulse would be ineffective. "

Wünnemann would like to know more about this balance of buoyancy and strength, he says. That is why the measurement of an artificial crater under the conditions of weightlessness is so important. Material behaves “very strangely” with the low attraction of small bodies.

Alan Harris from the German Aerospace Center is someone who has been giving serious thought to appropriate countermeasures for decades. Although now retired, he works as a consultant. Most recently, he headed the NEO Shield project, which brought together experts from different research groups. He says the size of asteroids also determines the method by which they are rendered harmless. Up to a size of around 500 meters, it could be pushed off course by an impactor. Nuclear explosives would probably be necessary up to a size of 1,000 meters. With bodies more than a kilometer in diameter, “we have problems,” says Harris. Not only are they big and heavy, they also rush up at 50,000 kilometers per hour. "We could try staggering multiple nuclear warheads, but I'm not very optimistic."

However, Harris says, people would be warned of impact for decades if not centuries. Because objects are moving in circles, a hit was usually preceded by close pass-bys. In the year 2029 the asteroid Apophis will fly so close to the earth that with its 300 meter diameter it can be seen very well with the naked eye. And 70 years later he's back. The meeting is scheduled for 2135 for the "doomsday asteroid" Bennu. In 2880 an asteroid with the identification 1950 DA could plunge into the sea off the American east coast. So you have time, says Harris, to arm yourself and take countermeasures.

The NEO-Shield group has also looked for minimally invasive intervention options. In the case of smaller bodies, one idea is to bring the asteroid surface to a boil at certain points with a jet of laser, so that the steam jet acts like a steering nozzle. Ion beams could have a similar effect. Small forces would act over a longer period of time. "Maybe that's better than a big bang after which you can't do anything."

Fire and brimstone

While the practitioner speaks like this, the theorist Wünnemann is fascinated by the cosmic dimension of the catastrophe high above the dinosaur skeletons. Collisions are "the dominant event in our solar system," he says. Without the collision with another body as large as Mars, the moon would not have broken out of the earth. And the fact that oxygen reached the planet in the form of ice crystals was also likely due to the impact of a foreign body. Impacts have sown earthly life, says Wünnemann, "but they also destroy it".

Memories of this cosmic connection can be found at a central point in the history of civilization. The downfall of Gomorrhah, this biblical city of sin, could refer to a meteorite fall, cloaked as a divine judgment: “The Lord let sulfur and fire rain from heaven” (Genesis 19.25). In fact, archaeologists found traces of a thermal explosion at the Dead Sea in 2018 that had glazed ceramic shards, requiring temperatures of 4,000 degrees. All life was wiped out on the spot in an area of ​​500 square kilometers. Up to 65,000 people could have died.

In the New Testament it is angels (“the Lord's clarity shone around them”) and the star of Bethlehem who descend from heaven. Perhaps only a comet or meteorite was seen. And the Kaaba in Mecca, the most important place of pilgrimage in Islam, has the black stone, which is said to consist of fragments of a meteorite, as a mystical reference point. It has never been scientifically investigated.

The thing that fell from the sky

The Nageswaran family of Freehold, New Jersey, believed they had narrowly escaped disaster when the lump of metal broke through their roof. Expertises from Rutgers University metallurgists and geologists were obtained, US aviation experts assured it would not be a part used in aircraft construction, and so it was on display at the university's geological museum. It was a big attraction there. But when scientists at the American Museum of Natural History put it under an electron microscope three months later, they found nothing of what an iron meteorite should contain. Instead: components of high-quality, rust-free chrome steel. The thing, whatever it was, might have fallen from the sky, but it did not come from beyond it.

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