Killer asteroids or "just passing by"?

Posted in: Out in space

“Doom!” always makes good headlines, and some British tabloids fretted about the recent passage of an asteroid close to Earth on January 2nd, and the fact it was only detected 6 days before. But what are the actual risks? How often does this happen?? And is anyone doing anything about it?

First, let us look at the numbers. Asteroid 2017 YD7 missed Earth by 1.8 million kilometres, close to 5 times the distance from the Earth to the Moon. So, even if it was within the zone classified as “potentially hazardous” by planetary scientists, its size (between 6 and 21 metres according to NASA) and its trajectory did not make it that much of a threat. Even if it had reached the Earth, the risks would have depended on what it was made of, how fast it travelled and whether it would give a glancing blow or fall vertically. The physics is well known, and you can play around with real impact simulators at and to assess different scenarios.

Large asteroid impact
University impact, but not as we would want it ... (from a test of

Simulating a 100-m wide rock falling on my office at hypersonic velocities, its fragments would destroy the entire campus, creating a crater 331 m deep and a magnitude 6 earthquake. Down the hill in Bath, collapsing buildings would be accompanied with widespread burning. So much for University impact ... But fun aside, it is even likely? After all, the asteroid would need to be large, dense, travelling extremely fast and impacting head on our planet.

How often does this happen? The good news are: “rarely and less and less often”. The number of asteroids impacting planets (and the Earth) has very sharply decreased after the “Late Heavy Bombardment” of several billion years. Most asteroids now are small, at least in potentially Earth-crossing orbits. In 2013, the Chelyabinsk meteor exploded above Russia, creating shock waves that injured around 1,000 people but created little destruction. But what about the others?

Are there “dinosaur-killers” still hurling through space? As they get disturbed by large planets or chaotic interactions with other bodies, are some of them likely to head our way? We know about close to 500,000 asteroids known, most orbiting far away between Mars and Jupiter. Telescope surveys can detect the largest objects, provided they are reflective enough. And common sense would hint that, if they are too small to detect easily, maybe they won’t create as much risk. Still, we want to know where they are, and what they are. This is where radar and radio-telescopes come to the rescue. By reflecting radio waves on asteroids, it is possible to learn about their size, their shape, how they tumble, and get ideas of what they are made of (hard rock or soft grains). And we have strong incentives to map those closer to Earth, as they contain materials useful for future interplanetary flights and mining (in 2016, I participated for example in the first conference between scientists and asteroid miners).

Once we know an asteroid is on its way to Earth, more frequent surveys enable scientists to better understand its trajectory. In most cases, it would disintegrate into the atmosphere, creating “shooting stars” or falling into empty areas (the Chelyabinsk fragments fell into a lake, for example). If it is large enough to be worrying, there are no proven technologies at the moment to destroy it, or even just re-direct it. If there is enough time, putting solar sails on its surface might work. And if not, more aggressive approaches can be used, like kinetic impacts (the joint NASA/ESA mission DART, for Double Asteroid Redirection Test, intends for example to do just that for a test scheduled in 2022).

So there is still work to do. But it is not as bleak as the tabloids would have us believe. And there is very interesting science to do out there in outer space ...

Posted in: Out in space


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