A flaming asteroid, yesterday

Could we avert a major asteroid strike?

Image credit: Science Photo Library

Earth’s atmosphere is regularly peppered by asteroids, most of which do little harm. However, once every few million years something much bigger comes along, putting life on Earth at risk. Could we stop one?

On the morning of 15 February 2013, just as people were starting work, a blinding light tore across the winter skies of south-western Russia. An explosion equivalent to 440,000 tonnes of TNT generated a shockwave that blew out windows and damaged buildings over an area of 200 square miles. The house-size Chelyabinsk asteroid, which flared 14 miles above ground, left 1,600 people with injuries, mainly caused by broken glass.

Asteroid strikes have played a major role in our planet’s history. The mass extinction that brought about the end of the dinosaurs some 66 million years ago was caused by a miles-wide (10-15km) asteroid that slammed into modern-day Mexico. It’s been theorised that asteroids brought water to planet Earth and even the basic organisms which led to life as we know it. Some theologians suppose that St Paul’s conversion to Christianity after seeing a blinding light on the road to Damascus was caused by an asteroid falling to Earth – perhaps changing the course of human history.

There are basically two methods to deal with an asteroid hurtling towards Earth. One is to try and hit it before it hits us, using some kind of nuclear device. Alternatively, we could attempt to deflect it, pushing it off course so it passes our planet without doing any harm.

Blowing up an asteroid appears the most straightforward solution. Anyone who’s seen the Bruce Willis classic ‘Armageddon’ will know the premise. One could either place a bomb directly on (or inside) the asteroid or set the device off near the surface, hoping the heat generated would cause much of the rock to vaporise while also changing its trajectory. But it’s an option full of risks.

“I wasn’t really thinking about planetary defence to begin with,” says Dr Charles El Mir, a mechanical engineer and professor at St Louis University in Madrid. El Mir was researching how certain materials shatter at Maryland’s Johns Hopkins University and became interested in what it would take to fragment an asteroid. While researching the topic he came across a modelling tool that could test his theories, and he and a team of other PhD students used the model to estimate the force required to shatter such a rock.  

One of the most compelling findings of the study is that simply hitting the rock with a bomb might not have the intended effect. “After about six hours, you notice that the fragments start to move back together in the model.”

El Mir’s findings indicate that in many cases, the core of the asteroid would not disintegrate – and so gravity would draw much of the fragmented parts of the asteroid back together. This potentially suggests that ‘blowing up’ an asteroid might fail to achieve its purpose.

How to...

Knock an asteroid off course

The gravity tractor method

In this approach, a spacecraft is sent to an asteroid and placed into orbit around the rock. By using a small amount of thrust, the spacecraft could gradually influence the direction in which the asteroid is moving.

Laser ablation

With this method, a spacecraft could be fitted with a laser that heats up part of the asteroid’s surface and vaporises the material. As material is ‘ejected’ in one direction, it would push the asteroid on a different course.

A robot swarm

One of the more outlandish ideas is to attack the asteroid with a swarm of autonomous robots that could drill into the asteroid and eject chunks of it into outer space, thereby reducing its size until it is no longer a threat.

Massimiliano Vasile is a professor of space systems engineering at Strathclyde University. “The problem is we know very little about what is below the surface of most asteroids,” so it would be very unpredictable to know what would happen if we applied a nuclear explosion to one. “You may have unwanted fragmentation, whereby the fragmentation generates more problems than a single asteroid on its own.”

The alternative, then, would be to try and push the rock off its current path. Caught early enough, even a tiny deflection of a few millimetres in outer space could result in the asteroid eventually giving planet Earth a wide berth.

“The most generally accepted and best studied approach,” says Vasile, “is a kinetic impactor.” This approach would involve a spacecraft ramming into an asteroid at high velocity – perhaps at a speed of 10km/s (bullets travel at around 1km/s for comparison). “The idea of most asteroid deflection methods is to put the asteroid out of synchronisation.”

Soon we will see whether this is an option that might just work. Launching in July 2021, Nasa’s Double Asteroid Redirection Test (DART) will be the first attempt at changing the flight path of an asteroid when it collides with an asteroid some 11 million kilometres from Earth in 2022. The target is a ‘moonlet’, which is a rock about the side of an Egyptian pyramid orbiting the larger Didymos asteroid.

The project is something of a collaboration with the European Space Agency (ESA), which is launching its own Hera spacecraft to monitor the after-effects of the impact. Mariella Graziano of GMV, a Spanish firm which is working with ESA, has helped build the software and hardware going into Hera. The Hera satellite will reach Didymos in 2026 and will provide high-resolution mapping of the ‘moonlet’ and DART’s impact. It will also attempt to place an autonomous satellite on its surface to collect more data. “We have built two ‘cube satellites’, which will be released from the main Hera spacecraft and conduct further research up close,” she explains.

“The technology for deflecting an asteroid is not complicated in the end,” says El Mir. More problematic, however, is humanity’s preparedness to respond and co-ordinate. “Humanity and politics are the real threat,” jokes Graziano, pointing to the inevitable disagreements about how to respond to such a risk.

Nonetheless, the Chelyabinsk asteroid strike has served as a wake-up call and a reminder of the possible dangers facing us. If it helps you sleep any easier at night, El Mir notes that in the past five years much more time and resources have been put into detecting asteroids than ever before, and the number we know about has grown dramatically since 2013. And happily, we haven’t discovered any on a collision course with Earth... yet.

Legal issues

Asteroid destruction

The law is clear: sending nuclear weapons into outer space is illegal. James Green, a professor of law at the University of Reading, is the author of a recent paper on sending nuclear arms to space. “Most lawyers would agree that you can’t station nuclear weapons in outer space,” he notes. “There’s also a treaty that says you can’t conduct a nuclear explosion there either.”

In his paper Green argues an exception needs to be made in international law for the purpose of destroying asteroids. With the threat of extinction facing us, it might seem unnecessary to worry about the legal ramifications of breaking today’s laws, but Green points out that “it is better if we have a clear, limited exception in law, rather than a scenario where we’ve got, say, a year or two to react and no legal clarity”.  

Having laws in place could make it easier to act. If one country decided to go it alone, then others might see it as a possible threat. What’s more, there’s always the risk that sending a nuclear device into space could go wrong. It would, therefore, be useful to have a legal framework around that would be responsible in the case of an altruistic mission failing.


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