Evading Asteroid Extinction

When we see a picture of the Earth, it can look very lonely – a blue marble floating in a sea of black space.

But the Earth has many more neighbours than you might think and they’re not all benign!

In this blog, we explore what’s out there, what the risk of collision is, and how we’re preparing for the next big impact.

What's out there?

Our Solar System isn’t just made up of planets and the Sun. Scattered amongst the planets, there are countless smaller objects of all shapes and sizes.

Check out our handy guide: A Beginner’s Guide to Meteorites, Asteroids, Comets, & Meteors

There are many types including asteroids, meteoroids, and comets, but due to their extremely high speeds, they can all pack a punch if they’re not slowed down before they collide with Earth. Asteroids typically move between 12 and 20 kilometres per second relative to Earth, comets are more unpredictable and can be much faster.

Some of these objects are created from collisions on rocky worlds such as Mars and the Moon, but the majority (such as the asteroids in the Asteroid Belt) are the leftover debris from the very formation of the Solar System 4.6 billion years ago.

When a rocky object has an orbit that brings it within about 200 million kilometres of the Sun (1.3 times Earth’s orbital radius), it’s called a ‘Near Earth Object’ or NEO for short. This doesn’t mean that NEOs are always near the Earth, but that sometimes their path brings them very close to the Earth.

Sometimes these objects collide with the Earth, but risk greatly depends on size.

What's the risk?

The Earth is bombarded with more than 90 tonnes of dust and sand-sized particles every day, while rocks the size of cars hit the Earth’s atmosphere about once a year. Luckily, objects smaller than about 25 metres across will most likely burn up as they pass through the Earth’s atmosphere producing a spectacular streak of light across the sky and causing little or no damage.

However for objects larger than 25 metres, the journey through the atmosphere may not entirely burn them up. The chart on the left shows how often objects hit the Earth’s surface, depending on size of that object.

Every 2,000 years or so, a football-pitch-sized rock manages to get through the atmosphere and collides with Earth, causing significant damage.

Very rarely, about once every few million years, an object hits Earth that is large enough to cause widespread devastation to our civilisation.

So there’s no reason to hold your breath waiting for the next big impact, but even an incredibly rare mass extinction event could happen at any time. And we think it’s happened before…

Plight of the dinosaurs

About 66 million years ago there was a sudden mass extinction of about 75 percent of the plant and animal species on Earth – this included most of the dinosaurs! The leading theory is that this extinction was caused by an asteroid or comet about 10-15 kilometres wide crashing into the Earth.

A father-and-son team based their theory on the presence of an element called iridium in a layer of clay found all over the Earth that dated from the time of the extinction event. Iridium is rare on Earth but it is found in abundance in most asteroids and comets.

And in the 1990s, scientists found another smoking gun to support the theory – the 150-kilometre-wide Chicxulub crater hidden beneath the Yucatán Peninsula in Mexico. The crater is just the right size and age that we expect for the collision proposed by the impact theory.

While rare, a large asteroid or comet impact is one of the most dangerous threats currently facing humanity, so it is vital to work out how we might be able to protect ourselves from ending up like the dinosaurs.

Dodging disaster

To be able to protect ourselves, we first need to know what’s out there.

In 1705, astronomer Edmund Halley published his orbit calculations for the object which eventually became known as Halley’s Comet. The 1758-1759 return of Halley’s Comet marked the first appearance of a comet predicted in advance.

Read more: How Halley’s Comet Shaped History

Monitoring the position of NEOs nowadays follows a similar process. Many organisations across the world, including the European Space Agency, conduct sky surveys using both ground and space telescopes to find new NEOs. Once an object’s location is known, it can be tracked until an orbit can be determined to predict its future movement in space.

If we find an object larger than 140 kilometres across, with an orbit that could make it come within 7,480,000 kilometres of Earth (equal to about 20 times the distance to the Moon), it is labelled as a Potentially Hazardous Object since it poses the most risk of a destructive impact. We keep an especially close eye on these objects to make sure they’re not on a collision course with Earth.

At some point, the inevitable will happen and a massive asteroid will hurtle towards the Earth. If we manage to detect it, what can be done to protect ourselves?

If you’ve watched Armageddon or Deep Impact, you may think that it’s as simple as sending a nuclear missile to break up the object mid-flight. In reality it’s much more complex.

In some cases, this method could be helpful but the last thing we’d want is for the debris to cause more damage than the original object!

So we’re coming up with lots of methods we could use to tackle different situations, including more indirect methods such as using lasers to vaporise the surface of the object. This would turn it into a mini-rocket as the gas escaping off the surface provides a thrust to change the direction of movement.

At the moment there are many different ideas for how we could intercept an incoming object but none have been properly tested.

But recent missions like Rosetta, Hayabusa2, and OSIRIS-REx have actually visited comets and asteroids, and are giving us practice in rendezvousing and landing on speeding rocks.

In the future, NASA is planning a space probe called DART which will attempt to move an asteroid in space by crashing into it at a speed of approximately 6 kilometres per second.