Planetary securityHow Do We Know Whether an Asteroid Headed Our Way Is Dangerous?

By Jonathan O’Callaghan

Published 12 April 2021

There are a lot of things that pose a threat to our planet – climate change, natural disasters, and solar flares, for example. But one threat in particular often captures public imagination, finding itself popularized in books and films and regularly generating alarming headlines: asteroids.

In our solar system there are millions of space rocks known as asteroids. Ranging in size from a few meters to hundreds of kilometers, these objects are mostly left over from the formation of our planets 4.6 billion years ago. They are building blocks that didn’t quite make it into fully fledged worlds.

Asteroids and other objects that make a closest approach to our sun of less than 1.3 astronomical units (1 astronomical unit, AU, is the Earth-Sun distance) are known as near-Earth objects (NEOs). These are objects deemed to pose the greatest risk to our planet.

It is not uncommon for asteroids to hit Earth. Hundreds of meteorites reach the surface of our planet every year, most too small to be of any concern. But occasionally, large rocks can hit and cause damage. In 2013, the Chelyabinsk meteor exploded over Russia, injuring hundreds. At the extreme end of the scale, 66 million years ago, an asteroid wiped out the dinosaurs.

Now scientists are trying to work out how much danger we might be in from future asteroids, and what we can do to prevent considerable damage to our planet. And while no known asteroids currently pose any significant threat to Earth (in late March 2021, one of the largest and best known asteroids on a possible collision course, Apophis, was ruled out as being a potential danger for at least 100 years thanks to better pinpointing of its orbit), the race is on to make sure we’re ready if or when one does.

As our methods of surveying the solar system improve, more and more asteroids are being discovered –with about 3,000 NEOs found in 2019. But there are important gaps in our knowledge that still need to be answered, namely, if we spot an asteroid coming our way, how do we know if it is a threat?

While most asteroids larger than one kilometer in size are accounted for, and their orbits known not to impact Earth, smaller asteroids are less well monitored. Even a rock tens of meters across can cause significant damage if it hits a populated area.

The time between spotting a new asteroid and it hitting our planet can be a matter of days and such an asteroid is known as an ‘imminent impactor’.