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Let’s rock: Happy International Asteroid Day!

Dinosaurs might not share our enthusiasm, but here at Cosmos we thought we’d take the opportunity to celebrate International Asteroid Day and those cool hunks of space rock whizzing through our solar system.

We’ve reported a lot on asteroids over the years, but the last year has been especially interesting. It saw the culmination of the Japanese Aerospace Exploration Agency’s Hayabusa2 sample-return mission from the asteroid 162173 Ryugu. In December 2020, five precious grams of the asteroid touched down at the Woomera Test Range in the South Australian desert, and planetary scientists around the world are currently scrutinising the sample for hints about how our solar system formed.

Recovery of the Hayabusa1 capsule on 14 June 2010 near Woomera, seven years and two billion kilometres after its journey began. Credit: JAXA

Hayabusa2 isn’t the only mission touching down on these rocky worlds. NASA’s OSIRIS-REx spacecraft has been gallivanting around the asteroid Bennu, and the samples it successfully took last October will return to Earth in 2023.

But maybe we’re going too fast. Maybe you’re still wondering: what the heck is the difference between an asteroid and a comet anyway? Don’t worry; even scientists get confused sometimes – like the time the interstellar interloper ‘Oumuamua turned out to be a comet instead of an asteroid, and that time we thought a weird comet was actually two asteroids circling each other.

Artist’s impression of interstellar asteroid 1I/2017 U1 (‘Oumuamua). Credit: European Southern Observatory / M. Kornmesser

For the record, asteroids and comets are both rocky bodies that are much smaller than planets, but asteroids are composed of rock and metal, while comets consist of ice, rock, gas and dust. Asteroids mainly orbit in the belt between Jupiter and Saturn, while comets orbit in the cold reaches of the solar system beyond Neptune. (Bonus: meteoroids are the debris of asteroids of comets. When they burn up in Earth’s atmosphere they’re called meteors, and if they reach Earth’s surface they’re called meteorites.)

Of course, as the dinosaurs would tell us if they were still alive, asteroids can also pose a risk to life on Earth – but luckily most of them miss the planet or burn up in our atmosphere.

If you’re worried, scientists have actually developed risk scales to tell whether an incoming space rock is going to kill us. Unfortunately, no risk assessment can tell us how to act if an asteroid is on course with the planet – that terrifying and crucial choice is completely up to us. In 2019, NASA actually used an international conference to war-game a fictitious scenario in which an asteroid was about obliterate New York City.

How flying reptiles might have viewed large asteroid slamming into the Earth – and all that followed. Credit: Science Photo Library/Getty Images

One prevalent idea is to nuke an incoming asteroid to destroy it before it hits us, but maybe that’s not so smart – lawyers would certainly have something to say about its legality.

However you feel about the possibility of a massive space rock slamming into our planet, and however you choose to spend this hallowed day of the asteroid, just know that they might be the reason we exist in the first place.