So, today is Asteroid Day! Perhaps it’s me, but there is a sort of menacing feeling when you talk to people about asteroids. The pertinent question seems to be: Will they get us the way they got the dinosaurs? You know the sort of thing. And of course it is natural to worry about such existential threats. As a community, perhaps planetary scientists are guilty of playing up this very remote possibility. It captures peoples’ imaginations, as the hit movie Don’t Look Up demonstrated.

So, what else is there about asteroids besides their baddie roll as potential planet pulverisers?  

As discussed in my short article on the Open University website, asteroids may have been the carriers of water and other volatiles to the inner Solar System. That is certainly a strong possibility.  But we could be accused of bigging-up that aspect of asteroids too. There are those that argue that there was enough water in the inner Solar System to more than account for Earth’s oceans. There remains no certainty that primitive asteroids were needed to get life going on Earth.

So, if the threat from killer asteroids has been overstated and they didn’t carry water and organics to Earth, what use are they? Not a lot you might think. Interesting curiosities at best, lumps of ugly rock at worst? If you did think that (and I’m sure you don’t) you would be very wrong. Because asteroids are the real deal. They are truly fascinating and amazing objects, no ifs, no buts. And here is why.

Unlike the planets, asteroids are old. Planets like Earth change and rejuvenate. They evolve and renew themselves. Erosion by wind and water, deposition by rivers, eruption of volcanoes, the slow building of mountain chains. Planets are constantly evolving. But asteroids are witnesses to the Solar System’s distant past. They formed within a few million years of Solar System formation and have changed very little since.

In fact a unique set of objects within some primitive asteroids, known as calcium aluminium-rich inclusions, CAIs for short, record the oldest Solar System ages. When scientists tell you that our Solar System is 4,567 million years old that age came from dating cm-sized CAIs, found within pieces from asteroids that land on Earth as meteorites.  

But that’s not all.

One group of asteroid fragments, known as CI chondrites, have a composition that matches that of the Sun. And as the Sun is more than 99% of mass of the Solar System it means that these asteroid fragments are basically the stuff from which the Solar System formed, apart from gases such as hydrogen. So, when you hold such a meteorite you basically have our Solar System in your hands.  

Unfortunately, this type of asteroid is very soft and crumbly and only very rarely survives atmospheric entry so that it can be collected as a meteorite. Plus when it does arrive from space it quickly interacts with Earth’s atmosphere and that changes its composition, a bit.

So, you can imagine the shock when unexpectedly the Japanese Space Agency’s (JAXA) Hayabusa2 mission returned samples from an asteroid that is composed of this rare type of asteroid material.

Launched in December 2014, the Hayabusa2 spacecraft reached asteroid 162173 Ryugu in June 2018. For the next year and a half it studied the asteroid in great detail and even collected two surface samples. It left Ryugu in November 2019 and returned a sample canister to Earth on 5^th December 2020. Inside was 5.3 g of ultra-clean asteroid dust. The canister was safely recovered in the Australian desert and transported to Japan for detailed scientific studies.

In early 2021 I was privileged to be invited to become a member Team Kochi led by Dr Motoo Ito. Based at the Kochi Institute, part of JAMSTEC, this is one of a number of teams that have been intensively studying these incredible samples.  

The results obtained from the study of asteroid Ryugu particles have been astonishing. It turned out that asteroid Ryugu was much richer in water and organic compounds than the spacecraft measurements had indicated. It seemed that the Hayabusa2 spacecraft had hit the jackpot and returned material from a very rare type of asteroid.

But is it so rare?

One possibility is that this type of asteroid is much more common than we once thought. They just don’t make it to down to the surface of the Earth as large lumps because they are so weak and fragile. Hayabusa2 is likely to change many of our ideas about the importance of these old, water and carbon-rich asteroids. And with this precious material safely in our laboratories we have a much better chance of working out whether such asteroids really did bring the ingredients of life to Earth.

Interesting curiosities, planet killers or bringers of life? I give you asteroids.

Happy Asteroid Day.