An asteroid is heading straight for Earth… and other fun space rock facts

In just over a fortnight, a shiny metal capsule will drop from space into the Utah desert. 

It is a human craft, but will be filled with alien material – samples scooped up from the dark and dusty surface of a near-Earth asteroid.

Named Bennu, it is currently a relatively close 75 million miles away, and will shed new light on the formation of our solar system 4.5 billion years ago.

However, we already know quite a lot about the early years of our neighbourhood thanks to another type of cosmic object – meteorites.

Meteorites are basically space rocks, ranging in size from dust particles to larger chunks of asteroid, that survive the fiery passage through Earth’s atmosphere.

And they’re amazing.

Nuclear scientist and meteorite fan Tim Gregory shares nine fascinating facts about these marvellous space travellers.

They’re basically time capsules

When the solar system formed, it was giant cloud of gas and dust – a nebula – similar to all these beautiful photos Hubble and the James Webb Space Telescope send back.

We were one of these giant interstellar clouds. And then, about four and a half billion years ago, a small clump of that cloud began to collapse in on itself. 

As the centre of that cloud became denser and denser, its gravitational field strength grew and grew, and so the collapse spiralled in a runaway. Most of that material, about 99.9% of it, formed the Sun. The leftovers formed the planets and everything else – moons, comets, and asteroids. 

But that’s just the general picture – there are a lot of nitty gritty details in how you get from a nebula to a solar system. There are two ways we can investigate that: telescopes, and meteorites.

Meteorites are the leftover debris from that era of planet formation. They’re small fragments of asteroids. They pre-date the planets, making them the oldest rocks in the solar system.

And in physics, what’s true for a cup of tea is true for an asteroid. Small objects cool down much quicker than big objects. Asteroids, therefore, lost their heat very quickly after they formed at the beginning of the solar system. Since then, they’ve remained virtually unchanged.

Meteorites are geological time capsules, and we can date them using the ‘radioactive clocks’ that exist naturally in all rocks. Radioactive elements – like uranium – decay into new elements at a rate determined by their half-lives. By measuring how much radioactive decay has happened in a meteorite, you can figure out its age. Using this method, we determined the solar system’s age: 4.6 billion years old.

Sometimes the Moon throws rocks at us

Almost all meteorites come from asteroids, but it’s very hard to say which particular meteorite came from which particular asteroid. 

However, sometimes we know exactly where they came from, like in the case of lunar meteorites. As the name suggests, they come from the Moon.

And the reason we know they came from the Moon is because we can compare them like-for-like to the rocks the Apollo astronauts brought back. We can compare the geology – and they turn out to be a perfect match.

But how do they get here? Because the Moon pretty much lacks an atmospheres, it’s a sitting duck for asteroid and meteorite strikes – as you can see from Earth by the many craters covering its surface. When it is struck, chunks of the Moon fly off, and if the angle is right, they crash down to Earth.

Some are more exotic

The second type of meteorite that we have a pretty good idea of where they came from are Martian meteorites. Yes, meteorites from Mars.

Now, while we don’t have samples of Mars to compare them to on Earth (yet), there are two major lines of evidence to tell us where they came from.

Firstly, their age. They’re much, much younger than almost all other meteorites, sometimes billions of years younger. They must have come from a celestial body that was geologically active in the relatively recent past – and there aren’t many of those in the solar system.

Secondly, although we haven’t brought samples of Mars back from missions yet, we’ve sent our robotic ambassadors to the surface of Mars: the landers and rovers. They’ve tested the chemistry of the atmosphere there, and it perfectly matches the chemistry of the gas in these meteorites.

I’ll never forget the first time I held one. I always thought it would be a future astronaut who touched the surface of Mars first, but we’ve done it decades earlier, which is kind of cool. 

The ingredients for life?

There is a rare type of meteorite called carbonaceous chondrites. Plain chondrites are actually the most common type of meteorite, the most primitive kind.

But carbonaceous chondrites are a very, very important type of meteorite – and my favourite. 

As the name suggests, they’re often rich in carbon, and the form of carbon we find in them is varied and complex.

But the most unexpected type of chemistry we find in them are amino acids.

And amino acids are the building blocks of life on Earth.

Now that’s not to say this is evidence of extraterrestrial life, although people suspected that when they were first discovered. But it does mean two things.

Firstly, meteorites could have delivered the building blocks of life to Earth from space.

And secondly, the building blocks of life are abundant in the solar system. To my mind, this vastly increases the probability that there’s life elsewhere in our solar system.

They smell

Nine years ago, I was an intern at Nasa’s Johnson Space Center, and my summer project was to study one of these carbonaceous chondrites, found in Antarctica in 1994.

I’d read accounts of how these meteorites can smell, so as soon as I cut it open, I took a deep breath – and it really did smell! It smelled kind of organic-y, actually a bit like a damp towel, a very musty sort of smell. 

Meteorites could be the waiter of the world

Anyone for a drink? 

There’s a prominent theory that the water on Earth was delivered by comets, but the isotopic composition of the ice found on them doesn’t match what we have in our rivers, oceans, and taps.

However, carbonaceous chondrites are chock full of water, and its composition is very, very similar to water on Earth.

Some of these meteorites are maybe 10 or 20 per cent water. But not water as we’re used to it – you can’t wring them out like a flannel. It’s bound up in crystal structures within the meteorite, but there’s still enough that they likely delivered water to the Earth which, of course, is vital for life. 

We have a lot to thank carbonaceous chondrites for!

A right royal head scratcher

Now, here’s a curious one. When they cracked open Tutankhamun’s tomb just over 100 years ago, wrapped alongside him in his linen was a foot-long dagger encased in this beautiful golden sheath.

But the blade from the dagger was made from iron, which was a bit of a head scratcher for the archaeologists who found it – because it predates the Iron Age.

And even stranger, the chemistry of the dagger perfectly matches the chemistry of iron meteorites.

This leads to the bizarre but inescapable conclusion that Tutankhamun’s dagger is made from an iron meteorite.

We’ll never know for sure if the person who made the dagger knew the piece of iron came from space. But what we do know is that, around the same time Tutankhamun lived, a new hieroglyph came into common usage – and its literal translation is ‘iron from the sky’.

There are literally tonnes of meteorites heading our way

What is the chance of being hit by a meteorite? Basically zero. 

However, about 40,000 tonnes of extra-terrestrial rock falls to Earth every year. Thankfully though, Earth is huge, so that 40,000 tonnes is spread very thinly.

Of course, sometimes big ones do hit the Earth and they can cause damage, such as the Chelyabinsk meteorite over Russia in 2013. 

And occasionally absolutely massive ones strike, perhaps most famously 66 million years ago in the Yucatán peninsula, which completely finished off the dinosaurs.

Another big one could be on the way

Extinction level asteroids are very infrequent, but there is the chance of a large one hitting Earth in the second half of next century. That sounds a long time in the future, but it’s not that far off, only about 150 years – a couple of generations.

The culprit? Asteroid Bennu, which Nasa has just visited to collect samples from. There’s about a 1 in 3,000 chance it will hit Earth. 

That would be like detonating many, many thermonuclear weapons simultaneously. All in one place. It would be pretty pretty bad.

But what’s almost poetic is that Bennu is a carbonaceous asteroid.

So carbonaceous asteroids and meteorites probably delivered the building blocks of life to Earth, and another might snuff quite a bit of it out.

Tim Gregory is a nuclear scientist working full-time in the heart of the British nuclear industry at Sellafield. He will be speaking at New Scientist Live at ExCeL London on Saturday, October 7. His debut book, Meteorite: How Stones from Outer Space Made our World, is available to buy now

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