What are near-Earth objects?

Asteroids and comets that pass close to our planet are called near-Earth objects or NEOs. These entities date back to the origins of the Solar System and have shaped the Earth’s past, sparked scientific curiosity, and even provided the inspiration for some Hollywood blockbusters. But what exactly and where are they? And do any of them have any real chance of slamming into Earth in the near future, and what can we do about it? That’s what we’re exploring this week, starting with Richard Binzel, professor of planetary sciences at the Massachusetts Institute of Technology…

Richard - A near-Earth object is pretty much what it says. It's an object or an asteroid, could be a comet, that happens to come passing near the Earth. When we say near the Earth, that means somewhere closer than the orbit of Mars and a bit closer than the orbit of Venus. So, a near-Earth object is just something that comes to our neighbourhood.

Chris - Of any size?

Richard - Yes, of any size.

Chris - Now, you mentioned two words there that are quite commonly used: asteroid, comet.
They are different. What's the distinction?

Richard - Well, first off, it's the appearance that will distinguish what we call an asteroid and what we call a comet. If it looks like a tiny little star, or star-like, it's an asteroid. But if it's fuzzy, we call it a comet. But generally speaking, they have two different origins. Asteroids tend to come in from the asteroid belt, which is between Mars and Jupiter. And comets tend to come from way out in the far reaches of our Solar System. They've been stored out in the deep freeze, if you will. And then these comets, which are now icy, they come in close to the sun, they heat up, the ices melt, and the gases that come off give them the fuzzy appearance.

Chris - And how big are these bodies, the asteroids and the comets, and how numerous are they?

Richard - The Earth sweeps up tonnes and tonnes of dust every day. So, it's innumerable particles of stuff out there that are dust-sized. Pebble-sized things fall through the atmosphere every hour. A few times a year, there's a big enough chunk, maybe the size of a small car, that enters the atmosphere. The atmosphere shields us. It's the friction of the body moving through the atmosphere at high speed. That friction basically vaporises the outer layers and breaks the object apart. Something big enough to give us meteorite samples falls a few times per year on the Earth. Of course, most of the Earth is water, so getting one to land in your backyard is incredibly rare, but it happens.

But when we get to objects the size of our recent friend, 2024’s YR4, that's about 50 metres or so. If you sat it next to Nelson's Column at Trafalgar Square, it would be just about the same height. We think an object like that, that size, 50 metres, comes wandering by the Earth-Moon system many times per year. So it's not uncommon that these space rocks come wandering by.

Chris - And what pulls them onto an Earth-crossing orbit? Because obviously we've been here for over four and a half billion years, so it's slightly surprising there's still stuff for us to run into.

Richard - That's a really interesting question because the asteroid belt, where most of these come from, is between Mars and Jupiter. But Jupiter's kind of the big bully in the Solar System. It's the most massive planet. Jupiter is always tugging on those orbits of those asteroids, thousands and millions of asteroids.And occasionally it will tug on one just right, so that instead of being in a mostly circular orbit, an asteroid might get nudged out of the asteroid belt, mostly by Jupiter's gravity, into a more elongated path or elliptical path. That path, as it goes around the Sun, can come closer to the Sun than the Earth, and that lets it come into our neighbourhood. We would call that a near-Earth or Earth-crossing asteroid.

Chris - What sort of damage can these things do relative to their sizes, and how do we know that?

Richard - Things that are as small as a car and smaller burn up very effectively in the atmosphere. When they're a bit larger, like Chelyabinsk in 2013, it may have been something 20 metres across or so. That's an object where the atmosphere shields us, but the object breaks apart close to the surface. And even though we don't get pummeled to make a crater, the pressure wave, that meteoroid, if you will, has been pushing through the atmosphere. That pressure wave will hit the ground and break windows, as happened in Russia in 2013. But things large enough, maybe 100 metres or a few hundred metres across, which would make a crater if they landed, that may happen once every few thousand years somewhere on the Earth.
We'd rather not have it land over a populated area, but again, those are incredibly, incredibly rare.

Chris - And the one that did for the dinosaurs, that caused the Chicxulub impact 65, 66 million years ago, how big was that?

Richard - Yes, at the top end of the scale, something about 10 kilometres across, is what was the bad day for the dinosaurs 65 million years ago. In that case, the impact did make a crater off of the Yucatan Peninsula of Mexico. And the consequence of such a large impact was that dust and debris filled the atmosphere. So, it wasn't just that you were in the wrong spot, but so much dust and debris was kicked out of the crater that that 10 kilometre impact probably made the Earth go dark. The Sun was blocked for months - or maybe a couple of years - and if you were a plant eating animal, you didn't survive. And if you were somebody who fed off of plant eating animals, you didn't survive either. It turns out mammals at that time were burrowing creatures that could survive off of seeds and scavenge some good old dinosaur meat lying around. The mammals survived. And so the fact that the dinosaurs didn't have a space programme is why we're here today.

Chris - I suppose in some respects, that's a good thing. It gave us our break, didn't it? But do we also have these incoming bodies to thank for the fact that we actually have the environment on the Earth we have at all? Because the Earth when it first formed was pretty hot and pretty dry. And some people have put it to me that most of the water arrived from space on these bodies.

Richard - Well, for certain, the water and basically everything we have in our chemistry that makes all of life came from outer space, and those are the ingredients that form the planets. So, it's a question of: what was the last layer that came in that gave us water, gave us the organic chemistry for life? And that may well have been meteorites. It may have well been the Earth cleaning up the leftover debris from the formation of the planets. We're talking four and a half billion years ago. They could have been the ingredients that allow us to have a conversation today and sip our morning coffee.