It was 2013 and an asteroid the size of an Olympic swimming pool was hurtling very close to Earth. The chance of a collision had been ruled out, but it was a big deal for astronomers. Scientist Paul Chodas was set to appear on a live NASA TV show to explain how the rock was about to whiz by, closer than many satellites and clearly visible through telescopes. An asteroid such as this one, called 2012 DA14, came this close only once every 40 years.

Then, just hours before the expected sighting, an enormous fireball formed in the morning sky over a city in central Russia. The blinding flash was 17 times brighter than the sun. A minute later an explosion shook the ground, shattering thousands of windows and blowing down doors. Glass and flying debris sprayed the people of Chelyabinsk. Out of nowhere, a different asteroid had struck.

By the time Chodas, who leads NASA’s Centre for Near Earth Object Studies, sat down in front of the TV cameras, the script had been scuttled by a once-in-a-lifetime impact. “It felt like we were in a cosmic shooting gallery,” he says.

No one had seen the rock careering towards Chelyabinsk. It had approached Earth from the direction of the sun; while telescopes searching for asteroids scan only the nighttime sky, this had arrived in the daytime. As large as the Chelyabinsk blast was – more than 1600 people were injured, mostly due to broken glass – there are other objects out there that pose even bigger concerns. The US government has tasked NASA with tracking the kinds of asteroids that could take out a city or change the climate.

What happens if a fragment of one of these things comes our way? And what is the difference between, say, a piece of falling space junk and a meteor? In Melbourne in August, flashes across the night sky turned out to be the remnants of a Russian rocket re-entering the atmosphere. But in May, it was a meteor exploding over the Gulf of Carpentaria that cast the night sky in green and orange – the biggest such blast over Australia in at least 30 years, even if not on the same scale as Chelyabinsk.

How likely is another meteor to strike? Why does space junk come back to Earth? And how prepared are we?

First, what’s a meteor?

Hadrien Devillepoix scanned the dusty earth of the Nullarbor Plain. It was weeks after a network of cameras had picked up a light “as bright as the full moon” falling in the vicinity of a Western Australian roadhouse. The astronomer was on reconnaissance looking for a black rock – easily confused with kangaroo droppings. “On this little track in the middle of nowhere I basically almost stepped on it,” Devillepoix says. “I thought my colleague had played a big prank on me and just planted a black rock.”

The rock was the size of an orange. It had an unmistakable sheen. Devillepoix picked it up and carefully placed it in a sampling bag. It felt far heavier than a typical rock – a sign of extraterrestrial origin.

Once objects like this land on the ground, they’re called meteorites. As they speed through the atmosphere – when they’re generally known as shooting stars – they’re meteors. In space, if a rock is smaller than roughly a metre, it’s a meteoroid. If larger, it’s an asteroid (more on those later). These aren’t to be confused with comets, icy objects that begin to melt as they come closer to the sun, trailing tails that can be visible from Earth.

If you’re lucky enough to see a shooting star, what you’re seeing is light as a meteor hits the atmosphere. “You have all this motion and kinetic energy of the rock, and that material evaporates, creates a gas. The gas collides with the air and you see the radiation,” explains Peter Jenniskens, a meteor astronomer at the SETI Institute in California. The effect occurs because the object hits our atmosphere at speeds of between 11 and 72 kilometres per second (or, Melbourne to Sydney in as fast as 10 seconds). That’s a lot of friction. Says Chodas: “At those speeds, the atmosphere acts almost as a solid wall, at least for small objects.“

Much of the rock disintegrates and vaporises, turning to dust. Anything the size of a pebble or larger that makes it to the ground is generally less than 1 per cent of the rock that entered the atmosphere. The Chelyabinsk object – 20 metres in diameter – scattered small meteorites across snow-covered fields that people were later plucking out like popsicles, says Jenniskens, who visited the site three weeks after the explosion.

But the main remnant was a 60-centimetre lump that slammed into an ice-covered lake. A major shockwave was felt through the city. “It’s the moment when it breaks up and it creates all these small pieces that are stopped by the atmosphere, that’s when all that kinetic energy goes into a shockwave,” says Jenniskens.

The fireball in North Queensland in May was hundreds of times brighter than the moon. It detonated between 20 and 30 kilometres above the ground. People reportedly felt their homes shake in the town of Croydon, near the Gulf of Carpentaria. “It was in the top 20 largest events ever in the last 30 years. And in the top seven largest over land,” says the director of the Curtin University’s Desert Fireball Network, Eleanor Sansom. “No one knew it was coming.”

She oversees 51 cameras covering about a third of Australian skies, but none where this meteor arrived. Her cameras record about 10 meteors dropping a meteorite on the ground per year. For all of Australia, Sansom estimates about 30 meteorites larger than a golf ball arrive each year. “We’re not talking about massive metre-sized rocks, we’re talking about quite small things.”

Where do these rocks come from?

About 4½ billion years ago, our early solar system was a great cloud of dust. As the material spun around the sun over millions of years, pieces stuck together to form planets. “These bodies were getting bigger and bigger, breaking up but also coming together,” says Sansom. But some didn’t stick: asteroids. “It is rubble, basically,” she says. “A lot of it is older than the Earth and the leftovers of planetary formations.”

These rubble piles are loosely bound and sometimes passing by our planet, but mostly don’t collide with us. “They go around the sun, the Earth goes around the sun, and we’re not on the same tracks,” Chodas says. The biggest concentration in the solar system is in the main asteroid belt between Mars and Jupiter. It contains between 1.1 and 1.9 million asteroids bigger than a kilometre in diameter, and millions more of smaller sizes. “That is basically an area that [scientists] think was probably a failed planet,” says Sansom.

Over millions of years, rocks collide and forces such as gravity from Jupiter (the largest planet in our solar system) can cause them to veer into “highways” that eject them from the belt and towards different regions or even other planets. “They’re not in there and safely out of the way,” Sansom says. “They are slowly migrating.”

More than 70,000 meteorites have been found on Earth, but scientists know where just a handful originated from in the solar system. When Sansom’s team detects meteors, they aim to record their trajectories and link them with asteroid families. Without doing this, Sansom says, “it’s a bit like giving a geologist a bucket of rocks and saying, ‘Hey, go explain the theory of plate tectonics or volcanism.’ You can’t do that without telling people where those rocks came from.”

As it turns out, the rock that Devillepoix found is possibly older than the Earth, he says, thought to have broken away from the main asteroid belt some 50 million years ago. It was made of a material known as chondrite – the building blocks of planets.

In some cases, meteorites can show what the core of planets might look like. “We can’t get to our own core here on Earth,” Sansom says. “But sometimes [asteroid] cores come to Earth as meteorites, and we can study those.”

What’s the risk of an asteroid hitting us?

The asteroid Apophis is named after an ancient Egyptian spirit of evil and chaos. On Friday, April 13 in 2029, it will pass Earth closer than many weather and communications satellites. When the 340-metre-wide asteroid was first discovered two decades ago, scientists thought it had a 2 to 3 per cent likelihood of colliding with Earth. “We had never seen a probability that high for such a large object,” Chodas says. “An object that large would cause a lot of damage should it hit. But we [now] know it’s not going to.” As Apophis orbits the sun and back towards Earth, collisions have also been ruled out in 2036 and 2068.

To put the concern in perspective, asteroids of this medium size hit Earth every 10,000 or 20,000 years. There are 190 known crater sites on the planet; the oldest, from an impact about 2.2 billion years ago, is the Yarrabubba meteor crater in Western Australia. (Unlike on the pocked surface of the moon, weather on Earth has eroded the remnants of many craters.)

In the extremely unlikely event that an asteroid the size of Apophis did collide with Earth today, Chodas says, the rock would stay mostly intact and create an explosion that could impact for “many kilometres”. It could wipe out a city or cause a tsunami. “These things come in with so much energy, the modelling says, that we would not be able to survive if you were anywhere near an impact of a medium-sized asteroid.”

Then there’s the even more far-flung possibility of a large asteroid – a kilometre or more in diameter – wreaking havoc. Fortunately, asteroids this big hit the Earth once every few million years. The most famous, which carved out a 300-kilometre-wide crater on what’s now Mexico’s Yucatan Peninsula about 60 million years ago, is believed to have brought on the extinction of dinosaurs. “Those ones, if they should impact the Earth, would change the climate,” Chodas says.

NASA scientists have been searching for these larger asteroids in the skies for 25 years, finding more than 850 of them. Chodas says this is about 95 per cent of the total large asteroid population. “We are tracking them. We know they are not going to hit the Earth in the next 100 years. We’re safe from the big ones.” His team has also found several thousand medium-sized asteroids of between 140 metres and a kilometre in diameter – only about 40 per cent of what is believed to be out there of this size. “It’s like sand on the beach, the small stuff is more numerous,” Chodas says. “There’s more work to be done to discover those because you never know if there’s one that could, you know, impact the Earth in the next 25 years or whatever.”

Apart from Chelyabinsk, the only other large meteor impact recorded in modern times is the Tunguska impact of 1908, where it’s thought a space rock 36 metres or so wide exploded over a Siberian forest, killing hundreds of reindeer and felling 80 million trees while leaving those in the epicentre of the blast upright but stripped of branches. The shockwaves were felt in England. Considering how “very infrequent” asteroid collisions with Earth are, they’re not something to lose sleep over, Chodas says. In his line of work, though, “you still want to make sure you have found all of those and are tracking them”.

What’s being done to defend against asteroid strikes?

Eleven million kilometres from Earth, a refrigerator-sized spacecraft collided with an asteroid last year – marking a new era. NASA hailed the deliberate impact, called the Double Asteroid Redirection Test, or DART, a “watershed moment for planetary defence”. It successfully shifted the orbit of Dimorphos, a moon circling a larger asteroid called Didymos. Neither asteroid posed a threat to Earth, but the spacecraft was able to shorten Dimorphos’ orbit around Didymos by 32 minutes and, in doing so, show proof of concept. “This mission shows that NASA is trying to be ready for whatever the universe throws at us,” said NASA chief Bill Nelson. Astronomers have since been analysing data from the mission to assess how a spacecraft could protect the Earth from a hazardous asteroid.

Each year, Chodas – in a role he calls “threat master” – prepares hypothetical scenarios for scientists to consider how to defend the planet from a medium-sized or larger asteroid. A kinetic “impactor” such as was used in the DART mission is currently the preferred option for preventing an impact. Depending on the size of the asteroid, he says, this option could require a warning time of about 10 years or more to be effective. “The idea is to change the velocity, and that would change the trajectory and make it, if there’s enough time, miss the Earth,” Chodas says.

Another solution that scientists plan to investigate is to send a different kind of spacecraft called an ion-beam deflector to rendezvous with the asteroid and fly in formation with it. It would direct the exhaust of one of its ion thrusters at the asteroid, altering its course over time. “We’ve simulated that process and it does seem to work pretty well – not quite as effective as a kinetic impactor but it has fewer complicating issues,” Chodas says. “Again, you would need time, probably a warning of, like, 20 years or so.”

There’s no such thing as a crack squad when it comes to asteroids. With a mission prepared, a spacecraft would need time to cover the millions of kilometres to reach the object (DART took 10 months to arrive at Dimorphos) and once the deflection occurs it could take several years for the change in velocity to significantly alter the asteroid’s course. “[Overall] if the warning is less than about seven years, it’s getting dicey,” Chodas says. “If it’s just a couple of years, there’s really no easy way to deflect the asteroid.” Other options canvassed include nuclear explosive devices, which Chodas considers a last resort, not to mention that it “violates a lot of treaties”.

Should more be done to prepare now? Probably not, says Chodas. “The impact of a 100-metre asteroid is extremely rare, roughly once every 10,000 years. It’s difficult to justify putting a lot of infrastructure in place to deflect a threat that could be many decades or centuries in the future. We certainly want to demonstrate we could do it, as we did with the DART mission, for example, and maybe should test other techniques such as ion-beam as well.”

Chodas, a mathematician, marvels at how far we’ve come. He recalls predicting when and where a series of comets would hit Jupiter in the 1990s and seeing the impacts happen within minutes of when he predicted – no small achievement back then. “All of that can be applied to the Earth and that’s what I’ve worked on for the rest of my career. We’ve predicted several little asteroids hitting the Earth, often accurate to a few seconds … Being able to predict the future is the coolest thing.”

Why do we keep hearing about space junk?

When Skylab, NASA’s first space station, began to descend from orbit in 1979, no one could predict exactly where its re-entry into Earth’s atmosphere would be. Just a year earlier, radioactive debris from a Russian satellite had been strewn across Canada. In the US, some people painted bulls-eyes on T-shirts or the roofs of their homes. In Australia, emergency services were on alert. As word came that Skylab’s re-entry was imminent, Stan Thornton, then 17, rushed to a lookout in Esperance, on Western Australia’s south coast. Just after midnight, he saw objects burning up in the sky, causing flashes of red, orange and green. “There was a sonic boom afterwards,” he recalls.

His mum, who stayed at home, heard noises on the roof of their chicken shed. The next day Thornton found bits of “charcoal” spread across their yard. His first thought was they could be radioactive. “We took it to the state emergency service in Esperance,” he says. Days later he was in San Francisco delivering them to the Examiner newspaper to receive a $US10,000 prize for the first pieces of the space station to be returned to the States. “It was a good thing,” he says. “It was probably something I’d never be able to afford to do in my life again.”

Today, there are more than 27,000 human-made objects being tracked in space, including 13,700 pieces of debris and more than 2000 used rocket bodies. This is only a fraction of the 130 million pieces of “junk” thought to be in space that are a millimetre or more in size. “Most of those millions of pieces are so tiny they will burn up in the Earth’s atmosphere and are never a problem. You probably don’t even see it,” says ANU astrophysicist Brad Tucker. “We worry about the small objects because of other spacecraft running into it and creating this proliferation effect.” Thousands of tracked pieces stem from China exploding an old weather satellite in 2007 and a defunct Russian spacecraft colliding with a functioning US spacecraft in 2009, according to NASA. Even a fleck of paint can cause damage to a quadruple-glazed craft window, as happened to the International Space Station in 2016.

Most objects orbiting the Earth are actually falling back down due to gravity, but because the Earth is also moving they are constantly missing it. “They fall again and they miss the Earth, and they fall again,” Tucker says. “[It’s] this nearly infinite cycle.” What slowly breaks this cycle is a little bit of atmosphere causing drag and slowing the objects as they orbit. As the objects fall lower, their orbits become closer until they encounter the denser levels of the lower atmosphere, where most small objects disintegrate. Sometimes this takes decades: “The higher you are, it takes longer to do that,” Tucker says. Some higher objects in the geosynchronous orbit, 36,000 kilometres from Earth, such as many weather and communications satellites, won’t re-enter the atmosphere at all, but “the vast majority of our satellites, which are in low Earth orbit, will,” Tucker says. In July, about 120 tracked objects re-entered the atmosphere.

Some objects are deliberately brought back to Earth and directed into the ocean. There’s even a watery graveyard for them. More than 250 retired space objects have splashed into the furthest point from any landmass in the Pacific Ocean named Point Nemo, after the submarine captain in Jules Verne’s novel Twenty Thousand Leagues Under the Sea. “Even if you have debris that falls a little bit west or east, you’re not going to hit anything,” Tucker says.

Occasionally, space junk that lands in oceans washes up on beaches, such as the metal dome found north of Perth in July that India confirmed was from an object used to launch satellites into orbit. Space junk burning up at night was visible in Melbourne in August hours after a rocket launch in Russia. Remnants of the Soyuz-2 rocket were planned to enter into the ocean south-east of Tasmania. “That was predicted, although we didn’t think you’d see it from Melbourne,” says Christopher De Luis, the Australian Space Agency’s general manager of the office of the space regulator.

Possibly more than half of the large space debris that returns to Earth doesn’t involve a controlled re-entry, Tucker estimates. “Part of what we’ve always banked on is a volume and numbers game. We aren’t sending that many things up and there aren’t many launches, and most of Earth is uninhabited,” he says. “But that small fraction of a risk is now becoming a larger fraction of a risk.” There are about 11,000 satellites in orbit, 40 from Australia. Another 20,000 are forecast to be launched worldwide before the end of this decade, says Australia’s space agency.

Still, space junk landing in populated areas is rare. In 2020, after China launched Long March 5B, pieces of the rocket reportedly fell in western Africa, causing property damage. The next year, a US farmer in Washington state found a pressure vessel from a SpaceX rocket in a field. In Australia, at least six pieces of space junk fell across several farms in NSW following a SpaceX crew launch in 2022. The biggest was more than two metres, says Tucker, who visited the site. “This was the trunk of the crew capsule,” he says. “Had someone, a sheep, or something been in the wrong place at the wrong time, we’d be having a very different discussion.”

Australia’s legislation for its own space debris requires operators to have a strategy to mitigate litter before permission is granted to launch spacecraft. “If it is to re-enter, and come down to the ground, it must be done in the safest manner possible,” says De Luis. “You can stop all space debris by stopping launches ... but you need to balance the benefits that come from accessing space while doing it safely.” The government’s crisis management framework also designates roles for officials during a “space weather, asteroid or extra-terrestrial body impacting on the Earth” or an incident involving space junk.

For stargazers, how would you even tell the difference between a meteor and a hunk of space junk? While a meteor would come streaking at speed, says Jenniskens, “a typical spacecraft re-entry looks like a slew of small fragments moving very majestically, very slowly, along the sky.”

In 2015, he was among scientists who watched a falling piece of space junk re-enter the atmosphere up close. They chartered a plane to Sri Lanka to a location where the debris would be visible from the air. At first, there was thick cloud. “The fog lifted, replaced by a bright blue sky and a minute later there was suddenly a dot of light,” Jenniskens recalls. The debris appeared like “a daytime meteor”. One day, they hope to observe an asteroid from an aircraft too. For now, this remains the closest they’ve got. “It gradually brightened and then broke into a slew of fragments,” he says. “It was surreal for it to happen right on cue.”

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