Probably my Instagram recommendations were being guided by some mysterious third force last week — or maybe the universe simply decided we hadn’t had quite enough bad news. Either way, here we are with a headline I (and, I imagine, you as well) have stumbled across at least several times recently: NASA has said that most “city-killer” asteroids are still undetected — meaning some could arrive with very little warning.

Which, admittedly, sounds terrifying. But is it just the drama of a headline, or is this actually something real to add to our already long list of fears? Let’s figure it out together.

In a broad sense, yes — but not quite in the way it sounds. We don’t “scan” space like a radar looking for danger. What we actually do is observe and track movement.

Astronomers use wide-field telescopes equipped with CCD sensors to repeatedly photograph the same sections of the sky, comparing those images over time. The logic is simple: stars stay still, asteroids don’t.

If a point of light shifts position between images, it is flagged as a moving object — and potentially an asteroid. From there, scientists calculate its trajectory: where it is heading, how fast it is moving, and, crucially, whether it might intersect with Earth’s orbit. If it does, it is classified as a potential threat. If not, it is simply logged and tracked.

Because, like most systems, this one has its blind spots. Overall, there are three main limitations — and they explain why these headlines keep appearing:

The honest answer is: anywhere from decades to just a few hours. But don’t panic — it really comes down to the asteroid’s size and how visible it is.

Not quite. We can’t really eliminate an asteroid — but we might be able to deflect it. And this isn’t just theory. It was tested in 2022 during the DART mission, when a spacecraft was deliberately crashed into an asteroid called Dimorphos.

Small, you might think — but not really. At around 160 metres wide, it sits firmly in the “city-killer” category. The impact didn’t destroy it, but it did shift its orbit by about 32 minutes — which, on a cosmic scale, is significant.

And while this wasn’t about saving the planet — Dimorphos was never a threat — it proved something important: if we spot an asteroid early enough, we might be able to move it just enough to miss us entirely. The catch, of course, is timing. Ideally, we would need around 10 to 20 years’ notice. A few years might still work, but with far less certainty. A few months, realistically, wouldn’t be enough.

Yes — they can, just not in an end-of-the-world way. These mid-sized asteroids are large enough to seriously damage a city, with shockwaves, fires, and local disruption. It is significant, but not global.

In many cases, we would likely get some warning — sometimes months, sometimes at least a few days. But not always. Certain objects can still slip through undetected, which means that window can shrink quite quickly.

So the risk is real, but uneven — less about constant threat, more about the occasional blind spot.

In reality, small objects reach Earth all the time — most of them burn up in the atmosphere and go completely unnoticed.

The most notable recent event was in 2013, over Russia — the Chelyabinsk meteor. It was only around 17 to 20 metres wide, but still powerful enough to create a shockwave that shattered windows across the city, injuring around 1,500 people, mostly from flying glass.

A bigger — and far more dramatic — example is the Tunguska Event in 1908. Estimated at around 50 to 60 metres across, it flattened roughly 2,000 square kilometres of forest. It happened over a remote area, which is really the only reason it didn’t turn into a human disaster. Had it occurred over a city, the outcome would have been very different.

These are in a completely different category. The last known extinction-level event was the Chicxulub impact around 66 million years ago, when about 75% of all species went extinct, including the dinosaurs.

But we already know that the largest asteroids are also the easiest to detect and track, which is why agencies like NASA have already mapped the vast majority of them.

And the good news: there is no extinction-level asteroid currently detected. So breathe out — we should be alright.