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In December, an asteroid the size of the Eiffel Tower will approach Earth. Should you be concerned?

Whether or not NASA conducts a mission, thousands of people could be killed by a completely unpredictable asteroid impact.

Our ability to detect asteroids before they collide with the Earth is still in its infancy, owing in part to physical laws.


Before we get too excited about NASA's asteroid deflection test, it's important to remember that humanity is terrible at predicting impacts.

More than 1,200 asteroids larger than a metre in size have struck our planet since 1988. These were not large enough to completely destroy the planet or cause an extinction event (scientists are confident that nothing like that will occur in the next 100 years), but they were certainly large enough to cause some damage.

However, humanity only predicted five of those impacts in advance - possibly six if we are generous and include the object known as A106fg, which may simply have been a close approach - but this accounts for less than 0.42 percent of all strikes on the planet. We cannot deflect asteroids that we are unaware of.

Even the five asteroids that were discovered before they collided with Earth were discovered with only a few hours to spare. Our detection systems have never provided more than a day's notice in advance, though scientists hope and expect them to improve.

The five years between the DART mission's NASA approval and its scheduled rendezvous with Dimorphos next year provide significantly less wiggle room.

With such short notice, we would not only be unable to deflect an asteroid, but we would also be unable to evacuate people who were expected to be affected by the impact.

Our ability to detect asteroids before they collide with the Earth is still in its infancy, owing in part to physical laws.

Surveying asteroids in our solar system in the dark depends on them reflecting light towards us, which is determined by the direction of their approach relative to the sun and the phase of the moon.

If they approach from an angle where we can't see them, they will strike without warning and have the potential to cause massive damage.

Asteroid impacts are so powerful that when they occur over the ocean or in remote parts of the world, they are typically detected by equipment used to monitor for secret nuclear testing.

However, when they occur in populated areas, they are sometimes detected by the people who are directly affected by the impact.

A meteor exploded in the atmosphere near Chelyabinsk, Russia, in 2013, causing an enormous fireball, shattering windows, and potentially causing over a thousand people to seek medical treatment for their indirect injuries.

That asteroid was estimated to be about 20 metres in size and went completely unnoticed before entering the atmosphere.

It burned up in the skies over the Ural region and exploded with 30 times as much energy as the Hiroshima nuclear bomb at an altitude of 30km, briefly outshining the sun.

The Chelyabinsk meteor caused an airburst and shockwave that hit six cities across the country, serving as a stark reminder that dangerous objects can enter the Earth's atmosphere at any time.

Astronomers estimate that there are tens of thousands of near-Earth asteroids 500ft (150m) wide or larger, large enough to cause regional devastation if they collide with Earth.

The Chelyabinsk object was only about 60ft (18m) wide, demonstrating that even small asteroids can be dangerous – and emphasising the importance of real-world testing of space-based planetary defence systems.

While the incident demonstrated that small asteroids can be dangerous, there is still a long way to go before missions like DART suggest that our planetary defences are capable of protecting them.

A similar explosion in a city like New York or London could kill thousands of people, especially if the structural integrity of a large building is compromised. We simply wouldn't see it coming at the moment.


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