Humanity has many issues to worry about – climate change, nuclear escalation, scarce resources, and overpopulation, to list just a few. An asteroid attack is another worry, arguably the most abrupt and outside our control.

We, after all, would not like human civilisation to end as the age of the dinosaurs did. The most significant asteroid attack known to humankind is the Chicxulub impactor, estimated to be about 10 kilometres in diameter, which around 66 million years ago slammed into Earth off the coast of Yucatán Peninsula, roughly where Mexico is today.

The impact threw vast amounts of debris into the air and caused massive tidal waves. The debris blocked sunlight, inhibiting plant growth, which in a mere nine months led to the extinction of many dinosaur species, especially the large ones.

The question which sparks our imagination is: could history repeat itself?

Comets and asteroids are leftovers from the formation of our universe. The difference is that asteroids are rocks, and comets consist of frozen gas and dust. Meteoroids form when either asteroids or comets smash into one another – they are much smaller than typical asteroids or comets, and their orbits are less predictable.

All of the above orbit the Sun in our solar system, with most in the asteroid belt between Jupiter and Mars. Their trajectory can overlap with the one of the Earth. If that happens, and the object is large enough to survive the travel through Earth’s atmosphere, it will hit the surface of our planet. It would then be called a meteorite.

Both the moon and Mars are found to have thousands and thousands of these impact craters, whereas our Earth has only about 170 documented craters (experts disagree on the exact number). How did we get so lucky and have so few craters despite our planet being much larger than the moon?

This is because of our atmosphere. If you have ever seen a shooting star, these are the meteoroids that enter our atmosphere and light up. After they enter the atmosphere, they are called meteors. This is because the atmospheric layers above the mesosphere – the exosphere and the thermosphere – do not contain a lot of particles, so the meteor passes through them easily. The mesosphere contains enough particles to cause friction between atmospheric molecules and the meteor, so the celestial body burns up.

The atmosphere of the moon is practically non-existent. “The density of the atmosphere at the moon’s surface is comparable to the density of the outermost fringes of Earth’s atmosphere where the International Space Station orbits,” the US National Aeronautics and Space Administration (NASA) explains.

The atoms and molecules in the lunar atmosphere seldom collide, meaning the moon has virtually no atmosphere protecting it, so meteors experience very little resistance and hit the moon undisturbed.

In the case of our Earth, the situation is entirely different. Meteors travel through our atmosphere for 10 to 15 seconds at tremendous speeds of about 7 to 45 miles per second.

The immense amount of friction the meteor receives heats it to around 1,800 degrees Celsius, this only occurs on the meteor’s surface as the heat does not have enough time to reach the insides of the rock. The rest of the meteorite would still be at its original temperature. Meteors arrive from deep space, where they spend most of their time at near absolute zero temperatures – effectively, meteorites are cold after impact.

According to NASA, Earth receives about 48.5 tons of meteoritic material each day. Almost all of this is vaporised in our Earth’s atmosphere, consequently causing us no harm. However, some meteorites are much larger and much more harmful. These larger meteorite occurrences happen roughly once every 2,000 years.

The biggest impact crater on Earth is the Vredefort Crater in Johannesburg, South Africa. It is 180-300 kilometres wide and formed about two billion years ago when an asteroid – estimated to be around ten kilometres long – launched towards Earth.

The most recent significant asteroid impact occurred in 2013 in Chelyabinsk, Russia. According to the European Space Agency (ESA), the meteorite disintegrated (due to ablation) 30 km above the ground at speeds of 18 kilometres per second. Its explosion released around half a megaton of energy – ESA calculated it was equivalent to ‘35 Hiroshima- sized bombs.’

“The relatively small rock approached Earth from very near the direction of the Sun, exploding in the atmosphere and creating a shockwave that damaged thousands of buildings, breaking windows and injuring roughly 1,500 people, mostly because of flying shards of glass. It was the largest asteroid to strike Earth in over a century”, ESA reported.

“Asteroids of this size ‘strike Earth roughly every 50-100 years,” said Richard Moissl, ESA’s Head of Planetary Defense.

Long before then, space agencies worldwide were at work to design planetary defence systems. In 2022, NASA successfully tested a project called the Double Asteroid Redirection Test (DART), marking humanity’s first-ever attempt to alter the course of a celestial body – and giving us a chance to defend Earth from unwanted space incursions.

The DART mission aimed to change the trajectory of an asteroid moonlet, Dimorphos, with a diameter of 160 metres, which orbits a larger, 7800-metre asteroid called Didymos. Both pose no threat to Earth.

A box-shaped spacecraft weighing 570 kg hit Dimorphos after travelling 11 million kilometres from Earth. The impact successfully slowed down Dimorphos from its initial velocity of 14,000 miles per hour and altered its trajectory. NASA is still closely observing Dimorphos and Didymos to measure the effectiveness of this impact.

Yet, the mission is considered a success – the ability to change the trajectory of the small moon means that we now have a dependable form of defence from future asteroids.