OPINION
Early in its history, the Earth was routinely bombarded with meteorites and even other planets. Thankfully for those of us who call it home, that cosmic smash-up derby has eased off over time. This is partly due to outer planets like Jupiter, and even our own Moon, acting as something like gigantic cosmic vacuum cleaners and sucking up incoming objects via their gravitational attraction.
That doesn’t stop every errant space rock, of course. Just ask the dinosaurs. There are plenty of near-Earth objects (NEOs) still whizzing around.
And even when the Moon takes a hit for its parent, it can carve off an NEO of its own.
A near-Earth asteroid has been traced to a huge crater on the Moon.
Space rock 2016 HO3, also known as Kamo’oalewa, is thought to have been blasted off the lunar surface, leaving the 14-mile-wide Giordano Bruno crater on the moon’s far side.
How did they come to his conclusion? For a start, Kamo’oalewa has a similar orbit around the Sun as the Earth, unlike most asteroids, which hail from the famous asteroid belt between the orbits of Mars and Jupiter.
Selected as the target of China’s Tianwen-2 mission, Kamo’oalewa measures between 150 and 190 feet in diameter, making [it] about half the size of the “London Eye” Ferris wheel.
The study, lead by Yifei Jiao, a visiting scholar at the University of Arizona Lunar and Planetary Laboratory, used modelling to essentially backtrack the asteroid’s origin.
According to the simulations, it would have required an impactor of at least 1 kilometer (0.6 mile) in diameter to launch a large fragment like Kamo’oalewa beyond the moon’s gravitational pull.
While the lunar surface is riddled with thousands of craters from impacts spanning the moon’s 4.5 billion history, only Giordano Bruno with its 14-mile diameter and estimated 4 million years of age fits the bill in terms of size and age, making it the most probable source of Kamo’oalewa’s origin.
KVPI
But Kamo’oalewa is very far from the only NEO. How many are there? And just how much of a threat are they likely to be?
In the past decade alone, Earth has had several close encounters with asteroids. The Chelyabinsk meteor event in 2013 is a haunting reminder of these cosmic threats. An asteroid, approximately 65 feet (20 meters) in diameter, entered Earth’s atmosphere and exploded over Russia, causing more than 1,500 injuries as well as widespread property damage.
Another significant near-miss was 2019 OK, a city-block-sized asteroid that zipped within 45,000 miles (72,000 kilometers) of Earth in 2019 – a cosmic hair’s breadth in the grand scheme of things. Traveling at some 55,000 mph (88,500 km/h), if the asteroid had struck our planet, it could have really wreaked havoc.
Another uncomfortably close encounter was the Tunguska Impact of 1908, which highlights the potential devastation even a relatively small asteroid can cause. In this event, an explosion over Siberia flattened an estimated 800 square miles (2,000 square kilometers) of forest, equivalent to about 80 million trees, yet left no crater. The explosion is believed to have been caused by the airburst of a meteoroid or comet fragment to be about 100 to 200 feet (30 to 60 meters) in size.
Of course, Earth gains about 100 tons of material every day – but that’s mostly tiny fragments no bigger than grains of dust. These are the stuff of the shooting stars you can see on any given night if you’re patient enough.
Fortunately, according to NASA, asteroids larger than about 330 feet (100 m) in diameter are only expected to strike Earth roughly once every 10,000 years. Meanwhile, nearly apocalyptic asteroids, those larger than about 0.6 miles (1 km) wide, are expected to strike our planet once every several hundred thousand years or so.
We have a rough guide to how much damage an asteroid can do, depending on its size.
Small asteroids: Less than 80 feet (25 m)
These usually burn up upon entering the atmosphere, occasionally resulting in meteors, or, at worst, small meteorites.
Medium-sized asteroids: 80 feet (25 m) to 360 feet (140 m)
An asteroid in this category can cause significant local damage. The Chelyabinsk event falls into this category.
Large asteroids: 360 feet (140 m) to 0.6 miles (1 km)
These could cause regional destruction, with such a collision causing devastating consequences, from fires to tsunamis to potential climate changes. Fortunately, these events are rare, estimated to occur every 10,000 years or so.
Massive asteroids: Greater than 0.6 miles (1 km)
These are the most terrifying. A collision with an asteroid of this size would be globally catastrophic, potentially causing mass extinctions. For instance, the asteroid impact that is believed to have helped cause the demise of the dinosaurs was roughly 6 miles (10 km) in diameter. Thankfully, these strikes are exceedingly rare, with a significant impact estimated to occur only once every million years or so.
Astronomy
Of course, those are probabilities. Statistically, the odds of rolling a one on dice is one in six, but that doesn’t mean that every sixth throw will be a one. Similarly, we might get really unlucky and get a big asteroid long before it’s “due”.
Which is exactly why it’s so important for bodies like NASA to keep a watch for the buggers. We’ll need all the time we’ve got, after all, to train a team of miners to be astronauts.