Ever since we discovered in the 1980s that a monstrous asteroid wiped out the dinosaurs,
we’ve been obsessed with the idea that the same thing could happen again. After all, the solar
system contains hundreds of millions of asteroids, and it seems like at any moment one could tumble
out of orbit and hurtle towards us. But how much time would we have to prepare if that happened?
This is Unveiled, and today we’re answering the extraordinary question:
how long would it take an asteroid to hit Earth?
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No asteroid could ever destroy Earth, thankfully, because none of them are big enough to
cause a planet-shattering impact. But many could wipe out life as we know it in the short term,
much like the asteroid that killed the dinosaurs 66 million years ago. That asteroid,
the Chicxulub Impactor, which landed on the coast of what is now Mexico’s Yucatan Peninsula,
was only about 6 miles wide going by the size of the crater, which is frighteningly small
considering just how many asteroids of this size or significantly bigger there are out
there. And some of the potentially hazardous asteroids watched by NASA, in particular one
called Apophis which has made headlines more than once for approaching Earth uncomfortably closely,
is only 1100 feet wide – that’s just 0.2 miles. NASA says there’s no danger of Apophis hitting us
in the next 100 years, but it’s true that a large asteroid definitely will hit Earth again one day.
But where might this asteroid come from? Well, excluding objects from beyond the solar system,
the absolute furthest point an asteroid could originate from is the Oort Cloud.
The Oort Cloud is a theoretical sphere of planetesimals beyond even the heliosphere,
surrounding the solar system like a shell of debris. Most long-period comets come from the Oort
Cloud and can take thousands or even millions of years to complete their orbits around the sun. The
Oort Cloud primarily contains ice and comets, but it does contain some heavy asteroids as well and
even a possible dwarf planet named Sedna – though there’s some debate about how to classify Sedna,
given its distance from the Sun. The inner edge of the Oort Cloud is thought to begin anywhere
between 2,000 and 50,000 astronomical units from the sun, and extend to between 100,000 and 200,000
astronomical units from the sun. One astronomical unit is the average distance between the sun and
Earth, which is roughly 93 million miles. 200,000 astronomical units is over three lightyears away!
So, how long would it take an asteroid coming from the outer edge of the Oort
cloud to crash into us here on Earth? Well, asteroids orbit the sun on average
at around 15.5 miles per second, or 55,800 miles per hour. Since, again, 1 AU is 93 million miles,
it would take an asteroid traveling at 55,800 miles an hour 1,666 hours or 69 days to go
1 AU. That means it would take 6.9 million days to travel from the edge of the Oort Cloud
if it’s 100,000 AU away, or 18,891 years. If the edge of the Oort Cloud is 200,000 AU away,
then doubling that number gets you the most extreme estimate: 37,782 years. That’s a very long
time for us to prepare, and by that point, it’s easy to imagine humans could have died out from
something else or even left Earth completely. At the nearest possible edge of the Oort Cloud, just
2,000 AU away, it would take a comparably fast 378 years. But that’s still plenty of time for us
to develop a countermeasure. So, if an asteroid fell out of its orbit in the Oort Cloud today,
and was traveling at an average speed, it would still take anywhere between 38,000 years and 380
years to reach Earth. Admittedly, it could travel faster or slower than this, though more likely
faster if you account for how acceleration speeds up the closer an object is to the sun.
Far closer to us is the Kuiper Belt. Unlike the Oort Cloud, the Kuiper Belt – like the
planets – orbits the sun on a relatively flat plane, rather than being a spherical cloud of
celestial bodies. It’s also significantly closer, ranging between 30 and 1,000 AU from the sun.
This means that at most, it’ll take 189 years for a Kuiper Belt asteroid to reach us,
and at the shortest, less than 6 years. Considering the Kuiper Belt is beyond Neptune,
that’s very worrying when we take a look at the solar system’s primary source of asteroids:
the asteroid belt between Mars and Jupiter. The asteroid belt is between 2.2 and 3.2 AU from
the sun, making it 1.2 and 2.2 AU away from Earth on average. At the shortest, that’s about 83 days,
and the longest, 152 days, or 5 months. That’s a lot more frightening than even the Kuiper Belt’s
6 years, which is starting to look like a very reasonable amount of time to prepare
for an asteroid impact. We also need to take into consideration exactly where Earth is in its orbit
relative to where the asteroid’s journey begins; for instance, if an asteroid 2.2 AU away from
the sun begins to head towards us, it’s possible that when it crosses Earth’s orbit, Earth might
be on the far side of the sun, an additional 2 AU away, and it would keep going straight into the
sun. It’s also worth noting that Earth’s orbit is huge; each year, Earth travels 584 million miles,
and more than 99.99% of Earth’s orbital path is empty at a given moment, which gives an asteroid
millions of miles of leeway. It’s been estimated that we’d need an
asteroid with a diameter of 60 miles to completely wipe out life on Earth. But the Chicxulub Impactor
was only 6 miles wide, so even a much smaller asteroid would do incredible damage to the planet.
And unfortunately for us, there are lots of asteroids floating around large enough to do
this – NASA estimates that there are as many as 1.9 million asteroids in the asteroid belt
with a diameter larger than 0.6 miles. And bear in mind, once again, that the tabloids’
favorite deadly asteroid, Apophis, is three times smaller than NASA’s lower limit. The asteroid belt
also has many objects that are hundreds of miles wide, including Ceres, the dwarf planet – though
it’s not likely that Ceres is going to fall into the sun anytime soon, or even ever.
With so many asteroids out there, we will have another large impact one day, even if it’s not
anytime soon, and we’ll need to be prepared when it happens. Luckily, not only are NASA and other
space agencies tracking asteroids but there are lots of publicly funded non-profit projects
aiming to fill in the gaps, like the Sentinel mission. Sentinel uses an infrared telescope
and boasts many people who previously worked for NASA and prestigious colleges.
Somebody will spot a world-destroying asteroid ahead of time. But one coming loose from the
Kuiper Belt still wouldn’t give us much of a heads-up if it came straight for Earth.
Many scientists are working on predictions of what would happen if an asteroid hit and
developing designs for countermeasures, but at the moment, the only realistic countermeasure
that we could pull off in the shortest time span – 83 days – involves nuclear bombs.
Blowing up a hazardous asteroid with enough nuclear weapons
is the only asteroid countermeasure currently in our wheelhouse and that could be deployed in such
a short amount of time. For once, Earth’s huge stockpiles of nuclear weapons will be beneficial;
the world has, in total, close to 14,000, and since an asteroid impact would affect everybody
it’s not hard to imagine the planet’s nuclear superpowers would work together to deal with
the threat. We even already have plans for absolutely huge nuclear devices solely for
the purpose of blasting a dangerous asteroid away. Other methods are more science-fiction,
with many researchers actually advocating for the use of a gravity tractor. We haven’t yet developed
any gravity tractors, which are large, artificial objects that would use their own gravitational
field to redirect a problem asteroid, but with enough R&D they could become viable options.
Despite the vast distances in space, many of the asteroids close to Earth
could find our planet worryingly quickly, and we don’t have much of a plan for when
the inevitable happens. And that’s how long it would take for an asteroid to reach Earth.
What do you think? Is there anything we missed? Let us know in the comments, check out these other
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