Were you one of the stargazers that spotted some of the beautiful Perseids this weekend? If so, we are here to sadly inform you that the annual event – one the most spectacular meteor showers of the year – may actually be a harbinger of doom.
The comet that these incandescent fragments come from, Swift-Tuttle, may end life on Earth as we know it. Ho hum. This is far from certain, by the way, and the chances of it ever happening are extremely low. So what's the deal?
Ol’ Swifty, discovered back in 1862, orbits our Sun once every 133 years. The comet actually gets closer to the Sun than the Earth does during its closest approach (perihelion), which ultimately means that it’s a potential impact risk for our pale, blue dot.
The next perihelion will be on July 26, 2126. Its stable orbit means it won’t be a threat then, but a few millennia or so later, we may be in trouble.
Forbes’ Ethan Siegel is suggesting that if Jupiter’s enormous and somewhat unpredictable gravitational field gives Swift-Tuttle the slightest of nudges, then it could adjust its orbit just enough for it to actually slam into the Earth. Considering the comet’s heart – its nucleus – is 26 kilometers (16 miles) across, this would be nothing short of apocalyptic.
So what would happen if a comet like Swift-Tuttle actually hit us? Would it be worse or better than an asteroid impact?
The worst possible scenario is a fast-moving, dense object hitting another dense object’s rocky surface; the lack of an atmosphere would not put much of a break on the incoming impactor, and a steeper angle would reduce the chances of it being a “mere” glancing blow.
Comets are arguably more dangerous for life on Earth than asteroids though. Asteroids generally travel at speeds of around 25 kilometers (16 miles) per second. Comets, on the other hand, rocket along at speeds of up to 70 kilometers (44 miles) per second. Assuming the impact site is the same, that extra speed means they have a lot more momentum, and therefore a lot more destructive power.
Here’s a good comparison: the asteroid that killed the dinosaurs, which was around 15 kilometers (9.3 miles) across unleashed the same amount of energy as roughly 100 million tonnes of TNT. That sounds like a lot, but it’s nothing compared to Shoemaker-Levy 9, the comet that hit Jupiter in the early 1990s.
This comet, which was originally “just” 5 kilometers (3.1 miles) across broke into multiple fragments. The largest fragment, G, slammed into Jupiter on July 18, 1994 at such a remarkable speed that this impact alone was 60,000 times more powerful than the asteroid that ended the age of the dinosaurs.
Swift-Tuttle is not only moving at typical comet speeds, but as aforementioned, it’s also quite sizeable. If it hit us, the energy unleashed would be around 10-28 times that of the dino-killing asteroid impact. It’s no Shoemaker-Levy 9, but it’s still catastrophic.
If you’re feeling glum about our prospects, then here’s the good news. The odds of it even hitting Earth during each orbit, interfering Jupiter or no, is one-in-500,000.
That makes it more likely than Yellowstone supervolcano erupting in any given year in some form of another, but here’s a strange stat to end on: if you’re American, you’re 14 times more likely to be run over by your own lawnmower than you are to see Swift-Tuttle wipe out humanity. So, all in all, we would advise enjoying the annual meteor shower and not worrying so much about space-borne killers.