As much as the science room diagrams and quaint ball and stick models of our solar system make it look like we’re all alone out here, that couldn’t be farther from the truth. The orbital disk around our sun is a busy place full of planets, moons, asteroids, and an unfathomable amount of space debris. Comets zoom past on highly-elliptical orbits, slingshotting around the sun and launching themselves out past Pluto. With kilometers upon kilometers of space between objects, most comets have a reasonably unobstructed path, but sometimes, things get rough.
A dramatic breakup
Back in the 1990s, Jupiter made the news as the comet Shoemaker-Levy 9 passed a little too close to the gas giant on its way to the center of the solar system. The planet’s gravitational pull ripped the comet apart, shattering the hunk of rock and ice and sending it cascading in hundreds of pieces into the swirling clouds of our most massive solar sibling. For many, that cataclysmic impact was a wake-up call: What if it happens to us?
Granted, Jupiter has a lot stacked against it that Earth doesn’t have to worry about. For starters, we sit comfortably close to the center of the solar system, protected by an orbiting wall of rocks called the asteroid belt. Not only would a comet have to make it through several other obstacles, including Jupiter, before it could take a whack at us, but it would have to hit a much smaller target. Jupiter is 2.5 times larger than every other planet in the solar system combined. Earth, by comparison, is a tiny speck with a pathetically small magnetic influence on the space around it.
Though the odds of Earth colliding with a comet are infinitesimally small, the chances are nonzero. Every day, tiny pieces of rock and debris hit our atmosphere. If you go outside on a clear, dark night and stare up at the sky, you’ll almost certainly see at least one “shooting star.” These meteorites are often too small to make it to the surface, and the elegant displays of light we see in the sky are the trails left behind as they disintegrate in the atmosphere. Meteor showers are a great time to see this phenomenon. More flashy examples are fireballs, which can sometimes be seen during the day. These spectacular lights are caused by large meteorites passing through the Earth’s atmosphere, and many of them make it to the surface. While they pose little threat, there are bigger things out there that could cause a lot more damage than a tennis-ball-sized hunk of space rock.
Earth is surrounded by over 20,000 potentially-problematic bits of rock. More than 800 of them are over a kilometer long. NASA keeps tabs on these near-Earth objects, and have been working on plans for how to deflect these asteroids so that humanity doesn’t go the way of the dinosaurs in an uncomfortably literal sense. If the Shoemaker-Levy 9 impact had happened to Earth, it would have caused a mass extinction like the planet hasn’t seen in 66 million years. In 2018, the comet 46P/Wirtanen made its closest approach yet, sliding past us at 30 lunar distances. Earth is in for several other close encounters, some even more daring than 46P/Wirtanen, in the coming decades. Looking to avoid a Shoemaker-Levy 9 event here on Earth, scientists want to know how we can prevent a potential doomsday event.
The first line of defense against humanity-annihilating asteroids and comets is identifying them before they’re too close to stop. NASA’s Center for Near Earth Object Studies does just that. Potential hazards are identified, cataloged, and tracked, so we know at all times where those bits of rock are and what, if anything, we need to do about them. Second is the DART project. The Double Asteroid Redirection Test is our second stage of defense against wayward space rocks. A probe will be launched in 2021 on an intercept course with the binary asteroid system, Didymos. Its goal is to study orbital patterns of asteroids around the sun by using the small-scale model provided by Didymos A and its moonlet, Didymos B. NASA scientists hope to learn how we can best redirect the orbital course of an Earthbound asteroid by making a series of calculations on this distant system and extrapolating to understand a larger-scale scenario.
What are the odds?
Despite the alarmingly large number of giant rocks careening through space in orbit around our sun, most are too small or soft to pose a threat. Plenty of impacts happen every year, though most are never recorded as they touch down somewhere in the ocean or remote areas such as northern Siberia and Antarctica. Despite the frightening-sounding numbers, an impact from a meteor between 330 feet and half a mile in size wouldn’t pose much of a threat to humanity unless it touched down in a major metropolitan area. An asteroid about a mile in diameter, however, could send enough dust into the atmosphere to cause climate disruptions similar to a massive volcanic eruption, leading to crop failures. Extinction-level impacts require objects at least three miles in diameter. Although there are about 15 comets that travel through Earth’s orbit ranging from half a mile to six miles long, none of them are of any present threat to us. If they manage to pass too close to Jupiter, the gas giant’s gravitational pull could potentially swing them our way and spell bad news. The chances of that happening are slim to none, so you can rest easy knowing that humanity probably won’t be wiped out by a meteor anytime soon.