How Often Do Space Objects Hit Earth? A Primer

The year 2020 is clearly out to make its mark in a big way: a global pandemic, massive wildfires across the Western United States,  huge demonstrations for social justice around the globe.

Here’s another one: a record observed near-miss of Earth by a rock from space. 

On Aug. 15, at 9:08 p.m. PDT, the robotic sky-scanning survey telescope at the NSF/NASA-funded Zwicky Transient Facility at Palomar Observatory in California captured an image of a previously unknown asteroid, 10-20 feet in diameter, whizzing by Earth at a speed of 8 miles per second. The image was taken only six hours after the rock’s closest approach, 1,830 miles from Earth’s surface over the southern Indian Ocean, closer than any previously known near-Earth asteroid, or NEA.

A student in India, examining images captured by the ZTF telescope in California, first spotted and reported the object.

Too Close for Comfort, Too Small to Notice?

The asteroid, named 2020 QG, set a record for the nearest miss of the Earth ever observed — just 1,830 miles or about a quarter of Earth’s diameter — yet it wasn’t spotted until after it passed!

This is normal for encounters with near-Earth asteroids of this size. Too small to be discovered until getting breathtakingly close to the Earth, these car-sized chunks of rock, often fragments from collisions between larger asteroids much farther away that took place long ago, lurk invisibly throughout the solar system. Estimates place their population in the hundreds of millions, though most of them pass no closer to Earth than the distance to the moon.

Asteroid 2020 QG may be small enough to sneak up on us unnoticed, but it would also do little damage, if any, if it did hit Earth. It would mostly burn up and disintegrate during its high-speed dash through our atmosphere, with possibly some small fragments reaching the ground. Since three-quarters of Earth’s surface is covered by ocean, such remnants often fall into water.

The Chelyabinsk meteor that exploded and mostly disintegrated in the sky over Russia in 2013 was at least three times the size of 2020 QG. It caused a powerful shock wave that broke windows, tumbled brick walls, and injured almost 1,500 people. Luckily, there were no fatalities. Despite these effects, only a few small fragments survived to reach the ground.

The Chelyabinsk meteor, by the way, was not detected until it entered our atmosphere and announced itself in an aerial blast with an estimated explosive power between 400-500 kilotons. 

Space Stuff Hitting Earth: How Concerned Should We Be?

It might surprise you to learn that space rocks and other debris fly close to and even impact the Earth all the time.

Every day about a 100 tons of space rock filters down to Earth’s surface, most of it in the form of dust grains that vaporize in the atmosphere and rain down as microscopic specks. You can see the larger particles flash through the night sky as meteors if you’re patient enough, but most of this space debris showers down on us unseen and unfelt.

Larger chunks of rock and metal that reach the ground before burning up completely are called meteorites, and are prized finds by collectors who can distinguish them from Earth rocks. Some meteorites can fetch a good price from the right buyer.

Scientists from the Jet Propulsion Laboratory say that about every 10,000 years, on average, an asteroid in the 100-meter (328 foot) class strikes the Earth, causing big problems in the region it hits: a huge impact blast and shock wave, or a tsunami, if the object hits the ocean. The famous “Meteor Crater” in northern Arizona, east of Flagstaff, is a near mile-wide, 600-foot-deep impact hole. It was formed 50,000 years ago when an asteroid measuring about 160 feet across hit the ground. Though this asteroid would have wreaked havoc across the local Pleistocene landscape, there were likely no global effects from the blast.

Every few hundred thousand years a larger object, half a mile or more across, collides with the Earth. Objects of this size produce global complications, throwing dust and other debris into the atmosphere around the planet, which can block off sunlight, cause acid rain, and ignite firestorms with the heat of reentering debris. These larger collisions also cause devastating shock waves and tsunamis.

The global effects of these major impacts have caused mass extinctions of plant and animal species. Take it from the dinosaurs, the poster-children of global collision catastrophe, who were wiped out by the impact and aftermath of a six-mile-or-more wide object that struck the northern tip of the Yucatan Peninsula about 65 million years ago, forming the Chicxulub impact crater, now mostly buried under sediment.

What’s to Be Done?

With all that space rock flying around out there, are we doing anything to protect us from it?

In short, yes. Since at least 1994, NASA has worked to discover and characterize asteroids and comets that have the potential to collide with Earth and inflict significant damage.

In 2005 the U.S. Congress handed NASA the goal of finding 90% of all potentially hazardous near-Earth asteroids, ones larger than 460 feet across, by the end of 2020. NASA’s NEO (Near-Earth Object) Observations Program is still working toward this target, using evolving technologies.  Fortunately, asteroids of this size are much easier to detect than small ones like 2020 QG, and they can be discovered and tracked years before coming close to Earth.

This gives us time to predict future collisions, and possibly do something to prevent them.  

On the ground, advance warning of the location and magnitude of a projected impact by an incoming asteroid could help us prepare, by evacuating the threatened region, for instance. 

Scientists are also thinking how we might alter the course of a threatening asteroid, turning a predicted collision into a near-miss scenario. With enough advance notice of a likely major impact — and we’re talking years — even a relatively small “nudge” to an asteroid’s trajectory could ultimately make the difference between hit and miss here on Earth. 

This may sound like something out of science fiction, but one concept being explored is the “gravity tractor,” a massive robotic spacecraft placed near an asteroid, which gives it a small but constant pull via its gravitational attraction. Flying alongside an asteroid, the spacecraft would use low-powered engine thrust to gradually “tug” the rock with mutual gravitational attraction, slowly steering the asteroid away from its Earth-bound path — kind of like a tiny tugboat guiding a huge ship onto a safe course.  

Sleep Well Tonight

Small asteroids like 2020 QG will continue to buzz and even hit the Earth multiple times each year. They will also often fly by or disintegrate in our atmosphere unnoticed.

But, NASA’s ongoing observation of near-Earth objects is a powerful tool for predicting when an asteroid or comet might impact the Earth. The good news is that no major impacts are foreseen anytime soon. 

Coming up in November: Election Day Near-Earth Asteroid.