Asteroid ‘Out of Nowhere’ Flew By Only 45,000 Miles From Earth

Visual of orbits of Earth (blue), the moon around Earth and Asteroid 2019 (white line).

On July 25 a hunk of space rock named Asteroid 2019 flew uncomfortably close to Earth. And no one seemed aware it was coming.

The rock was about 187 to 427 feet wide and it moved on a course that brought it within 45,000 miles of the Earth’s surface. That’s one-fifth of the distance to the moon. One scientist stated: “It’s probably the largest asteroid to pass this close to Earth in quite a number of years.”

As the Washington Post reported:

This asteroid wasn’t one that scientists had long been tracking, and it had seemingly appeared from “out of nowhere,” Michael Brown, a Melbourne-based observational astronomer. … According to data from NASA, the craggy rock was large, … and moving fast along a path that brought it within about 73,000 kilometers (45,000 miles) of Earth. That’s less than one-fifth of the distance to the moon and what Duffy considers “uncomfortably close.”

“It snuck up on us pretty quickly,” said Brown, an associate professor in Australia with Monash University’s School of Physics and Astronomy. He later noted, “People are only sort of realizing what happened pretty much after it’s already flung past us.”

The asteroid’s presence was discovered only earlier this week by separate astronomy teams in Brazil and the United States. Information about its size and path was announced just hours before it shot past Earth, Brown said.

So why was the asteroid not seen?

First, there’s the issue of size, Duffy said. Asteroid 2019 OK is a sizable chunk of rock, but it’s nowhere near as big as the ones capable of causing an event like the dinosaurs’ extinction. More than 90 percent of those asteroids, which are more than half a mile wide or larger, have already been identified by NASA and its partners.

“Nothing this size is easy to detect,” Duffy said of Asteroid 2019 OK. ?You’re really relying on reflected sunlight, and even at closest approach it was barely visible with a pair of binoculars.”

Brown said the asteroid’s “eccentric orbit” and speed were also likely factors in what made spotting it ahead of time challenging. Its “very elliptical orbit” takes it “from beyond Mars to within the orbit of Venus,” which means the amount of time it spends near Earth where it is detectable isn’t long, he said. As it approached Earth, the asteroid was traveling at about 24 kilometers per second, he said, or nearly 54,000 mph. By contrast, other recent asteroids that flew by Earth clocked in between 4 and 19 kilometers per second (8,900 to 42,500 mph).

“It’s faint for a long time,” Brown said of Asteroid 2019 OK. “With a week or two to go, it’s getting bright enough to detect, but someone needs to look in the right spot. Once it’s finally recognized, then things happen quickly, but this thing’s approaching quickly so we only sort of knew about it very soon before the flyby.”

The last-minute detection is yet another sign of how much remains unknown about space and a sobering reminder of the very real threat asteroids can pose, Duffy said.

“It should worry us all, quite frankly,” he said. “It’s not a Hollywood movie. It is a clear and present danger.”

Duffy said astronomers have a nickname for the kind of space rock that just came so close to Earth: “City-killer asteroids.” If the asteroid had struck Earth, most of it would have probably reached the ground, resulting in devastating damage, Brown said.

“It would have gone off like a very large nuclear weapon” with enough force to destroy a city, he said. “Many megatons, perhaps in the ballpark of 10 megatons of TNT, so something not to be messed with.”