Apophis will pass Earth closer than any asteroid in decades, but don't panic

There are a lot of asteroids in the news lately, but the asteroid has been gaining considerable notice since its discovery in 2004. The asteroid, which is more than 1,100 feet (about 340 meters) across and is traveling more than 10,000 miles an hour, will pass closer to Earth than any asteroid this decade.

Understandably, this is cause for alarm for many who have grown up on stories of dinosaur-killers and civilization-ending asteroid impacts from science fiction, something that doesn't come purely from our imagination.

Given that Apophis will come within around 20,000 miles of Earth, which is closer than many geostationary satellites currently in orbit, it's natural to ask, will the Apophis asteroid hit Earth? If it did, when, where, and what impact would it have, and why do we believe that we're safe for the next few decades, at the very least? 

Will Apophis asteroid hit Earth?

The short answer is no, asteroid Apophis will not hit the Earth, and it's important to get that out of the way early.

Near-Earth Objects (NEOs) are high-priority research for government agencies around the world like NASA and the ESA, which is understandable given their potentially apocalyptic impacts. Even "modestly" sized asteroids of less than 100 meters long can hit the Earth with the force of nearly two hundred atomic bombs detonated in a single instant, and there is ample evidence that such impacts were very common throughout Earth's history.

This is one of the reasons why astronomers keep such a close eye on near-Earth asteroids, namely those that pass within about 1 million miles of Earth, but especially those that come closer than the orbit of the Moon. Known as a Lunar Distance (LD), this is about 239,000 miles or about 384,000 kilometers, and while that sounds like it's really far away, this is nothing in cosmic terms.

Asteroids passing within this distance are especially dangerous because they obviously pass through the influence of Earth's gravity and so have a likelihood of being pulled in towards the Earth's surface. How much they are pulled in could be the difference between life and civilization-ending catastrophe, so any identified potential threat is actively monitored.

Fortunately, this is why we know that we don't have to worry about asteroid 99942 Apophis, as it is officially known, for at least the next century. When it was first discovered, its orbit was predicted and showed it was coming uncomfortably close to Earth in 2029, making it something of a doomsday rock in the popular consciousness.

"When I started working with asteroids after college, Apophis was the poster child for hazardous asteroids," said Davide Farnocchia, of NASA's Center for Near-Earth Object Studies (CNEOS), part of NASA's Jet Propulsion Laboratory (JPL), in a NASA statement announcing the downgraded risk.

"With the support of recent optical observations and additional radar observations," "the uncertainty in Apophis' orbit has collapsed from hundreds of kilometers to just a handful of kilometers when projected to 2029. This greatly improved knowledge of its position in 2029 provides more certainty of its future motion, so we can now remove Apophis from the risk list."

What does NASA say about Apophis?

As of now, NASA says that Apophis won't pose a threat to Earth until well after its 2068 approach. "A 2068 impact is not in the realm of possibility anymore," Farnocchia said, "and our calculations don't show any impact risk for at least the next 100 years."

"Although Apophis made a recent close approach with Earth," said JPL scientist Marina Brozovic, who led the radar campaign at the Deep Space Network's Goldstone Deep Space Communications Complex in California that refined Apophis's orbital characteristics, "it was still nearly 10.6 million miles (about 17 million kilometers) away. Even so, we were able to acquire incredibly precise information about its distance to an accuracy of about 490 feet (150 meters)."

"This campaign not only helped us rule out any impact risk," Brozovic added, "it set us up for a wonderful science opportunity."

Using Goldstone in collaboration with the Green Bank Telescope in West Virginia to better image Apophis at a distance of 44 LD, the asteroid hunters were able to capture a relatively clear picture of Apophis at a resolution of a little less than 39 meters per pixel, "which is a remarkable resolution, considering the asteroid was 17 million kilometers away, or about 44 times the Earth-Moon distance," Brozovic said. "If we had binoculars as powerful as this radar, we would be able to sit in Los Angeles and read a dinner menu at a restaurant in New York."

The researchers hope that this radar and newer equipment will help them get an even clearer look at the asteroid as it comes within about 18,000 miles of Earth in 2029. At this distance, Apophis will be visible to the naked eye to observers on the ground in the Eastern Hemisphere.

What would happen if asteroid Apophis hit Earth?

Ok, but what would happen if asteroid Apophis hit Earth? At more than 1,100 feet across, according to recent estimates, this would be a very devastating impact, but not the worst the Earth has experienced—or even humans for that matter.

For context, the Tunguska impactor, which is believed to have been an asteroid about 90 feet across that exploded over Russian Siberia in 1908, produced a roughly 12-megaton blast that flattened Siberian forests for hundreds of miles around ground zero. This is the energy of about 185 Hiroshima bombs, and even though it was an atmospheric detonation, it would have been catastrophic for anyone caught in the radius of the blast that exploded toward Earth like a cosmic shotgun. 

Possibly the only eye-witness to the event, who was sitting in a chair in front of a shop about 40 miles away from ground zero, saw the explosion light up the sky before being blown out of his chair by a blast of air so hot he believed he'd caught fire.

"Suddenly in the north sky… the sky was split in two," he told investigators, "and high above the forest the whole northern part of the sky appeared covered with fire… At that moment there was a bang in the sky and a mighty crash. The crash was followed by a noise like stones falling from the sky, or of [artillery] firing."

This was forty miles away from a suspected asteroid that was just 90 feet across. Apophis is 340 feet across or nearly four times the size, which would produce an explosion of just around 850 megatons. Not all of that energy would hit the Earth, as much of it would produce an atmospheric shockwave, but this would be one of the most—if not the most—devastating explosions in the historical record.

Still, this wouldn't be anything close to the dinosaur killer that hit off the Yucatan peninsula about 65 million years ago. That impactor was several miles across at least, and even though it wiped out the larger non-avian dinosaurs, life on Earth still survived. Human civilization would almost certainly survive an impact from asteroid Apophis, though we definitely wouldn't be in the best shape afterward, especially in areas around the impact itself. 

While it's entertaining to play out an impact scenario like this, it's important to remember that while NASA takes these NEOs seriously, Apophis at least is not a threat to humans for a long while yet. There may be other dangers lurking out there unseen, but Apophis isn't one of them.