On Friday, 13, 2029, Earth will have a fascinating and intimidating visitor in the form of the “God of Destruction” asteroid Apophis. This asteroid — named after the Egyptian serpent god of chaos and destruction Apep — is so large and will pass within 19,000 miles (30,600 kilometers) of Earth, so close to our planet that it can be seen with the naked eye.
New research suggests that if any other much smaller space rocks collide with Apophis, the asteroid, which is about as wide as the Empire State Building, could be redirected, and might narrowly miss Earth during later passes or, in extreme circumstances, even during the 2029 pass. But don't panic just yet.
The study's author, Paul Wiegert, an astronomer at the University of Western Ontario, told Space.com that the chance of Apophis being struck by a dangerous asteroid is very low. “The chances are essentially 1 in a million that an asteroid strike could deflect Apophis enough to put it at risk of a future collision after 2029, and only 1 in a billion that it could force Apophis to collide with Earth in 2029,” he said.
“I have calculated the chances that the asteroid Apophis, whose current path is calculated to safely pass by our planet in 2029, could be deflected onto a more dangerous path by an unexpected small asteroid impact,” Wiegert explained. “These are the same kind of small asteroids that sometimes appear as 'shooting stars' or 'fireballs' in our atmosphere and could strike Apophis unexpectedly.”
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Wiegert said that in his research, he had asked the question whether an asteroid would hit us during the period from 2021 to 2027, when our telescopes would not be able to observe Apophis, and whether this would deflect it enough to become dangerous.
He added, “Asteroid Apophis is essentially unobservable from now until 2027, because it is in the daytime sky, and so it could hit us without us even being immediately aware of the event.”
A chaotic event could mean the rise of Apophis
Discovered in 2004, Apophis (full name Apophis 99942) instantly shot to the top of tables that measure the threat of so-called potentially hazardous asteroids (PHAs) — asteroids 460 feet (140 meters) or more in width that come within 20 lunar distances of Earth.
Because of Apophis' size and its trajectory, it has topped the European Space Agency's (ESA) PHA “impact risk list” and NASA's Sentry risk table for nearly two decades.
That was until the asteroid's very close pass in March 2021 led NASA scientists to realize that Apophis wouldn't actually hit Earth for at least 100 years.
Now, this new research shows that chance encounters with other space rocks could redirect the 1,000-foot-wide (305-meter-wide) Apophis, putting it on a collision course with Earth in 2029 or beyond, meaning it could return to the top of the threat table.
Wiegert calculated what the impact would be if objects of different sizes hit Apophis, and then determined the probability of such an impact using the number of objects of these sizes that hit Earth each year.
“An asteroid about 60 centimeters (24 inches) in diameter could, if it hit Apophis from just the right direction, put Apophis on a collision course after 2029. But to raise the risk of a collision in 2029, an asteroid about 3 meters (10 feet) in diameter would have to hit Apophis, again, in just the right direction,” Wiegert said. “Such collisions are extremely rare.”
When considering the angle needed for Apophis to hit Earth in the future, Wiegert found that the chance of the asteroid turning to hit our planet after 2029 is 1 in 1 million. The chance of an asteroid about 3 meters wide turning around to hit Apophis in 2029 and hitting Earth was 1 in 2 billion.
These low odds are fortunate because Apophis would cause enormous damage if it hit Earth. The Planetary Society has estimated that if Apophis hit Earth it could release energy equivalent to more than 1,000 megatons of TNT, the equivalent of exploding tens or hundreds of nuclear weapons.
This could cause devastation within a radius of hundreds of miles. While it would be nowhere near the devastation caused by the asteroid that wiped out the dinosaurs and two-thirds of all species on Earth 65,000,000 years ago, millions of people would die if Apophis hit a densely populated metropolitan area.
If we know that Apophis is going to hit Earth during its next pass in 2029, 2036 and 2068, we will have some options to deflect it. For example, just as a small impact can move the God of Destruction space rock toward Earth, another small collision can shift its trajectory away again if space agencies have enough warning.
In 2022, NASA will try this idea, striking the asteroid system Dimorphos and Didymos with the Double Asteroid Redirection Test (DART) impactor and measuring the change in its momentum.
“It's possible that a mission like DART could be used to guide Apophis back on a safe path, but exactly how and whether this would work is still to be worked out,” Wiegart said.
Other deflections range from the forceful, such as attacking the asteroid with a nuclear weapon, to the spectacular, such as painting one side of the asteroid black, causing it to absorb more solar radiation, shift its center of mass, and alter its trajectory. These methods are scientifically well established, but they remain to be tested.
“It's hard to say what other methods will work, but scientists are seriously considering our options,” Wiegert said.
Although Apophis will not be able to live up to its fearsome name by colliding with Earth in 2029, its scientific impact will be immeasurable. Plans have already been made to use spacecraft and satellites to meet this asteroid on its closest approach to our planet.
“The close passage of Apophis in 2029 is a natural milestone for our planet, as we enter a time where we as a global community have the ability to avoid catastrophic asteroid impacts, such as the one that wiped out the dinosaurs,” Wiegert concluded. “The astronomical community as a whole continues to think about hazardous asteroids like Apophis, as well as potential next steps.”
Wiegart's Apophis research was published in The Planetary Science Journal.