In May, a group of international scientists assembled near Washington, D.C., to tackle an alarming problem: what to do about an asteroid hurtling toward Earth.
Astronomers at a mountaintop observatory in Hawaii had spotted an 800-foot-wide asteroid, dubbed 2019 PDC, when it was 35 million miles away. By asteroid standards, it was relatively small—not even close to the six-mile-wide piece of space rock believed to have wiped out the dinosaurs 65 million years ago. Still, this asteroid was traveling at 31,000 miles an hour, and if it hit Earth, the impact could release the equivalent of 500 megatons of TNT—about 10 times more powerful than the largest nuclear weapon ever built.
Scientists at NASA’s Jet Propulsion Laboratory calculated that the big rock was headed toward Denver. Unless the asteroid could be deflected, two million people would have to be relocated, and the city would be obliterated.
All of this was hair-raising but, fortunately, not real: The scientists were participating in a highly dramatized but scientifically plausible “hypothetical asteroid impact scenario” at the International Academy of Astronautics’
sixth Planetary Defense Conference, held in College Park, Md.
The sky wasn’t falling this time, but the underlying questions are still urgent. Many scientists argue that the most effective way to deal with a threat from a small asteroid would be to send up an unmanned spacecraft armed with a nuclear explosive device (they hesitate to call it a bomb) to blow it up or nudge it off course. Nuking an incoming asteroid is also the preferred Hollywood method—it worked spectacularly well for Bruce Willis in the exciting but scientifically challenged
1998 film “Armageddon”—but the nuclear option faces serious hurdles in the real world. Sending nuclear weapons into space, even to save Denver, makes lots of people nervous and could violate international treaties governing the militarization of space.
Bruce Willis tries to save the Earth from an incoming asteroid in ‘Armageddon’ (1998). Photo: Touchstone Pictures/Everett Collection
So after some heated debate, the scientists assembled in Maryland decided to deploy a fleet of unmanned, nonnuclear
“kinetic impactor” spacecraft against the incoming asteroid. Kinetic impactors are essentially cannonball technology: You pack a spacecraft with a payload of solid metal and then crash it head-on into the asteroid, in hopes not of destroying it but of reducing its speed by a tiny fraction. That way, by the time it reaches its predicted rendezvous point with Earth, our planet will have already moved on in its orbit, and the asteroid will fly harmlessly by.
At least in theory. In the Maryland scenario, NASA, the European Space Agency, Japan, Russia and China all launched hastily designed and untested kinetic impactor ships. Three of them smashed into the asteroid. The main body of the asteroid was deflected and would miss Earth. Denver was saved! Unfortunately, one of the kinetic impacts inadvertently broke off a 200-foot-wide chunk of the asteroid—and that hurtling fragment was now on track to hit New York City.
The only hope was to destroy the fragment with a nuclear device. But existing ground-launched, nuclear-armed ballistic missiles weren’t designed to take on an asteroid in space, and there simply wasn’t time to launch a nuclear-armed spacecraft to intercept the asteroid chunk. New York would just have to take the hit. Millions of people were evacuated, the asteroid exploded in a fireball over Central Park—and Manhattan was wiped off the map.
Mercifully, Manhattan is still very much with us. But the war game was a reminder that asteroid defense isn’t science fiction but a serious and necessary venture.
True, the chances of a civilization-destroying asteroid impact are exceedingly small, at least in the foreseeable future. Asteroid strikes that cause regional devastation and catastrophic global climate change occur, on average, only about once every 100,000 years or more. On the other end of the scale, Earth is routinely bombarded by small asteroids that almost always burn up or blow up high in the atmosphere, creating meteors or fireballs that are visually spectacular but pose little or no danger. In December 2018, for example, a 30-foot-wide asteroid exploded in the atmosphere over the Bering Sea with the explosive force of a dozen Hiroshima atomic bombs—but except for a few satellites and sensor systems, no one noticed.
The most immediate threat isn’t from the largest or smallest asteroids but from those in between. Over the past two decades, asteroid hunters with NASA and other international space agencies have identified and tracked the orbits of more than 20,000 asteroids—also known as near-Earth objects—that pass through our neighborhood as they orbit the sun. Of those, about 2,000 are classified as potentially hazardous—asteroids that are large enough (greater than 150 yards in diameter) to cause local destruction and that come close enough to Earth to someday pose a threat.
The good news is that scientists don’t expect any of these known asteroids to collide with Earth within at least the next century. Some will come pretty close, though:
On an unlucky Friday the 13th in April 2029, the thousand-foot-wide asteroid Apophis will pass a mere 19,000 miles from Earth—closer than the satellites that bring us DISH TV.
But here’s the bad news: Hundreds of thousands of other near-Earth asteroids, both large and small, haven’t been identified. We have no idea where they are and where they are going.
On Feb. 15, 2013, a relatively small, 60-foot-wide asteroid traveling at 43,000 mph exploded in the atmosphere near the Russian city of Chelyabinsk, sending out a blast wave that injured 1,500 people. No one had seen the asteroid coming.
We need to find and track these unknown invaders as soon as possible. But while NASA’s “planetary defense” budget has been steadily increasing over the past decade, the $150 million allocated in 2019 for asteroid detection, asteroid tracking and related programs amounts to less than 1% of the space agency’s $21.5 billion budget.
Nor is it clear that we could deflect a small but dangerous asteroid heading our way even if we did spot it. No asteroid-deflection method has ever been tested in real-space conditions—and, as the conference’s war game demonstrated, using untested technology always entails a risk that the mission could go terribly wrong.
In 2021, NASA intends to launch its
Double Asteroid Redirection Test mission to try the kinetic impactor deflection method against a nonthreatening asteroid called Didymos. More tests will be required before we can achieve even a modest planetary defense capability. (Because of legal and political objections, NASA has no plans to test nuclear-explosive asteroid-deflection methods in space.)
Over its 4.5 billion-year history, Earth has been hit millions of times by powerful asteroids, and it will inevitably be hit again—whether two centuries from now or next Tuesday.
So it isn’t a question of whether humankind will have to confront the prospect of a destructive asteroid hurtling our way; it is only a question of when.