Less than eight years from now, on Friday the 13th of April 2029, a 370-meter-wide asteroid called Apophis will pass by the Earth, coming nearer to our planet than geosynchronous satellites. But despite the calendrical bad omen, this will be a lucky day: Apophis will not strike our planet—this time, anyway (its orbit ensures Apophis will visit us again in 2036, 2051, 2066 and so on). In 2029, this asteroid’s passage will instead be a cosmic close shave, the equivalent of a speeding bullet brushing against the hairs on your head—in which the “bullet” carries the equivalent impact energy of all the world’s nuclear arsenals combined.

Such dangerous liaisons are shockingly frequent. On September 30, 2054 and September 23, 2060, an even larger asteroid packing an even more potent wallop, the half-kilometer-wide Bennu that was recently visited by NASA’s OSIRIS-REx spacecraft, will also swoop close to Earth.

Neither Bennu nor Apophis are large enough to be existential threats—their impacts could destroy cities and devastate geographical regions but would not send humanity spiraling into extinction like the 10-kilometer-wide impactor that snuffed out the dinosaurs some 66 million years ago. Still, these asteroids remain especially worrisome because the further into the future we look, the harder it becomes to know with certainty whether or not any particular encounter will result in a disastrous impact event. Both objects are so-called “gravitational keyhole” threats—the possibility that during a close pass they will traverse a small, specific region of near-Earth space in which our planet’s gravity tweaks their trajectories just so to cause a future encounter to end in an Earth impact. In short, they each pose a chronic-but-hazy threat, one so insidious that it could lull us into a state of false complacency about the all-too-real risks.

We do not have to accept this anxiety-inducing status quo. Rather than merely biting our nails each time these and other potentially hazardous space rocks fly by, we should consider another option, a “plan B.”

A Last Line of Defense

Our current approach to planetary defense boils down to wishful thinking that nothing bad will happen soon and that we will eventually figure out a solution. So far, we have been focused on “situational awareness” in order to understand the threats. This is necessary but not sufficient for actually protecting the Earth from asteroids. And the standard next step—deflecting potential threats so they will not hit us—has problems of its own, chiefly that successful deflection often requires intervention many years in advance. In this mode, many space rocks found hurtling toward imminent impact with Earth would already have slipped through all our defenses. There is, however, another way, one that promises to radically change our ability to protect ourselves.

The basic principle is simple to understand. Imagine you are Roger Rabbit playing a dangerous game of chance, choosing between two unopened doors. Behind door number one you get a 500-kilogram grand piano being dropped on your head from a height of one kilometer. Behind door number two you get 500 kilograms of foam balls dropped on you from the same height. Which do you choose? If you are Roger you might choose door number one, but a Scientific American reader would choose door number two. Why? Both possess the same mass and potential energy, but basic intuition suggests that large numbers of foam balls will not cause the same damage to you as one piano. Fragmenting the mass into smaller portions ensures that each will carry far less energy, and will also allow the atmosphere to more effectively slow each fragment’s fall. This is a rather precise analogy to our proposed planetary-defense method, which we affectionately call “PI” (pronounced like π), which is short for “Pulverize It!”

A diagram of the authors’ proposed “Pulverize It!” planetary defense system.
A diagram of the authors’ proposed “Pulverize It!” planetary defense system. Rocket-launched “interceptors” (left) deploy ahead of an incoming asteroid (right), breaking it into smaller fragments that then disintegrate and burn in Earth’s upper atmosphere (bottom). Credit: Alexander N. Cohen (UCSB), Peter Cohen

Our idea (which is detailed in several technical papers submitted for peer-reviewed publication and available on our Web site and arXix) is to effectively pulverize any threatening asteroid into a large number of smaller fragments circa 10 meters or less in diameter. This is possible because asteroids have low surface gravity and most are easy to breakup and disperse. For all but the largest impactors (greater than a kilometer wide), such fragmentation could be achieved using barrages of nonnuclear interceptors launched from Earth or its vicinity using existing launch systems and associated technologies. Our same system using small nuclear penetrators is also an option for large threats.

Once fragmented, the incoming impactor’s energy would be efficiently converted into heat, sound and light by the Earth’s atmosphere, which would act much like a bulletproof vest absorbing a blast of buckshot. Our analysis shows this approach  works incredibly well at mitigating imminent threats: An impactor the size of the 20-meter-wide space rock that broke up over Chelyabinsk, Russia in February 2013 could be intercepted a mere 100 seconds prior to impact, whereas one the size of the Tunguska impactor (50 meters in diameter) would require interception some five hours prior to impact. Something the size of Apophis could be dealt with 10 days prior to striking Earth, and something as large as Bennu would need to be fragmented 20 days in advance. These are extraordinarily short intercept times compared to deflection approaches. Even shorter times would be enabled with more energetic interceptors if required.

Of course, a knowledgeable reader may realize we have not told the entire story. Both the previously mentioned Chelyabinsk and Tunguska impacts were airburst events, after all, and in both cases surrounding natural and artificial structures sustained significant damage. This damage chiefly came from the sonic-boom-like acoustic blast waves emitted by the bodies as they broke apart in the atmosphere.

Our PI approach would not eliminate airbursts, but by shattering incoming bodies before they enter the atmosphere the resulting small fragments would be spread out over larger geographical areas and would each produce much smaller blast waves and critically that arrive at different times. Just as you’d expect to have bruising and soreness from a bulletproof vest absorbing a buckshot blast, so too would one expect some damage to still occur on the ground from the acoustic shock wave and associated flash of light and heat as a threatening asteroid’s tumbling fragments burned up in the skies overhead. But this damage would be slight in comparison to the alternative; for a Chelyabinsk-like impactor, a person on the ground would experience a series of loud “booms” and see a series of optical flashes—a “sound and light show” with some broken windows, rather than a cataclysm that lays waste to a city, region or country.

Demonstrations and Detections

Despite our system’s ability to leverage existing technologies and launch vehicles, its creation would nevertheless require major investments. In short, this would be expensive. But even so, the cost-benefit ratio is remarkably favorable given the almost incalculable damage that would be associated with failing to prevent an asteroid strike.

Furthermore, its creation would allow us more flexibility in dealing with known impact threats, now and on into the distant future. Much as mass vaccination programs are used to proactively prevent against pandemics, PI offers a way to proactively address many asteroids that, while potentially hazardous in their Earth-crossing orbits, pose no immediate threat. While likely a controversial approach, it is little different from other proactive threat management we practice in life. We could mitigate threats such as Apophis and Bennu on any given close pass before they spark full-blown emergencies. It is within our power to do so. Whether we do so or not is not just a technical issue but one of policy and cooperation and common agreement. This is an area where international cooperation could benefit the entire planet; much like the current emphasis on collectively solving Earth’s climate and pandemic crisis, we come together to solve the “impact” crisis, too.

Mitigating a Chelyabinsk-size threat could be done using a relatively small rocket that is not much larger than those developed to intercept intercontinental ballistic missiles. Mitigating Apophis or Bennu can be done with a single larger launcher such as NASA’s forthcoming Space Launch System, SpaceX’s Starship rocket, or even smaller vehicles carrying high-speed upper stages for rapid transit beyond the vicinity of Earth’s moon. Multiple interceptors would be desirable to boost chances of success. A future planetary defense system might deploy interceptors in orbit or on or around the moon for an “always at the ready” rapid response approach. In this sense a planetary defense system could be analogous to existing national missile defense systems.

PI has a logical test path, from ground demonstrations using asteroid “mock-ups,” to in-space testing on “synthetic targets,” all the way to disruption attempts for small, minimally threatening asteroids and other validating exercises before any actually threatening target is engaged and mitigated.

However, we cannot mitigate that which we cannot see. NASA and other space agencies are doing an excellent job of finding and tracking those asteroids that are significant threats, but currently these efforts are generally limited to objects typically larger than Apophis. There are many smaller as-yet-undetected threats that exist, as the Chelyabinsk airburst showed so clearly in 2013. Without a suitable, separately developed “early warning system,” PI and any other planetary defense method would offer suboptimal protection. PI is just one piece of this urgent puzzle: To properly protect the Earth, we must fully open more eyes on the skies.

For more information: www.deepspace.ucsb.edu/projects/pi-terminal-planetary-defense.