Imagine a scenario where we detect an asteroid heading straight for Earth. Imagine that it will arrive in a couple of days, or worse, only a few hours. What could be done to put a stop to it?

With such short notice, we may be able to protect ourselves and the planet. But first, we’d have to test and create the necessary infrastructure.

When it comes to asteroids, humanity is engaged in a process of discovery. We send spacecraft to visit them, and those missions occasionally return samples to Earth. NASA’s Lucy mission has recently begun, with the goal of visiting eight asteroids—one in the main asteroid belt and seven Jupiter trojans. We’re also collecting as many asteroids as we can. We want to know which ones pose a threat to Earth.

But, as we’re doing this important work, how are we preparing for an asteroid to hit us? There’s lots of talk and lots of research into this. The Planetary Defence Conference is a bi-annual conference centered on threatening asteroids. The most recent one was conducted in April of 2021, and it was the seventh one.

A mock asteroid threat is established at each of these Planetary Defence conferences, and during the course of the five-day meeting, increasingly comprehensive mock observations of the asteroid are released as fictional months pass. Teams of attendees follow the developments and come up with mitigation strategies. At the 2021 conference, the final mock radar observations revealed that the incoming asteroid was 105 meters across and would strike an area bordering Germany, the Czech Republic, and Austria. What mitigation strategies did the attendees come up with?

Map of recent 873 events greater than 0.073 Kt from April 15, 1988, to Sept 29, 2021 air burst impacts from atmospheric infrasonic sensors. The Chelyabinsk 500 Kt event on February 15, 2013, is easily seen over Russia. It is important to note that the energy ranges of several of these asteroid crash events are in the tactical to strategic nuclear weapon ranges. The white spots lack altitude data. Image Credit: P. Lubin/Experimental Cosmology Group, UCSB. Data from Alan B. Chamberlin (JPL/Caltech) https://cneos.jpl.nasa.gov/fireballs/.

Surprisingly, not much. There was nothing that could be done to mitigate the impact, and instead, the focus was turned towards civil defence and evacuations. Is the status quo adequate? A 105-meter asteroid will not destroy civilization, but it will cause significant damage. A 105-meter asteroid with the energy of a modern, big thermonuclear weapon can reach Earth with up to 30 Mt (megatons of TNT).

In a paper titled “PI: Terminal Planetary Defense,” Philip Lubin outlines what’s needed to protect Earth from asteroids without much lead time. Lubin is a physics professor at UC Santa Barbara and the paper’s sole author. The acronym PI stands for “Pulverize It.” It’s available on the pre-press site arxiv.org.

“So far humanity has been spared large scale catastrophe as was visited upon our previous tenants but counting upon being “lucky” is a poor strategy in the longer term.”

Lubin, Philip

We know that some asteroids are planned to pass close to Earth. Apophis was a concern in December 2004 when it looked like it had a 2.9% chance of striking Earth on Friday, April 13th, 2029. It’s about 370 metres (1,210 feet) in diameter and could do an enormous amount of damage. Asteroid Bennu, which measures 490 meters (1608 feet) in diameter, will also approach close to Earth in 2036.

We know that they won’t hit Earth. At least not this time around.

However, an asteroid that approaches that near to Earth can pass through what are known as “gravitational keyholes.” These are regions where Earth’s gravity can nudge an asteroid and send it on a direct collision course with Earth.

“If it goes through the gravitational keyhole, it will generally hit Earth on the next round,” Lubin said in a press release.

According to Lubin, we can prepare for this. Because an asteroid will surprise us at some point.

Lubin claims that in an emergency asteroid situation, humanity can still rely on Earth’s atmosphere for some protection. The issue is busting an approaching asteroid into small enough pieces that they’ll burn up in the atmosphere without causing a massive shockwave or triggering a “nuclear winter” scenario.

Lubin proposes intercepting an incoming asteroid with a device that strikes it with an array of penetrating rods. Some of the rods would contain explosives. The goal is to pulverize the bolide into little fragments of 15 meters (45 ft) or less in diameter. These fragments would combine to form a debris cloud. Some of the fragments will strike Earth, but hopefully, many will not. This is determined by the detection time, the size of the impactor, and the angles and velocities.

In the Pulverize It scenario, an asteroid heading for Earth would be struck with an impactor that had an array of rods, some with explosives. The asteroid or comet would be broken into tiny pieces that would pose less of a threat. Image Credit: Lubin/Experimental Cosmology Group, UCSB.

According to Lubin, the fragments that collide with Earth’s atmosphere will move at hypersonic speeds of around Mach 60. At that speed, the fragments would be heated by extreme friction and would begin to break apart high in the atmosphere, anywhere between 30 and 50 kilometres (18 and 31 miles).

Each of the fragments is still carrying a significant amount of energy. According to Lubin, each is comparable to a current thermonuclear weapon. But rather than a single air burst or impact, the shower of fragments would be similar to multiple thunder and lightning events.

This figure from the study shows two fragment clouds resulting from successful interceptions. The top asteroid is a smaller asteroid, and the bottom asteroid is a larger asteroid. With a larger asteroid, some of the bits would totally miss Earth. Image Credit: Lubin 2021.

There would be a lot of noise, but not all at once. There’d be flashes, but not one enormous one. There would also be blast waves, but instead of a single large blast and damaging shock wave, a number of smaller ones that did not overlap. Finally, there would be dust. But even the amount of dust is not likely to be dangerous.

This graphic shows how observers would feel the blasts separately instead of as one big, destructive explosion. Image Credit: Lubin/Experimental Cosmology Group, UCSB.

Lubin brings our attention to the asteroid Apophis in his work. Apophis will come very close to Earth on Friday, April 13th, 2029. It will be 31,000 km closer to the Earth’s surface than geosynchronous satellites (19,300 mi). At 370 meters in diameter, it will carry enough energy to power humanity’s entire nuclear arsenal. The threat is real.

Most asteroid strike prevention talk centers on advance warning. With ample warning, months or even years, a relatively tiny mass kinetic projectile can be launched to intercept an approaching asteroid. If the asteroid is far enough away from Earth when the projectile strikes it, then a little nudge is all that’s needed.

However, the PI approach was simulated with very little lead time. “The PI program is remarkable in the extremely short response times needed for mitigation,” Lubin writes. Lubin and his colleagues did thousands of simulations for various sized asteroids, and the results look promising.

Lubin urges a proactive stance on asteroids, especially on “repeat offenders” like Apophis and Bennu. He argues that asteroid dangers require both a formal decision-making procedure and a technological implementation of the PI approach. And it has to be international.

“We could eliminate threats such as Apophis and Bennu with PI by intercepting them on a close approach and eliminating them as a future threat. This is both a policy and a technical area to be discussed,” Lubin writes.

“Ideally, the area of planetary defence would transcend national borders as the threat respects no borders. An ideal scenario would be to bring together an international effort to defend our planet for future generations.”

Apophis is coming, and Lubin believes it will be a good, though controversial, occasion to put the PI method to the test.

Is it time? What are we waiting for?

“While we often say that nothing in life is certain but death and taxes, we can certainly also add human extinction to this list,” Lubin said. “There is a large asteroid or comet lurking in our solar system with ‘Earth’ written on it. We just do not know where it is or when it will hit.”