What Are Your Options When you’ve Only Got Hours or Days to Prevent an Asteroid Impact?

Imagine an asteroid headed straight for Earth. It could arrive within a few days or even less. What can be done to stop it from happening?
It may be possible to save the planet and ourselves in such a short time. We'd need to test the infrastructure and create it.

Humanity is actively engaged in the discovery of asteroids. Spacecraft are sent out to explore them and, sometimes, those spacecraft bring back samples. NASA's Lucy mission launched just recently. Its mission is to visit eight asteroidsone within the main asteroid belt and seven Jupiter trojans. We are also busy cataloguing as many asteroids as we can. We want to find out which asteroids pose a threat for Earth.

While we are doing this important work, how do we prepare for an asteroid headed right for us? This topic is the subject of much research and much talk. The Planetary Defence Conference is a bi-annual conference that focuses on dangerous asteroids. The seventh conference was held in April 2021.

Each conference features a mock threat to an asteroid. Over the course of five days, detailed mock observations are made of the asteroid. Participants form teams to follow developments and devise mitigation strategies. The final mock radar observations at the conference in 2021 revealed that the incoming asteroids was 105m across. It would strike an area bordering Germany and the Czech Republic. What mitigation strategies were the attendees able to come up with?

Map showing 873 recent events greater than 0.03 Kt, from April 15, 1988 to September 29, 2021. Over Russia, the February 15, 2013 Chelyabinsk 500 Kt incident can clearly be seen. Important to remember is that many of these asteroid strikes occur in the range of strategic to tactical nuclear weapons energy. Altitude data is not available for white-colored points. Image credit: P. Lubin/Experimental Cosmology Group at UCSB. Data from Alan B. Chamberlin (JPL/Caltech) https://cneos.jpl.nasa.gov/fireballs/.

It is troubling to say that not much. There was no way to reduce the impact so the attention was switched to civil defense and evacuations. Is it enough to keep the status quo? Although an asteroid of 105 meters won't wipe out civilization, it will still do a lot. A 105-meter asteroid could strike Earth with 30 Mt (megatons TNT), which is the equivalent of a large, modern thermonuclear weapon.

Philip Lubin, a physics professor at UC Santa Barbara, outlines the steps needed to protect Earth against asteroids in a paper called PI: Terminal Planetary Defense. Lubin, a physics professor at UC Santa Barbara, is the only author of this paper. Pulverize it is the PI in the title. You can find it on arxiv.org.

Although humanity has so far avoided large-scale catastrophe like that which was inflicted upon our former tenants, it is not wise to count on being fortunate for the long term. Philip Lubin

We know that there are asteroids scheduled to have close encounters with Earth. Apophis was an issue in December 2004, when it appeared that it would strike Earth at 2.9% probability. It measures in at 370m (1,210ft) in diameter and could cause massive damage. The asteroid Bennu (490m 1608ft) in diameter will also be close to Earth in 2036.

We know they won't reach Earth. They won't, at least not this time.

The problem is that an asteroids that passes so close to Earth could pass through what are known gravitational keyholes. This is where Earth's gravity can push an asteroid into a collision course with Earth.

Lubin stated in a press release that if it passes through the gravitational keyhole it will usually hit Earth the next round.

According to Lubin, we can prepare for it. We will be taken by surprise when an asteroid strikes us.

Lubin states that in an emergency situation involving an asteroid, we can still depend on Earth's atmosphere to provide some protection. It is important to break down an approaching asteroid into small enough pieces so that they can burn up in the atmosphere and not cause a huge shockwave or trigger a nuclear winter.

Lubin suggests intercepting an incoming asteroids with a device that strikes it with an array penetrating rods. Some rods could be filled with explosives. The goal is to reduce the bolide into smaller pieces that measure approximately 15 meters. The diameter of the bolide should be no more than 45 feet. These fragments would create a cloud of debris. Some fragments may hit Earth, but most will not. This depends on the time of detection and the size of impactors, as well as the angles and velocities.

The Pulverize it scenario would see an asteroid headed for Earth being struck by an impactor with a variety of rods and explosives. The impactor would break down the asteroid or comet into smaller pieces that pose less threat. Image credit: Lubin/Experimental Cosmology Group (UCSB).

According to Lubin, the fragments will travel at supersonic speeds. They could reach Mach 60. The fragments would travel at that speed and be heated by extreme friction, breaking apart high up in the atmosphere at 30km to 50km (18 to 31 miles) altitude.

Each fragment still contains a lot energy. According to Lubin, each fragment is very similar to a modern thermonuclear weapons. However, rather than one air blast or impact, the showering of fragments would be comparable to multiple thunder and lightning events.

The study's figure shows two fragment clouds that resulted from successful interceptions. The top shows a smaller asteroid, while the bottom shows a larger one. You can see how fragments could miss Earth completely if there was a larger asteroid. Image credit: Lubin 2021.

It would make a lot of noise but not all at once. There would be flashes but not one large flash. There would also be blast waves. But instead of one huge blast and destructive shock wave there would be many smaller ones. There would also be dust. However, even a small amount of dust is unlikely to pose a danger.

This graphic illustrates how observers might feel the blasts as separate events, rather than one large, destructive explosion. Image credit: Lubin/Experimental Cosmology Group (UCSB).

Lubin's work draws our attention on the asteroid Apophis. Apophis will be very close to Earth on Friday, April 13, 2029. It will be closer to the surface of Earth than geosynchronous satellites, at 31,000 km (19.300 mi) It will carry enough energy to power the entire human nuclear arsenal at 370 meters. This threat is real.

Asteroid strike prevention talks mainly revolve around advance warning. A kinetic projectile with a small mass can be launched to intercept an incoming asteroid if there is enough notice. A little push is enough if the asteroid is not far enough from Earth when it is struck by the projectile.

The PI approach was simulated using very short lead times. Lubin writes that the PI program has remarkable response times for mitigation. Lubin and his collaborators have performed thousands of simulations on a variety of asteroids of various sizes and the results are promising.

Lubin calls for a proactive approach to asteroids, particularly with repeat offenders such as Apophis or Bennu. He argues that both a formal decision-making and technological implementation of the PI approach are necessary to deal with asteroid threats. It must be international.

By intercepting potential threats like Apophis or Bennu with PI, we could eliminate them as a threat and remove them from our future plans. Lubin writes that this is both a policy issue and a technical topic to be discussed.

As the threat does not respect borders, it would be ideal that the area of planetary defense extends beyond national borders. A perfect scenario would be for an international effort to protect our planet for future generations.

Lubin believes that Apophis will be coming and it could be a great opportunity, even controversial, to test the PI method.

Is it the right time? Are we still waiting?

Lubin stated that while we say death and taxes are certain in life, human extinction can be added to that list. A large comet or asteroid lurks in the solar system, with Earth written on its surface. It is not known where or when it will strike.

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