Scientists from the Massachusetts Institute of Technology (MIT) have come up with a new solution to the problem of what to do if asteroid threatens to smash into Earth.
The researchers have devised a decision map to help determine the best course of action, describing their method as a "preventative strike" as opposed to a "last-minute deflection."
The planet is currently surrounded by thousands of Near-Earth Objects (NEOs). These are comets and asteroids with orbits that cross Earth's, so have the potential to collide with us.
"Surveys that search for these objects discover typically two to three new NEOs every night," Detlef Koschny, European Space Agency (ESA) Project Scientist, told Newsweek.
The probability of one of these NEOs hitting Earth any time in the near future is practically zero, and most are too small to require a deflection method. Even so, a major collision is a case of when, not if.
"At some point, one of these objects will hit us," said Koschny.
Current methods of asteriod deflection are "somewhat behind," Gregory Brown, Astronomer at Royal Observatory, Greenwich, told Newsweek. "Determining the most effective route for asteroid deflection under different circumstances is one step on the road to preparing our defenses," he added.
Several methods have been put forward for deflecting NEOs, including the use of nuclear bombs. However, there are concerns nuclear detonation may not successfully deflect of the target asteroid. There are also safety and political issues involved.
Gravity tractor techniques—a spacecraft that deflects an object via its gravitational field rather than contact—also come with problems. This includes technology is not advanced enough to carry it out in the near term.
As a result, the MIT team say a kinetic impactor—a projectile that deflects an object through impact—is the only method that is do-able in the near-future.
This is where MIT's decision map comes in. The map, described in the journal Acta Astronautica, considers the asteroid's mass, momentum, trajectory and how much warning time scientists have before collision. All these factors have degrees of uncertainty that must be added into the equation to identify the best method of deflecting an incoming asteroid.
Crucially, the study's authors considered their proximity to what is known as a gravitational keyhole.
A gravitational keyhole is a usually narrow region that, if an asteroid were to pass it, would put said asteroid on a collision course with Earth. "A keyhole is like a door—once it's open, the asteroid will impact Earth soon after, with high probability," lead author Sung Wook Paek told MIT News.
Paek's aim is to close that gravitational keyhole.
"Getting to the asteroid before this time might make for an easier deflection than waiting until it is already on its final approach," said Brown.
"In most deflection methods, the changes in course are typically very small, often expecting only changes in velocity of a few millimetres or centimetres per second," he added. "So early detection is key to enable the chance to act well ahead of the collision and ensure success."
Paek and colleagues applied their decision map to two near-Earth asteroids: Bennu and Apophi, chosen because scientists know the locations of their gravitational keyholes in respect to Earth.
Using simulations, they tested three alternative missions. The first involved a kinetic impactor—a projectile that interrupts the path of an approaching near-Earth object to knock it off track. The second considered sending a "scout" before the projectile to measure the asteroid and advise scientists on what type of projectile to use. And the third sending two scouts—one to advise specs and one to nudge the asteroid slightly off course before the projectile is brought in. The last requires more warning time, but it would have a greater chance of missing Earth.
According to their calculations, it would be possible to apply the third mission to Apophis if scientists had five years of warning before it passes a keyhole. One scout and a projectile could be sent if scientists had between two and five years of warning. Less than that and scientists would have to carry out the first mission involving one projectile and no scouts. Under a year and there might not even be time for a projectile, the scientists found.
Because Bennu's material composition is better understood, the researchers say it may not be as necessary to use scouts prior to sending the projectile.
"People have mostly considered strategies of last-minute deflection, when the asteroid has already passed through a keyhole and is heading toward a collision with Earth," said Paek. "I'm interested in preventing keyhole passage well before Earth impact.
"It's like a preemptive strike, with less mess."
According to Brown, Paek's method would work best on medium sized NEOs between extinction level asteriods like Chicxulub, which is thought to have caused the extinction of the dinosaurs, and smaller objects like the Tunguska and Chelyabinsk meteors, which take place every few decades and cause relatively minor damage. Those that are "common enough to be worth putting effort in to redirect, yet large enough to be of serious concern if they hit."
"When events like this would happen is uncertain, but it certainly is a case of when not if," said Brown. "That said, the vast majority of Near Earth Asteroids are in no immediate danger of crashing into the Earth and regularly pass by at relatively close distances with no ill effects.
"There is, after all, far more space to miss the Earth than there is Earth to actually hit."
In the future, Paek hopes to experiment with different types of projectile—how do multiple smaller projectiles compare to one large one, for example. And what would happen if you changed the site of the launch from Earth to the moon?
