Nuclear Bombs and Gravity Tractors: The Weird Ways Earth Could Be Saved From an Asteroid

Today is International Asteroid Day—a United Nations (UN)-sanctioned event observed annually on June 30 that aims to raise awareness about the risks of asteroid impacts.

Large near-Earth objects (NEOs)—asteroids or comets that pass close to our orbit—represent "potentially catastrophic" threats to our planet, according to the UN. With this in mind, scientists around the world are investigating how we might able to protect the Earth in the unlikely event that a space rock large enough to cause serious damage was headed right for us.

Two upcoming space missions are hoping to shed light on this question—the Double Asteroid Redirection Test (DART) and Hera, managed by NASA and the European Space Agency (ESA), respectively.

The aim of these missions is to gain an understanding of how hard it is to deflect an asteroid. The DART mission, scheduled to launch later this year, will deliberately crash a special spacecraft—known as a "kinetic impactor"—into an asteroid called Dimorphos in September, 2022, at a speed of nearly 15,000 miles per hour. Scientists are hoping that the impact will push the asteroid slightly away from its current trajectory.

Dimorphos is estimated to measure 160 meters (525 feet) across and is similar to the handful of asteroids that are most likely to cause us concern in the next 100 years or so. If a space rock of this size were to strike the Earth it would totally devastate a region within tens of miles of the impact site.

The goal of the Hera mission, which launches in 2024 and arrives at Dimorphos in 2026, is to observe how the space rock responded to being hit by the DART spacecraft.

According to Alan Fitzsimmons from the Astrophysics Research Centre at Queen's University Belfast, U.K., the two missions represent humanity's first practice run in planetary defence.

"We expect that we would have years or even decades of warning when we do discover the next impactor that can cause local devastation on Earth," Fitzsimmons told Newsweek.

"In theory, we can move the asteroid onto a different orbit so that it doesn't strike us. But we don't know exactly how much we can move even a small asteroid, because our models give us a wide range of possible outcomes, and we've never tried it in real life. DART and Hera will solve this problem for us."

Scientists hope the two missions will validate this deflection technique so that the knowledge gained can be applied to potential future scenarios where there is a real threat to the Earth.

Patrick Michel, the principal investigator for Hera, told Newsweek the missions will "offer us a fully documented deflection test and impact experiment, which we can use to validate our numerical models of impacts. So far [these have been] validated by impacts performed in laboratories on centimeter-size targets.

"We need an experiment at the actual scale of an asteroid, because we know that the extrapolation from small to large targets is not a simple problem. So, we will be able to determine how an object of 160 meters in diameter responds to an impact, in terms of how much of the momentum is transferred to it and what is the size of the crater."

Furthermore, Hera will perform measurements that will cast new light on the internal structure of these space rocks—an important factor in how they might respond to an impact from a spacecraft like the one used in the DART mission.

"With the full understanding of the result of the DART impact, we will be much better placed to optimize the design of an actual deflection mission," he said.

Despite the ambitious nature of these missions, this deflection technique is relatively simple. Nevertheless, it is the most advanced technology we have when it comes to protecting our planet from impacts, according to Fitzsimmons. But there are some potential drawbacks to this method, although we will understand more about its limitations following the completion of DART and Hera.

"The cons are that it's not clear how large an asteroid we can move using a kinetic impactor like DART, and it depends a lot on the warning time we have," Fitzsimmons said. "The other problem is we don't know much about how small asteroids vary in structure, and the response of another asteroid to DART may be different. But we'll be on firmer ground for calculating that variation after DART and Hera."

While no other comparable planetary defence tests are currently being planned in practice, there are several other techniques for protecting the Earth from NEO collisions that have been and are being studied on paper.

Experts told Newsweek that among the most viable is the gravity tractor method. This technique involves placing a massive spacecraft next to the asteroid. The idea is that the force of gravity produced by that mass will very slowly pull the target asteroid away from its original trajectory.

Thomas Statler, program scientist for DART, told Newsweek: "This requires many years of lead time to be effective, but there's none of the ambiguity associated with the kinetic impactor because we know exactly how gravity works."

However, this technique would only work for small asteroids—of the order of 100 meters (328 feet) in diameter—due to limits on the mass of objects we can send into space, according to Michel.

For the largest asteroids—those greater than one kilometer (0.6 miles) in diameter—the only option may be to detonate a very large explosive device, such as a nuclear bomb, close to the space rock. The aim would not be to blow it apart (think the 1998 science fiction film Armageddon) but to vaporize part of its surface, Statler told Newsweek.

"The force of this hot vapor jetting away from the asteroid would push it in the opposite direction. But the possibility of launching a very large explosive device on a rocket and detonating it in space would be a complex social and geopolitical issue," Statler said.

Fortunately, the risk of kilometer-size asteroids striking the Earth is very low, occurring on average once every 500,000 years, according to Michel.

More speculative ideas have also been discussed, for which we do not currently have the technological capabilities. These often involve placing various types of propulsive devices on the target asteroid, or altering its reflective properties when it comes to visible light or thermal energy, which can have an effect on the trajectory, Andy Cheng, DART investigation team lead at John Hopkins University, told Newsweek.

Fitzsimmons said: "There's a number of other ideas that have been proposed, from using a swarm of kinetic impactors, to continuously firing a laser or ion beam at an asteroids surface to slowly move it.

"There's even been the idea of painting one side of an asteroid, which theoretically would very, very slowly change its orbit. But any tests of these techniques are in the far future.

"We are grateful that we have the opportunity with DART and Hera to test the kinetic impactor technique."