How Earth Might Be Saved if an Asteroid Was Headed Straight for Us

NASA is preparing to test Earth's defenses by smashing a spacecraft into an asteroid system. While neither Dimorphos nor Didymos, the target of the Double Asteroid Redirection Test (DART) spacecraft, present a threat to Earth, the mission will see if such an impact could divert an asteroid that is on a collision course with our planet.

This isn't the only method suggested for the diversion of an asteroid, however. Space agencies have a range of potential diversion methods from the seemingly obvious, like blasting an asteroid with a nuclear missile, to obscure ideas like painting its flank.

Professor Bong Wie, director of the Asteroid Defense Research Collaboration at Iowa State University, told Newsweek two things are key to the success of such missions: "The warning time or mission lead time, as well as the asteroid size, plays the most crucial role in asteroid diversion."

The first stage in an asteroid impact scenario would be the detection and identification of an approaching object. This is the remit of NASA Jet Propulsion Laboratory's (JPL) Center for Near-Earth Object Studies (CNEOS).

Once an NEO is identified, its orbit is computed by CNEOS and calculated up to a century into the future. This is how CNEOS calculations impact risks, with potentially hazardous objects (PHOs) placed on JPL's Sentry impact monitoring system table. In the event of a predicted impact, CNEOS calculates the impact time, location, and geometry.

Objects spotted on a collision course with Earth by CNEOS could potentially provide us with lead times ranging from years, to decades or even centuries to divert it before an impact.

Paint It Black

Timescale: Centuries to Decades

In 2012, Sung Wook Paek, a graduate student in MIT's Department of Aeronautics and Astronautics, suggested that spacecrafts could pelt an approaching asteroid with paintballs in an effort to divert it.

The paint would have the effect of either increasing or decreasing the amount of sunlight reflected or absorbed by the asteroid. Because photons from the sun carry momentum, they could act as tiny rocket boosters shifting the trajectory of an asteroid, altering its course and preventing an impact.

Paek estimated that it would take at least 20 years and a lot of paint to divert an asteroid of a size capable of causing significant damage to Earth. And an even larger asteroid could take centuries to shift off course.

Additionally, Texas A&M aerospace engineer David Hyland believes that the use of traditional paint may not be possible because it would simply vaporize in the near-vacuum of space. Hyland and his team instead suggest the use of a dry powder sprayed onto an asteroid.

Gravitational Tow

Timescale: Decades to Years

Another asteroid diversion measure involves the use of a tractor to tow the space rock to safety using gravity as a bond rather than a physical tether.

Gravity Tractor
Illustration shows one of the methods humanity could use to divert an asteroid. Placing a spacecraft alongside an asteroid creates a gravitational drag that could pull the asteroid off course. NASA

The gravitational tractor method makes use of Newton's Third Law of Motion, which says that for every action or force there is an equal and opposite reaction or force.

When a spacecraft orbits the Earth, the Earth's gravity tugs on the spacecraft to keep it in orbit. Newton's Third Law tells us that the spacecraft also gravitationally tugs on the Earth.

The difference here is that the spacecraft has a negligible mass compared to the mass of the Earth. A gravity tractor spacecraft would fly alongside an asteroid, introducing a gravitational influence that gradually pulls the asteroid off course.

This technique may not work for asteroids with a diameter above 500 meters, the objects that pose the most significant risk to Earth, and it would still require us to have advance warning of an impact ranging from decades to years.

The Direct Approach

Timescale: Years

The DART mission, set to launch on November 23 at 10:20 p.m. PDT (11/24/21 01:20 a.m. EDT), is an example of a kinetic impactor, the use of brute force to slam an asteroid off course.

Kinetic Impactor
An illustration showing how DART, an example of a kinetic impact used to divert an asteroid should work. The mission aims to alter the asteroid's orbit by just 1%. NASA

"The basic physics principle is sort of like playing billiards," Paek, who has also produced work on the use of impacts as a diversion method, explained.

Deflecting an asteroid with a kinetic impactor may also depend heavily on the composition of the space rock, as deflecting a loose collection of rocks is a very different proposition from deflecting a cluster of smaller bodies.

The National Academy of Sciences says that the diversion of a smaller asteroid would require a warning time of at least one to two years. But, for the deflection of a larger asteroid, 100s of miles in diameter, decades would still be needed for this direct approach to be successful.

The Nuclear Option

Timescale: Years to Months

In a study published earlier this year, researchers concluded that if Earth was threatened by a sizable asteroid impact with less than a decade's warning, the only chance of avoiding a massive impact would be the nuclear option.

"For less than a 10-year lead time from detection to Earth impact even for a 150-meter asteroid, a deflection is not a viable option," Bong agreed. "The disruption of fragmentation approach is the only option, other than evacuation."

This brings the risk of Earth being pelted with fragments left over from the blast. "The use of nuclear devices as the last resort is intended mainly for disruption or fragmentation, not deflection," Bong said. "Some portion of fragmented pieces will enter the atmosphere as the side effects."

Fortunately, in a simulation of such a situation, the authors found that over 99 percent of the blasted asteroid's mass didn't reach the ground. This was the case even when nuclear weapons were deployed with a lead time of less than a month.

Evacuation is The Only Option

Timescale: Weeks to Days

The Sentry Impact table currently lists no objects with a high probability of hitting the Earth, that doesn't mean such objects aren't out there, however. The responsibility of spotting a hitherto-undetected asteroid on a collision with Earth missed by CNEOS would likely fall to the Asteroid Terrestrial-impact Last Alert System (ALTAS).

ATLAS is a robotic astronomical survey and early warning system optimized for detecting smaller near-Earth objects a few weeks to days before they impact Earth. Currently, the most likely amount of warning time provided by ATLAS is between one and a few days.

In such short-lead time scenarios, NASA's Earth Planetary Defense Coordination Office (PDCO) says that only adequate emergency response would involve mass evacuations and preparing buildings and other structures for the impact.

Fortunately, ATLAS adds that this short notice likely only applies to smaller objects with a diameter of around 50 meters as larger objects are more likely to be spotted years ahead of impact.

Winning Asteroids
In this Feb. 15, 2013 photo provided by Chelyabinsk.ru, a meteorite contrail over the Ural Mountains' city of Chelyabinsk, about 930 miles east of Moscow, Russia. After a surprise meteor hit Earth at 42,000 mph and exploded over a Russian city in February, smashing windows and causing minor injuries, scientists studying the aftermath say the threat of space rocks hurtling toward our planet is bigger than they had thought. Meteors like the one that exploded over Chelyabinsk _ and those that are even bigger and more dangerous _ are probably four to five times more likely to hit Earth than scientists thought before the February mid-air explosion, according to three studies released Wednesday in the journals Nature and Science. Yekaterina Pustynnikova/Chelyabinsk.ru/AP

These smaller objects can still cause significant risk to both people and property, as seen in February 2013 during the Chelyabinsk event, the explosion of an asteroid 14 miles above Russia. The event still caused 1,600 people to be injured and blew out windows in a 200-square mile radius.

A stark reminder of our vulnerability to asteroid impact is buried beneath our feet in the geological record.

The Chicxulub impactor hit Earth around 66 million years ago, leaving a 93-mile wide crater and triggering a mass extinction that killed three-quarters of all life on the planet, including most of the dinosaurs. Diversion strategies may be vital if humanity aims to avoid the same fate.

Asteroid Impact
A stock image of an asteroid impact devastating the Earth. NASA will launch the DART mission to test a potential asteroid diversion method. But humanity has a few more tricks up its sleeve. ratpack223/Getty