Staving off asteroids – what can we do?

An asteroid named Florence whizzed past Earth on Friday, safely passing us by at the distance of 7 million kilometers (4.4 million miles) from the surface. According to NASA, Florence was "the largest asteroid to pass this close to our planet since the first near-Earth asteroid was discovered over a century ago."

With its diameter of 4.4 kilometers, NASA classifies the object as "potentially hazardous." An impact with an asteroid this size would cause unprecedented destruction. In 2013, a much smaller and previously undetected meteor exploded over Chelyabinsk, releasing the energy equivalent around 30 Hiroshima atom bombs, according to NASA. As the blast happened over 23 kilometers above ground, most of the energy was absorbed by the atmosphere and nobody was killed. Still, the resulting shockwave knocked people off their feet, shattered windows in six Russian cities, and the fireball temporarily blinded scores of observers on the ground. Scientists estimate that the object was 59 feet (18 meters) wide.

The events such as the Chelyabinsk blast, or the more devastating Tunguska incident in 1908, remain very rare. Next time, however, we might be ready for it - with scientists across the globe developing numerous strategies for planetary defense.

Nuclear option

The science fiction notion of blasting an asteroid out of the sky might not be enough to protect humanity, researchers say. Our nuclear missiles could be effective against smaller asteroids, but any object big enough to threaten our civilization would be too big to be destroyed in such a way. Additionally, fragmenting an incoming asteroid could create a "shotgun effect" with many smaller pieces possibly dealing even more damage when hitting Earth.

A more advanced concept involving nuclear weapons was picked up by NASA in 2012 from Iowa State researcher Bong Wie and NASA engineer Brent Barbee. The duo assumed varying warning times to mount an anti-asteroid mission: ranging from several years to only days before the impact.

The risk of asteroids hitting Earth is "very real," Wie said in a paper published at NASA's website. "It is only a matter of when, and humankind must be prepared for it."

The researchers created a concept for a two-part spacecraft called Hypervelocity Asteroid Intercept Vehicle or HAIV. The vehicle would carry a nuclear bomb. Approaching the asteroid, the non-nuclear section of the HAIV would smash into it and create a crater. The nuclear device would then enter the crater and detonate, with the strength of the blast magnified multiple times underground. If done correctly, the blast might be enough to scatter asteroid fragments and reduce their chances of hitting Earth.

According to Wie and Barbee, their system would be capable of destroying an asteroid of up to 45 meters in size outside the orbit of the Moon, providing a one-week warning. Larger objects would demand longer warning periods.

However, it would first take several years to build such a system, and its components still need to be experimentally tested.

Read more: Meteorite explosion puts space on UN agenda

Kinetic impactor technique

With HAIV still on the drawing board, both NASA and their European colleagues in ESA are already preparing missions to test the kinetic impactor technique - hitting asteroids with man-made objects to alter their course. Given long enough warning times, even a slight course correction could direct an asteroid safely past Earth.

NASA is currently designing the Double Asteroid Redirection Test (DART) spacecraft, expected to eventually rendezvous with an asteroid called Didymos. The asteroid would fly by Earth in 2022 and then again in 2024. Didymos is a binary system, consisting of a larger object, some 780 meters in size, and a smaller one, around 160 meters wide, which is orbiting its larger twin.

After catching up with Didymos, the refrigerator-sized DART will strike the smaller object while traveling at the speed around six kilometer per second, which is nine times faster than a bullet. The impact should change the orbit of smaller segment and provide data for such attempts on a larger scale.

ESA is responsible for the Asteroid Impact Mission (AIM) spacecraft, which would monitor the impact and record the results.

Read more: 'The dinosaurs were a little bit unlucky'

Gravity tractor

Arguably the most elegant strategy for knocking a potentially dangerous asteroid off course is the so-called gravity tractor. The concept is simple - a spacecraft flying alongside an asteroid for years or decades would have enough gravitational pull to change its path. However, the technique has never been tried in practice and would require "decades for building, launching, and carrying out a mitigation mission," according to NASA.

Scientists have also proposed various other options, such as attaching rocket engines or solar sails to asteroids, or even painting the objects white to change the amount of solar radiation, which would provide a small but steady push away from the impact zone.

Sky is clear for now

Scientists say that a car-sized asteroid hits the Earth atmosphere about once every year, and burns up before reaching the surface. However, NASA predicts that there is less than a 0.01 percent chance of a potentially hazardous asteroid making an impact in the next hundred years.

In 2135, a 500-meter asteroid Bennu will make a close pass to Earth, getting inside the Moon's orbit and potentially changing its path. Although we know that it would not hit Earth that time, its orbit might change just enough to collide with Earth several decades later. The chances for that are about one in 2,700. This would give the Earth time to prepare and launch one of its planetary defense missions.

Even if Bennu ends up colliding with Earth, the asteroid is not big enough to destroy human civilization.

Last year, NASA launched the OSIRIS-REx mission towards Bennu, which is scheduled to reach the object in 2018 and study it for two years before returning to Earth with a sample. The primary mission goals are to learn more about the evolution of Earth and the Solar System. However, scientists could also use this data to prepare for the potential impact in the future.