Spray paint killer asteroids could redirect them away from Earth

Asteroids pose an existential threat to humanity. A collision with a 6.2-mile (10-kilometer) asteroid caused the dinosaurs to disappear around 65 million years ago. Astronomers expect further collisions with asteroids of about 0.6 miles (1 km) every 500,000 years or so.

This is why NASA and other space agencies are trying to map the population of near-Earth asteroids. Today, only 40 percent of them have been spotted. But the goal is to paint a complete picture of the threats posed by asteroids up to a few tens of meters, in the coming decades.

This raises an obvious question: if we find an asteroid heading our way, what should we do next? Last month, NASA launched the Double Asteroid Redirection Test (DART) mission to test an idea. It involves crashing the spaceship onto an asteroid to change its course. Other options include attaching thrusters to the asteroid to deviate it from its path or even ablating the rock surface with a nuclear explosion.

Today, Jonathan Katz of Washington University in St. Louis, Missouri, says there is an easier and more efficient way to redirect asteroids by painting them with a metallic coating. The idea is that the coating changes the amount of sunlight reflected from the asteroid, its albedo, creating a thrust that redirects it. “Changing the albedo of an asteroid changes the strength of solar radiation on it, and therefore its orbit,” he says.

Force of light

This push would be minimal. But Katz points out that once a small asteroid has been identified, its trajectory can be determined centuries in advance, especially if transponders are placed on its surface to track it more accurately.

So the threat can be identified hundreds of years in advance and a small force operating on that timescale is all that would be needed.

Astronomers have long known that small asteroids are influenced by a similar phenomenon called the Yarkovsky effect. This is the result of the Sun heating an asteroid, which then re-emits this energy later, creating a small thrust. Others have suggested modifying this effect to redirect an asteroid away from Earth. Katz’s suggestion, on the contrary, generates an immediate push that is easier to calculate.

He points out that asteroids are generally dark. So coating one with lithium or metallic sodium would dramatically increase its reflectivity, turning it into an interplanetary disco ball. He calculates that about 2.2 pounds (1 kilogram) of metal could cover an entire asteroid with a layer a micrometer thick that would turn the asteroid into silver.

The increased boost in this reflectivity would be equivalent to changing the effective solar mass that the asteroid is experiencing. This in turn would change its orbit.

Katz calculates the effect of this approach. “A [164 feet (50 meters)] diameter of the asteroid can be deviated by [about 1,864 miles (3000 km)] in a century or [621 miles (1000 km)] in [some] 30 years, ”he says.

Interplanetary disco ball

More controversially, he says that would be enough to move a Tunguska-class impactor away from a city and to a less populated area, like an ocean.

The Tunguska event over Siberia in 1908 was a megaton explosion believed to have been caused by a [164 feet (50 m)] diameter comet disintegrating in the upper atmosphere or a larger asteroid grazing the edge of the atmosphere.

An alternative approach would be to cover half of the asteroid to generate a stronger directed force. “” The coating of a hemisphere of an asteroid in an elliptical orbit can produce a torque of solar radiation moving it one Earth radius in [about] 200 years, ”Katz says.

A spacecraft in a polar orbit above an asteroid that emits metal as vapor should be able to paint all or part of the body, Katz explains.

Ref: Painting asteroids for planetary defense: arxiv.org/abs/2112.03501

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