As a paleontologist and an avid fan of disaster movies, occasionally I spend time contemplating the likelihood of Earth experiencing a major asteroid strike within my lifetime. In general, asteroids 10 km (~6 miles) in diameter strike the Earth every 50 to 100 million years. The last known strike of that size was the impact that caused the extinction of the non-bird dinosaurs, which happened around 66 million years ago. Theoretically, that puts us in the golden window of expecting another extinction-level extraterrestrial impact, though it doesn’t mean one is going to appear out of nowhere just to keep the timetable.

A 10 km asteroid strike is devastating, but even asteroids 1 km in diameter can pack a pretty big wallop. The good news is that space agencies have already located 95% of the celestial objects of this size or greater in the near Earth area. Most of these asteroids are completely harmless neighbors in our celestial suburb. However, for about every thousand asteroids we can happily ignore, there’s one that could cause problems in the future. While we aren’t in any immediate danger of getting vaporized by one of those large asteroids, it’s good to have plans in place for what to do if one starts straying a bit too close to the Earth’s orbit.

One of these asteroid deflection plans will be immediately familiar to anyone who watches science fiction movies: Blow them up. NASA has explored the theoretical use of nuclear weaponry to destroy asteroids, but it is a surprisingly delicate proposition. If you blow up a massive asteroid, you might end up with a rain of smaller asteroid particles instead. How much damage could a smaller asteroid impact do? Apparently, a lot.

In 2013, an asteroid only about 20 meters in diameter burst above Chelyabinsk Russia. It exploded with the force of 30 Hiroshima bombs, causing injuries to over 1,400 people, and inflicting infrastructure damage over an area of ten miles. If you blow up one 1 km asteroid into dozens of 20 meter asteroids, you could be setting yourself up for disaster.

That said, there are plenty of ways one can deflect a potentially dangerous asteroid that don’t involve any explosions whatsoever. We often discover potentially dangerous asteroids decades, or even centuries in advance. If we use that time to our advantage, we can implement slow-acting deflection methods that are a little more subtle than a nuke. Some of these methods would require technology that we are ages away from developing, but others are charming in their simplicity. Some are even rather humorous, as far as ways to save the world go.

Next time there’s a blockbuster film about an asteroid heading towards Earth, imagine what might happen if they used one of these tactics instead.

Playing Paintball in Space

In 1902, a Russian civil engineer named Ivan Osipovich Yarkovsky came to realize that sunlight was having an effect on the movement of asteroids. When light from the sun hits the dark surface of an asteroid, the surface begins to heat up. As an asteroid rotates in space, the surface that was heated by the sunlight will turn away. As it does, the absorbed energy is radiated away in the form of infrared photons. This energy transfer acts like a tiny rocket thruster, altering the momentum and movement of the asteroid.

Since sunlight interacting with the surface of an asteroid affects its movement, altering the level of sunlight hitting an asteroid could change that asteroid’s trajectory. David Hyland of Texas A&M is leading a group studying how to harness the Yarkovsky effect to redirect potentially dangerous asteroids. His proposed method of changing the amount of sunlight an asteroid receives is simple: Paint the asteroid white.

So how would one paint an asteroid? Most earthly paints are oil or water-based, which would vaporize in the vacuum of space. To solve this issue, Hyland had to consider the other properties of asteroids. Asteroids are constantly being bathed in solar wind, a stream of charged particles emitted by the sun. This gives the surface of asteroids a slightly positive charge. Instead of using traditional paint, Hyland suggests that a negatively charged white powder could do the trick. The negative charge of the powder would be drawn to the positive charge of the asteroid, leading to a pretty white coating for the celestial delinquent.

Painting an asteroid white to redirect it is a quirky and interesting method, but there are a few downsides. First, no one has actually taken measurements to confirm that asteroid surfaces have a positive charge. Second, this is definitely a slow method. Painting an asteroid white would only alter its motion by a small degree, so this is something that would need to be set up years in advance of a potential collision. Still, it has real potential, and there’s an artistic sort of appeal to having a universe full of painted asteroids.

It’s a Wrap

Working off a similar principle as painting an asteroid white, another asteroid redirection method has been proposed by Australian PhD student Mary D’Souza. Rather than covering an asteroid with white powder, she suggested instead wrapping the asteroid in reflective sheeting.

The surface of an asteroid is naturally rather dull. A satellite orbiting an asteroid could be programmed to wrap it in ribbons of reflective mylar sheeting. Photons from sunlight reflecting off the sheeting would give the asteroid a slight push in the opposite direction, which could move the asteroid off course after a few years (or decades) of being shiny in space. There’s no need to wrap the whole thing, either. Just half of a wrapped asteroid could give it enough push.

NASA Double Asteroid Redirection Test (DART)

This next method is one that is being actively developed by NASA. Even though a massive asteroid impact is fairly unlikely to happen in our lifetimes, it’s probably inevitable in the long term unless something is done to stop it. The Double Asteroid Redirection Test (DART) is an ongoing project designed to forestall any asteroid apocalypse.

The target of DART’s initial test is a binary asteroid called Didymos. Didymos is two asteroids traveling as one. The larger asteroid is approximately 800 meters across, while the smaller is only 150 m. Though Didymos doesn’t pose an immediate threat to Earth, it is classified as “potentially hazardous” for possible future complications. It came within 7.18 million km of Earth in November of 2003 and will make its next close drive by in November 2123, when it is estimated to come within 5.9 million km. What NASA is trying to do is knock Didymos from being potentially hazardous to not at all hazardous by slamming into the smaller Didymos asteroid with a highly specialized spacecraft, the DART impactor.

If the DART impactor craft slammed into the smaller asteroid at the right point in its orbit, it could send it spinning off course, and its gravity would pull the larger asteroid off course as well. The DART craft is planned to be around the size of a refrigerator. This might not seem too impressive, but the plan would be for it to hit Didymos at 3.7 miles per second, which is 5-9 times faster than a speeding bullet. This could absolutely change the orbit of Didymos and give stargazers one less asteroid to worry about. Didymos is making distant approaches to Earth in 2022 and 2024, and NASA hopes to be able to test DART during one of these passes.

Asteroid Ida here is a friendly inhabitant of our solar system and not likely to bother any planets.

Activate the Tractor Beam

No one is going to be inventing a Star Wars-style tractor beam any time in the near future. Still, the natural properties of mass and gravity can be used to have a similar effect, if on a very small scale. Gravity is the force that causes all objects with mass to be drawn towards one another. On Earth, the planet’s own gravity tends to overwhelm the effects of any other object’s gravitational field. In space, the pull of smaller objects is more evident.

If a spacecraft flew next to an asteroid for years or decades, its gravitational pull could slowly tug the asteroid into a new path. Of course, the asteroid’s gravity would also be acting on the spacecraft. To counteract this, the spacecraft would need thrusters to give itself enough push to keep itself at a constant distant from the asteroid. This would create the net result of the asteroid being minutely accelerated towards the spacecraft.

Since the spacecraft could be programmed to have a very specific course, this method has the greatest level of control of the asteroid redirection proposals thus far. Not only could future scientists pull an asteroid out of a collision course, they could hypothetically slowly divert an asteroid into a commercial processing area too.

Another bonus of this method is that it would work on a very wide range of asteroid types. If an “asteroid” was a pile of rubble moving as a single object, it would be hard to paint it white or blow it up with an explosion device. However, its course could easily be changed by using a spacecraft to pull it with gravity. Swiftly rotating asteroids are problematic for some of the other methods too, but gravity doesn’t care about rotation.

Unfortunately, great as this method is, the technology needed to pull it off is still a long way from being developed. There are size cut-offs as well, and a 1 km asteroid is probably too big to be pulled away by a gravity tractor. The expense and technological expertise needed means that this method won’t be put into use anytime soon, though it has great potential when it is.

The solar system and some of our asteroid neighbors.

And Others

There are many other methods of asteroid deflection that have been suggested, some more feasible than others. Here is a selection of other scientifically-backed methods to get rid of local problem asteroids in ways more creative than using explosives:

Ion Beam Shepherd – This method involves a spacecraft moving alongside an asteroid and firing a high speed ion beam onto the asteroid’s surface. Energy would get transferred from the ion beam to the asteroid, which would modify the orbit without need for direct contact.

Solar beam – Have you ever wondered what would happen if you used a giant magnifying glass to shoot a beam of sunlight at an asteroid? I haven’t, but the people who came up with this asteroid deflection method have. A space station with gigantic lenses could focus a beam of solar energy on the surface of an asteroid. The transferred energy would cause matter on the surface of the asteroid to vaporize, creating thrust and sending the asteroid off course.

Rocket power – Attach a rocket to the surface of an asteroid and let rip.

Steam – Release a cloud of steam into the path of an asteroid and gently slow its speed, which will alter its course.

All tied up – Attach a tether to a large object, perhaps another asteroid, and attach it to the problem asteroid. This will change the center of mass of the asteroid and its trajectory as well.

In Closing

There really are a lot of ways we could save ourselves from asteroid impact, but we’re still not entirely in the clear. We’re very unlikely to be hit by surprise by a large asteroid, but what about smaller asteroid strikes? Still very much on the table. We’ve located 95% of large asteroids in our area, but for those 20 meters or smaller, that number is a mere 1%, and thousands of people could get injured or worse for every 20 meter asteroid strike.

So what are our options? NASA is now on a mission to map asteroids 140 meters or larger in the near Earth vicinity, and they have located 30% of them since 2005. That’s a start. The smaller the asteroid the less brightly it shines, but this sweep is picking up even some of the 20m ones.

Sudden death by random meteor strike is probably always going to be a possibility, but it’s not a reason to avoid paying your bills on time. Though people do get injured by meteors and their shockwaves every once in a while, there has been only one instance of someone actually getting killed by a meteorite in the last thousand years. One single meteor death, and NASA suspects the rock may have come from land-based explosion, not from space. There may in fact be zero people who have been killed by meteors in over a thousand years.

So while meteor strikes might make for a good film, the truth is that vending machines, falling coconuts, and shopping on Black Friday are much more likely to kill you than a space rock, and if one is looking to start trouble, all it may need is a fresh coat of paint.

- Kate Dzikiewicz, Paul Grisowld Howes Fellow