Among more than 12, 000 identified asteroids and comets traveling in our cosmic neighborhood, almost 900 of them have at least 1 km in diameter. Such an object would have a mass 1, 3 billion tons, and its impacting force would be equivalent to the explosion of 46, 300 megatons (millions of tons) of TNT. How many more, even larger ones, could come from greater distances, like the Kuiper belt?

Since its formation, Earth has been bombarded from space countless times. Approaching earthbound NEO (Near-Earth Object) or KBO (Kuiper belt object) may appear as a viable threat at any time. Although it might take years before an impact occurs, it might also take years before we develop technology to protect ourselves. What could we do to prevent the mass extinction scenario? We cannot build large-enough bulletproof vest to shield the Earth from a celestial bullet with its name on it. But there are some interesting ideas about intercepting earthbound asteroids and ‘convincing’ them to change their minds about hitting our planet.

According to some calculations, Apophis, an asteroid double in size of the one which hit Siberia in 1908, can come dangerously close to Earth in 2029 and 2036. The impact would have the power of 20-megaton bomb. As a comparison, the atomic bomb detonated on Hiroshima in 1945 had a power of 15 kilotons. Another asteroid, known as 2011 AG5, has also been identified as a potentially hazardous, with odds of hitting the Earth in 2040 set at 1 in 625. It is just a matter of time when a collision with a really big one would occur.
Reading some of the out-of-the world ideas, discussing possible mitigation techniques, can be very amusing. Wait a minute; all those ideas NEED to be outlandish! We’re talking about things happening hundreds of million miles away. But, some of them are more entertaining than others, like: fastening a solar sail to an asteroid, or mounting a probe equipped with a laser, which could heat up the rock and allow gasses to apply a tiny, but sufficient force, moving an asteroid off its course, or again, the probe with digging equipment to munch on it and eject its pieces far away in the space. In the case of Philae, we have witnessed how mounting anything on a speeding and tumbling space rock can go wrong. Only this time, we cannot afford to be wrong.

These kinds of threat, we have to say, were not taken so lightly by our politicians. In 1991 the U.S. Congress mandated NASA to investigate the gravity of the threat and propose solutions. Since then, NASA, ESA and other space agencies are working hard to devise a strategy that would deflect dangerous asteroids and comets from their collision course with Earth.

Nuclear solution
Nuking an asteroid while still at a respectful distance from our planet seems an obvious choice, since we still possess a respectful arsenal of 22, 300 nuclear warheads, and means to deploy them in space. Just a simple Kaboom!, and the problem solved. Oh, how we love when it ends “on that bombshell”. Where such detonation should occur, relative to an asteroid, and what would be the effects on its trajectory, would largely depend on its size and composition. In any case, NASA suggests, it is still the best option if we need to respond to such a threat on a short notice.

Hypervelocity kinetic impactors
Similar invasive strategy implies the use of kinetic impactors, like HAIV (Hypervelocity Asteroid Intercept Vehicle). Given that nuclear devices cannot hit an asteroid at great speeds, because of their rather sensitive fusing mechanisms, HAIV would serve as a high velocity armor-piercing bullet. Based on its sheer speed it would produce a tremendous impacting force. Depending on the size of an asteroid, one kinetic impactor could prove sufficient to deflect the trajectory of a small asteroid, or, in case of a larger one, it can be used in combination with nuclear devices. In such configuration, the impactor would slam into the asteroid at high enough velocity to create a deep crater, that would subsequently allow for more effective, sub-surface, detonations of nuclear warheads. The European Space Agency (ESA) in cooperation with NASA is planning two missions, Don Quijote and AIDA, which would test this defense option on 99942 Apophis and 65803 Didymos. Don Quijote’s launch date is scheduled for 2015, while AIDA’s is proposed for 2019.

Gravity tractor
Everything in the space loves to cuddle, even inanimate bodies. Love, pardon, gravity is ever-present force in everything. Gravitational tractor could be the most elegant solution, supposing that we have enough time to allow a spacecraft to sidetrack an asteroid and gradually alter its trajectory by a gravitational force. The main advantage, compared to other approaches, is that it would be effective on any type of asteroid or a comet, regardless of its composition. While the two above mentioned solutions could be effective on solid objects, they would have a rather limited results on a pile of rubble traveling together through space. The problem is how to prevent a spacecraft from crashing into the asteroid or a comet, once their gravitational forces begin to interact?

‘Artistic’ approach
Our next Bruce Willis-type of hero might be a painter. If you’re the one, don’t be surprised if NASA guys come knocking on your door one day, asking you a favor,”Could you, please paint an asteroid for us?” Please don’t say you’re busy. All jokes aside, but this is another viable option of deflecting an asteroid from its trajectory. Everybody knows bright surfaces reflect more light than the dark ones. If we could paint one side of an asteroid in bright colors, then solar radiation would push more against it, and divert an asteroid just enough to pass us by. A solar sail would function on a same principle, but it would be almost impossible to attach such a structure on a spinning and hurtling rock, traveling at speeds of 100, 000+ kilometers per hour.
There are other Earth-saving ideas out there, involving tugboats, ion thrusters, mirrors, and many more, which could prove functional in the long run, but bearing in mind the overall costs of a given mission and the fact that we wouldn’t have a chance for a re-attempt, it makes sense to stick to those solutions which sound more feasible.

Link: Movies about asteroids hitting Earth

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