An artist’s impression of the Deep Impact probe observing the impact of its projectile on the surface of comet Temple-1 in 2005. Image courtesy NASA.
Conventional Missile Impact Weapon
Tech Level: 10
Nuclear Impulse Weapon
Tech Level: 10
Surface Anchored Rocket Engine
Tech Level: 10
Surface Anchored Ion Engine
Tech Level: 10
Gravity Tug
Tech Level: 10
Solar Mirror Impulse Orbiter
Tech Level: 11
Surface Anchored Mass Driver
Tech Level: 12
Surface Anchored Solar Sail
Tech Level: 12
Directed Energy Impulse Weapon
Tech Level: 13
Asteroid Tug
Tech Level: 14
Billions of asteroids and comets of all sizes pepper the solar system, multi-megaton debris left over from the formation of the sun and its planets. A huge belt of rocky debris abides between the orbits of Mars and Jupiter, and some planetary moons, including Mars’ Phobos and Deimos, appear to be captured asteroids. In Earth’s neighborhood alone, thousands crisscross the planet’s orbit every year. These small but incredibly numerous bodies represent both an unprecedented threat to human civilization as well as an invaluable natural resource for future space-faring cultures.

Farther out in the solar system are immense conglomerates of water and methane ice, the comets. They are mostly located in an immense ring called the Kuiper Belt beyond Pluto, and in a widely dispersed sphere at the edges of interstellar space called the Oort Cloud. However, some of these find their way into the inner solar system to occasionally become spectacular astronomical light shows in our skies.

Of immediate concern is the danger these objects could potentially pose to humanity’s long-term survival. An impact of one even a few hundred meters across could hit like an atomic bomb and wipe out a city. Larger asteroids could cause immense environmental calamities rarely, if ever seen, in human history. An asteroid or comet about ten kilometers wide hit the Yucatan Peninsula with so much force 65 million years ago that the environmental aftermath wiped out not only the dinosaurs but seventy five percent of all species then extant.

Finding any asteroid or comet on a potential collision course with Earth is an ongoing effort engaged in by NASA and various observatories throughout the world. If one such “doomsday rock” is ever found, how best to neutralize its threat is a matter of some debate.

Most agree that the more lead time the world has–decades or even centuries would be ideal–the better. Also, deflection is considered much more desirable than blowing it into smaller pieces. Detonating the offending asteroid or comet would not decrease its speed or decrease its mass. Instead of a single large bullet aimed at Earth, we would now have a wide-angled shotgun blast aimed at our planet’s face. Which would be worse for the planet most experts aren’t sure.

However, if we only had a short amount of warning, say a few weeks to a few months, launching nuclear weapons at it as it approached may be the only viable alternative. As stated, its unsure if such a tactic would make things better or worse, but it might be the only option that we would have.

If we find out at least a few years, or even a few decades, in advance that an impact is imminent, our options open up considerably. Still, no matter what technique we use to redirect an incoming doomsday rock, it is still no easy matter altering the course of a body that could mass as much as a mountain–or even a whole mountain range.

For asteroids, redirection is complicated by the fact that some asteroids may not be all one solid mass. Some theories suggest some of them may be only loose conglomerates of small debris and dust held together by mutual gravitic attraction. Some techniques discussed for deflection, such as direct high speed impactors, may only nudge some of the asteroid off course but leave the main mass of it heading in our direction.

Comets have a different problem as far as redirection goes. Since most of them fall into the inner solar system from much farther out, they have more time to build much higher velocities than most asteroids. Because of this, they may prove much harder to deflect. Also, the outgassing associated with these bodies may make them harder to deal with, especially if within the solar system and they are experiencing enough outgassing to form a tail.

The technologies discussed in this article need not always have a doom and gloom application. In decades or centuries hence, when space travel is abundant and routine, mankind will start looking toward asteroids and comets both as harvestable resources and as potential real estate for outposts, bases, and colonies. If this is the case, “herding” these smaller bodies, moving them to more desirable orbits or to centralized processing centers and so on, may become much sought-after techniques.

Tech Level: 10
One of the most straight-forward ways of moving an asteroid is to smash it with a heavy, fast-moving mass. In fact, so far this is the only method that has been actually been tried in reality. On July 4, 2005, NASA’s Deep Impact probe slammed an impactor into comet Temple 1 in deep space. The experiment was designed to blast a crater into the side of the comet to better analyze its interior composition from the ejecta, but it also measurably altered the comet’s previous trajectory.

However, the relative speed between the comet and the probe was fairly close to each other; the objective was only to create a small crater, not do major damage. However, if one was seeking to deflect the comet with more force, a warhead from a missile could hit at a much higher relative velocity, of dozens of miles per second difference. A nuclear warhead would be superfluous at those speeds.

The problem with direct impact deflections is that it runs the risk of splitting or shattering the target without significantly altering its course. Like normal rocks and masses of ice, asteroids and comets are bound to have minute faults and cracks throughout their mass which could be catastrophically amplified by the impacting weapon. Most of the impact energy would go into deforming the mass instead of deflecting it. Also, for targets on the larger end of the spectrum, even megatons worth of such impacts may only have a negligible effect on trajectory.

Tech Level: 10
The potential danger of shattering an asteroid or comet without altering its course will be present in any attempt to move such a body. Even with careful scans of the target, hidden fault lines deep within may remain. Also, as stated previously, some asteroids may be only loose conglomerations of rubble, and hitting one small spot would only have negligible effect on the rest of the mass.

Such bodies would be needed to be pushed from all points on a facing side simultaneously to avoid potential splintering. One way to achieve this is to use a powerful nuclear explosion, not on its surface, but off to its side a few kilometers, so the radiation pressure and what there is of a shockwave will give it the gentle nudge needed to alter its trajectory. Conventional explosives are considered too weak for this technique to be effective, especially with the limited mass most spaceships are able to carry.

In space, with no atmosphere to absorb the energy, most of a nuclear warhead’s energy will manifest as radiation and heat. This radiation pressure will produce a propulsive impulse over the entire facing side of the asteroid or comet, as well as perhaps triggering some outgassing