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 events. For most massive targets, a single such blast from even a large nuke probably wouldn't be enough, but a series of such explosions would be enough to turn all but the most massive threatening bodies.

The nuclear warheads used need not be off-the-shelf models either. Theoretical research conducted for Project Orion (see the article on Nuclear Pulse Drive in the Deep Space Propulsion section) indicated it was possible to produce tamped nuclear explosives. Tamping the detonation means focusing a large proportion of its explosive energy in one general direction, which could prove very useful for moving multimegaton rocks. Also, for larger targets, radiation pressure alone may not be enough, so, like the Project Orion pulse bombs, the weapon may be coated with layers of polyethylene, which would break down into a ‘wave’ or high-speed hydrogen and carbon atoms upon detonation and deliver much more of a physical push to the asteroid or comet than the bomb’s radiation flash alone.

If the world detects any incoming asteroid or comet in the near future that needs to be deflected and there's at least a year or two lead time, this is the most likely method to be deployed to deflect the intruder.

A variation on this technique is used only with proven solid bodies, and is mentioned in science fiction stories dealing with far future asteroid mining as opposed to near-future deflection of possible impactors. A small nuke is used on the surface of the asteroid or comet in order to create a large crater. The crater is then used as a crude "rocket nozzle" to channel succeeding blasts and allow the body to build up speed on a predetermined trajectory, much like a crude nuclear impulse drive.

Tech Level: 10

This involves landing a conventional rocket-powered space ship on the surface of the asteroid or comet, anchoring it down, canting the motor up at a proper angle, and using the rocket engine's thrust to nudge the megaton ball of rock and ice to a new course.

Needless to say, this would only work with a comet or an asteroid that's solid all the way through. The engine would have to placed more or less in line with the body's center of gravity to be most effective. Also, because most small bodies are rotating at a significant clip, burns would have to be precisely timed to be effective.

The major drawback to this scheme is that conventional rocket engines are notorious fuel-guzzlers. In order to impart sufficiently strong enough impulse to induce a course correction, the engine may have to fire hundreds of times and be resupplied with fuel at regular intervals over months or years.

Tech Level: 10

Very similar to the rocket engine scheme outlined above, but would use an ion engine instead of a conventional rocket. Ion engines are much more fuel conservative and can fire continuously for many months without needing to be refueled. The bad thing about them is they can deliver only tiny amounts of thrust in the short term compared to normal rocket engines. Though the specific impulse of an ion engine will eventually surpass that of a conventional rocket engine thanks to its long endurance, it needs a considerable interval to do so.

Ion engines would be used to move asteroids only if there were no immediate hurry to give it a new trajectory. However, because of their very high fuel efficiency, they would also prove probably the cheapest way to move around such a rock with precision. So while this technique probably wouldn't be used for any near-future deflection missions, it could certain prove a very practical and economic means of herding asteroids in the far future for harvesting and exploitation.

Tech Level: 11

An idea originally proposed by US astronauts Edward Lu and Stanley Love. The two suggested that actually landing any kind of probe on the very uneven and pockmarked surface of an asteroid or comet would be very impractical, so their idea centers around a probe that would mass around 20 tons and use its own gravity to nudge asteroids up top 200 meters across onto new trajectories.

Basically the tug would orbit the target body, making course corrections with its engines, and literally tow the asteroid onto a new course with its slight gravity over the course of a year or more. The probe's thrusters would be angled away from the asteroid's surface so that they did not reduce the towing force. The nudge given would be very slight on all but the smallest bodies, and while that would be good enough to deflect would-be doomsday asteroids with plenty of advanced warning, it would be very impractical for any other application, such as asteroid herding or mining.

Tech Level: 11

A probe is sent out to the target body, which releases inflatable trusses with highly reflective mylar stretched between them. Once fully deployed, the probe forms a large parabolic solar mirror. The focus point of the mirror is trained on a small area of the asteroid of the comet, heating it and inducing outgassing. The probe slowly orbits the target rock, training the focus on the same spot with every pass, eventually creating enough thrust in the right direction to initiate a course change.

Like with the ion engine scheme, this option would be economical but very slow to produce any real results, and thus would most likely be deployed for low-priority asteroid and comet herding as opposed to any near-term deflection mission.

Tech Level: 12

A mass driver is another name for an electromagnetic launcher. This scheme uses a complex form of propulsion that has never been tested outside the laboratory, but there is every indication that the concept is sound.

The mass driver would consist of four major components: power supply (either nuclear or solar powered), drill/mass extractor, loading mechanism, and driver coils. The drill/extractor digs into the surface of the body and breaks apart small pieces, which are then fed into the driver coils. The driver coils would resemble a large, broad gun barrel sticking up away from the surface, and would function very similarly to a coil gun.

The debris is shot up through the barrel away from the asteroid, imparting some of that energy into the mass of the asteroid or comet. After many such shots, the body is slowly nudged onto a new trajectory. Much more fuel and energy efficient than standard rockets and packing substantially more oomph than ion engines or solar mirrors, this would probably be the method of choice for moving asteroids once the technology is better developed, for both deflection missions and asteroid and comet herding.

Tech Level: 12

Perhaps the most fuel and energy efficient of all the schemes mentioned here, this is simply anchoring a solar sail onto the surface of the asteroid, unfurling it, and letting pressure from sunlight very gradually push the intruder out of the way. This is one technique that would work much more effectively on asteroids than on comets. Solar sails are most effective in the inner solar system, and the copious outgassing from comets in that region would make it difficult at best to deploy and operate the sail efficiently, even if it was coupled with laser propulsion from a fixed point deeper into the sun's gravity well.

Of all the options discussed in this article, this is the one that will produce the most gradual change in direction and velocity and direction, probably requiring years to work to full effect. However, once set up, a surface-anchored solar sail would also be able to operate with minimal maintenance or oversight for that entire time.

One configuration for the solar sail for this scheme is for a large kite-shaped structure several dozen to several hundred meters across arranged on rigid struts perpendicular to the body's surface. As the body rotates, the sail's light-reflective side emerges at the same spot against and again, slowly pushing the asteroid or comet in the right direction.

An alternative design would use long rectangular vanes that could be unrolled along the asteroid's plane of rotation but perpendicular to its surface, basically turning the body into an oversized heliogyro solar sail. The large vanes can be oriented individually, greatly increasing the maneuverability of the modified body.

Tech Level: 13

A space-borne weapons platform, usually cited as being laser but could also be a maser or particle beam. There are two methods such technology could work to alter the trajectory of an asteroid or comet.

The most straight forward is to simply blast one small area of the body over and over again in rapid succession, forcing both outgassing and explosive force to nudge the rock on its way. Of course, the weapon would be calibrated not to be so powerful as to risk fracturing the target, and the exact target point would be carefully calculated to ensure maximum impact.

A less intuitive way of nudging the asteroid or comet is to use a laser from a fair distance off, hitting the broad face of the rock with a wide beam. Like with the nuclear impulse option, the radiation pressure over the entire side of the body will eventually nudge it into a new trajectory. The directed beam weapon cannot deliver as much energy in a single pulse as a nuclear detonation can, but the weapon can fire repeatedly over a given period of time, being able to eventually deliver just as much total energy to the target, if not more.

Tech Level: 14

An asteroid tug is any type of spaceship that is specially designed to attach itself to an asteroid or comet and use its engines to alter the rock’s course. Unlike other schemes, the tug would not be a one-time, one-rock machine; it would be reusable for any number of such missions. With ships like a tug, asteroid herding becomes much more economically viable, as large amounts of asteroids and comets can be moved with one machine.

Also, by Tech Level 14, it is assumed more powerful rocket engines such as plasma, fission, or fusion rockets will come online, making moving multimegaton masses much more viable for relatively small spaceships. Ships are also assumed to be operating automatically or being teleoperated by remote human operators, but asteroid tugs can be manned as well.

There are two general methods a tug can work. On is by landing on the asteroid or comet, carefully positioning itself and its thrust exhaust, anchoring itself down, and

firing away until the ball of rock or ice is locked onto a new course.

A second method is anchoring cables (most likely made up of carbon nanotube filament, like those being discussed for use in space elevators) to strategic points on the facing surface and literally pulling the body into a new orbit.

In neither scheme would the tug have to be permanently attached to the asteroid or comet, unless speed is a factor and the mass would have to be constantly accelerated/decelerated the entire way. Just a few initial boosts would likely be needed to send it where it needed to go. Either the mass could then be captured by a gravity well at its destination, or another tug or ship can rendezvous with it to slow it down into an appropriate orbit.


General Information on Asteroids and Comets

General articles on moving Comets and Asteroids

Very technical discussion about the physics of moving Asteroids.

Article added 03/07