With the spectacular impact of comet Shoemaker-Levy 9 on Jupiter, we start to wonder what would happen if a comet was headed towards our homes on Earth. If we discovered a comet was coming our way with only a few months warning (I'll call it six), could we render it harmless? I think we could, and I will propose a method (or two methods, combined) for accomplishing the feat.

The first thing I would _not_ do is try to divert the whole comet. Assuming that, unlike SL9, the threatening berg is still in one piece, it won't be once we start trying to install extractors and engines on its surface, apply thrust, or other disruptions. Only small tidal force was needed to rip SL9 to shreds; most comets and even some asteroids are also quite fragile. My own comet extractor design, for example, will require careful installation of stiffening structures and geosynthetic bags (kind of like the webworks used to hold back mud slides that you sometimes see on mountain roads). The design will require quite a bit of simulation and adjusting, and very good knowledge of the composition of the particular comet before it will work. Not the kind of stuff we can develop in six months.

The second thing I would not do is match velocities. Why waste free kinetic energy? Besides, in the worst case the comet could be retrograde like P/Halley, with rendesvous delta-v prohibitive.

That leaves hitting the comet in an intercept trajectory and trying to bust it into pieces small enough to burn up in Earth's atmosphere.

I'd take advantage of both kinetic energy of intersecting trajectories and the fragility of the comet. Two good ways to deliver kinetic energy are "needles" (long thin objects, biased to orient on the intercept velocity vector) and "threads" (a web oriented orthogonally to the intercept velocity vector). Specifically:

* A cloud of small pins or javelins. These would be designed to penetrate up to the entire diameter of the comet (parallel to the intercept velocity vector), causing cracks, and creating outgassing pressures (from heat dissipation) that further weaken and disrupt the structure. Explosive deployment would be timed so that the cloud would be roughly the diameter of the comet upon impact.

* Large gossamer webs, in the shape of a spider's web, made out of Kevlar. The deployment mechanism might be explosive, it might involve several Brilliant Pebbles orienting and thruster systems on the edges of the web that make it spread out, or it might involve an electromagnetic loop on the edge, so that repulsive force spreads the web. We have a wide variety of options for the deployment step. The web will have a slightly larger diameter than the comet (depending on our accuracy; if we hit it dead center it can be a bit smaller). We hit the berg with several webs, the first one very coarse (containing only a few very strong tethers) the last fine enough to break remaining pieces into chunks so small they will burn up high in Earth's atmosphere, their energy dissipated so that no blast reaches the ground.

The best method might be to hit with the pins first, to weaken the structure, and then hit with the webs. The net effect might be something like dropping a snowball ten stories onto an iron grate platform, only moreso. (A comet is much weaker than a snowball, and the collision velocity is on the order of 10 km/s rather than 100 m/s: 10,000 times the energy per unit mass).

Note that we need to know the position of both the comet and the intercepting spacecraft to within less than a single comet diameter. This will require extensive effort both in observation of the comet and spacecraft navigation. We had good experience with the Galileo observations of the asteroids Ida and Gaspra in this regard.

Automated deployment of long (10 km length) tethers is currently within the state of the art -- this was done by a student group off a Delta rocket last year. Deploying a web of such tethers is an added but not insurmountable challenge. With six month's notice, a wide variety of packages could be built and put on Atlas, Delta, Titan, Ariane, Proton, Cyclone, Zenit,Long March, and H-2 launches that were scheduled for other spacecraft within those six months (plus some that were scheduled for later if they can be expedited on an emergency schedule). We can probably get off 10-15 launches. A more powerful or extra upper stage, depending on the specific configuration, can be added to provide the extra boost to an interplanetary trajectory (about 9-25 km^2/s^2, depending on the specific cometary and intercept trajectories, more than GTO where most of the launchers were targeted before the emergency).

Design diversity would be provided by several teams in parallel designing payloads of different sizes (for a specific rocket's payload shroud) and different functionality (some pins, some webs, some nuclear devices to pick up any strong rocks that get through the pins and webs). This provides a good kind of redundancy -- we're less likely to get a design failure that effects all the misions. Also, should interception fail on the first attempt due to an error in navigation or deployment that can be corrected by on board reprogramming, the error could be corrected for the followup attempt(s), which would already be on their way, scheduled for a few days following the first attempt.