Comet Tempel 1 wins...I guess one rule one can draw apart from all the science value, is equipment and humans steer clear of comets and meteors tracks.
Well, in all honesty, Deep Impact was moving nearly 24,000 mph relative to comet Tempel 1 when it performed its function of ramming it. A human or probe equipped expedition could rendezvous with a comet with an approach speed of several miles per hour. Taking a week or so to pierce the coma and approach slowly enough so that impacting particles are moving slowly is the way to do it. Even ESA's Halley's Comet probe Giotto took some impressive hits when it flew by--buth the probe was moving at several kilomters per second relative to the comet. No attempt was made to do a slow flyby at nearly rendezvous speeds--because the delta-v requirements are much higher for rendezvous. A peopled nuclear powered mission should not have this as much of a constraint: rendezvous would be the obvious feature of such a mission. This automatically increases safety by decreasing relative speed between probe and comet.
I agree with Ty. I think John Hopkins U./NASA did manage an asteroid soft land on a small celestial solar system body. And no doubt a soft land on a comet/asteriod to allow more science comet assay is within reason, but a routine mining operation to process water seems harder to accomplish when easier water mining and processing can be accomplished on more stable celestial bodies like moons or planets.
Science value is necessary to possibly divert an asteriod/comet if in fact it was on a heading toward Earth. The fact remains, two massive celestial bodies like the Earth and Body X where to meet, one of the two or both would need to change course. Naturally, a comet of sufficient small size could be diverted, or sliced in pieces, but there is nothing humans could do in the quick short term to prevent a massive Km sized Celestial Body X from it's planned impact with Earth leaving most if not all humans in the aftermath gone. I think this scenario has been played out before with another species of life on Earth.
Time is of the essence when dealing with collision diversion. If a collision is detected sufficently far enough in advance, then something really low tech like an inflatable mirror focussing sunlight on a spot to create an artifical jet could do the trick. Even though the jet has little effect on the comet, the combined impulse over the course of a substantial part of its path near the sun could give enough impulse to nudge it into a slightly different trajectory--one that would miss the earth. Such a mission requires a potential collision to be detected years in advance---this is the scenario that gives us all that '...warm, fuzzy feeling...'
The more likely scenario would involve collision detection perhaps no more than a year out--and this requires substantially more drastic preperations. This is the one typical of the cliche movies where nuclear weapons are launched en masse toward the threatening body. Ironically, nuclear weapons might be more effective against comets than asteroids--only because the volitile fraction of a comet is presumaby much higher. A series of impulses involving the vaporization of some hundreds of millions of tons of comet material from a series of targeted nuclear detonations could provide the necessary delta-v to cause a miss.
The last scenario--I think of as a 'doomesday scenario' because it is both less likely and more likely to occur in certain respects. A 'doomsday scenario' involves a body with which we know nothing about. It is practically undetectable either by infra-red or radar, because the body is small, cold, and is naturally stealthy because of low albedo and naturally radar absorbing materials (iron oxides diffused within a matrix of silicate and ice would be a surprisingly good radar absorber. ) We generally only find out about these bodies after they pass us. Which is bad if you're trying to prevent a collision.
If the body is small (say 10 to 20 m across) and just happens to enter the earth's over a politically unstable spot--let's see there's the border between Chechneya and Russia; there's North and South Korea, and oh yeah, how about the border between Pakistan and India or the disputed Kashmir region. The resultant explosion would look very much like a high yield thermonuclear bomb (between 10-20 megatons) and would certainly flatten and pulverise any cities unfortunate enough to be beneath one. It could be understandable that a country might get 'confused' and without thinking, launch a retaliatory strike. Nuclear war.. Hopefully not global. And all because a chunk of space debris landed in somebody's backyard, and nobody thought to fly radiological instruments through the plume to prove its non-nuclear origin. Ooops. But dead is dead.
The other 'doomsday' scenario is essentially the same as above, only the impactor is say 2km or larger in size. Now the airburst will be collosal. Debris will probably strike the ground, maybe even an intact rock. In the water, Tsunamis may form from the dished crater of the impactor's 'splash.' On land, enormous amounts of dust and debris is ejected and rains down upon the land. Whole regions could be devestated. The globab climate could be modified. An impact in an area where substantial deposites of magnesium sulfate or calcium sulfate (such as in White Sands, New Mexico) would release vast quantities of sulfuric acid aerosols into the atmosphere causing substantial cooling.
Stopping such a large, stealthy impactor would be nearly impossible given the nearly zero lead time. So the only way we can protect ourselves is to actively search these things out--orbit the infrared telescopes that can do the sky scans. Maybe even building large, dedicated space and ground based radars with multiban detection. RADAR absorbing materials usually are fairly frequency specific. A multiban detection system with discrimination ought to be able to pull some signal out of the background noise. Deflecting a large impactor would also be nearly impossible given that the delta-v requirements steadily increase the closer in time and space it comes to the impact point. A 'terminal' interception system would likely require very large, gigaton (thousand megaton) yield thermonuclear bombs. Such weapons would probably mass around 25 to 50 tons each. These would probably more likely fracture the comet as deflect it--but if enough is vaporized the rest may scatter in such a way that the bigger pieces may miss. To completely vaporize the whole comet would take even more energy--hundreds of gigaton yield weapons, or a really big teraton (trillion ton) yield weapon. A teraton weapon would be a monster that would probably mass a couple of thousand tons! It begins to sound impractical.
As with cancer and other medical problems, the key for comet and asteroid deflection is early detection!
