When it contacts normal matter (such as a planetary atmosphere) it explodes in a nearly-total conversion of mass to energy with the atmospheric gas. The result is a really bright, really high-energy upper-atmosphere detonation with pretty much no lingering radiation. The effects of the blast itself depend on the amount of antimatter involved, the ability of the magnetosphere to turn aside the energy of the blast, and suchlike.

I expect Moon and Siren can both give a better estimation of how much energy would irradiate the planet's visible surface, though.

Upper-atmo nuclear explosion? Kindly let me get off planet first. kthxbai

Not nuclear; antimatter. There's really a huge difference in the quality of the explosion. Nuclear explosion, you get radiation from the blast, fallout particles, etc. Antimatter reaction, you get a lot less in the way of lingering radiation, because the mass gets converted into pure energy. A lot more bang for your buck, and it doesn't leave things uninhabitable afterwards.

That is a good point - although considering the velocity of a bolt launched from a ship-mounted railturret in hard vacuum, you might well just get a Column of Hard Energy as it tears down through the atmosphere and bleeds off mass in the process. Could be a hell of a way to sterilize things, really.

And sure, why not?

Fire a conventional(?) projectile just before the AM one to clear a path to get a closer to the ground detonation.

Nice weapon, reminds me a litte of my own Fission Cannon.

Drafting only does so much. The air will rush in right after it, you might extend the annialation plume down another hundred or two meters depending on air pressure and other factors.

You also have to remember that space is not empty. There will be interstellar gas particles, dust, and other such that will "degrade" the antimater before it hits the target.

Gas and dust is ridiculously thin once you get away from most distinct gravity wells, though; on the order of a long atom every cubic meter. The odds of measurably degrading the bolt in hard vacuum, unless you're utterly insane and fighting with this in the midst of a dark nebula or the like, are negligible; even if you do, the energy release of such a collision, while impressive from the viewpoint of those who have no clue how much energy the building blocks of matter can pack, is fairly minimal. You'd have more concern, I'd think, if you were fighting in the Kuiper Belt or the Oort Cloud, where there may still be moderate amounts of loose material to interfere. But then, again, if you fight in a 'crowded' (from a celestial viewpoint) location, you're either brave or an idiot; in the temperatures out there, the molecular bonds in water get amazingly strong, and I rather doubt many ships want to be hit by steel-hard iceballs wandering along at cometary speeds.

Reaction jets on a gauss weapon - particularly gauss ammunition made of solid antimatter, which would have to be precision-generated without every *touching* the thing - just makes my head hurt, Moon. By the time you can do that with any reliability and not lose labs in the process, you can probably fabricate exotic particles with properties far more useful as weapons - particularly since such a reaction jet would likely need to use antimatter in the jet, which sprays extra mass across the field, which, if you fire these at even half the typically presented rate of a gauss weapon, means you're replacing a normal gas cloud with an antimatter cloud that is much, much more dangerous.

Yes, there should be a variance if you fire it through a dense enough cloud of gas; however, given both the velocities involved and the extreme high-band energies released, you'd likely have more to worry about if you were trying to be stealthy, as firing this through a dense cloud would be much like firing a tracer round, leaving a highly excited trail of gas behind it. Of course, firing antimatter rounds isn't exactly something you do to be stealthy in the first place - that kind of explosion isn't exactly quiet.

And yes, assuming the Sand Caster can push enough material in the way to absorb or deflect the antimatter bolt far enough from the ship as to have it safe despite the released energies, it would be a very good countermeasure.

Oy.

No matter what happens, it is still many kilograms of mass moving at a very high speed. Whatever what is between here and there, unless it contain sufficient matter to either completely deflect or annihilate the antimatter projectile, it will strike it's target. This may be slightly less effective than a normal bolt of iron in the immediate strike - A bow-wave of gamma radiation from encountering matter in the path will, simultaneously, bleed off a small amount of kinetic energy from the piece, and serve to 'push' further intervening dust/light particulates from the path. Remember, even photons have a pressure associated with them, even if it is very low. You can use light to turn a propeller, you can use light to sail a starship. A pulse of really BRIGHT light may as well be an explosion. It bears all the same characteristics thereof.

If the target is really lucky, they're a small enough vehicle that this can punch through, with the addition of a lot of extra radiation and heat during the strike. If they're unlucky, it lodges. And it starts to react. Let me say this a few times, for emphasis: Solid Antimatter takes a long time to react with surrounding matter. Solid antimatter cannot immediately react in its entirety with local matter. The required distance between matter and antimatter for their mutual annihilation is extremely intimate, properly measured in the Planck Length, the smallest distance the universe itself bothers to track(Side note). Because of this, unless the antimatter projectile becomes lodged in a target, the majority of reaction will be between the positron shell structure of the anti-iron with the electrons of any near-by matter. This annihilation alone will suffice to push away fine particulate dust. The true ugly happens when the anti-iron becomes lodged in another piece of iron, and e+/e- annihilation is no longer sufficient to push it away. As the p/n pairs start annihilating, energy release will go up by a approximately 3 orders of magnitude, very likely causing the anti-iron to melt or vaporise, freeing it to pair annihilate more quickly, rapidly increasing the explosion's magnitude.

Side Note: Planck's Length is one potential answer to Zeno's Dichotomy Paradox, by denying the infinite divisibility of true space, we place a functioning limit on the infinite sum, making the full sum calculable by simple arithmetic means. Blam.

Kassil: Reaction Jets and Sand Casters would use conventional matter, not exotic matters. They will just produce "stuff" that will impact/ degrade the anti-mater some. Reaction jets because you will be manuvering in this environment will expell dense gas; Sand Casters because they are anti weapon systems spreading sand in a dense cloud. They both are (dirt) cheap anti-systems for anti-mater weapons (and sand casters are anti-laser defenders and can double as emergency reaction engines).

If you add something that will react and apply kinetic explosion/ reactions when high energy reactions occur around it it becomes more effective. These are not perfect anti weapons, but they will but a dent in them. And they are a ton cheaper than an antimater shell.

You might also have a remote that flies near your ship that will pull as an electromagnet helping to deflect these things (and other rail weapon rounds).

Besides, what about a laser tracking system that will simply shoot the thing as it is incomming.

There are always lovely (and cheap) anti weapons. systems.

I can picture that ammo for the EAR would cost a fortune; especially if it is fired in full auto, as the submission suggests.

Also, stray bullets would be a serious problem - they can stay as 'mines' on the battlefield long after the conflict has faded.