This is a very interesting question. Martial art, if done properly, is very effective in a zero-gravity field. All techniques applicable in a combat in a gravity field is equally available in a zero-gravity situation. However, these techniques need an impulsive change in trajectory right before impact. I will go through the three basics: strikes, holds and throws. If you think of any other, please comment of go ahead to create your own answer.
The physics is thus: When you throw weight in a straight line towards an obstacle, the equal and opposite reaction occurs along that same line. So conventional straight punches and straight kicks are almost useless for two reasons
- Most conventional strikes are done with the ground as a root. Unconsciously, martial artists use this third law of motion in their strikes by pushing up from the ground (which is sturdy) and translating or "channeling" that reaction energy through their extensions [; I will use "extensions" to mean anything you can strike with: fists, feet, shoulders, hips, head, and so on]. Since there is no sturdy ground in zero-gravity (I assume the two combatants are floating) there is nowhere to get this reaction energy from.
- When the strike makes impact -- and the force on impact is equal to the force it meets -- it can displace the striker into uncomfortable positions. This could either be spinning or directional displacement.
The momentum for a proper strike in a zero-gravity field should originate from one's core (right about where your navel is). This momentum is then translated to your extensions while they are lose. Immediately before impact, the martial artist should look to tense up the extension, and change the trajectory of the hit. This, if done correctly reduces the distance the momentum has to travel; thereby...
- creating an explosive effect on impact
- changing the trajectory of a the reactive force
The example hit is Bruce Lee's "One Inch Punch". The performer stands facing his target, with his body extension (fist) already stretched out and loosely clenched. When the momentum from the core reaches the fist, the fist is tensed and directed upwards. It is difficult to generate power from this technique but with practice, you get better.
This is the easiest way to win (and lose!) a fight in zero-gravity. If you are grabbed and the grab is a proper hold to choke, I do not think there is much to do in this situation. You can try to tuck or roll, to escape from this would require strength.
However, choking is not easier wither in zero-gravity. We must be aware of the function gravity plays in a choke -- especially when it is on the ground. Most submission holds has the performer exerting some downward force during his hold. The weight on his hold is greatly helped by gravity. In a zero-gravity space, the martial artist does not have this luxury and is therefore left to his strength alone to perform the hold. As the fight drags on, strength will wane.
This is the weakest form attack in zero-gravity. Recall the little splurge of physics I wrote about when explaining strikes. If one studies throws, one will readily notice that the thrower needs to be a rigid body OR backed by (or grounded on) a rigid body. This rigidity must have enough resistance to withstand the thrower's momentum and the weight of the "throwee". Such grounds do not exist in a zero-gravity field. Also, any momentum generated by the thrower will be easily rebounded based on Newton's Third Law.
Assuming a throw is properly executed... where to? Assuming a boundless zero-gravity field, a throw does not only provide the target with a means of escape, but it does not deliver any damage whatsoever.
Also, if a martial artist goes in for a throw (with enough momentum to do some damage) and he is held before he can execute the throw. Both practitioners will begin spinning; at which point the spar becomes a game of dance.
Concluding, I would say a spar session in zero-gravity will be a true test of martial arts. It allows the artist recede into himself, diminish his reliance on external factors and provides an environment for the artist to be the art which he practices: fine-tuning strikes and escape from holds.