With these long answer questions the examiners want to see both the quality of your English and your knowledge of the topic. Typically, this means using key words and phrases. The numbers in the brackets are what usually marks are awarded for.
Conservation of momentum always applies (1) in physical situations, and is considered one of the basic building blocks of physics. So therefore when Argon is fired from the tank, in order for the net change in momentum to be zero, then the probe and Argon most move in opposite directions (1). Following on from this it also stand to reason that the change in momentum of the probe must be the same as that of the Argon (1).
Another law in physics is the conservation of energy, of which energy must always be conserved (1). Energy must be put into the system, of the probe and Argon, to fire the Argon from the probe. This energy can then be converted to kinetic energy. Therefore the speed of the Argon and the probe can increase. (You can also state here other ways energy can be converted, but not sound as sound doesn't travel in space)