Grav-thrusters, generally used for acceleration, spout Higg's bosons in order to pull the ship forward. Alteration of the makeup of the hull plating allows these bosons to be 'recycled,' storing them in a state where they will take up no mass, and can be reused. Antigrav-thrusters, naturally, perform the opposite effect, and are mainly used to brake the ship. Of course, there are thrusters of both kind all over the ship, meaning that one form might supplant another in an emergency. However, due to the fact of the thrust that anti-grav drive provides, they are not recommended as a permanent source of forward thrust, as the fuel must be dumped behind the ship to push it forwards, requiring it to be left behind. As the trip is a long one, a recyclable source of fuel is a must.
However, as recyclable sources of fuel are few and far between, temporary sources of propulsion are available on board. Ion propulsion is the first backup used regularly. By shooting out ions from the rear of the ship, thrust is produced, although in a relatively small quantity. As this does require unrecyclable fuel to power, it is, of course, not usable as a permanent source of propulsion.
The most powerful means of transportation in space, at the time of the Amphodel story, is not by propulsion at all, but by the use of pseudofolding. While true folding involves folding space from the starting point to the target point, pseudofolding involves very small folds, one at a time, towards the target point. Creating any sort of fold takes months, so the time required to arrive by pseudofolding is much greater than that of a true fold, especially when a very large distance is to be crossed. Also adding to the time of the journey is the requirement that the ship making the fold be completely stopped. This means that once a ship exits and reconnects a fold, it must come to a come to a stop in order to make another and enter it, creating almost a full day of acceleration and braking for each fold.
The scientific community of humanity has, as one might expect, greatly advanced in the time since the present. During the time of Amphodel's setting, cybernetics is the field of the day. With regard to the Amphitrite project, several types of cyberneticists were called upon.
As for science's interaction with culture, one direct impact of the cybernetics revolution is the requirement of several times as many years of education, in many cases. Earth, even before the fold accident, still had many underdeveloped regions in Africa and South America. The exodus to Mars was expected to eliminate the disparity, as everyone would get a chance to inhabit equally developed areas. And although many of the ethnic groups tended to band together out of their perceived common bond, they had a common bond with everyone that came with them, and the progress was inescapable.
And thus at the time of Neptune II's launch, racial barriers are nonexistent, and educational equality is generally the case, wherever you look. This phenomenon resulted in the creation of a much larger scientific community than there had ever been on Earth, making scientific progress move faster than it otherwise would have.