At the same time, and quite distinct from the feelings of sadness and regret, there were episodes of brief but overwhelming depression, almost a physiological response, as bad as anything I’d experienced before.
-- Leigh (Chapter 2, Part 4)

Importance: As the spaceship Nereus passes beyond Pluto, the crew loses sight of Earth, leading to immune responses such as fevers, rashes and the depression Leigh describes here. In Florida, doctors name a new disease - Earth Loss - based on what Leigh describes in this quote. While science and speculative fiction is often preoccupied with the wonders of space, here MacInnes shows that long-term space travel would have real physiological and psychological effects.

Our velocity after acceleration is such that the smallest fleck of dust may destroy us. The propulsion system locks on to the exterior of the craft and runs, like the navigation, completely autonomously.
-- Leigh (Chapter 1 Part 4)

Importance: This quote shows the incredible power contained within the propulsion...