Final answer:
The tone of the passage from 'The Race to Space: Countdown to Liftoff' is best described as 'tense' due to the description of critical and potentially dangerous circumstances faced by the astronauts during their return to Earth.
Step-by-step explanation:
The passage from The Race to Space: Countdown to Liftoff describes a situation with astronauts facing a very critical and potentially dangerous moment as they return to Earth. The description of their surroundings, the mention of minimal electrical power, and the physical sensations of melting ice and water give the passage a very tense tone. It is not happy, as the situation is fraught with peril; not detached, as the passage delves into the specifics of the astronauts' experience; and not pessimistic, as it does not convey a sense of hopelessness or negative outcomes.