Scene Context: Les realizes that the Sagan cannot simply turn around and retrieve Sarah without dooming the entire mission.
Essence of the Phenomenon: A massive spacecraft is incapable of performing a rapid "drive-by" to pick up a person in open space, as often depicted in movies. Turning around, decelerating, altering trajectory, and returning to the landing profile requires a massive actual delta-v budget.
Scientific Basis: All orbital mechanics are governed by the law of conservation of momentum and a finite supply of reaction mass. If the spacecraft is only hours away from atmospheric entry, its trajectory is already rigidly bound to the mission's energy budget. Any major rescue maneuver means expending that budget.
Current Limitations: In such a scenario, attempting to save one person can automatically doom the entire crew. This is not a moral choice between cruelty and compassion, but a direct conflict between flight physics and human reaction time.