SPOCK: Doctor, assume this is the planet we’re on. This is the approaching asteroid. If we don’t get to that deflection point in time, it will become physically impossible to divert this asteroid. In that case, everyone on this planet will die, including the captain.
MCCOY: Can a few more minutes matter, Spock?
SPOCK: In the time it’s taken me to explain the problem, the asteroid has moved from here to here. The longer we delay, the less the likelihood of being able to divert it.
I love that Spock is functionally in command with Kirk missing, and he still takes the time to explain to McCoy why he’s making his orders. You can tell he’s changed a bit since The Galileo Seven.