As a probe approached a distant moon, Nova and Luna compared two descent plans on the control screen. Both graphs showed altitude in kilometers as a function of time in minutes. Plan A began at an altitude of 12 kilometers and decreased at a constant rate of 2 kilometers per minute, reaching altitude 0 at 6 minutes. Plan B also began at 12 kilometers, but after 4 minutes its altitude was still 6 kilometers. From that point, its graph became steeper and reached altitude 0 at 6 minutes. Luna noted that both plans had the same vertical intercept and landing-time intercept, but their rates of change behaved differently. Plan A provided a steady descent throughout the interval, while Plan B required a faster final drop. To avoid the sudden change in descent rate, Nova and Luna selected Plan A. The probe followed the steady path and landed at the planned time.