Phobos' orbit is roughly 30,000 kilometers in circumference, and it travels that distance more than three times a day, so its speed in its orbit around Mars is about 100,000 kilometers per day, or about 4000 kilometers per hour, or about one kilometer per second. Since it's about 30 kilometers in diameter, it takes about 30 seconds for it to cross any one point along its orbit. If MAVEN crossed the plane of Phobos' orbit at that point in any of those 30 seconds -- wham! Thinking about it, the fact that they're extending MAVEN's miss time by only 150 seconds is pretty amazing. The two bodies are going to pass very, very close.

We can predict Phobos' position so well thanks mostly to a long history of Earth-based observations, but orbit prediction has recently received a boost from some unlikely occasional astronomers: Mars rovers. Once in a while, you'll see cool videos shot from the surface of Mars, watching one moon or another pass across the face of the Sun. They have also observed Mars' moons passing by each other in mutual events, or passing by bright stars like Regulus and Aldebaran. These observations are cool in their own right, but they also permit astronomers to make very precise measurements of the positions of the moons.