I didn't see this before. Phobos, like the other Martian moon, Deimos, is an obvious "capture." If you were standing on the Martian equator, both moons would "rise" in the west, and set in the east. Deimos takes about 15 hours to cross your visual horizon. It is obviously the older captured object, and it is "slowing done." It is also more than 15,000 miles above the planet.
Phobos, however, is less than 6000 miles above the planet. It is moving much faster, too. It would cross your visual horizon in under four hours. If the Japanese Space Agency pulls this off, that will be pretty slick. Like landing on the ball in a pinball machine.