NASA has sent a whole host of spacecraft across the solar system and even beyond. They range from crewed ships to orbit and to the moon to robotic explorers. Among them are a range of mission classes from Flagships to Discovery Class programs. Now a new category has been announced: Probe Explorers. This new category will fill the gap between Flagship and smaller missions. Among them are two proposed missions; the Advanced X-ray Imaging Satellite and the Probe Far-Infrared Mission for Astrophysics.