Us cities to visit for Within the Jupiter family is the group of Encke-type comets. The shape and orientation of these comets’ orbits around the Sun are modiied by long-range interactions with Jupiter, but their orbits no longer bring them into close approaches to Jupiter. Indeed their entire orbits are entirely within Jupiter’s orbit. Over time, it seems that a comet’s nucleus may form a crust that seals its remaining volatiles inside. As a consequence, it may cease to react to the Sun’s heat and therefore no longer develop a coma and tails. Since the nucleus itself is darker than freshly laid asphalt, it ceases to be visible to naked-eye observers on Earth and is liable to be mistaken for an asteroid. Astronomers had classiied Wilson-Harrington an asteroid but then discovered some old images from decades beforehand that revealed that it had formerly sported a gas tail. Us cities to visit 2016.