Hoppers are a class of short-range, reusable manned rocket vehicles designed to operate solely on the moon. An updated off-shoot of lunar lander technology developed for the Apollo missions, hoppers would have two purposes: to bring crews and cargo up and back from lunar orbit, and to allow long-range excursions over the lunar surface farther than manned lunar rovers can reach.
Hoppers are so-called because they are designed to operate in short "hops"--either up to and back from orbit, or in parabolic sub-orbital arcs across the lunar surface. They are usually envisioned as small two- or three-manned vehicles, designed to carry payloads of only a few tons at most. They may be pressurized and provide life support, though some versions currently being discussed are unpressurized and would need passengers to wear pressure suits.
As an orbital vehicle, they work pretty straight-forward, relying on one or more powerful rocket motors for both ascent and to slow it down on descent and initiate a soft landing. Because the moon’s gravity is significantly less than Earth’s, the hopper’s engines do not have to be nearly as powerful to obtain orbit, as was evidenced by the modest lunar ascent vehicles on the Apollo missions.
For excursions across the surface, a hopper will move in long suborbital parabolic arcs over the surface of the Moon. The rockets will boost it upward at an angle a number of miles over the surface, then use its momentum to carry it the rest of the way as it decelerates for a soft landing. This way, astronauts in a hopper can cover distances of hundreds of kilometers in under an hour, compared to the days they may have to spend in a lunar rover to cover he same distances. The downside of course is that hoppers would be much more expensive to produce and maintain, and would consume far more fuel.
Because these rely purely on rockets for propulsion, fuel availability and conservation will always be a major concern with these vehicles, especially when the Moon is still sparsely settled and supply is always at a premium. For most routine tasks over the surface, unmanned teleoperated rovers will be used simply because of their greater economic advantage. Hoppers will be used for missions that are too far away over the surface, such as sending maintenance crews to a farside observatory, or where speed is an absolute necessity, such as rescue missions. They will be most useful in surface excursion mode in moving personnel between surface facilities quickly and easily.
NASA and the X-Prize Foundation are currently jointly sponsoring a $3 million "X-Prize Cup" to encourage private development of lunar hopper technology that may be used on future space missions.