Two techniques have been proposed (see figure) for controlling the buoyancies and thus the altitudes of robotic lifting balloons (aerobots) that would carry scientific instruments for exploration of Mars. Buoyancy-control techniques other than these have been, variously, used on Earth and/or proposed for use in exploring planets other than Mars, but have been found inadequate for providing the requisite altitude control in the thin Martian atmosphere and at the low nighttime Martian surface temperature. The proposed techniques could also be used on Earth; for example, for carrying instruments to perform surveillance, monitor weather, or measure pollution.
The first proposed technique pertains to solar hot-air balloons. The concept of solar hot-air balloons is not new in itself; toy solar-heated balloons have been commercially available for years, and experiments on solar-heated aerobots for planetary exploration have been performed in recent years. The novel aspect of the proposed technique lies in the addition of controllable vents to the tops of the solar-heated balloons, similar in function to the controllable vents on commercial combustion-heated balloons. By letting out heated air from a balloon, one could reduce buoyancy to obtain descent. Conversely, one could close the vent so that as solar heating continued, buoyancy would increase, causing the balloon to ascend. In the case of remote and/or automatic control, the vent could be, for example, a motorized, balanced butterfly valve similar to a carburetor air valve. Of course, the balloon would have to land at night. Using this technique, multiple controlled soft landings and re-ascents have occurred in test flights in the Mojave Desert and off Southern California's Catalina Island.
Another unusual advantage of solar-heated balloons on Mars is the ability to use the balloon instead of retro-rockets, to soft-land payloads. The balloons not only cost much less, but they can increase useable landed payloads from under 10 percent (Pathfinder) to about 50 percent of total atmospheric entry mass.
The second proposed technique would be implemented on balloons filled with low-density gases (e.g., helium) at slight overpressures. This technique would provide altitude control during the night as well as the day. A balloon would be equipped with a variable-emissivity surface to control its internal temperature, and thus its buoyancy, by controlling the balance of thermal radiation among the balloon, the ground below, and the sky above. In the Mars case, for example, a balloon might be coated with gold, and equipped with a gold-coated top cover that could be retracted to expose a white top surface to the radiant cooling of deep space. Thus, retraction of the cover would cause buoyancy to decrease.
This work was done by Jack A. Jones of Caltech for NASA's Jet Propulsion Laboratory. NPO-20360