Until now, the only practical balloon systems proposed to explore the martian atmosphere have been superpressure balloons, which fly at a constant altitude, or short-lived helium balloons, which precariously drag a snake through;all types of surface weather, or a day/night combination of the two. For the first time, two novel atmospheric balloon systems now appear quite viable for controlled balloon landings at selected martian surface locations. These balloons could softland payload packages, such as lightweight surface roving vehicles. The two balloon approaches and a land rover concept are described below, along with a combination of the two approaches. Solar Hot-Air Balloons: These "Montgolfiere" balloons are named after the 18th-century French brothers Joseph-Michel and Jacques-Etienne Mongolfier, who first flew hot-air balloons. Using entirely solar heat, they are ideal for landing at the martian poles during summer or for shorter flights at lower latitudes. Recent tests have already confirmed the ease of altitude deployment and filling of these solar hot-air balloons. Furthermore, actual landings and reascents of solar hot-air balloons have been recently demonstrated by JPL, using a novel, lightweight, top air vent that is radio controlled. One particularly useful application of these balloons is their use as a parachute to soft-land packages that are up to 50% of the total entry mass, which represents a fivefold improvement over present retrorocket landing-systems. Variable-Emissivity Balloons: A second atmospheric balloon system uses a variable-emissivity superpressure helium balloon that can land at night at any martian latitude. These balloons would be gold-coated, superpressure helium balloons during both night and day, They could land at prescribed targets by exposing a section of the upper white balloon surface to the radiant cooling of deep space during the night. This reduces the temperature and pressure in the balloon to create negative buoyancy, thus causing descent, while replacement of the gold top cover; causes reascent. Specific areas could be targeted for landings by using atmospheric currents at various altitudes, similar to techniques used by balloonists flying over the Earth. Inflatable Roving Vehicles: JPL has recently fabricated and tested a number of roving vehicles with large inflatable balloons that act as tires. One version, with 75-cm-diameter wheels, has already demonstrated the ability to make large traverses in JPL's simulated "Mars Yard." A full-scale version, with 1.5-m-diameter wheels, should be capable of climbing large rocks (less than or equal to 0.5 m), traveling reasonably fast (approximate to 500 m/h) and far (approximate to 10 km), and yet have very low mass (approximate to 6 kg). Low-Cost Combined Atmospheric/Surface Mission: A simple, solar hot-air balloon would act as a parachute to land a 6-kg inflatable rover. The balloon would then rise to a 3-km altitude while carrying a 2-kg camera/magnetometer/communications package for the remainder of daylight hours. The entire package would then soft-land at dusk. Total Mars entry mass would be about 20 kg, and the mission could be flown to Mars at very low cost (approximate to $5M total launch costs) via one of the CNES Ariane 5 GTO piggyback launches. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.
