Traditionally, planetary exploration uses landers and rovers for in situ measurements and orbiters for remote sensing. Landers and the first generation rovers can conduct studies of very limited areas of the planet: square metres for landers and square kilometres for rovers. The main driver for selection of landing sites is safety and the safest sites are usually flat and not scientifically interesting. Besides, even the best imaging from the orbit cannot guarantee an obstacle-free site needed for safe landing.

Robotic balloons (aerobots) may significantly change the future of in situ planetary exploration. Aerobots can be used to study eight solar system bodies with atmospheres: Earth, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Saturn's moon Titan. Besides the Earth, Venus, Mars and Titan are the prime candidates.

Venus is the closest and the easiest planet for aerobots. The first planetary balloons ever flown were part of the highly successful Soviet-led VeGa (Venus-Halley) mission in 1985 (Sagdeev et al., 1986; Kremnev et al., 1986; Blamont et al., 1993).

On Venus, aerobots may serve as the scientific platforms for in situ atmospheric measurement and for study of atmospheric circulation. They can be used to drop imaging and deep sounding probes at sites of interest and to acquire and relay high-rate imaging data. Balloon ascent from the surface is essential for a Venus surface-sample return mission.

On Mars, aerobots can fill the gap in resolution/coverage between orbiters and rovers. Powered aerobots (airships) can make controlled global flights for highresolution radar, visible, infrared, thermal, magnetic and neutron mapping. They can be used for deployment of a network of surface stations. Tethered balloons could provide ultra high-resolution imaging of local areas for navigation of rovers and data relay to the main lander station. Solar-heated balloons could be used as atmospheric decelerators for low-speed landing and to conduct studies in summer polar areas. In the more distant future, airships could be used for human transportation.

On Titan, powered aerobots, and to a lesser extent free balloons, can perform long-duration low-altitude global flight for surface mapping, in situ atmospheric measurements, take surface samples and deploy landers and rovers for in situ surface studies.

One attractive feature of aerobots is their capability for deployment of large-size (but light-weight) structures that can be used to increase resolution and sensitivity of science instruments exploring the surface and sub-surface of the planet, and to increase communication data rate.

Aerobot technologies have advanced in recent years as a result of progress in envelope materials and design - technologies driven primarily by the needs of scientific balloons for the Earth's stratosphere. Technologies for deployment and inflation, navigation, control, communication and power are also developing rapidly in response to planetary applications.

6.1.1 Balloon basics and planetary environments

Any lighter-than-air (LTA) vehicle can be described by Archimedes' 2000-year-old principle of flotation:

where B is the buoyancy force, V the volume of gas inside the balloon, and pa the atmospheric density. At equilibrium

g where M is the total mass of the aerobot, Mb the mass of the balloon envelope, Mg the mass of gas and Mp the mass of the payload. The denser the atmosphere, the smaller the volume of buoyant gas (and aerobot shell) needed to fly. Using the ideal gas law, Equations 6.1 and 6.2 can be written as

Mb + Mp = (pa-p)V = Vpa„i( 1 _pWa^ = pv( 1 -^„g^ (6.3) b p Pg) RTa { Pa„aTj ^ V Pa„jJ ^ '

where pg, Pg, „g and Tg are the density, pressure, molecular weight and temperature of the buoyant gas; Pa, „a and Ta are the pressure, molecular weight and temperature of the ambient atmosphere, and R is the gas constant. If the pressure inside the balloon exceeds the ambient pressure by AP (AP is called the superpressure) then

This basic equation describes all types of balloons. Their classification is illustrated in Figure 6.1.

One more balloon type - Rozier - is a combination of a light-gas and a Montgolfiere balloon. Cases with „g < „a and 1P = 0 describe light-gas zero-pressure balloons:

The buoyancy of these balloons increases with the temperature of the buoyant gas. In a steady flight of a balloon made of transparent film without an additional heat source, Ta = Tg and

For a fixed mass of gas, the inflated volume of zero-pressure balloons varies with ambient pressure, i.e. with altitude. If the zero-pressure balloon is displaced from its equilibrium altitude (where gas fills all balloon volume), e.g. by upward vertical convection currents or by heating, it has to vent gas to avoid stress in the envelope and maintain 1P = 0. When the disturbance action stops, ballast has to be dropped to bring the balloon to steady flight (now at higher altitude). Venting of gas and use of ballast significantly limits zero-pressure balloon lifetime and their use in planetary exploration.

The pressure of the buoyant gas of superpressure balloons in steady flight exceeds ambient pressure; the balloon envelope is filled completely and has a

Figure 6.1. Classification scheme for balloons.

fixed volume. These balloons can remain afloat for long periods of time. On Earth, some superpressure balloons have stayed aloft for several years. Use of strong (and heavier) materials and more demanding balloon design is the price to be paid for long-duration flight. A number of materials (polyester, Kapton, nylon, PBO films and different composites) can be used for superpressure balloons. A 'pumpkin' shape design, where tendons take most of the superpressure load, significantly relieves requirements on the balloon material and allows the use of weaker films.

Superpressure balloons are described by Equation 6.4. Superpressure AP can be calculated as

AP increases with the temperature of the buoyant gas and balloon performance is driven by the radiative environment and properties of the balloon material. Even visually transparent films significantly absorb infrared radiation. To design superpressure balloons, radiation fluxes in the planetary atmospheres should be known or carefully evaluated.

The buoyancy of ambient gas balloons („g = „a, Pg = Pa) is created by heating of the gas and depends on an overtemperature AT. The governing equation is

On Earth, balloons can be heated by solar radiation during the day and by Earth/ atmosphere infrared radiation at night. On Mars, buoyancy can be produced only by solar heating (see Chapter 8) and solar Montgolfière balloons can be used only for daytime missions. Concepts for Titan include using ambient air warmed by the waste heat from an RTG (see Section 9.3).

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