![]() Outer solar system missions also must carry their own power sources since the Sun is too far away to provide enough energy, or else they must have very large arrays of solar cells. Spacecraft to the outer solar system must therefore be highly reliable and capable of a greater degree of independence and autonomy. ![]() If a problem develops near Saturn, for example, a wait of hours for the alarm to reach Earth and for instructions to be routed back to the spacecraft could spell disaster. Even at the speed of light, messages take hours to pass between Earth and the spacecraft. Flight times to the giant planets are measured in years to decades, rather than the months required to reach Venus or Mars. The challenges of exploring so far away from Earth are considerable. Table 11.2 summarizes the spacecraft missions to the outer solar system. Exploration of the Outer Solar System So FarĮight spacecraft, seven from the United States and one from Europe, have penetrated beyond the asteroid belt into the realm of the giants. As a result, the compounds detected in the atmosphere of the giant planets are mostly hydrogen-based gases such as methane (CH 4) and ammonia (NH 3), or more complex hydrocarbons (combinations of hydrogen and carbon) such as ethane (C 2H 6) and acetylene (C 2H 2). In the early solar system, most of the oxygen combined with hydrogen to make H 2O and was thus unavailable to form the kinds of oxidized compounds with other elements that are more familiar to us in the inner solar system (such as CO 2). This hydrogen caused the chemistry of the outer solar system to become reducing, meaning that other elements tend to combine with hydrogen first. The chemistry for all four giant planet atmospheres is dominated by hydrogen. In the outer solar system, gases dominate the two largest planets, Jupiter and Saturn, hence their nickname “gas giants.” Uranus and Neptune are sometimes called “ice giants” because their interiors contain far more of the “ice” component than their larger cousins. “Rocks” are even less abundant than ices, and include everything else: magnesium, silicon, iron, and so on. Common ices are water, methane, and ammonia, but ices may also include carbon monoxide, carbon dioxide, and others. “Ices” means compounds that form from the next most abundant elements: oxygen, carbon, and nitrogen. The way it is used here, the term “ices” refers to composition only and not whether a substance is actually in a solid state. The “gases” are primarily hydrogen and helium, the most abundant elements in the universe. The material available to build these planets can be divided into three classes by what they are made of: “gases,” “ices,” and “rocks” (see Table 11.1). Jupiter alone exceeds the mass of all the other planets combined ( Figure 11.2). The giant planets hold most of the mass in our planetary system. Summarize the missions sent to orbit the gas giants.Chronicle the robotic exploration of the outer solar system.Provide an overview of the composition of the giant planets.By the end of this section, you will be able to:
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