{"id":429,"date":"2022-02-11T21:42:25","date_gmt":"2022-02-11T21:42:25","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/astronomy\/?post_type=chapter&#038;p=429"},"modified":"2022-04-28T21:27:37","modified_gmt":"2022-04-28T21:27:37","slug":"11-1-exploring-the-outer-planets","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/astronomy\/chapter\/11-1-exploring-the-outer-planets\/","title":{"raw":"11.1 Exploring the Outer Planets","rendered":"11.1 Exploring the Outer Planets"},"content":{"raw":"<div class=\"textbox textbox--learning-objectives\"><header class=\"textbox__header\">\r\n<h3 class=\"textbox__title\">Learning Objectives<\/h3>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n<p id=\"fs-id1168584108202\" class=\" \">By the end of this section, you will be able to:<\/p>\r\n\r\n<ul id=\"fs-id1168466150866\">\r\n \t<li>Provide an overview of the composition of the giant planets<\/li>\r\n \t<li>Chronicle the robotic exploration of the outer solar system<\/li>\r\n \t<li>Summarize the missions sent to orbit the gas giants<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<p id=\"fs-id1168469859612\" class=\" \">The\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term517\" class=\"no-emphasis\" data-type=\"term\">giant planets<\/span>\u00a0hold most of the mass in our planetary system. Jupiter alone exceeds the mass of all the other planets combined (Figure 11.2). The material available to build these planets can be divided into three classes by what they are made of: \u201cgases,\u201d \u201cices,\u201d and \u201crocks\u201d (see\u00a0Table 11.1). The \u201cgases\u201d are primarily hydrogen and helium, the most abundant elements in the universe. The way it is used here, the term \u201cices\u201d refers to composition only and not whether a substance is actually in a solid state. \u201cIces\u201d means compounds that form from the next most abundant elements: oxygen, carbon, and nitrogen. Common ices are water, methane, and ammonia, but ices may also include carbon monoxide, carbon dioxide, and others. \u201cRocks\u201d are even less abundant than ices, and include everything else: magnesium, silicon, iron, and so on.<\/p>\r\n\r\n<div id=\"OSC_Astro_11_01_Jupiter\" class=\"os-figure\">\r\n<figure data-id=\"OSC_Astro_11_01_Jupiter\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img id=\"2\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/61fc6b10a0c424d462be89f457c8294d3af553f6\" alt=\"Photograph of Jupiter. Taken from the Cassini spacecraft, the alternating light and dark cloud bands are visible over the entire planet. The Great Red Spot is at lower right. Also seen is the shadow of the moon Europa at lower left.\" width=\"975\" height=\"460\" data-media-type=\"image\/jpeg\" \/> Figure\u00a011.2\u00a0Jupiter.\u00a0The Cassini spacecraft imaged\u00a0Jupiter\u00a0on its way to Saturn in 2012. The giant storm system called the Great Red Spot is visible to the lower right. The dark spot to the lower left is the shadow of Jupiter\u2019s moon Europa. (credit: modification of work by NASA\/JPL) Abundances in the Outer Solar System[\/caption]<\/figure>\r\n<\/div>\r\n<div class=\"os-table os-top-titled-container\">\r\n<table id=\"fs-id1168466145735\" class=\"grid landscape aligncenter\" summary=\"Table 11.1 \"><caption>Table 11.1 Abundances in the Outer Solar System<\/caption>\r\n<thead>\r\n<tr valign=\"top\">\r\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Type of Material<\/th>\r\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Name<\/th>\r\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Approximate % (by Mass)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Gas<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Hydrogen ([latex]{{\\rm{H}}_2}[\/latex])<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">75<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Gas<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Helium (He)<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">24<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Ice<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Water ([latex]{{\\rm{H}}_2}{\\rm{O}}[\/latex])<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">0.6<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Ice<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Methane ([latex]{\\rm{C}}{{\\rm{H}}_4}[\/latex])<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">0.4<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Ice<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Ammonia ([latex]{\\rm{N}}{{\\rm{H}}_3}[\/latex])<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">0.1<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Rock<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Magnesium (Mg), iron (Fe), silicon (Si)<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">0.3<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<div class=\"os-caption-container\"><span style=\"text-align: initial;font-size: 1em\">In the outer solar system, gases dominate the two largest planets, <\/span><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term519\" class=\"no-emphasis\" style=\"text-align: initial;font-size: 1em\" data-type=\"term\">Jupiter<\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0and\u00a0<\/span><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term520\" class=\"no-emphasis\" style=\"text-align: initial;font-size: 1em\" data-type=\"term\">Saturn<\/span><span style=\"text-align: initial;font-size: 1em\">, hence their nickname \u201cgas giants.\u201d\u00a0<\/span><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term521\" class=\"no-emphasis\" style=\"text-align: initial;font-size: 1em\" data-type=\"term\">Uranus<\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0and\u00a0<\/span><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term522\" class=\"no-emphasis\" style=\"text-align: initial;font-size: 1em\" data-type=\"term\">Neptune<\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0are called \u201cice giants\u201d because their interiors contain far more of the \u201cice\u201d component than their larger cousins. The chemistry for all four giant planet atmospheres is dominated by hydrogen. This hydrogen caused the chemistry of the outer solar system to become\u00a0<\/span><em style=\"text-align: initial;font-size: 1em\" data-effect=\"italics\">reducing,<\/em><span style=\"text-align: initial;font-size: 1em\">\u00a0meaning that other elements tend to combine with hydrogen first. In the early solar system, most of the oxygen combined with hydrogen to make [latex]{{\\rm{H}}_2}{\\rm{O}}[\/latex] 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 [latex]{\\rm{C}}{{\\rm{O}}_2}[\/latex] ). As a result, the compounds detected in the atmosphere of the giant planets are mostly hydrogen-based gases such as methane ([latex]{\\rm{C}}{{\\rm{H}}_4}[\/latex]) and ammonia ([latex]{\\rm{N}}{{\\rm{H}}_3}[\/latex]), or more complex hydrocarbons (combinations of hydrogen and carbon) such as ethane ([latex]{{\\rm{C}}_2}{{\\rm{H}}_6}[\/latex]) and acetylene ([latex]{{\\rm{C}}_2}{{\\rm{H}}_2}[\/latex]).<\/span><\/div>\r\n<\/div>\r\n<section id=\"fs-id1168469838200\" data-depth=\"1\">\r\n<h3 data-type=\"title\">Exploration of the Outer Solar System So Far<\/h3>\r\n<p id=\"fs-id1168466338754\" class=\" \">Eight spacecraft, seven from the United States and one from Europe, have penetrated beyond the asteroid belt into the realm of the giants.\u00a0Table 11.2 summarizes the spacecraft missions to the outer solar system.<\/p>\r\n\r\n<div class=\"os-table os-top-titled-container\">\r\n<table id=\"fs-id1168466163447\" class=\"grid landscape aligncenter\" summary=\"Table 11.2 \"><caption>Table 11.2 Missions to the Giant Planets<\/caption>\r\n<thead>\r\n<tr valign=\"top\">\r\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Planet<\/th>\r\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Spacecraft<sup id=\"footnote-ref1\" data-type=\"footnote-number\"><a role=\"doc-noteref\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/11-1-exploring-the-outer-planets#fs-id1168466162585\" data-type=\"footnote-link\">1<\/a><\/sup><\/th>\r\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Encounter Date<\/th>\r\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Type<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr valign=\"top\">\r\n<td rowspan=\"9\" data-valign=\"top\" data-align=\"left\"><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term523\" class=\"no-emphasis\" data-type=\"term\">Jupiter<\/span><\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Pioneer 10<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">December 1973<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Pioneer 11<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">December 1974<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Voyager 1<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">March 1979<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Voyager 2<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">July 1979<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Ulysses<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">February 1992<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Flyby during gravity assist<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Galileo<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">December 1995<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Orbiter and probe<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Cassini<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">December 2002<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">New Horizons<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">February 2007<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Flyby during gravity assist<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Juno<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">July 2016<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Orbiter<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td rowspan=\"4\" data-valign=\"top\" data-align=\"left\"><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term524\" class=\"no-emphasis\" data-type=\"term\">Saturn<\/span><\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Pioneer 11<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">September 1979<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Voyager 1<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">November 1980<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Voyager 2<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">August 1981<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\">Cassini<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">July 2004 (Saturn orbit injection 2000)<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Orbiter<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\"><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term525\" class=\"no-emphasis\" data-type=\"term\">Uranus<\/span><\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Voyager 2<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">January 1986<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td data-valign=\"top\" data-align=\"left\"><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term526\" class=\"no-emphasis\" data-type=\"term\">Neptune<\/span><\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Voyager 2<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">August 1989<\/td>\r\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<div class=\"os-caption-container\"><span style=\"text-align: initial;font-size: 1em\">The challenges of exploring so far away from Earth are considerable. Flight times to the giant planets are measured in years to decades, rather than the months required to reach Venus or Mars. Even at the speed of light, messages take hours to pass between Earth and the spacecraft. 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. Spacecraft to the outer solar system must therefore be highly reliable and capable of a greater degree of independence and autonomy. Outer solar system missions also must carry their own power sources since the Sun is too far away to provide enough energy. Heaters are required to keep instruments at proper operating temperatures, and spacecraft must have radio transmitters powerful enough to send their data to receivers on distant Earth.<\/span><\/div>\r\n<\/div>\r\n<p id=\"fs-id1168469428508\" class=\" \">The first spacecraft to investigate the regions past Mars were the NASA Pioneers 10 and 11, launched in 1972 and 1973 as pathfinders to\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term527\" class=\"no-emphasis\" data-type=\"term\">Jupiter<\/span>. One of their main objectives was simply to determine whether a spacecraft could actually navigate through the belt of asteroids that lies beyond Mars without getting destroyed by collisions with asteroidal dust. Another objective was to measure the radiation hazards in the\u00a0<em data-effect=\"italics\">magnetosphere<\/em>\u00a0(or zone of magnetic influence) of Jupiter. Both spacecraft passed through the\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term528\" class=\"no-emphasis\" data-type=\"term\">asteroid belt<\/span>\u00a0without incident, but the energetic particles in Jupiter\u2019s magnetic field nearly wiped out their electronics, providing information necessary for the safe design of subsequent missions.<\/p>\r\n<p id=\"fs-id1168469450139\" class=\" \"><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term529\" class=\"no-emphasis\" data-type=\"term\">Pioneer 10<\/span>\u00a0flew past Jupiter in 1973, after which it sped outward toward the limits of the solar system.\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term530\" class=\"no-emphasis\" data-type=\"term\">Pioneer 11<\/span>\u00a0undertook a more ambitious program, using the gravity of Jupiter to aim for Saturn, which it reached in 1979. The twin Voyager spacecraft launched the next wave of outer planet exploration in 1977. Voyagers 1 and 2 each carried 11 scientific instruments, including cameras and spectrometers, as well as devices to measure the characteristics of planetary magnetospheres. Since they kept going outward after their planetary encounters, these are now the most distant spacecraft ever launched by humanity.<\/p>\r\n<p id=\"fs-id1168469874620\" class=\" \">Voyager 1 reached Jupiter in 1979 and used a gravity assist from that planet to take it on to Saturn in 1980. Voyager 2 arrived at Jupiter four months later, but then followed a different path to visit all the outer planets, reaching Saturn in 1981, Uranus in 1986, and Neptune in 1989. This trajectory was made possible by the approximate alignment of the four giant planets on the same side of the Sun. About once every 175 years, these planets are in such a position, and it allows a single spacecraft to visit them all by using gravity-assisted flybys to adjust course for each subsequent encounter; such a maneuver has been nicknamed a \u201cGrand Tour\u201d by astronomers.<\/p>\r\n\r\n<div class=\"textbox textbox--exercises\"><header class=\"textbox__header\">\r\n<h3 class=\"textbox__title\">Link to Learning<\/h3>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\nThe Jet Propulsion Laboratory has a nice video called\u00a0<a href=\"https:\/\/openstax.org\/l\/30JPLGrandT\" target=\"_blank\" rel=\"noopener nofollow noreferrer\">Voyager: The Grand Tour<\/a>\u00a0that describes the Voyager mission and what it found.\r\n\r\n<\/div>\r\n<\/div>\r\n<div class=\"textbox textbox--examples\"><header class=\"textbox__header\">\r\n<h3 class=\"textbox__title\">Making Connections<\/h3>\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n<h4 id=\"3\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Engineering and Space Science: Teaching an Old Spacecraft New Tricks<\/span><\/h4>\r\n<p id=\"fs-id1168466278077\" class=\" \">By the time\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term531\" class=\"no-emphasis\" data-type=\"term\">Voyager 2<\/span>\u00a0arrived at\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term532\" class=\"no-emphasis\" data-type=\"term\">Neptune<\/span>\u00a0in 1989, 12 years after its launch, the spacecraft was beginning to show signs of old age. The arm on which the camera and other instruments were located was \u201carthritic\u201d: it could no longer move easily in all directions. The communications system was \u201chard of hearing\u201d: part of its radio receiver had stopped working. The \u201cbrains\u201d had significant \u201cmemory loss\u201d: some of the onboard computer memory had failed. And the whole spacecraft was beginning to run out of energy: its generators had begun showing serious signs of wear.<\/p>\r\n<p id=\"fs-id1168469474648\" class=\" \">To make things even more of a challenge, Voyager\u2019s mission at Neptune was in many ways the most difficult of all four flybys. For example, since sunlight at Neptune is 900 times weaker than at Earth, the onboard camera had to take much longer exposures in this light-starved environment. This was a nontrivial requirement, given that the spacecraft was hurtling by Neptune at ten times the speed of a rifle bullet.<\/p>\r\n<p id=\"fs-id1168469408112\" class=\" \">The solution was to swivel the camera backward at exactly the rate that would compensate for the forward motion of the spacecraft. Engineers had to preprogram the ship\u2019s computer to execute an incredibly complex series of maneuvers for each image. The beautiful Voyager images of Neptune are a testament to the ingenuity of spacecraft engineers.<\/p>\r\n<p id=\"fs-id1168469384909\" class=\" \">The sheer distance of the craft from its controllers on Earth was yet another challenge. Voyager 2 received instructions and sent back its data via on-board radio transmitter. The distance from Earth to Neptune is about 4.8 billion kilometers. Over this vast distance, the power that reached us from Voyager 2 at Neptune was approximately [latex]{10^{ - 16}}[\/latex]\u00a0watts, or 20 billion times less power than it takes to operate a digital watch. Thirty-eight different antennas on four continents were used by NASA to collect the faint signals from the spacecraft and decode the precious information about Neptune that they contained.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<div id=\"page_d9cfd957-5241-4611-abff-426e7b4f7a0c\" class=\" chapter-content-module\" data-type=\"page\" data-cnxml-to-html-ver=\"2.1.2\"><section id=\"fs-id1168469440698\" data-depth=\"1\">\r\n<h3 data-type=\"title\">Enter the Orbiters: Galileo, Cassini, and Juno<\/h3>\r\n<p id=\"fs-id1168469613798\" class=\" \">The Pioneer and Voyager missions were flybys of the giant planets: they each produced only quick looks before the spacecraft sped onward. For more detailed studies of these worlds, we require spacecraft that can go into orbit around a planet. For Jupiter and Saturn, these orbiters were the Galileo, Cassini, and Juno spacecraft. To date, no orbiter missions have been started for Uranus and Neptune, although planetary scientists have expressed keen interest.<\/p>\r\n<p id=\"fs-id1168466109936\" class=\" \">The Galileo spacecraft was launched toward Jupiter in 1989 and arrived in 1995. Galileo began its investigations by deploying an entry probe into Jupiter, for the first direct studies of the planet\u2019s outer atmospheric layers.<\/p>\r\n<p id=\"fs-id1168469470254\" class=\" \">The probe plunged at a shallow angle into Jupiter\u2019s atmosphere, traveling at a speed of 50 kilometers\u00a0<em data-effect=\"italics\">per second<\/em>\u2014that\u2019s fast enough to fly from New York to San Francisco in 100 seconds! This was the highest speed at which any probe has so far entered the atmosphere of a planet, and it put great demands on the heat shield protecting it. The high entry speed was a result of acceleration by the strong gravitational attraction of Jupiter.<\/p>\r\n<p id=\"fs-id1168469472634\" class=\" \">Atmospheric friction slowed the probe within 2 minutes, producing temperatures at the front of its heat shield as high as 15,000 \u00b0C. As the probe\u2019s speed dropped to 2500 kilometers per hour, the remains of the glowing heat shield were jettisoned, and a parachute was deployed to lower the instrumented probe spacecraft more gently into the atmosphere (Figure 11.3). The data from the probe instruments were relayed to Earth via the main Galileo spacecraft.<\/p>\r\n\r\n<div id=\"OSC_Astro_11_01_Galileo\" class=\"os-figure\">\r\n<figure data-id=\"OSC_Astro_11_01_Galileo\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img id=\"5\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/98c33e0a19b62c1008d75155abf3502dd16a85a7\" alt=\"Artist\u2019s depiction of the Galileo Probe entering Jupiter\u2019s atmosphere. At upper left the parachute is seen, connected by a cable to the spherical-shaped probe near the center of the illustration. At lower right, the protective heat shield falls ahead of the probe, protecting it from the heat of entry.\" width=\"975\" height=\"382\" data-media-type=\"image\/jpeg\" \/> <strong>Figure\u00a011.3<\/strong>\u00a0Galileo Probe Falling into Jupiter. Galileo Probe and Juno Image of Jupiter\u2019s South Pole.\u00a0(a) This artist\u2019s depiction shows the Galileo probe descending into the clouds via parachute just after the protective heat shield separated. The probe made its measurements of Jupiter\u2019s atmosphere on December 7, 1995. (b) This Juno image, taken in 2017 from about 100,000 kilometers above the cloudtops, shows the south polar region of Jupiter with its dramatic complex of storms and clouds. The enhanced-color image was processed for NASA\/JPL by citizen scientist John Landino. (credit a: modification of work by NASA\/Ames Research Center; credit b: modification of work by NASA\/JPL-Caltech\/SwRI\/MSSS\/John Landino)[\/caption]<\/figure>\r\n<\/div>\r\n<p id=\"fs-id1168469521131\" class=\" \">The probe continued to operate for an hour, descending 200 kilometers into the atmosphere. A few minutes later the polyester parachute melted, and within a few hours the main aluminum and titanium structure of the probe vaporized to become a part of Jupiter itself. About 2 hours after receipt of the final probe data, the main spacecraft fired its retro-rockets so it could be captured into orbit around the planet, where its primary objectives were to study Jupiter\u2019s large and often puzzling moons.<\/p>\r\n<p id=\"fs-id1168466282630\" class=\" \">The\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term533\" class=\"no-emphasis\" data-type=\"term\">Cassini<\/span>\u00a0mission to\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term534\" class=\"no-emphasis\" data-type=\"term\">Saturn<\/span>\u00a0(Figure 11.4), a cooperative venture between NASA and the European Space Agency, was similar to Galileo in its two-fold approach. Launched in 1997, Cassini arrived in 2004 and went into orbit around Saturn, beginning extensive studies of its rings and moons, as well as the planet itself. In January 2005, Cassini deployed an entry probe into the atmosphere of Saturn\u2019s large moon,\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term535\" class=\"no-emphasis\" data-type=\"term\">Titan<\/span>, where it successfully landed on the surface. (We\u2019ll discuss the probe and what it found in the chapter on\u00a0Rings, Moons, and Pluto.)<\/p>\r\n<p id=\"eip-173\" class=\" \">The Voyager and Galileo missions to Jupiter were primarily designed to study the moons and the atmosphere of the planet. The next NASA mission, an orbiter called Juno, arrived at Jupiter in July 2016. In order to meet its objectives of studying the jovian magnetosphere, it has a very elongated (eccentric) 55-day orbit, that takes it from 4 thousand kilometers above the cloud tops out to 76 thousand kilometers. The orbit takes the craft over Jupiter\u2019s poles, giving us remarkable close-ups of the polar regions (previous spacecraft viewed the planet from lower latitudes).<\/p>\r\n<p id=\"eip-307\" class=\" \">Juno was originally designed without a camera, but fortunately scientists rectified this omission, adding a simple downward-looking color camera to use during close passes by Jupiter. Recognizing the value of such images, both scientific and artistic, it was decided to post the raw images and encourage \u201ccitizen scientists\u201d to process them. The product has been many dramatic, brightly colored views of Jupiter, such as\u00a0Figure 11.3.<\/p>\r\n\r\n<div id=\"OSC_Astro_11_01_ECassini\" class=\"os-figure\">\r\n<figure data-id=\"OSC_Astro_11_01_ECassini\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"731\"]<img id=\"7\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/161d087a9e166eb100eb87be4e1fff279d9d9bec\" alt=\"Image of the Earth from Saturn. The Earth appears in the distance as a pale blue dot (arrowed at lower right) below Saturn\u2019s rings in this dramatic image from the Cassini orbiter. The night side of Saturn is seen at upper left, with the backlit rings covering the remaining top half of the image.\" width=\"731\" height=\"492\" data-media-type=\"image\/jpeg\" \/> <strong>Figure\u00a011.4<\/strong>\u00a0Earth as Seen from Saturn.\u00a0This popular Cassini image shows Earth as a tiny dot (marked with an arrow) seen below\u00a0Saturn\u2019s rings. It was taken in July 2013, when Saturn was 1.4 billion kilometers from\u00a0Earth. (credit: modification of work by NASA\/JPL-Caltech\/Space Science Institute)[\/caption]<\/figure>\r\n<\/div>\r\n<\/section><\/div>\r\n<div data-type=\"footnote-refs\">\r\n<h3 data-type=\"footnote-refs-title\">Footnotes<\/h3>\r\n<ul data-list-type=\"bulleted\" data-bullet-style=\"none\">\r\n \t<li id=\"fs-id1168466162585\" data-type=\"footnote-ref\"><a role=\"doc-backlink\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/11-1-exploring-the-outer-planets#footnote-ref1\">1<\/a> <span data-type=\"footnote-ref-content\">Both the Ulysses and the New Horizons spacecraft (designed to study the Sun and Pluto, respectively) flew past Jupiter for a gravity boost (gaining energy by \u201cstealing\u201d a little bit from the giant planet\u2019s rotation).<\/span><\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/section>\r\n<div class=\"textbox\">This book was adapted from the following: Fraknoi, A., Morrison, D., &amp; Wolff, S. C. (2016). 11.1 Exploring the Outer Planets. In <i>Astronomy<\/i>. OpenStax. https:\/\/openstax.org\/books\/astronomy\/pages\/11-1-exploring-the-outer-planets under a <a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\/\" target=\"_blank\" rel=\"noopener noreferrer\">Creative Commons Attribution License 4.0<\/a><\/div>\r\n<div>Access the entire book for free at <a href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/1-introduction\">https:\/\/openstax.org\/books\/astronomy\/pages\/1-introduction<\/a><\/div>","rendered":"<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<h3 class=\"textbox__title\">Learning Objectives<\/h3>\n<\/header>\n<div class=\"textbox__content\">\n<p id=\"fs-id1168584108202\" class=\"\">By the end of this section, you will be able to:<\/p>\n<ul id=\"fs-id1168466150866\">\n<li>Provide an overview of the composition of the giant planets<\/li>\n<li>Chronicle the robotic exploration of the outer solar system<\/li>\n<li>Summarize the missions sent to orbit the gas giants<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<p id=\"fs-id1168469859612\" class=\"\">The\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term517\" class=\"no-emphasis\" data-type=\"term\">giant planets<\/span>\u00a0hold most of the mass in our planetary system. Jupiter alone exceeds the mass of all the other planets combined (Figure 11.2). The material available to build these planets can be divided into three classes by what they are made of: \u201cgases,\u201d \u201cices,\u201d and \u201crocks\u201d (see\u00a0Table 11.1). The \u201cgases\u201d are primarily hydrogen and helium, the most abundant elements in the universe. The way it is used here, the term \u201cices\u201d refers to composition only and not whether a substance is actually in a solid state. \u201cIces\u201d means compounds that form from the next most abundant elements: oxygen, carbon, and nitrogen. Common ices are water, methane, and ammonia, but ices may also include carbon monoxide, carbon dioxide, and others. \u201cRocks\u201d are even less abundant than ices, and include everything else: magnesium, silicon, iron, and so on.<\/p>\n<div id=\"OSC_Astro_11_01_Jupiter\" class=\"os-figure\">\n<figure data-id=\"OSC_Astro_11_01_Jupiter\">\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"2\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/61fc6b10a0c424d462be89f457c8294d3af553f6\" alt=\"Photograph of Jupiter. Taken from the Cassini spacecraft, the alternating light and dark cloud bands are visible over the entire planet. The Great Red Spot is at lower right. Also seen is the shadow of the moon Europa at lower left.\" width=\"975\" height=\"460\" data-media-type=\"image\/jpeg\" \/><figcaption class=\"wp-caption-text\">Figure\u00a011.2\u00a0Jupiter.\u00a0The Cassini spacecraft imaged\u00a0Jupiter\u00a0on its way to Saturn in 2012. The giant storm system called the Great Red Spot is visible to the lower right. The dark spot to the lower left is the shadow of Jupiter\u2019s moon Europa. (credit: modification of work by NASA\/JPL) Abundances in the Outer Solar System<\/figcaption><\/figure>\n<\/figure>\n<\/div>\n<div class=\"os-table os-top-titled-container\">\n<table id=\"fs-id1168466145735\" class=\"grid landscape aligncenter\" summary=\"Table 11.1\">\n<caption>Table 11.1 Abundances in the Outer Solar System<\/caption>\n<thead>\n<tr valign=\"top\">\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Type of Material<\/th>\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Name<\/th>\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Approximate % (by Mass)<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Gas<\/td>\n<td data-valign=\"top\" data-align=\"left\">Hydrogen ([latex]{{\\rm{H}}_2}[\/latex])<\/td>\n<td data-valign=\"top\" data-align=\"left\">75<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Gas<\/td>\n<td data-valign=\"top\" data-align=\"left\">Helium (He)<\/td>\n<td data-valign=\"top\" data-align=\"left\">24<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Ice<\/td>\n<td data-valign=\"top\" data-align=\"left\">Water ([latex]{{\\rm{H}}_2}{\\rm{O}}[\/latex])<\/td>\n<td data-valign=\"top\" data-align=\"left\">0.6<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Ice<\/td>\n<td data-valign=\"top\" data-align=\"left\">Methane ([latex]{\\rm{C}}{{\\rm{H}}_4}[\/latex])<\/td>\n<td data-valign=\"top\" data-align=\"left\">0.4<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Ice<\/td>\n<td data-valign=\"top\" data-align=\"left\">Ammonia ([latex]{\\rm{N}}{{\\rm{H}}_3}[\/latex])<\/td>\n<td data-valign=\"top\" data-align=\"left\">0.1<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Rock<\/td>\n<td data-valign=\"top\" data-align=\"left\">Magnesium (Mg), iron (Fe), silicon (Si)<\/td>\n<td data-valign=\"top\" data-align=\"left\">0.3<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<div class=\"os-caption-container\"><span style=\"text-align: initial;font-size: 1em\">In the outer solar system, gases dominate the two largest planets, <\/span><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term519\" class=\"no-emphasis\" style=\"text-align: initial;font-size: 1em\" data-type=\"term\">Jupiter<\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0and\u00a0<\/span><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term520\" class=\"no-emphasis\" style=\"text-align: initial;font-size: 1em\" data-type=\"term\">Saturn<\/span><span style=\"text-align: initial;font-size: 1em\">, hence their nickname \u201cgas giants.\u201d\u00a0<\/span><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term521\" class=\"no-emphasis\" style=\"text-align: initial;font-size: 1em\" data-type=\"term\">Uranus<\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0and\u00a0<\/span><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term522\" class=\"no-emphasis\" style=\"text-align: initial;font-size: 1em\" data-type=\"term\">Neptune<\/span><span style=\"text-align: initial;font-size: 1em\">\u00a0are called \u201cice giants\u201d because their interiors contain far more of the \u201cice\u201d component than their larger cousins. The chemistry for all four giant planet atmospheres is dominated by hydrogen. This hydrogen caused the chemistry of the outer solar system to become\u00a0<\/span><em style=\"text-align: initial;font-size: 1em\" data-effect=\"italics\">reducing,<\/em><span style=\"text-align: initial;font-size: 1em\">\u00a0meaning that other elements tend to combine with hydrogen first. In the early solar system, most of the oxygen combined with hydrogen to make [latex]{{\\rm{H}}_2}{\\rm{O}}[\/latex] 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 [latex]{\\rm{C}}{{\\rm{O}}_2}[\/latex] ). As a result, the compounds detected in the atmosphere of the giant planets are mostly hydrogen-based gases such as methane ([latex]{\\rm{C}}{{\\rm{H}}_4}[\/latex]) and ammonia ([latex]{\\rm{N}}{{\\rm{H}}_3}[\/latex]), or more complex hydrocarbons (combinations of hydrogen and carbon) such as ethane ([latex]{{\\rm{C}}_2}{{\\rm{H}}_6}[\/latex]) and acetylene ([latex]{{\\rm{C}}_2}{{\\rm{H}}_2}[\/latex]).<\/span><\/div>\n<\/div>\n<section id=\"fs-id1168469838200\" data-depth=\"1\">\n<h3 data-type=\"title\">Exploration of the Outer Solar System So Far<\/h3>\n<p id=\"fs-id1168466338754\" class=\"\">Eight spacecraft, seven from the United States and one from Europe, have penetrated beyond the asteroid belt into the realm of the giants.\u00a0Table 11.2 summarizes the spacecraft missions to the outer solar system.<\/p>\n<div class=\"os-table os-top-titled-container\">\n<table id=\"fs-id1168466163447\" class=\"grid landscape aligncenter\" summary=\"Table 11.2\">\n<caption>Table 11.2 Missions to the Giant Planets<\/caption>\n<thead>\n<tr valign=\"top\">\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Planet<\/th>\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Spacecraft<sup id=\"footnote-ref1\" data-type=\"footnote-number\"><a role=\"doc-noteref\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/11-1-exploring-the-outer-planets#fs-id1168466162585\" data-type=\"footnote-link\">1<\/a><\/sup><\/th>\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Encounter Date<\/th>\n<th scope=\"col\" data-valign=\"top\" data-align=\"center\">Type<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr valign=\"top\">\n<td rowspan=\"9\" data-valign=\"top\" data-align=\"left\"><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term523\" class=\"no-emphasis\" data-type=\"term\">Jupiter<\/span><\/td>\n<td data-valign=\"top\" data-align=\"left\">Pioneer 10<\/td>\n<td data-valign=\"top\" data-align=\"left\">December 1973<\/td>\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Pioneer 11<\/td>\n<td data-valign=\"top\" data-align=\"left\">December 1974<\/td>\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Voyager 1<\/td>\n<td data-valign=\"top\" data-align=\"left\">March 1979<\/td>\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Voyager 2<\/td>\n<td data-valign=\"top\" data-align=\"left\">July 1979<\/td>\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Ulysses<\/td>\n<td data-valign=\"top\" data-align=\"left\">February 1992<\/td>\n<td data-valign=\"top\" data-align=\"left\">Flyby during gravity assist<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Galileo<\/td>\n<td data-valign=\"top\" data-align=\"left\">December 1995<\/td>\n<td data-valign=\"top\" data-align=\"left\">Orbiter and probe<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Cassini<\/td>\n<td data-valign=\"top\" data-align=\"left\">December 2002<\/td>\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">New Horizons<\/td>\n<td data-valign=\"top\" data-align=\"left\">February 2007<\/td>\n<td data-valign=\"top\" data-align=\"left\">Flyby during gravity assist<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Juno<\/td>\n<td data-valign=\"top\" data-align=\"left\">July 2016<\/td>\n<td data-valign=\"top\" data-align=\"left\">Orbiter<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td rowspan=\"4\" data-valign=\"top\" data-align=\"left\"><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term524\" class=\"no-emphasis\" data-type=\"term\">Saturn<\/span><\/td>\n<td data-valign=\"top\" data-align=\"left\">Pioneer 11<\/td>\n<td data-valign=\"top\" data-align=\"left\">September 1979<\/td>\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Voyager 1<\/td>\n<td data-valign=\"top\" data-align=\"left\">November 1980<\/td>\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Voyager 2<\/td>\n<td data-valign=\"top\" data-align=\"left\">August 1981<\/td>\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\">Cassini<\/td>\n<td data-valign=\"top\" data-align=\"left\">July 2004 (Saturn orbit injection 2000)<\/td>\n<td data-valign=\"top\" data-align=\"left\">Orbiter<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\"><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term525\" class=\"no-emphasis\" data-type=\"term\">Uranus<\/span><\/td>\n<td data-valign=\"top\" data-align=\"left\">Voyager 2<\/td>\n<td data-valign=\"top\" data-align=\"left\">January 1986<\/td>\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td data-valign=\"top\" data-align=\"left\"><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term526\" class=\"no-emphasis\" data-type=\"term\">Neptune<\/span><\/td>\n<td data-valign=\"top\" data-align=\"left\">Voyager 2<\/td>\n<td data-valign=\"top\" data-align=\"left\">August 1989<\/td>\n<td data-valign=\"top\" data-align=\"left\">Flyby<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<div class=\"os-caption-container\"><span style=\"text-align: initial;font-size: 1em\">The challenges of exploring so far away from Earth are considerable. Flight times to the giant planets are measured in years to decades, rather than the months required to reach Venus or Mars. Even at the speed of light, messages take hours to pass between Earth and the spacecraft. 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. Spacecraft to the outer solar system must therefore be highly reliable and capable of a greater degree of independence and autonomy. Outer solar system missions also must carry their own power sources since the Sun is too far away to provide enough energy. Heaters are required to keep instruments at proper operating temperatures, and spacecraft must have radio transmitters powerful enough to send their data to receivers on distant Earth.<\/span><\/div>\n<\/div>\n<p id=\"fs-id1168469428508\" class=\"\">The first spacecraft to investigate the regions past Mars were the NASA Pioneers 10 and 11, launched in 1972 and 1973 as pathfinders to\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term527\" class=\"no-emphasis\" data-type=\"term\">Jupiter<\/span>. One of their main objectives was simply to determine whether a spacecraft could actually navigate through the belt of asteroids that lies beyond Mars without getting destroyed by collisions with asteroidal dust. Another objective was to measure the radiation hazards in the\u00a0<em data-effect=\"italics\">magnetosphere<\/em>\u00a0(or zone of magnetic influence) of Jupiter. Both spacecraft passed through the\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term528\" class=\"no-emphasis\" data-type=\"term\">asteroid belt<\/span>\u00a0without incident, but the energetic particles in Jupiter\u2019s magnetic field nearly wiped out their electronics, providing information necessary for the safe design of subsequent missions.<\/p>\n<p id=\"fs-id1168469450139\" class=\"\"><span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term529\" class=\"no-emphasis\" data-type=\"term\">Pioneer 10<\/span>\u00a0flew past Jupiter in 1973, after which it sped outward toward the limits of the solar system.\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term530\" class=\"no-emphasis\" data-type=\"term\">Pioneer 11<\/span>\u00a0undertook a more ambitious program, using the gravity of Jupiter to aim for Saturn, which it reached in 1979. The twin Voyager spacecraft launched the next wave of outer planet exploration in 1977. Voyagers 1 and 2 each carried 11 scientific instruments, including cameras and spectrometers, as well as devices to measure the characteristics of planetary magnetospheres. Since they kept going outward after their planetary encounters, these are now the most distant spacecraft ever launched by humanity.<\/p>\n<p id=\"fs-id1168469874620\" class=\"\">Voyager 1 reached Jupiter in 1979 and used a gravity assist from that planet to take it on to Saturn in 1980. Voyager 2 arrived at Jupiter four months later, but then followed a different path to visit all the outer planets, reaching Saturn in 1981, Uranus in 1986, and Neptune in 1989. This trajectory was made possible by the approximate alignment of the four giant planets on the same side of the Sun. About once every 175 years, these planets are in such a position, and it allows a single spacecraft to visit them all by using gravity-assisted flybys to adjust course for each subsequent encounter; such a maneuver has been nicknamed a \u201cGrand Tour\u201d by astronomers.<\/p>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<h3 class=\"textbox__title\">Link to Learning<\/h3>\n<\/header>\n<div class=\"textbox__content\">\n<p>The Jet Propulsion Laboratory has a nice video called\u00a0<a href=\"https:\/\/openstax.org\/l\/30JPLGrandT\" target=\"_blank\" rel=\"noopener nofollow noreferrer\">Voyager: The Grand Tour<\/a>\u00a0that describes the Voyager mission and what it found.<\/p>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<h3 class=\"textbox__title\">Making Connections<\/h3>\n<\/header>\n<div class=\"textbox__content\">\n<h4 id=\"3\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Engineering and Space Science: Teaching an Old Spacecraft New Tricks<\/span><\/h4>\n<p id=\"fs-id1168466278077\" class=\"\">By the time\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term531\" class=\"no-emphasis\" data-type=\"term\">Voyager 2<\/span>\u00a0arrived at\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term532\" class=\"no-emphasis\" data-type=\"term\">Neptune<\/span>\u00a0in 1989, 12 years after its launch, the spacecraft was beginning to show signs of old age. The arm on which the camera and other instruments were located was \u201carthritic\u201d: it could no longer move easily in all directions. The communications system was \u201chard of hearing\u201d: part of its radio receiver had stopped working. The \u201cbrains\u201d had significant \u201cmemory loss\u201d: some of the onboard computer memory had failed. And the whole spacecraft was beginning to run out of energy: its generators had begun showing serious signs of wear.<\/p>\n<p id=\"fs-id1168469474648\" class=\"\">To make things even more of a challenge, Voyager\u2019s mission at Neptune was in many ways the most difficult of all four flybys. For example, since sunlight at Neptune is 900 times weaker than at Earth, the onboard camera had to take much longer exposures in this light-starved environment. This was a nontrivial requirement, given that the spacecraft was hurtling by Neptune at ten times the speed of a rifle bullet.<\/p>\n<p id=\"fs-id1168469408112\" class=\"\">The solution was to swivel the camera backward at exactly the rate that would compensate for the forward motion of the spacecraft. Engineers had to preprogram the ship\u2019s computer to execute an incredibly complex series of maneuvers for each image. The beautiful Voyager images of Neptune are a testament to the ingenuity of spacecraft engineers.<\/p>\n<p id=\"fs-id1168469384909\" class=\"\">The sheer distance of the craft from its controllers on Earth was yet another challenge. Voyager 2 received instructions and sent back its data via on-board radio transmitter. The distance from Earth to Neptune is about 4.8 billion kilometers. Over this vast distance, the power that reached us from Voyager 2 at Neptune was approximately [latex]{10^{ - 16}}[\/latex]\u00a0watts, or 20 billion times less power than it takes to operate a digital watch. Thirty-eight different antennas on four continents were used by NASA to collect the faint signals from the spacecraft and decode the precious information about Neptune that they contained.<\/p>\n<\/div>\n<\/div>\n<div id=\"page_d9cfd957-5241-4611-abff-426e7b4f7a0c\" class=\"chapter-content-module\" data-type=\"page\" data-cnxml-to-html-ver=\"2.1.2\">\n<section id=\"fs-id1168469440698\" data-depth=\"1\">\n<h3 data-type=\"title\">Enter the Orbiters: Galileo, Cassini, and Juno<\/h3>\n<p id=\"fs-id1168469613798\" class=\"\">The Pioneer and Voyager missions were flybys of the giant planets: they each produced only quick looks before the spacecraft sped onward. For more detailed studies of these worlds, we require spacecraft that can go into orbit around a planet. For Jupiter and Saturn, these orbiters were the Galileo, Cassini, and Juno spacecraft. To date, no orbiter missions have been started for Uranus and Neptune, although planetary scientists have expressed keen interest.<\/p>\n<p id=\"fs-id1168466109936\" class=\"\">The Galileo spacecraft was launched toward Jupiter in 1989 and arrived in 1995. Galileo began its investigations by deploying an entry probe into Jupiter, for the first direct studies of the planet\u2019s outer atmospheric layers.<\/p>\n<p id=\"fs-id1168469470254\" class=\"\">The probe plunged at a shallow angle into Jupiter\u2019s atmosphere, traveling at a speed of 50 kilometers\u00a0<em data-effect=\"italics\">per second<\/em>\u2014that\u2019s fast enough to fly from New York to San Francisco in 100 seconds! This was the highest speed at which any probe has so far entered the atmosphere of a planet, and it put great demands on the heat shield protecting it. The high entry speed was a result of acceleration by the strong gravitational attraction of Jupiter.<\/p>\n<p id=\"fs-id1168469472634\" class=\"\">Atmospheric friction slowed the probe within 2 minutes, producing temperatures at the front of its heat shield as high as 15,000 \u00b0C. As the probe\u2019s speed dropped to 2500 kilometers per hour, the remains of the glowing heat shield were jettisoned, and a parachute was deployed to lower the instrumented probe spacecraft more gently into the atmosphere (Figure 11.3). The data from the probe instruments were relayed to Earth via the main Galileo spacecraft.<\/p>\n<div id=\"OSC_Astro_11_01_Galileo\" class=\"os-figure\">\n<figure data-id=\"OSC_Astro_11_01_Galileo\">\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"5\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/98c33e0a19b62c1008d75155abf3502dd16a85a7\" alt=\"Artist\u2019s depiction of the Galileo Probe entering Jupiter\u2019s atmosphere. At upper left the parachute is seen, connected by a cable to the spherical-shaped probe near the center of the illustration. At lower right, the protective heat shield falls ahead of the probe, protecting it from the heat of entry.\" width=\"975\" height=\"382\" data-media-type=\"image\/jpeg\" \/><figcaption class=\"wp-caption-text\"><strong>Figure\u00a011.3<\/strong>\u00a0Galileo Probe Falling into Jupiter. Galileo Probe and Juno Image of Jupiter\u2019s South Pole.\u00a0(a) This artist\u2019s depiction shows the Galileo probe descending into the clouds via parachute just after the protective heat shield separated. The probe made its measurements of Jupiter\u2019s atmosphere on December 7, 1995. (b) This Juno image, taken in 2017 from about 100,000 kilometers above the cloudtops, shows the south polar region of Jupiter with its dramatic complex of storms and clouds. The enhanced-color image was processed for NASA\/JPL by citizen scientist John Landino. (credit a: modification of work by NASA\/Ames Research Center; credit b: modification of work by NASA\/JPL-Caltech\/SwRI\/MSSS\/John Landino)<\/figcaption><\/figure>\n<\/figure>\n<\/div>\n<p id=\"fs-id1168469521131\" class=\"\">The probe continued to operate for an hour, descending 200 kilometers into the atmosphere. A few minutes later the polyester parachute melted, and within a few hours the main aluminum and titanium structure of the probe vaporized to become a part of Jupiter itself. About 2 hours after receipt of the final probe data, the main spacecraft fired its retro-rockets so it could be captured into orbit around the planet, where its primary objectives were to study Jupiter\u2019s large and often puzzling moons.<\/p>\n<p id=\"fs-id1168466282630\" class=\"\">The\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term533\" class=\"no-emphasis\" data-type=\"term\">Cassini<\/span>\u00a0mission to\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term534\" class=\"no-emphasis\" data-type=\"term\">Saturn<\/span>\u00a0(Figure 11.4), a cooperative venture between NASA and the European Space Agency, was similar to Galileo in its two-fold approach. Launched in 1997, Cassini arrived in 2004 and went into orbit around Saturn, beginning extensive studies of its rings and moons, as well as the planet itself. In January 2005, Cassini deployed an entry probe into the atmosphere of Saturn\u2019s large moon,\u00a0<span id=\"d9cfd957-5241-4611-abff-426e7b4f7a0c_term535\" class=\"no-emphasis\" data-type=\"term\">Titan<\/span>, where it successfully landed on the surface. (We\u2019ll discuss the probe and what it found in the chapter on\u00a0Rings, Moons, and Pluto.)<\/p>\n<p id=\"eip-173\" class=\"\">The Voyager and Galileo missions to Jupiter were primarily designed to study the moons and the atmosphere of the planet. The next NASA mission, an orbiter called Juno, arrived at Jupiter in July 2016. In order to meet its objectives of studying the jovian magnetosphere, it has a very elongated (eccentric) 55-day orbit, that takes it from 4 thousand kilometers above the cloud tops out to 76 thousand kilometers. The orbit takes the craft over Jupiter\u2019s poles, giving us remarkable close-ups of the polar regions (previous spacecraft viewed the planet from lower latitudes).<\/p>\n<p id=\"eip-307\" class=\"\">Juno was originally designed without a camera, but fortunately scientists rectified this omission, adding a simple downward-looking color camera to use during close passes by Jupiter. Recognizing the value of such images, both scientific and artistic, it was decided to post the raw images and encourage \u201ccitizen scientists\u201d to process them. The product has been many dramatic, brightly colored views of Jupiter, such as\u00a0Figure 11.3.<\/p>\n<div id=\"OSC_Astro_11_01_ECassini\" class=\"os-figure\">\n<figure data-id=\"OSC_Astro_11_01_ECassini\">\n<figure style=\"width: 731px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"7\" src=\"https:\/\/openstax.org\/apps\/archive\/20210823.155019\/resources\/161d087a9e166eb100eb87be4e1fff279d9d9bec\" alt=\"Image of the Earth from Saturn. The Earth appears in the distance as a pale blue dot (arrowed at lower right) below Saturn\u2019s rings in this dramatic image from the Cassini orbiter. The night side of Saturn is seen at upper left, with the backlit rings covering the remaining top half of the image.\" width=\"731\" height=\"492\" data-media-type=\"image\/jpeg\" \/><figcaption class=\"wp-caption-text\"><strong>Figure\u00a011.4<\/strong>\u00a0Earth as Seen from Saturn.\u00a0This popular Cassini image shows Earth as a tiny dot (marked with an arrow) seen below\u00a0Saturn\u2019s rings. It was taken in July 2013, when Saturn was 1.4 billion kilometers from\u00a0Earth. (credit: modification of work by NASA\/JPL-Caltech\/Space Science Institute)<\/figcaption><\/figure>\n<\/figure>\n<\/div>\n<\/section>\n<\/div>\n<div data-type=\"footnote-refs\">\n<h3 data-type=\"footnote-refs-title\">Footnotes<\/h3>\n<ul data-list-type=\"bulleted\" data-bullet-style=\"none\">\n<li id=\"fs-id1168466162585\" data-type=\"footnote-ref\"><a role=\"doc-backlink\" href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/11-1-exploring-the-outer-planets#footnote-ref1\">1<\/a> <span data-type=\"footnote-ref-content\">Both the Ulysses and the New Horizons spacecraft (designed to study the Sun and Pluto, respectively) flew past Jupiter for a gravity boost (gaining energy by \u201cstealing\u201d a little bit from the giant planet\u2019s rotation).<\/span><\/li>\n<\/ul>\n<\/div>\n<\/section>\n<div class=\"textbox\">This book was adapted from the following: Fraknoi, A., Morrison, D., &amp; Wolff, S. C. (2016). 11.1 Exploring the Outer Planets. In <i>Astronomy<\/i>. OpenStax. https:\/\/openstax.org\/books\/astronomy\/pages\/11-1-exploring-the-outer-planets under a <a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\/\" target=\"_blank\" rel=\"noopener noreferrer\">Creative Commons Attribution License 4.0<\/a><\/div>\n<div>Access the entire book for free at <a href=\"https:\/\/openstax.org\/books\/astronomy\/pages\/1-introduction\">https:\/\/openstax.org\/books\/astronomy\/pages\/1-introduction<\/a><\/div>\n","protected":false},"author":33,"menu_order":11,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[48],"contributor":[],"license":[],"class_list":["post-429","chapter","type-chapter","status-publish","hentry","chapter-type-numberless"],"part":427,"_links":{"self":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters\/429","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/users\/33"}],"version-history":[{"count":9,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters\/429\/revisions"}],"predecessor-version":[{"id":930,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters\/429\/revisions\/930"}],"part":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/parts\/427"}],"metadata":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapters\/429\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/media?parent=429"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/pressbooks\/v2\/chapter-type?post=429"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/contributor?post=429"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/astronomy\/wp-json\/wp\/v2\/license?post=429"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}