Since the 1960s dozens of unmanned space missions have visited the other planets of the solar system. All the planets but Pluto have been visited and probed at close range. The impact of these missions on our understanding of our planetary system has been nothing short of revolutionary. In the next few chapters we will see many examples of the marvelous images radioed back to Earth. Here, we focus on just a few of these remarkable technological achievements.
In 1974, the U.S. spacecraft Mariner 10 came within 10,000 km of the surface of Mercury, sending back high-resolution images of the planet. These photographs, which showed surface features as small as 150 m across, dramatically increased our knowledge of the planet. For the first time, we saw Mercury as a heavily cratered world, in many ways reminiscent of our own Moon.
Mariner 10 was launched from Earth in November 1973 and was placed in an eccentric 176-day orbit about the Sun, aided by a gravitational assist (see Interlude 6-2) from the planet Venus (Figure 6.9). In that orbit, Mariner 10's nearest point to the sun (perihelion) is close to Mercury's path, and its farthest point away (aphelion) lies between the orbits of Venus and Earth. The 176-day period is exactly two Mercury years, so the spacecraft revisits Mercury roughly every 6 months. However, only on the first three encounters—in March 1974, September 1974, and March 1975—did the spacecraft return data. After that, the craft's supply of maneuvering fuel was exhausted. In total, over 4000 photographs, covering about 45 percent of the planet's surface, were radioed back to Earth during the mission's active lifetime. The remaining 55 percent of Mercury is still unexplored.
Figure 6.9 The path of the Mariner 10 probe to Mercury included a gravitational boost from Venus. The spacecraft (inset) returned data from March 1974 until March 1975, providing astronomers with a wealth of information on the planet Mercury.
In all, some 20 spacecraft have visited Venus since the 1970s, far more than have spied on any other planet. The Soviet space program took the lead role in exploring Venus's atmosphere and surface, while American spacecraft have performed extensive radar mapping of the planet from orbit. The American Mariner 2 and Mariner 5 missions passed within 35,000 km of the planet in 1962 and 1967, and Mariner 10 grazed Venus at a distance of 6000 km en route to Mercury.
During roughly the same period, the Soviet Venera (derived from the Russian word for Venus) program got under way, and the Soviet Venera 4 through Venera 12 probes parachuted into the planet's atmosphere between 1967 and 1978. The early Venera probes were destroyed by enormous atmospheric pressures before reaching the surface. Then, in 1970, Venera 7 (Figure 6.10) became the first spacecraft to soft-land on the planet. During the 23 minutes it survived on the surface, it radioed back information on atmospheric pressure and temperature. Since then, a number of Venera landers have transmitted photographs of the surface back to Earth and have analyzed the atmosphere and the soil. None of them survived for more than an hour in the planet's hot, dense atmosphere. The data they sent back make up the entirety of our direct knowledge of Venus's surface. In 1983 the Venera 15 and Venera 16 orbiters sent back detailed radar maps (at about 2-km resolution) of large portions of Venus's northern hemisphere.
Figure 6.10 One of the Soviet Venera landers that reached the surface of Venus. The design was essentially similar for all the surface missions. Note the heavily armored construction, necessary to withstand the harsh conditions on the planet's surface.
The U.S. Pioneer Venus mission in 1978 placed an orbiter at an altitude of some 150 km above Venus's surface and dispatched a "multiprobe" consisting of five separate instrument packages into the planet's atmosphere. During its hour-long descent to the surface, the probe returned information on the variation of density, temperature, and chemical composition with altitude in the atmosphere. The orbiter's radar produced images of most of the planet's surface.
The most recent U.S. mission was the Magellan probe (shown in Figure 6.11), which entered orbit around Venus in August 1990. The spacecraft began sending back spectacular data (see Chapter 9) in September 1990. It completed its first 243-day mapping cycle (the time required for Venus to rotate once beneath the probe's orbit) in May 1991. Magellan's spatial resolution was at least 10 times better than the best images previously obtained. It could distinguish objects as small as 120 m across and measure vertical distances to within less than 50 m. The probe covered the entire surface of Venus with unprecedented clarity, rendering all previous data virtually obsolete. Many theories of the processes shaping the planet's surface have had to be radically altered or abandoned completely because of Magellan's data.
Figure 6.11 The U.S. Magellan spacecraft is launched from the space shuttle Atlantis in May 1989.
Both NASA and the Soviet (now Russian) space agency have Mars exploration programs that began in the 1960s. However, the Soviet effort was plagued by a string of technical problems, along with a liberal measure of plain bad luck. As a result, almost all the detailed planetary data we have on Mars has come from unmanned U.S. probes.
The first spacecraft to reach the Red Planet was Mariner 4, which flew by Mars in July 1965. The images sent back by the craft showed large numbers of craters caused by impacts of meteoroids with the planet's surface, but nothing of the Earthlike terrain some scientists had expected to find. Flybys in 1969 by Mariner 6 and Mariner 7 confirmed these findings, leading to the conclusion that Mars was a geologically dead planet having a heavily cratered, old surface. Studies of Mars received an enormous boost with the arrival in November 1971 of the Mariner 9 orbiter. The craft mapped the entire Martian surface at a resolution of about 1 km, and it rapidly became clear that here was a world far more complex than the dead planet imagined only a year or two previously. Mariner 9's maps revealed vast plains, volcanoes, drainage channels, and canyons. All these features were completely unexpected, given the data provided by the earlier missions. These new findings paved the way for the next step—actual landings on the planet's surface.
The two U.S. Viking spacecraft arrived at Mars in mid-1976. Viking 1 and Viking 2 each consisted of two parts. An orbiter mapped the surface at a resolution of about 100 m (about the same as the resolution achieved by Magellan on Venus), and a lander (see Figure 6.12) descended to the surface and performed a wide array of geological and biological experiments. Viking 1 touched down on Mars on July 20, 1976. Viking 2 arrived in September of the same year. By any standards, the Viking mission was a complete success; the orbiters and landers returned a wealth of long-term data on the Martian surface and atmosphere. Viking 2 stopped transmitting data in April 1980. Viking 1 continued to operate until November 1982.
Figure 6.12 A Viking lander, here being tested in the Mojave Desert prior to launch.
In August 1993 the first U.S. probe since Viking—Mars Observer, which was designed to radio back detailed images of the planet's surface and provide data on the Martian atmosphere exploded just before entering Mars' orbit. However, since that time, the U.S. Mars program has moved into high gear (see Interlude 6-3). A replacement for Mars Observer, called Mars Global Surveyor, was launched from Earth in 1996, arriving at Mars in late 1997. This spacecraft is currently orbiting the planet, scanning it with cameras and other sensors, and charting the Martian landscape.
Mars Global Surveyor was followed (and in fact overtaken) by Mars Pathfinder, whose launch date was moved up from 1998 to December 4, 1996. Pathfinder arrived at Mars in late June, 1997. On July 4 it parachuted an instrument package to the Martian surface. Near the ground, the parachute fell away and huge airbags deployed, enabling the robot to bounce softly to a safe landing. Side panels opened and out came a small six-wheeled minirover, called Sojourner, (shown in Figure 6.4). For almost three months Sojourner roamed the Martian countryside controlled from Earth, at a rate of a few meters per day, taking geological and atmospheric measurements far and wide near the mother ship.
NASA also has ambitious plans for manned missions to Mars. However, the enormous expense (and danger) of such an undertaking, coupled with the belief of many astronomers that unmanned missions are economically and scientifically preferable to manned missions, make the future of these projects uncertain at best.
Two pairs of U.S. spacecraft launched in the 1970s—the Pioneer and Voyager missions—revolutionized our knowledge of Jupiter and the jovian planets. Pioneer 10 and Pioneer 11 were launched in March 1972 and April 1973, respectively, arriving at Jupiter in December 1973 and December 1974. The Pioneer spacecraft took many photographs and made numerous scientific discoveries. Their orbital trajectories also allowed them to observe the polar regions of Jupiter in much greater detail than later missions would achieve. In addition to their many scientific accomplishments, the Pioneer craft also played an important role as "scouts" for the later Voyager missions. The Pioneer series demonstrated that spacecraft could travel the long route from Earth to Jupiter without colliding with debris in the solar system. They also discovered—and survived—the perils of Jupiter's extensive radiation belts (somewhat like Earth's Van Allen belts, but on a much larger scale). In addition, Pioneer 11 used Jupiter's gravity to propel it along the same trajectory to Saturn that the Voyager controllers planned for Voyager 2's visit to Saturn's rings.
The two Voyager spacecraft (see Figure 6.13) left Earth in 1977, reaching Jupiter in March (Voyager 1) and July (Voyager 2) of 1979 to study the planet and its major satellites in detail. Each craft carried sophisticated equipment to study the planet's magnetic field, as well as radio, visible-light, and infrared sensors to analyze its reflected and emitted radiation. Both Voyager 1 and Voyager 2 used Jupiter's gravity to send them on to Saturn. Voyager 1 was programmed to visit Titan, Saturn's largest moon, and so did not come close enough to the planet to receive a gravity-assisted boost to Uranus. However, Voyager 2 went on to visit both Uranus and Neptune in a spectacularly successful "Grand Tour" of the outer planets. The data returned by the two craft are still being analyzed today. Like Pioneer 11, the two Voyager craft are now headed out of the solar system, still sending data as they race toward interstellar space. Figure 6.14 shows the past and present trajectories of the Voyager spacecraft.
Figure 6.13 The Voyager spacecraft. Voyager 1 andVoyager 2 (shown here) were identical.
Figure 6.14 The paths taken by the two Voyagerspacecraft to reach the outer planets. Voyager 1 is now high above the plane of the solar system, having been deflected up and out of the ecliptic plane following its encounter with Saturn. Voyager 2 continued on for a "Grand Tour" of the four jovian planets. It is now outside the orbit of Pluto.
The most recent mission to Jupiter is the U.S. Galileo probe, launched by NASA in 1989 (see Figure 6.4).It arrived at its target in 1995 after a rather roundabout route (shown in Figure 6.15) involving a gravity assist from Venus and two from Earth itself. In the process, Galileo provided astronomers with spectacular views of Earth and Moon, as well as the only close-up photographs of asteroids ever obtained.
Figure 6.15 Galileo's path to Jupiter included one flyby of Venus, two of Earth, and two trips through the asteroid belt, before reaching its destination in 1995. The inset shows the Galileo probe being launched from the space shuttle Atlantis in 1989.
The Galileo mission consisted of an orbiter and an atmospheric probe. The probe descended into Jupiter's atmosphere, slowed by a heat shield and a parachute, making measurements and chemical analyses as it went. The orbiter executed a complex series of gravity-assisted maneuvers through Jupiter's moon system, returning to some moons already studied by Voyager and visiting others for the first time. Some of Galileo's main findings are described in Chapter 11.
Shortly after launch, Galileo mission controllers discovered that the craft's main antenna, needed to radio its findings back to scientists on Earth, had failed to fully deploy. Despite repeated efforts to open it, the instrument remains jammed in an almost closed position. As a result, all data had to be transmitted via a small secondary antenna, greatly reducing the amount of information that could be returned. Nevertheless, with improved data processing techniques, most of the scientific objectives of the mission were still met. So successful was the original mission, in fact, that it has now been extended for two more years, mainly to study Jupiter's innermost moons Io and Europa.
In October 1997 NASA launched the Cassini mission to Saturn. The launch (from Cape Canaveral) sparked controversy because of fears that the craft's plutonium power source might contaminate parts of our planet following an accident either during launch or during a subsequent gravity assist from Earth in 1999—one of four needed for the craft to reach Saturn's distant (9.5 A.U.) orbit. (Two of the other assists will come from Venus, the final one from Jupiter.) Once Cassini arrives at Saturn in 2004, it will dispatch a probe into the atmosphere of Titan, Saturn's largest moon, and orbit among the planet's moons for 4 years, much like Galileo at Jupiter. If experience with Galileo is any guide, Cassini will likely resolve many outstanding questions about the Saturn system, but, is sure to pose many new ones too.
