Interplanetary¶
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Interplanetary
Not all of NASA’s efforts were focused entirely on manned spaceflight, although to most of its dedicated critics, it seemed that way. In the 1960s, America began exploring its planetary sisters, concentrating on Venus and Mars. However in the 1970s, NASA took more concentrated and sophisticated looks at Venus and Mars, and began initial exploration of the larger outer planets and Mercury.
NASA had planned later in the decade to land a probe on Mars, but it needed a detailed survey of the planet to determine a proper site. Previous Mars probes (Mariners 4, 6 and 7) had imaged less that 10% of the planet. The Mariner 71 series of two orbiter spacecraft (eventually Mariners 8 and 9) were managed by the Jet Propulsion Laboratory (JPL). After Mariner 8 experienced a launch failure, Mariner 9 was reprogrammed to survey the whole planet. Lift-off was on May 1971, and it successfully entered Mars orbit in November of the same year, becoming the first interplanetary probe to orbit another planet. Expectations were that imaging would begin immediately, but unusual dust storms covering the whole planet, the worst in a century, prevented this for two months. Since the planet could not be viewed for awhile, its moons, Deimos and Phobos, were photographed. By February 1972, the dust had settled, and the prime mission began. Volcanoes and valleys larger than anything on earth were discovered. The Valles Marineris canyon was found to be four to five times deeper than our own Grand Canyon, and could fit in the entire United States. The volcano Olympus Mons, which could be seen from Earth, was higher than any mountain here on our planet, with its base big enough to stretch from Los Angeles to San Francisco. Mariner 9 had collected 7300 photographs of Mars covering the whole planet during a 349 day mission.
The U.S. now turned to Jupiter and Saturn. One of the prime barriers to exploring these large outer planets was the asteroid belt between the orbits of Mars and Jupiter. Pioneers 10 and 11, launched in 1972 and 1973 respectively, were appropriately named and proved that spacecraft could safely travel this belt. Pioneer 10 would lead the way to Jupiter, while Pioneer 11, following in its predecessors footsteps, would also flyby Jupiter. In addition to passing by Jupiter and using a new technique of gravity assist, Pioneer 11 would then continue on to the first look at the planet Saturn. This gravity assist lets a spacecraft increase its energy and be deflected toward a different trajectory when passing by another planet due to the planet’s gravity combined with it motion around the sun. Both missions were successful beyond expectations. Pioneer 10 returned the first close-up pictures, approximately 300 medium resolution images, of Jupiter and its moons before being whipped into a trajectory taking it past the Planet Pluto by the late 1980s. Thus, it became the first interplanetary spacecraft to leave our Solar System. Pioneer 11 encountered Saturn in 1979 after a voyage of six years, and passed within 20,000 miles of Saturn’s outer ring. Its major discoveries included an additional outer ring of Saturn, and a new moon. Indeed, the Pioneers had paved the way for the future Voyager spacecraft.
The Planet Mercury has been visited by only one unmanned probe so far, Mariner 10 from 1974 through 1975. Though Pioneer 11 was launched before it, Mariner 10 was the first interplanetary spacecraft to use the gravity assist technique when it explored the Planet Venus one time, and then revisited Mercury three times. Most Mariner missions consisted of dual spacecraft, but Mariner 10 was a solo shot. Also, to hold down additional costs, the program used an Atlas booster with the Venus gravity assist, instead of the more expensive Titan III launch vehicle and a direct Mercury trajectory. Mariner 10 was launched on November 3, 1973, and when it passed Venus, it was slowed down, heading toward the Sun, and its intended target, Mercury. As it passed by Venus, the spacecraft took the ultra-violet images of that planet, showing for the first time, the fast wind speeds circulating around Venus. These extraordinary winds were later confirmed by other American and Soviet probes. Five months later (March 1974), Mariner 10 went by Mercury for the first of three visits, each about six months apart. Mercury turned out to be a heavily cratered place similar to our own Moon, and after three fly-bys , the spacecraft was able to photograph about 35% of Mercury’s surface. Mariner 10 still visits Mercury every 176 days even though its payloads are long dead, and there are no immediate plans do revisit the planet in the immediate future.
During the 1960s, America had soft-landed an unmanned spacecraft (Surveyor) on the moon, but had made no such attempts on any of its sister planets. It had accomplished flybys to Venus, Mars and Mercury (in that order), but the 1970s would change all that with the Viking project and Mars was the target. Viking has started out as part of the more ambitious Voyager program which was canceled in 1967 for cost reasons (not to be confused with the Grand Tour Voyager spacecraft of the 1970s). The Vikings were resurrected in 1968 for launch in 1975. The space probes each consisted of an orbiter and lander, with a prime mission to seek out life on Mars. Viking 1 was launched in August 1975, and originally scheduled to land on Mars on July 4, 1976 (the 200th anniversary of the birth of the United States). However, once in Martian orbit, photography from the orbiter showed the original landing site to be too rocky. Other sites were examined, and the Viking 1 lander successfully arrived on the surface of the Red Planet on July 20, 1976. America’s intense fascination with our planetary neighbor truly began. Viking 2 arrived at Mars one month later, and its lander duplicated Viking 1’s feat by setting foot on the surface on September 3, 1976, some 7500 miles from its sister spacecraft. Joint operations began. Preliminary photographic scans of the Martian surface from both landers showed it to be an intensely rock covered with a rusty brown color. The pictures also showed no evidence of planet life, similar to our own Sahara desert. The Viking landers then began the second part of their job, examining Mars for microscopic life with its attached biology laboratory. The laboratory contained three experiments to probe for any life forms, but results were inclusive. Both sets of orbiters and landers continued their wide-ranging meteorological, geological, chemical and seismic studies for several years after landing. The last vehicle to be shut down was the Viking 1 lander in November 1982.
The most spectacular interplanetary missions to date have been unquestionably the flights of Voyagers 1 and 2. Launched in 1977, they did not complete their heavenly odysseys until 1989 when Voyager 2 flew-by the Planet Neptune finishing a journey that had its genesis in the 1960s. Scientists had earlier noted that he Outer Planets (Jupiter, Saturn, Uranus, Neptune and Pluto) had an unusual planetary alignment that occurred only once every 172 years, allowing one or two spacecraft to visit all of them at a minimum cost and time. One of these opportunities would occur in the 1976-1980 timeframe. From the “Mariner-Jupiter-Saturn 1977” studies of the late 60s, the “Grand Tour” was born. The original plan was to launch one spacecraft in 1977 with flybys of Jupiter (1979), Saturn (1980) and Pluto (1986), with another launch in 1979 going to Jupiter (1981), Uranus (1985) and Neptune (1988). Unfortunately, due to public apathy, escalating program costs (nearly $1 billion), and increasing Space Shuttle developmental costs, the Grand Tour was canceled in 1972.
The Voyagers were resurrected later in the year with the following restrictions - they were to based on the successful Mariner-Viking designs, were to cost only half as much as the original Grand Tour project, and were to focus only on a more intense investigation of Jupiter and Saturn and its moons. However, JPL engineers and scientists planned for the possibilities of further explorations if funds held out, and they got their wish. What the World got was a voyage through our whole outer planetary system except for Pluto. Voyager 2 was launched before Voyager 1 in August 1977, and was directed toward a more thorough examination of Jupiter and Saturn. Voyager 2 began its journey in September 1977 and ended its mission in 1989 when it explored Uranus and Neptune. What both spacecraft discovered was breathtaking. Voyager 1 discovered that Jupiter had rings like Saturn and Uranus, while finding two new additional Jovian moons, raising the total to 16. Upon passing Jupiter’s moon Io, Voyager revealed active volcanoes, making Io one of the most geologically active moons in the solar system. Upon reaching Saturn, Voyager 1 discovered three new moons of that planet, further investigated the intricacies of its rings, and thoroughly examined the Saturnian moon Titan. After that, Voyager 1 had finished its mission and became and interplanetary sojourner. Voyager 2 followed Voyager 1 in exploring Jupiter and Saturn, discovering additional rings around Jupiter. Voyager 2 then proceeded to Uranus and Neptune. At Uranus, Voyager discovered ten new moons, and two new rings. Finally 12 years after its launch, Voyager 2 flew by the planet Neptune. It soon joined the other interplanetary travelers, Pioneers 10 and 11, and Voyager 1 in their paths out of our solar system.
The last mission for the Golden Age of Interplanetary Exploration was Pioneer-Venus 1 and 2, also sometimes called Pioneers 12 and 13. These spacecraft were our nation’s only dedicated flights to the Planet Venus in the 1970s, even though Mariner 9 and had used a gravity assist from Venus on its way to explore Mercury. Pioneer-Venus consisted of two spacecraft, each one launched by a Atlas-Centaur booster. Pioneer-Venus 1 was an orbiter vehicle, also called Pioneer-Venus Orbiter or PVO, whose prime mission was to study the Venusian atmosphere long term and map the surface using a radar instrument. Pioneer-Venus 2 was a multi-atmospheric probes, consisting of a bus with four probes which would penetrate the Venus atmosphere and sample the high-wind, high-pressure environment before burning up during their descent. Pioneer-Venus 1 (PVO) was launched on May 20, 1978 and entered Venusian orbit on December 4, 1978. The radar mapper on the PVO allowed a construction of a topographical map of Venus with a 75 km resolution. It showed the planet to have no polar flattening or equatorial bulge like Earth, due to Venus’s slow rotation rate of 243 Earth-days. The PVO also examined the Venusian atmosphere down to 31 km (the limit of its capability), and found that most of the cloud cover probably consisted of sulfuric acid. Pioneer-Venus 2 (the atmospheric probes) was launched on August 8, 1978, and all probes entered Venusian atmosphere on December 9, 1978. Two the probes entered the day side of the planet, while the other two entered the night side. The Pioneer-Venus 2 probes discovered that the upper atmosphere rotated once every four days around the planet, and the cloud covers definitely consisted of sulfuric acid droplets. Thus ended the flight of America’s last interplanetary mission launched in the 1970s.