Distant Giants

Shortly after the dawn of the Space Age, it was realized that a planetary alignment would occur in the 1970s that would not be repeated for 179 years. A properly aimed spacecraft could fly past each of the gas giant planets in our solar system—Jupiter, Saturn, Uranus, and Neptune—using gravity assists to increase spacecraft orbital energy and deflect the trajectory at each encounter in preparation for the next.

NASA planned four missions to accomplish this task. Before the end of the Pioneers 10 and 11 and Voyagers 1 and 2 missions, humanity had photographed at close range each of the giants and many of their satellites and gathered valuable information on planetary and interplanetary environments. After the final planetary flyby, as the craft all headed into the interstellar abyss, one turned its camera sunward and photographed its distant world of birth, swimming in a band of scattered sunlight like a "pale blue dot.''

Mission planners realized that there was a chance, however slight, that these tiny probes could be intercepted by spacefaring extraterrestrials as they cruised the galactic wilderness. Therefore, each was equipped with a message plaque—a "message in a bottle'' dropped by us into the galactic ocean.

Pioneer 10 entered Jovian space in December 1973. After conducting a preliminary survey of the giant planet's cloudbanks, rings, satellites, and radiation belts, Pioneer used Jupiter's gravitation field to alter its trajectory and propel it toward the stars.

One year later, Pioneer 11 encountered giant Jupiter. This time, however, the trajectory was altered so that the probe would flyby Saturn on its way out of the solar system.

The preliminary findings and images from the Pioneers gave mission planners the opportunity to meticulously plan the missions of the much more capable Voyagers. Voyager 1 encountered Jupiter in March 1979 and flew by Saturn in November 1980. The final grand-tour probe, Voyager 2, also flew by Jupiter and Saturn. It then encountered Uranus in 1986 and Neptune in 1989.

These craft should be considered extrasolar rather than interstellar probes. They will leave the region of space influence of the Sun, but the spacecraft are not intended or designed for travel to another star. The fastest of them, Voyager 1, is cruising through the interstellar night at about 17 kilometers per second or 3.5 AU per year. If it were directed toward our Sun's nearest stellar neighbor (which it is not), it would get there in approximately 70,000 years.

The photographic survey of the outer planets conducted by these four craft has been invaluable, as have the particles and fields measurements of the outer interplanetary and near-interstellar environments. Of most significance to future mission planners are the imaging of Jupiter's large satellite Europa, which indicated the presence of a water ocean beneath a thick ice crust. Another large Jovian satellite, Io, was found to be in a state of perpetual volcanic activity. Studies of Titan, the largest satellite of Saturn, revealed that this object has a dense atmosphere and surface conditions that might resemble those of an early Earth.

To further study these enigmatic satellites and the environments oftheir parent worlds, two ambitious follow-on missions were planned. Galileo would be directed to enter orbit around Jupiter, while Cassini would become a satellite of Saturn and deposit the Huygens probe on Titan's surface.

Galileo was originally scheduled to be launched by a Centaur high-energy upper stage deployed in Earth orbit from a space shuttle. But after the Challenger accident of 1986, it was decided that the use of Centaur might pose risks to a shuttle crew. As a less energetic rocket was substituted, multiple passes of Earth and Venus were therefore required before Galileo could be injected into a trans-Jupiter trajectory.

During its sojourn in the inner solar system and its flight to the giant planet, Galileo imaged Earth and Moon from deep space, as well as asteroids Gaspra and Ida. It was discovered that Ida has a small satellite asteroid that has been dubbed Dactyl.

Finally arriving at Jupiter in December 1995, Galileo entered orbit around the giant planet. Before it adjusted its interplanetary trajectory, Galileo dispatched a subprobe which entered Jupiter's atmosphere and

FIGURE 5.3 Yet another New World was seen as the Huygens Probe descended to the surface of Saturn's moon Titan. (Courtesy NASA)

radioed data from a depth of a few hundred kilometers below the visible cloud deck.

During its years of operation, Galileo surveyed jupiter's large and small satellites, its rings, and accurately mapped the radiation environment of this giant world. Its most significant results in the long term may be close-up photographs of Europa, revealing that there are apparent cracks in the ice sheet. If liquid water exists near the surface of this satellite, future probes may find life there.

On july 1, 2004, the Cassini spacecraft entered orbit around Saturn and relayed to Earth many beautiful pictures of that planet and its ring system. A spectacular accomplishment of the mission was the successful operation of the Huygens Titan lander. This subprobe survived a perilous descent to the surface of Saturn's moon, Titan, which has a hydrocarbon atmosphere about as dense as the Earth's. Although most terrestrial life forms would quickly expire in Titan's harsh environment, some form of life may be possible in liquid methane "springs" and "lakes" on this remote and very distant surface (Figure 5.3).

Small Worlds

Although the robotic exploration of the planets has gathered much attention, missions have also been conducted to smaller solar-system bodies—comets and asteroids. Space scientists and mission planners are interested in these diminutive denizens of the solar system for several reasons. First, they contain vital clues about the origin and evolution of the solar system. Second, they contain materials that may provide a resource base for an expanding human presence in space. Finally, these objects occasionally collide with Earth—with devastating consequences. Only in situ measurements can help us to devise the best methods to mitigate these catastrophes.

In 1986, one of the most famous of these objects paid a periodic visit to the inner solar system. During the 1986 apparition of Halley's Comet, it was visited by a host of spacecraft launched by Europe, japan, and the USSR. Europe's Giotto entered the comet's coma and rapidly cruised past its nucleus, transmitting magnificent images and miraculously surviving the perilous passage to encounter a second comet sometime later.

In 1996, NASA launched NEAR (Near-Earth Asteroid Rendezvous) toward asteroids Mathilde and Eros. After flying past Mathilde, NEAR continued on toward Eros. It orbited this near-Earth asteroid and soft-landed on the surface at the conclusion of its mission (Figure 5.4).

FIGURE 5.4 Asteroid Mathilde, as photographed by the NEAR spacecraft. (Courtesy NASA)

Building upon NEAR's success, NASA launched Deep Space 1 in 1998. Conceived as a technology demonstrator of solar-electric propulsion (SEP) and other technologies, Deep Space 1 imaged asteroids and a comet on its journey through the inner solar system.

The following year, NASA's stardust probe rocketed toward a rendezvous with Comet 81P Wild 2, in the first attempt to return comet samples to Earth. Stardust, and its celestially harvested samples, returned to Earth in 2006. NASA's most recent comet venture, Deep Impact, dispatched a subprobe to impact Comet P/Temple 1 on July 2005. For the first time, space scientists gathered information about a comet's interior.

Europe and Japan have not been idle in the exploration of small solarsystem objects. Japan's Hayabusa was launched in 2003. Equipped with solar-electric propulsion, this craft was directed toward asteroid 25143 Itokawa. Samples from this asteroid are to be collected and returned to Earth in 2007. In March 2004, Europe's Rosetta was launched toward a 2014 rendezvous with Comet 67/P Churyumov—Gerasimenko. If all goes according to plan, Rosetta will orbit the comet and deposit a lander on its surface.

Future scientific missions to small solar-system objects are planned. At least one of them may be privately funded.

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