Some of NASA's most exciting and productive missions have been conducted by robotic spacecraft that are able to venture much farther into the cosmos than humans dare. These missions explore the planets, comets, and other objects of our solar system as well as the characteristics of the interplanetary medium that lies between them.
There have been many such missions, but several stand out.
Every day, Voyager 1 extends our reach deeper into space than any other human-made object. The craft launched three decades ago and is now more than a hundred times farther from the sun than our own Earth—over 9 billion miles (15 billion kilometers).
Voyager 1 is at the very fringe of our solar system. It is traveling 1 million miles (1.6 million kilometers) a day and, in the next decade, may pass beyond the frontier of the sun's heliosphere and become the first human-made object to reach interstellar space.
Launched in 1977, Voyager 1 and 2 have studied the planets Jupiter, Uranus, Saturn, and Neptune, explored their moons, examined their rings, and revealed much about interplanetary elements such as solar wind.
Now, from deep space, the two craft continue to return data. Scientists expect both Voyagers to function until sometime around the year 2020—an incredible accomplishment for a mission originally designed to run just four years.
Another long-lived spacecraft, Pioneer 10, launched in March 1972 and later that year became the first craft to fly through the asteroid belt, a swath of rocky debris between Mars and Jupiter. Pioneer 10 reached Jupiter in 1973 and captured close-up images of the planet, as well as valuable data on its radiation and magnetic fields.
At the time of its last transmission on January 22, 2003, Pioneer 10 was 82 times farther from the sun than Earth—so far that even a signal traveling at the speed of light took more than 11 hours to reach Earth.
The Cassini-Huygens mission is an ongoing international effort to explore Saturn and its largest moon, Titan. The mission combines the resources of NASA, the European Space Agency, the Italian Space Agency, and other international experts.
Scientists hope that Titan, as well as Saturn and its rings, may help them understand more about the birth and evolution of our entire solar system.
Titan is one of the solar system's most intriguing destinations. The moon has an Earthlike atmosphere rich in organics and, as revealed by Cassini, it may have oceans or lakes of liquid ethane or methane.
The mission, which was launched in 1997, entered Jupiter's orbit and used the massive planet's gravity to slingshot it faster on its route to Saturn. The spacecraft reached Saturn in 2004 and began orbiting the giant planet. Huygens, the detachable probe, landed on the surface of Titan early in 2005 and recorded data on that moon's clouds and atmosphere before landing and beginning surface study.
In recent years, interplanetary missions have also targeted comets. Many scientists believe that the material that makes up comets is relatively unchanged from the time during which they were formed, billions of years ago when the solar system was young. Because of this ancient nature, comets could provide valuable clues about the origins of planets.
NASA's Stardust mission was the first to visit a comet and return to Earth with samples. In 2004 the spacecraft flew by the comet Wild 2 at a distance of only about 149 miles (240 kilometers), trapping tiny particles from the comet as well as interstellar dust. Though the recovered remnants are small, they could have enormous importance for theories of the solar system's creation.