Saturn has become one of the best-known of the gas giants because of the lengthy and continuing Cassini mission, so in a way it is surprising that relatively few space probes have actually reached this majestic planet. On the other hand, of course, the challenges are much greater: Saturn is roughly twice as far away from Earth as Jupiter. In the history of space flight, only four spacecraft have visited this planet, only one of which entered orbit: Pioneer 11, Voyager 1 and 2, and Cassini-Huygens.
– Pioneer 11 (1979) –
One of the last of NASA’s successful Pioneer series of probes, Pioneer 11 passed Saturn in September 1979, at a perilously close altitude of just under 15,000 miles from its atmospheric clouds. Mission planners had not originally intended to take such a route but, since both Voyager probes were also en route, decided to send the older and cheaper Pioneer spacecraft on a dangerous close flyby to see whether buildups in particles near Saturn would pose a risk to the incoming spacecraft. (Fortunately, they did not.)
In doing so, however, Pioneer 11 did unknowingly veer perilously close to a different fate: collision with the previously undiscovered moon of Epimetheus, which it ultimately missed by a few thousand miles. In addition to spotting a few other undiscovered moons (at a much safer distance), Pioneer 11 then made a groundbreaking and unexpected discovery: Saturn’s large moon Titan had an atmosphere thick enough that it could plausibly support life, if not for the fact that it was so extremely cold.
Pioneer was not intended to orbit Saturn, merely to visit it. After this flyby it continued on its path out of the solar system. Contact was lost in 1995 after the onboard nuclear generator was exhausted.
– Voyager 1 (1980) –
In November, Voyager 1 sped by Saturn, coming closer than 100,000 miles to the edge of the massive gaseous atmosphere and taking beautiful pictures of the planet’s rings and moons. The decision to send Voyager so close to Saturn was a fateful one: the gravity well disrupted its trajectory so that it could no longer continue on to Uranus and Neptune as originally intended. Mission operators had been aware of this risk, but considered it worthwhile: a close approach meant new pictures of Titan (whose atmosphere had just been discovered by Pioneer), and Voyager 2 would still be able to make the intended trip to the outer planets.
– Voyager 2 (1981) –
Voyager 1’s sister probe reached Saturn in August of the following year. Based on the knowledge scientists had gained from the two previous missions, scientists used Voyager 2 to study temperature differences in Saturn’s atmosphere. However, following Voyager 1’s diversion, the chief priority for Voyager 2 was to prepare it for a study of Uranus and Neptune.
Both Voyager probes survived and are currently on their way out of the solar system. They are gradually losing power to operate scientific instruments, but will be sending at least some data back to Earth until the 2020s.
– Cassini-Huygens (2004-present) –
The last and most important mission to Saturn was Cassini-Huygens, the main probe of which continues to orbit in the Saturn system and will send back data until it is destroyed in 2017 or so. A joint NASA and European Space Agency missions, Cassini was perhaps the most ambitious space probe ever launched when it shot into the sky in 1997. Intended for an extremely long, multi-year study of the Saturn ring system, several of its most important moons (especially Titan, which has a unique methane atmosphere and may be a candidate for life), and Saturn’s own massive atmosphere. The total cost was over $3 billion.
Cassini reached Saturn in 2004 and, after entering orbit, proceeded to set its first major record: sending a special lander, the Huygens component of the probe, to make a landing on the surface of Titan. This was the first spacecraft to land on a moon other than our own, and was able to survive on the surface for an hour and a half, sending back images of a desolate orange-tinged surface. Since then, Cassini has continued to orbit Saturn and study its moons, in the process finding what seems to be an underground water source on a much smaller moon, Enceladus.
Cassini’s original and extended mission objectives have now been met. Recently, NASA agreed to extend funding until 2017, so that further research can be conducted. At that time the spacecraft will fire its thrusters and descend directly into the Saturnian atmosphere, burning up and being crushed. As with the Galileo probe at Jupiter, the decision to destroy the spacecraft in this way was made so that there was no chance of it crashing into a life-bearing moon (like Titan) and contaminating the surface with bacteria from Earth which may have survived on the interior of the probe.
