Ask Dr. SETI ®
by Larry Klaes
In the first decade of the Space Age, humanity had succeeded in sending a handful of robotic space probes to Earth's two nearest planetary neighbors, Venus and Mars. The voyages of these mechanical vessels, which only took a matter of months, were brief in their visits to these alien worlds. Nevertheless, these new kinds of explorers gave scientists their first knowledge of the true natures of these places after centuries of speculation.
Much farther beyond, where the Sun is eventually reduced in appearance to just a very bright star, is the realm of the outer gas giant worlds. These planets are many times larger than all of the inner terrestrial globes put together and lack solid surfaces in the same sense as our Earth and its celestial brethren. The Jovian planets also keep in their mighty gravitational grips collections of moons and rings of debris that would qualify them as whole solar systems in their own right.
But for humanity in the early days of space exploration, these alien places were very far away and full of unknowns, including whether a fast-moving spacecraft could navigate the natural boundary between the terrestrial and Jovian realms known popularly as the Asteroid Belt without being smashed to pieces by potentially deadly dust and meteoroids. In addition, a spacecraft of that era would take decades to reach all the outer worlds; such vessels were still on the proverbial drawing boards, while most of the actual probes which did reach the nearest worlds in functioning order often did so with a lot of luck and engineering skill.
In that same time, when the two main players of the Space Age were preparing to see who could place the first humans on the lunar surface, it was determined that the outer planets would align in such a way in their solar orbits in the late 1970s that they could be reconnoitered by a quartet of nuclear-powered space probes flying past each world in just one decade. The plan and the mission were appropriately named the Grand Tour.
Early on the project was threatened with termination, not by some hazard in space but by budgetary problems on Earth. To stay alive in NASA, the Grand Tour was scaled back to explore just the two nearest gas giants, Jupiter and Saturn. The vessel numbers were reduced from four to two: The remaining probes were christened Mariner 11 and 12, following in the line of American space probes that had opened the way to understanding the inner Solar System. By the time the vessels that remained from the initial outer worlds exploration plan were ready to be launched into the heavens in the late summer of 1977, there were a number of further significant changes to the mission.
Up front, the twin spacecraft had their names changed from Mariner 11 and 12 to Voyager 1 and 2. This was done both to reflect their expanded designs and goals beyond what the earlier Mariners had accomplished and to make the probes and their missions more exciting to the public. The Voyager team also hoped that, though the craft were still officially meant to explore just Jupiter and Saturn, they would be strong and adaptable enough to complete most of the original Grand Tour plan by reaching Uranus and Neptune just over one decade hence.
Finally, just months before the two Voyagers would leave Cape Canaveral in Florida aboard separate powerful Titan 3E/Centaur rockets, a small group of far-seeing individuals convinced NASA to place a sampling of sights and sounds of our world and our species engraved onto two golden records which were subsequently attached to the sides of the Voyagers. These discs would accompany the probes past the outer worlds into the wider realm of the Milky Way galaxy. These artifacts would serve as a long-lasting record and tribute to the beings who built and launched these early interstellar wanderers and as a greeting for either their distant children or other intelligences that may move among the stars.
With their missions spanning the second decade of the Space Age, the two Voyagers truly revolutionized our understanding of the outer Solar System, in spite of the fact that they were not the first vessels from Earth to explore that region of our celestial neighborhood. That honor went to Pioneer 10 and 11, which flew past Jupiter in 1973 and 1974, respectively, with Pioneer 11 going on to flyby Saturn in 1979. The Pioneer probes then headed off into interstellar space carrying golden plaques engraved with basic information about humans, our Solar System, and our place in the galaxy. Nevertheless, the improved technologies aboard the Voyagers allowed scientists to surpass what was seen and found at and about those enormous globes by either the Pioneers or Earth-bound astronomers of the era.
At their first destination, Jupiter, the Voyagers revealed the incredibly complex patterns of the planet's cloud patterns, including the Great Red Spot, which was confirmed to be a hurricane system three times the size of Earth that has been churning in the Jovian atmosphere for at least four centuries. Amazing as this was, what captured even more attention from the scientists, media, and public alike were the four large Galilean moons that circled Jupiter, collectively named after the Italian astronomer who discovered them in 1610. These moons were truly worlds in their own right and not the relatively sedate places initially thought to be.
The innermost of the Galilean moons, named Io, turned out to have highly active volcanoes spewing molten sulfur all over its surface and far into space. Alien volcanoes had been seen before, on the planet Mars, but Io's were anything but extinct, to say nothing of being almost completely unexpected before the Voyagers came on the scene in 1979. The next moon nearest to Io, called Europa, was a contrast: The moon's surface was icy and smooth, populated by long dark lines across its face, with only a few impact craters large enough to be visible to Voyagers' cameras. But underneath Europa's covering of ice appeared to be a different story: A global ocean of briny liquid water perhaps sixty miles deep with twice the volume of all the water on Earth! Though certainly not visible to the instruments of its mechanical discoverers, serious speculations on the possibility for living creatures and what forms they might take in the distant waters of Europa wasted little time in appearing.
Thanks to the Voyagers, worlds that were once hardly even considered as abodes of geological activity and life were now seen as even better prospects for living organisms than the traditional worlds in those categories. Voyagers' discoveries at Jupiter, perhaps more than any other place the probes would fly past on their journeys out of the Solar System, truly changed humanity's perspectives on the alien realms inhabiting the outer reaches of our celestial neighborhood. Witnessing the truly dynamic nature of our Solar System through the Voyagers also enriched and expanded our thinking about worlds and beings around other suns, made all the more plausible by the discoveries of extrasolar planets in the years since the primary Voyager missions, of which most of those found so far appear to be similar to Jovian worlds.
Thirty-three years after leaving Earth and twenty-one years after Voyager 2 had flown past the last of the gas giant planets, Neptune, both Voyagers continue to function and return priceless data on regions of the outer Solar System where no human-made spacecraft has ever been before. This area, known as the heliosphere, is considered part of the cosmic boundary between our Solar System and where true interstellar space lies. Perhaps before they expire around 2025, one or maybe both of the Voyagers will last long enough to perform one more scientific mark by revealing the constituents of deep space beyond the influence of our Sun.
The very fact that the Voyagers would be propelled into the Milky Way galaxy by their interactions with the giant planets of the outer Solar System is what inspired the late Cornell astronomer and science popularizer Carl Sagan and others to create what has become known as the Voyager Interstellar Records. While he and others knew the odds of the Voyagers ever being found by other intelligences were small, the fact that the probes would be only our third and fourth artifacts sent to the stars compelled Sagan and his companions to utilize this opportunity to preserve something of ourselves where it could last far longer than anywhere on Earth, perhaps one billion years or more.
Most importantly, while the Voyagers were built and launched by the United States of America, the golden records were designed to represent as much of our whole human species and the rest of life on Earth as possible in images, words, sounds, and music. Scanning through the contents of the records, which can be done from this Web site: http://goldenrecord.org one gets a definite sense of our being one species on just one world among hundreds of billions of stars in a vast Universe composed of billions of galaxies. To quote Carl Sagan in the Epilogue of the 1978 book on the Voyager Interstellar Record titled Murmurs of Earth: "But one thing would be clear about us: No one sends a message on such a journey, to other worlds and beings, without a positive passion for the future. For all the possible vagaries of the message, they could be sure that we were a species endowed with hope and perseverance, at least a little intelligence, substantial generosity and a palpable zest to make contact with the cosmos."
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this page last updated 4 September 2010
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