GRAND TOUR OF THE SOLAR SYSTEM

There’s a NASA website where you can follow the two most distant human artifacts as they sail into the void of space. The real-time odometers for the Voyager 1 and Voyager 2 spacecraft flick silently upward. Single kilometers are a blur; even the tens of kilometers digit changes too fast to follow, while the hundreds of kilometers digit ratchets up by one every few seconds. These large and rapidly growing numbers are mesmerizing in the same way as counters of the national debt or the world’s population; numbers this large are difficult to fathom. By late-2012, Voyager 1 was 18.4 billion kilo­meters or 11 billion miles from Earth, and its near-twin Voyager 2 was 15 billion kilometers or 9 billion miles from Earth. Their feeble radio signals take more than a day to reach the Earth as the probes streak through space at approximately 58,000 kilometers per hour, or roughly 36,000 miles per hour.1

To see why these spacecraft represented such a leap in our voy­aging through space, consider a scale model of the Solar System where the Earth is the size of a golf ball. On this scale, the Moon is a grape where the two objects are held apart with outstretched arms. That gap is the farthest humans have ever traveled, and it took $150 billion at 2011 prices to get two dozen men there.2 Mars on this scale is the size of a large marble at the distance of 1,100 feet at its closest approach. As we’ve seen, it took an ar­duous effort spanning more than a decade before NASA success­fully landed a probe on our nearest neighbor. A very deep breath

is needed to explore the outer Solar System. In our scale model, Jupiter and Saturn are large beach balls 1.5 and 3.5 miles away from Earth, respectively, and Uranus and Neptune are soccer balls 7 and 12 miles from the Earth. This large step up in distance was a great challenge for spacecraft designers and engineers. On this scale, the Voyager 1 and 2 spacecraft are metallic “motes of dust” 48 and 37 miles from home, respectively.

The great thirteenth-century polymath and Dominican friar Albertus Magnus, like many before him, wondered about other worlds. He framed the issue in a way that would be familiar to a modern scientist, saying it was “ . . . one of the most noble and ex­alted questions in the study of Nature.”3 Before the Voyager space­craft did their “Grand Tour” of the outer Solar System, the gas giant planets were ciphers, barely resolved by the largest ground – based telescopes. Imagine trying to see details on beach balls and soccer balls that are miles away. Appetites had been whetted by the Pioneer 10 and 11 probes, which flew by Jupiter in 1973 and 1974, with Pioneer 11 going onward to Saturn in 1979, but the twin Voyagers promised to send back much sharper pictures. In the 1970s, theory suggested that the gas giants were spheres of hydrogen and helium similar in composition to the Sun. If they had solid cores at all, the surfaces would be at temperatures of tens of thousands of degrees and pressures many millions of times that at the Earth’s surface.4 Their moons were assumed to be inert and uninteresting rocks like Mercury or the Moon. The word “world” comes from the Old English woruld, referring to human existence and the affairs of life. Yet the outer Solar System seems inhuman and inhospitable for life.

Or is it? Just a year before the launch of Voyager, Cornell Uni­versity astronomers Carl Sagan and Ed Salpeter published a pro­vocative paper in which they argued that free-floating life-forms might populate the temperate upper reaches of the gas giants.5 The authors pushed the concept of life far beyond the bounds of ter­restrial biology; aerial “gas bags” sounded like a conceit of sci­ence fiction, but at the time no one could prove them wrong. Like explorers venturing into terra incognita, nobody knew what the Voyagers might find.