POSTED 6 SEPTEMBER 2007
Meeting our strange solar system
Long before they reached the edge of the solar system, the Voyager spacecraft earned their keep by ogling the outer planets and their many strange moons. Today, with portraits of those bodies in all the astronomy books, it may be hard to remember how mysterious they were when the Voyagers blasted off in 1977.
What Voyagers viewed:
A few greatest hits from the longest journey (a slideshow)
The crafts were the first to visit Uranus and Neptune, which orbit far beyond Saturn. Even after the Voyagers observed Jupiter and Saturn's loony lunar zoo, the surprises continued, says Anne Verbiscer, then a graduate student at Cornell University and now a scientist at the Cassini Imaging Laboratory at the Space Science Institute in Boulder, Colo. As Voyager 2 was about to shoot the first close-ups of Neptune's moon Triton, "People were still going back and forth, would we see the surface or not? We did not know how thick or thin the atmosphere would be. ... But we had a great encounter, saw lots of features on Triton's surface."
Detail of photo by Calvin J. Hamilton
Triton followed the mold by breaking the mold, she adds. "The amazing thing about Triton, like many of these bodies, is that it did not look like anything else. Before Voyager, we expected all the moons to look like our moon, except maybe icy; with craters but not much else. A great thing about that mission is it showed us that each one of these worlds is unique, wonderful in its own right."
Intrigued by Enceladus
Extra enchantment was elicited by Enceladus, an icy moon of Saturn. Voyager data suggested that the tiny moon (diameter: 500 kilometers) had recent geologic activity.
"We did not know Enceladus was active, until Cassini so beautifully demonstrated that," says Verbiscer, who wrote her Ph.D. thesis about the moon. "In the Voyager pictures, there are large areas that looked as if they did not have any craters; they had big cracks, lots of folds and rifts, evidence of a lot of geologic activity. It looked as if this body had been heated up, and whatever craters were there had been erased by a number of processes."
A recent inspection by the spacecraft Cassini, which is now orbiting Saturn, showed that Enceladus is spewing out icy particles. Cassini also confirmed that Enceladus has deep canyons on the surface, a hot spot at the south pole, and a covering of ice that could have formed within the past few hours or decades. The smooth plains seen by Voyager now appear, in the Cassini close-ups, to have small fractures. And the water ice spewed out by Enceladus is apparently the source of Saturn's faint E ring.
Enceladus is among a small handful of solar-system bodies with any geological activity. Why would a tiny moon in a frigid region of the solar system be so active?
That's TBD: To be decided.
Just as Pioneer 10 taught the Voyager designers the danger of radiation in space, so Voyager taught the Cassini designers which instruments to bring to Saturn. "Voyager was the initial reconnaissance of the solar system," says Verbiscer. "Voyager showed us what the major moons of Saturn looked like, and the rings, and we knew what we wanted go back and look at." Voyager 1 found that Saturn's moon Titan was obscured under a thick, hazy atmosphere. "We wanted to go back and bring the right instruments, now that we knew we can't see Titan's surface in visible light, so we brought a near-infrared mapping spectrometer as well as radar, and produced incredible images of the surface of Titan."
By now, the Voyager twins have certainly earned the name their original name, "Grand Tour." In reading the following hosannas, remember that all of our sources were associated with Voyagers. Many had focused their careers on the space twins, and thus could hardly be called unbiased observers.
Nevertheless, here's what they told us about the significance of the Voyagers.
Ed Stone of Caltech, who has been chief Voyager scientist since the early 1970s, says the missions bagged many firsts: "The atmosphere on Titan, nitrogen, with a liquid methane rain; the rings of Jupiter, imaged for the first time; the rings of Saturn, where the wakes of the moons are causing ripples in the rings; Triton, the last object we visualized, is only 40 degrees above absolute zero, but it has geysers of nitrogen. That's a real puzzle, what is the source of the energy that is causing it to erupt? Every time we looked we found things we could not imagine."
Krimigis, who is principal investigator of a cosmic-ray experiment on Voyagers, says, "What became my love affair with Voyager began more than 37 years ago, when ... I was appointed a member of the Mission Definition Team. Little did I know at the time what an adventure this would turn out to be. I have now been with the project for most of my career in space science, and I consider Voyager the highlight of my entire existence as a scientist. Little did I know, as I was growing up on the Greek island of Chios, that some day I would be one of the few lucky people to represent humanity on the first mission that would travel beyond our solar system. This is the kind of stuff that dreams are made of!"
It would be difficult or impossible to dig up a planetary scientist not impressed by the Voyagers, so we did not waste much time scrounging for a counterpoint. (We certainly did hear many remarks about the relative benefits of robotic exploration of space, compared to human exploration.)
According to Daniel Baker, whose laboratory at the University of Colorado has an instrument on each Voyager, "These are extraordinary creations of human beings, that with very primitive processors, a very small amount of memory ... are still doing great work after decades." The space twins have returned a "vision of all these different worlds, these planetary systems ... with dozens of moons, and each different, unique. The pictures that were returned, the opening of our eyes!"
Voyager's many discoveries will not, Baker admits, "affect the price of gasoline," but they may have inspirational value. "Going someplace we have never been before -- it's hard to put a value on that. If you do these things, and it stimulates students to be more interested in science, and they go on to more technological careers, and that helps build the country, the economy. ... You could say that's a little far-fetched and not very measurable, but I believe even the most esoteric things we do in science often contribute in remarkable ways to stimulate us."
Just as modern telescopes have revealed a universe full of bizarre, fascinating objects undreamt of 40 years ago, the Voyagers turned planetary science topsy-turvy. "Every place they went, they found new, unexpected phenomena, characteristics, objects, processes," says Baker. "Yogi Berra [the legendary Yankee catcher] said you can observe a lot just by looking. By sending a general, nonspecific reconnaissance vehicle to these places, and going with an open mind and a wide range of instruments, that has allowed people to see what the magnetic fields were like, the particles, moons, and atmospheres of these different objects were like. Voyager rewrote the textbooks on every one of the outer planets."
Our bibliography: No textbook!