ANNALS OF EXPLORATION
by Michael Benson
For the past eight years, the vintage spacecraft known as the Galileo Orbiter has been tracing a complex path between Jupiter’s four large moons. During this time, it has made detailed scientific observations and taken thousands of high-resolution photographs, beaming them to Earth, half a billion miles away. On September 21st, Galileo’s extended tour of Jupiter’s satellites will end, and it will hurtle directly toward the immense banded clouds and spinning storms of the largest planet in the solar system.
As the orbiter plummets toward Jupiter’s atmosphere, several of its observational instruments will send a live transmission to Earth, and this data stream could prove highly illuminating. Galileo may be able to confirm the existence of a rocky ring close to the planet—a feature that has long been suspected. Other instruments will convey information about the density and composition of the mysterious, smokelike “gossamer rings” suspended inside the orbit of Amalthea, a moonlet near Jupiter.
At 2:57 p.m. Eastern Daylight Time, Galileo will be travelling at a speed of thirty miles per second, and its boxy octagonal frame will start glowing red. Seconds later, it will be white-hot. By 3 p.m., many of its eighty-five thousand components will have separated from each other, and will continue to break up, becoming a hail of rapidly liquefying shrapnel. By the time the spacecraft’s remains are three hundred miles inside Jupiter’s atmosphere, where the temperature is twelve hundred degrees, all its aluminum components will have vaporized. At six hundred miles, its titanium parts will disintegrate. Jupiter is a gaseous planet, with a radius of forty-four thousand miles—big enough to contain all the other planets and moons of the solar system—and Galileo will have hardly penetrated its outermost atmospheric layer. Having just crossed Jupiter’s threshold, it will vanish, leaving no clues of its earthly origin or its complicated mission.
Obliteration is precisely what NASA intends for the spacecraft. The reason is that Galileo may still harbor some signs of life on Earth: microorganisms that have survived since its launch from the Kennedy Space Center, in Florida, in 1989. If the orbiter were left to circle Jupiter after running out of propellant (barring an intervention, this would likely happen within a year), it might eventually crash into Europa, one of Jupiter’s large moons. In 1996, Galileo conducted the first of eight close flybys of Europa, producing breathtaking pictures of its surface, which suggested that the moon has an immense ocean hidden beneath its frozen crust. These images have led to vociferous scientific debate about the prospects for life there; as a result, NASA officials decided that it was necessary to avoid the possibility of seeding Europa with alien life-forms. And so the craft has been programmed to commit suicide, guaranteeing a fiery, spectacular end to one of the most ambitious, tortured, and revelatory missions in the history of space exploration.
Although Europa wasn’t the only target of Galileo’s camera during its years in space, its pictures of this weirdly fissured sphere—many of which show icebergs that apparently rafted into new positions before being refrozen into the moon’s ice crust—produced euphoria among planetary scientists in the late nineties. They now speculate that Europa’s global ocean may be more than thirty miles deep, which would mean that the moon has considerably more water than Earth. As Richard Terrile, a member of the NASA division that designed Galileo, has said, “How often is an ocean discovered? The last one was the Pacific, by Balboa, and that was five hundred years ago.”
The orbiter also conducted forty flybys of planets and moons, far more than any other spacecraft. It was the first to swing close to an asteroid; the first to orbit one of the outer planets; the first to document fire fountains erupting from the surface of Jupiter’s volcanic moon, Io; and the first to fly through a plume from Io, a lurid yellow-orange sphere with an estimated three hundred volcanoes erupting at any given time. In July, 1994, Galileo provided direct observation of fragments of the Shoemaker-Levy 9 comet slamming into Jupiter; these collisions produced explosions more powerful than that of the largest H-bomb.
In recent years, when the mission was directed from Earth by a skeleton crew on a low budget and had absorbed more than four times as much of Jupiter’s fierce radiation field as it had been designed to withstand, Galileo’s systems faltered frequently, but it continued to make discoveries. Last November, for example, its scanner registered the presence of up to nine tiny moons orbiting close to Jupiter. In June, 2000, it oddly failed to recognize the bright star cluster Delta Velorium, which flickers in Vela, a constellation that can be seen in the Southern Hemisphere. Subsequent observations from Earth confirmed that this group of five stars contains a dual-sun system, with one of its component parts periodically eclipsing the other, resulting in the variable light output that puzzled the spacecraft’s instrument. Galileo thus became the first interplanetary space mission ever to make an interstellar discovery.
Conceived by NASA in the early seventies, Galileo had a rocky beginning; its early history was marked by a series of delays. Its entire flight plan had to be redesigned five times, both because its technical specifications kept changing and because the positions of the planets shifted between launch dates. It was trucked back and forth between California and Florida, and was disassembled, cleaned, stored, and then reassembled. Although the orbiter was an extremely sophisticated piece of technology for the seventies, when it finally went into space, in 1989, many of its systems were already out of date. (Its main processors were rebuilt versions of the RCA 1802 chip, which was used to run primitive video games like Pong.)
Galileo’s most critical pre-launch problem was a woefully underpowered solid-fuel booster that could barely propel the craft out of Earth’s orbit. It was able to get as far as Mars or Venus, but reaching the outer planets appeared to be impossible. Galileo had been specifically designed for shuttle deployment; after the explosion of the space shuttle Challenger in January, 1986, a newly safety-conscious NASA had decided that the orbiter’s original, liquid-fuelled booster—which was more powerful but also potentially more dangerous than a solid-fuel device—couldn’t be lofted alongside the shuttle’s human cargo. The spacecraft seemed to be on the verge of a one-way trip to the Smithsonian.
Trajectory specialists at NASA’s Jet Propulsion Laboratory set to work, attempting to figure out how to get Galileo to Jupiter with what amounted to a lawnmower engine under the hood. The man who eventually solved this puzzle was Roger Diehl.
I spoke with Diehl, who still works at the Jet Propulsion Laboratory, in July. He told me that his first idea was to get the spacecraft to Mars, and then use that planet’s gravity to hurl it all the way to Jupiter. “I would go to bed at night, and my wife said she could even hear me talking about trajectories in my sleep,” he recalled. But he eventually realized that because Mars had swung from its ideal position during one of Galileo’s launch delays, that approach wouldn’t work. “It turns out that Mars is so small that if you go out of your way to fly by Mars to get a gravity assist you usually won’t get a benefit,” Diehl said. “So then I said, ‘Well, let’s launch to Venus.’ ”
This was hardly an obvious solution. Venus is in the inner solar system, and Jupiter is very far in the opposite direction. Moreover, this approach posed a significant thermal problem: Galileo had not been designed to travel closer to the sun before heading toward the frigid space around Jupiter. “If anyone had talked to a spacecraft person, there would have been a reluctance. They would have said, ‘No, don’t do that,’ ” Diehl said, laughing.
But he came up with a daring new flight plan anyway. Galileo would fly to Venus, curve back, swing around the Earth, then fly around Earth a second time exactly two years later; this trajectory would act like a slingshot, flinging Galileo all the way to Jupiter. Diehl realized that such a course would take several more years than the original plan, but he was undeterred. “I said to myself, ‘I’m going to think of the problem as doing a tour of the planets of the solar system, with the goal of getting to Jupiter,’ ” he recalled. “I didn’t care how many years it would take.”
Diehl presented his boss, Bob Mitchell, with the unlikely scheme in August of 1986. Mitchell approved the concept, which was dubbed veega, for “Venus Earth Earth Gravity Assist.” Within days, Jet Propulsion Laboratory designers came up with a way to save Galileo from the harsh temperatures near Venus: they could attach lightweight, strategically placed sun shields that would protect it from intense heat.
In the next several months, another scientist at the Jet Propulsion Laboratory, Lou D’Amario, substantially improved Diehl’s initial concept; for example, he expanded Galileo’s itinerary, modifying its trajectory to make it fly past two asteroids. In the end, the veega approach would require six years to propel the spacecraft to Jupiter, double the flight time of Galileo’s original plan.
Diehl considers his revision of Galileo’s trajectory, which effectively saved the mission, the highlight of his career. “My car license plate says ‘veega,’ ” Diehl said. “Every morning, I go out and I see the word.”
Galileo was successfully deployed from the space shuttle Atlantis on October 18, 1989, seven years after its original launch date. It spent the next year making its detour to Venus. In December, 1990, Galileo began its “Earth-1” maneuver: the first Earth flyby. This happened to coincide with the buildup to the first Gulf War. NASA had to inform the North American Aerospace Defense Command that the blip that would appear on its radar screens on December 8th—an incredibly fast-moving object that might well seem to originate from the Middle East—was not an enemy missile but a robotic spacecraft coming from Venus.
Throughout the entire first part of its journey into space, Galileo’s umbrella-shaped high-gain antenna, intended to be its main communications link to the Earth from Jupiter, had remained snugly folded at one end of the craft. It was the largest such device ever to have been sent out of Earth’s orbit. The plan was to deploy it only after the orbiter had receded far enough from the sun—because it, too, had originally been designed to operate in the bitter-cold temperatures of the outer solar system. In the meantime, the spacecraft would rely on a smaller, much slower antenna that was intended to be used only close to Earth.
In April, 1991, when Galileo was nearing the cooler climes of the asteroid belt, which is between Mars and Jupiter, the time had come to open the high-gain antenna and begin pulsing data toward Earth, at an optimal rate of a hundred and thirty-four kilobytes per second. Galileo was designed to have enough bandwidth to fire home one picture per minute, while also transmitting information from its other instruments.
But when the Jet Propulsion Laboratory finally ordered Galileo to open this key device, it stuck. Scientists running the mission were devastated: without a means of sending back high volumes of data, Galileo would be severely hobbled. Within a week of the antenna failure, two engineering teams were formed at the Jet Propulsion Laboratory. One was dedicated to getting the high-gain antenna unstuck. The other had to figure out how to rescue the mission without the use of the antenna; it was made up primarily of telecommunications specialists from the Deep Space Network. This division often provided NASA with a “million-mile screwdriver”—that is, a way of fixing a spacecraft by sending radio signals from Earth.
Leslie Deutsch, then the head of research and development for the Deep Space Network, is a garrulous but precise mathematician. “This was a crisis,” he recalled in a recent conversation. “I got together with a few people, and we did some brainstorming. First, we said, ‘Suppose we don’t change anything. What’s the data rate going to be when we get to Jupiter, if we have to use this low-gain antenna?’ ” The answer was ten bits per second, which translated to about one picture a month—and then only if Galileo’s ten other scientific instruments weren’t in use. Such a data rate was pitifully inadequate; in space, complex phenomena must often be photographed many hundreds of times before they can be properly understood.
Instead of attempting to change the spacecraft’s hardware, the Deep Space Network rescue squad began thinking about how it could improve Galileo’s information-processing capabilities. There was one possibility: Galileo’s fundamental software could be rewritten. To accomplish this feat, the onboard computer had to be powerful enough to handle the more advanced algorithms employed in the updated code. “The computer system on Galileo was ancient,” Deutsch said. “So we looked into what kind of microprocessors were on board, and how much memory there was. And there was good news and bad news.”
The bad news was that Galileo’s computer processors were so old that their original designers would need to be brought out of retirement for consultation. The good news was that, shortly before being launched into space, Galileo had been outfitted with twice as many memory chips as its designers originally intended; engineers had been worried that they were vulnerable to damage by radiation absorbed during the long journey in space. But after nineteen months in flight all the orbiter’s memory chips were still functioning, which allowed Deutsch’s team to do something that had never been attempted: change all a spacecraft’s software applications in midflight. Updating the software would enable the team to introduce advanced data-compression techniques, which would help make it possible for Galileo to send useful pictures and other valuable information from Jupiter over the low-gain antenna. Galileo would now be capable of sending more than two hundred pictures per month, along with other data. This rate was considerably slower than originally planned, and some of Galileo’s objectives would have to be modified or abandoned. But the mission could still accomplish more than seventy per cent of its goals.
It took years, but by the time the orbiter completed its first sweep around Jupiter its software had been fully replaced. It was a move with unprecedented risks—“a complete brain transplant over a four-hundred-million-mile radio link,” as one team paper put it—and any error could have meant losing the spacecraft. But the update was necessary, and the code transfer was flawless.
One problem remained: Galileo could collect information much faster than it could send it back. Its designers needed to find a way to store images, so that they could be slowly transmitted back to Earth. The orbiter, it turned out, had a tape recorder on board. Manufactured by the Odetics Corporation, in California, it was practically indistinguishable from the reel-to-reel recorders that were attached to the higher-end stereo systems of the sixties and seventies. Though the machine was practically obsolete, it became one of Galileo’s most important features.
The recorder had been incorporated into the orbiter’s design for one reason: to back up data from its atmospheric probe, which was scheduled to tunnel into Jupiter’s clouds in 1995, when Galileo arrived at the planet’s doorstep. This snub-nosed device would release its heat shield, deploy a parachute, and transmit information about Jupiter’s atmosphere back to the orbiter as it sank into oblivion. The whole procedure was supposed to unfold over the course of an hour. During that time, the probe’s findings would be relayed from Galileo back to Earth.
After the failure of the high-gain antenna, however, the tape recorder became a critical instrument. Galileo’s handlers at the Jet Propulsion Laboratory realized that it would be necessary to store all the incoming images and other scientific data gathered by its instruments during its flybys of Jupiter’s moons. That information could then be fed into Galileo’s computers (using the new data-compression software) and slowly transmitted back to Earth during the months-long lulls when the craft was travelling between Jupiter’s moons.
The magnetic tape spooled in Galileo’s tape recorder became a thread on which the mission’s destiny hung. The entire system had been jerry-rigged, but it worked. Galileo began slowly transmitting spectacular images of Jupiter and its moons to Earth, where an upgraded antenna system picked up the spacecraft’s slow, faint signals.
In March, 1996, the Jet Propulsion Laboratory team assigned the task of fixing the jammed antenna finally gave up. One analysis attributes the malfunction to a design flaw that was exacerbated by vibrations sustained when the antenna was hauled repeatedly between Florida and California during the years of launch delays.
Even before the scientists at the Jet Propulsion Laboratory finished updating Galileo’s software, the orbiter was not completely useless. In October, 1991, it took the first high-resolution images of an asteroid. Because the process of downloading photographs was so slow, it was instructed to send them back in fragments. Paul Geissler, a planetary geologist then at the University of Arizona’s Lunar and Planetary Laboratory, was one of the few researchers allowed to view them as they came in, bit by bit. “It was wonderful—we were locked into a room and sworn to silence,” Geissler said. “Because we didn’t have the high-gain antenna, the data came in as what we call ‘jail-bars.’ Galileo would send down a line, and then skip twenty lines, then send down another line, and then skip twenty lines and send down another line, and the issue was, is the asteroid in the frame at all, and should we use our precious bits to send down this frame or should we save it for the next frame?”
Geissler recalled the moment when his team realized that there was a tiny moon orbiting the asteroid. “In one of these jail-bars you could see Ida, and then it dropped off back into space again, and then there was another little blip. That’s all we had. These particular jail-bars had three lines and then skipped a bunch, and this blip was in all three of the lines, so we were dead certain that it wasn’t a cosmic-ray hit or anything like that. We knew there was something there. But we waited until another instrument on Galileo had a confirmation of it, and then we announced it.”
Although astronomers had long believed that some asteroids have moonlets, this was hard proof. It was also a reassuring illustration of what could still be achieved by Galileo, even in its extremely compromised state.
I asked Geissler, one of the leading image processors among planetary scientists, what it was like to see such unprecedented pictures before anyone else. He told me a story about the first complete shots of Ida, which had trickled in slowly, over a period of months in early 1994. “We had gotten two pictures of Ida up close, from different perspectives,” he said. “As the spacecraft flew past the asteroid, it snapped a picture, at high resolution, and then it flew a little bit farther and then snapped another picture of the same region.” Geissler realized that this separation allowed for the creation of a stereo image, which, when viewed properly, can give an object vivid three-dimensional form. “So I processed those pictures, and shot negatives of them, and brought them home—that was late on a Friday,” he told me. “I had a darkroom at home, and later that night I made eight-by-tens of these two, and I had pinched a stereoscope from work. I popped in these two wonderful eight-by-tens and became the first human being to see a stereo image of an asteroid at high resolution!” Geissler chuckled. “That entire weekend, anyone who came close to my door was dragged over—‘Look at this!’ You know, the mailman, the babysitter. That was really a thrill.”
For decades, scientists have known that three of Jupiter’s four large moons have high concentrations of frozen water. But only the hardiest optimists among them dared to speculate that liquid water could exist that far from the Sun. Europa’s average surface temperature is estimated to be two hundred and sixty degrees below zero.
In 1979, the twin Voyager space probes flew past Jupiter at approximately ten times the speed of a rifle bullet. The closest they got to Europa was about a hundred thousand miles; Galileo has veered to within a hundred and twenty-four miles of the moon. Despite being so far away, the Voyager probes compiled a photographic record suggesting—indirectly—that Europa might be warmer below its icy surface. The most obvious clue was to be found on images of Io, Jupiter’s innermost large moon. Firmly gripped by the tidal pull of its parent planet’s gravity, yet yanked the other way by the shifting gravitational fields of two of its three sister moons, Io produces seemingly endless chains of active volcanoes. At three thousand degrees, they are far hotter at their source than any volcano on Earth. Io is the most volcanic object in the solar system; the mere proximity of such an excitable object to Europa suddenly rendered the idea of subsurface water more imaginable. If such active volcanism was present on Io, why couldn’t there be similar eruptions on Europa’s seabed?
The other Voyager-era clue was more subtle and mysterious. Long, looping chains of scalloped cracks snake across large spans of Europa’s surface. These unusual patterns, which extend for hundreds of miles across the crystalline topography encircling the moon’s poles, were already clearly visible in the Voyager images. In 1996, Galileo began taking highly detailed photographs of Europa. Scientists concluded with excitement that the fissures on Europa—which were dubbed “arcuate ridges”—were unique in the solar system.
Meanwhile, a handful of planetary geologists struggled to sort out what the curved lines on Europa’s surface signified. One of them was Randy Tufts, a geologist at the University of Arizona. Tufts had been fascinated by those eerie ridges even before Galileo reached Jupiter. In a conversation I had with him a few years ago, he recalled that in the early nineties he had printed multiple copies of the low-resolution Voyager pictures and handed them out to his nonscientist friends—hoping that one of them might miraculously intuit the cause of the surface cracks. He had even taken the pictures to a glassblowing studio in downtown Tucson and asked the workers there if they had ever seen similar patterns. (They hadn’t.) “I was just casting about for any kind of analogue,” he told me.
In 1998, Tufts discovered an immense, gently curved fault line in the southern hemisphere of Europa. Galileo photographs revealed that the crack, which was subsequently named the Astypalaea Linea, extends about six hundred miles, which is comparable to the San Andreas Fault. This feature offered clear evidence that parts of Europa’s crust were slowly moving—perhaps even floating.
That summer, it occurred to Tufts that the curvature exhibited by both the Astypalaea Linea and the arcuate ridges could be caused by the immense gravitational pull of Jupiter, which has three hundred times the mass of Earth. The linked curves of the arcuate ridges, he realized, could be explained by the fact that Jupiter does not exert a consistent amount of force on Europa. The planet pulls more strongly on the moon when the two bodies happen to be closer together. “Since Europa’s elliptical orbit sometimes takes it farther away from Jupiter, the amount of stretching it undergoes kind of relaxes a little bit,” he explained. Cracks start propagating—but then, as Europa recedes from Jupiter, they stop. Because Europa’s Jupiter-facing hemisphere rocks back and forth during each orbit, by the time the gravitational stresses pick up again they’re oriented in a slightly different direction.
With the help of Greg Hoppa, an orbital-dynamics specialist, Tufts plotted the effect of these fluctuating force levels; he ended up with looping cracks that look just like the ones on Europa. That was quite a breakthrough, but the team’s next insight was even more significant: the whole process couldn’t happen without the existence of a large body of subsurface water to exert tidal pressure from below. Ice crusted on solid rock could never be affected so much. The tides on Europa are much higher than those on Earth, reaching almost a hundred feet; when Jupiter pulls these enormous subsurface bulges of water in its direction, Tufts concluded, the ice on the surface begins to crack.
In the end, Tufts’s insight is appealingly straightforward. By studying the elegant shapes on Europa’s surface, he divined what lay beneath. “Later, I found myself sort of apologizing for its simplicity,” he said. “And people said, ‘Well, you know, some of the best ideas in science are very simple ones.’ They’re often so simple that everyone sees right through them.” Tufts was excited by the idea that life might exist on Europa. “It always seemed to me that if we found life someplace else it would give us a vastly new perspective on existence,” he told me. “And we would probably realize that we weren’t quite so important as we thought we were.” He frowned thoughtfully. “I mean, it might take us down a peg, which always could be useful.”
Randy Tufts died last year, at the age of fifty-three, from a bone-marrow disorder. Not long before his death, he was working with scientists on plans for an orbiter that would investigate Europa’s ocean more closely. In 2002, the project was cancelled, owing to budget cuts.
In late July, I called Arthur C. Clarke at his home in Colombo, Sri Lanka, and asked him to comment on Galileo’s impending death. Clarke has long been fascinated by Europa; it figures prominently in the sequels to “2001: A Space Odyssey.” In particular, I wondered if he shared NASA’s concern that if Galileo were to crash into the distant moon it could transfer microbes from Earth to Europa’s ocean. Clarke didn’t answer directly, instead suggesting that I read an old tale of his, “Before Eden,” written in 1961. “It’s all about the danger that we might contaminate new worlds,” he said.
The story is about a scouting expedition to the South Pole of Venus, which is described as being “a hundred degrees hotter than Death Valley in midsummer.” The expedition leaves behind a single human artifact—a bag of waste. It ends up contaminating a strange Venusian life-form that the expedition had discovered there, thus ending its evolution. I concluded that Clarke probably endorsed NASA’s plan to destroy Galileo.
I spoke with Leslie Deutsch about his reaction to the decision, and he said he was initially angry, though he understood the rationale. Over the years, Deutsch admitted, he had become emotionally attached to the distant robot emissary, adding that this would be only the second time that NASA had deliberately destroyed a functioning spacecraft. When I asked Bill O’Neil, Galileo’s long-serving project director and one of the key architects of the effort to save the mission, what his reaction had been when he’d heard of the decision, he mulled it over for a few days, then sent me an e-mail. Galileo’s end would bring a personal sense of satisfaction at what had been achieved, he wrote. Still, he found it ironic that “Galileo Galilei only got house arrest by his sponsor the Roman Catholic Church for discovering things they didn’t want to be true, whereas our Project Galileo gets a death sentence from NASA for its greatest discovery: the prospect of life on Europa.” ?