The Last World

Way out at the far edge of the solar system sits odd, mysterious unexplored pluto. After 5 billion years the tiny planet is overdue for some visitors.

By Dava Sobel|Saturday, May 01, 1993
From its outpost on the solar system’s far frontier, Pluto tantalizes would-be explorers with its remoteness and mystery. This is the maverick planet: the farthest, the smallest, the darkest, the coldest, and arguably the strangest. Oddball Pluto is neither a terrestrial world (like Mercury, Venus, Earth, and Mars) nor a giant ball of gas (like Jupiter, Saturn, Uranus, and Neptune) but the sole member of its own planetary category--an ice dwarf fashioned from the dregs of the great whirling nebula of gas and dust that condensed to form the sun and the rest of the planets some 5 billion years ago.

Pluto is the last unexplored planet in the sun’s entourage. But now, at long last, NASA visionaries propose to send a low-cost, high- audacity mission--the Pluto Fast Flyby--to scrutinize the planet from close range. At least one scientist at the Jet Propulsion Laboratory (JPL) in Pasadena has fondly described the flyby as a cannonball carrying a camera and a radio. In fact, there will be two cannonballs. The Pluto Fast Flyby will be a double-duty mission consisting of twin spacecraft that will rendezvous with Pluto within one year of each other, taking two looks at a world overlooked for too long. Lean and hell-bent as marathon runners, the tiny spacecraft must race through the planning and design stages to the launchpad before the end of this decade and then beat a direct trajectory to Pluto in six or eight years--quick--before the atmosphere that currently envelops the planet freezes and falls to the surface as some exotic breed of snow.

If the mission doesn’t fly before the snow falls, astronomers will miss altogether the chance to study the atmosphere. It will have vanished. Astronomers base this bizarre weather forecast on Pluto’s present motion away from the sun: in the course of the planet’s 248-year orbit, an atmosphere of methane and nitrogen apparently bubbles out of the surface when the sun is close by, only to freeze solid again soon afterward. Pluto thus behaves somewhat like a comet, sprouting new parts near the sun’s light and heat. No other planet does that. No wonder such excitement attends the effort to go there.

The impetus for the Pluto Fast Flyby came from a postage stamp, of all things. In 1991 a series of stamps commemorating U.S. space exploration matched the planets with their spacecraft visitors: Mars- Viking, Jupiter-Pioneer, Neptune-Voyager, and so on. Only Pluto, engraved in seafoam green, hung alone against the black backdrop of space, above the legend pluto not yet explored.

That bothered me a lot, recalls Robert Staehle, now the Pluto team’s manager at JPL. He immediately began pushing for a mission to explore Pluto and discovered that he was not alone in his wish to visit the planet--that there was, in fact, an active Pluto underground of researchers scattered across the country, dreaming up mission strategies and itching to go. Staehle became the catalyst for this community. An engineer committed to NASA’s renewed interest in small, speedy spacecraft that can be built on the cheap, he was able to pull together the ideas of many people to shape a plausible Pluto initiative.

Last August, in a courtly gesture, the 37-year-old Staehle telephoned then-86-year-old Clyde Tombaugh, who discovered Pluto in 1930, and formally asked permission to visit his planet.

I told him he was welcome to it, Tombaugh confirmed good- naturedly from his office at New Mexico State University, though he’s got to go one long, cold trip.

Tombaugh declined Staehle’s invitation to meet with the project team, on the grounds that commuting between New Mexico and California would be too arduous. Three times in the past, however, he joined expectant throngs at JPL as spacecraft returned the first detailed views of Mars, Saturn, and Neptune. When I made the Pluto discovery, Tombaugh recalled, only a few individuals dreamed of going to the planets--and even we didn’t expect to see this happen in our lifetime.

Tombaugh didn’t just stumble on Pluto; he hunted it down. By the middle of the nineteenth century astronomers had deduced the existence of unseen planets from the wobbly orbit of Uranus, a planet discovered accidentally by William Herschel in 1781. In finding Uranus, Herschel had doubled the size of the solar system--his new planet lay almost 2 billion miles from the sun, compared with less than a billion miles for Saturn, the most distant planet the ancients had known. In the furor following Herschel’s discovery, astronomers tracked Uranus’s position and compared its observed orbit with the path predicted by Kepler’s and Newton’s laws. Discrepancies measured in the thousands of miles suggested that another planet, even farther from the sun, must be tugging Uranus off course. These calculations led to the discovery of Neptune--on paper--in 1845, fully a year before anyone actually located the blue planet in the sky. But even Neptune could not account for all the displacement in the orbit of Uranus, and so the planet hunt continued.

American astronomer Percival Lowell sought what he called Planet X in vain until his death in 1916. Fourteen years later Tombaugh, then a youth of 24 working at Lowell’s own observatory in Arizona, found the elusive planet through dogged persistence--and new techniques. Knowing where to look wasn’t enough. Tombaugh needed to use a gadget that had been developed in Germany for discerning faint moving objects at great distances. This blink comparator enabled him to superimpose two images of the same area of the heavens taken several nights apart. In these matching views, each of the hundreds of thousands of stars would appear in exactly the same place, and so the combined stellar images would look no different from those of either individual photographic plate. But anything like a faint planet would move from night to night, and this motion would show up in the matching process and betray the planet’s existence.

Tombaugh spent nearly a year making these tedious comparisons. Finally he identified the moving star that we now call Pluto--4,000 times dimmer than the faintest star we can see with our naked eyes. That’s it! Tombaugh recalls exclaiming at the moment of discovery.

The planet proved to have the most eccentric orbit in the solar system. All the other planets follow fairly round, concentric orbits in a flat plane around the sun, as though tracing ripples on the surface of a pond, ringing the spot where a stone has hit the water. Pluto’s orbit, however, is a grossly exaggerated ellipse that dips inside the orbit of Neptune on one side of the sun and stretches far beyond Neptune’s orbit on the other. The Pluto-to-sun distance thus varies from a minimum of 2.8 billion miles to a maximum of 4.6 billion. What’s more, Pluto’s path tilts away from the rest of the solar system at a crazy 17-degree angle, as though some cataclysmic collision had knocked it out of kilter. If the other planets trace ripples on a pond, Pluto is a fish leaping out of the water beyond the outermost ripple. (Pluto also tilts far over on its axis at an exaggerated 58-degree inclination--two and a half times Earth’s inclination of 23 degrees.)

For four decades after Pluto’s discovery, the world’s largest telescopes strained to see the surface of this tiny distant body--smaller even than Earth’s moon. None could discern any details beyond the planet’s greenish-pinkish color and some vague dark splotches, which showed up as a periodic drop in the amount of light gathered during observations. These dark regions helped establish Pluto’s rotation period, because they rolled into view once every six and one-third days.

Then, in the mid-1970s, sensitive new infrared detectors came along that enabled observers to identify the spectral lines of methane in the sunlight reflected from Pluto’s surface. Astronomers had predicted a mixed composition of frozen methane and ammonia on Pluto, given the abundance of these chemicals in the outer solar system, but this confirmation of methane was the first real evidence they had.

Soon they learned of an even more exciting discovery. On June 22, 1978, James Christy, working at the U.S. Naval Observatory in Washington, D.C., serendipitously found that tiny Pluto had its own moon.

Christy had been poring over telescope images in an attempt to measure Pluto’s orbit precisely by tracking the planet’s motion against a familiar grid of background stars. In the process he noticed that Pluto’s shape didn’t look quite round, but as though there were a bump on its perimeter, and he noted further that the bump changed its position from one photograph to another. Christy determined within a few hours of his initial observation that this moving lump was actually an orbiting moon. He later christened it Charon, a name that had both mythological and personal significance: Pluto was the Greek god of the underworld, and Charon was the boatman who ferried the souls of the dead to that realm. Charon also sounded much like Christy’s wife’s name, Charlene, which is shortened to Char.

Charon, roughly 740 miles in diameter, is half the size of Pluto, which has a diameter of about 1,460 miles. This makes Charon the largest satellite, relatively speaking, of any planet. Earth’s moon is the second largest, with a diameter one-quarter that of Earth’s 8,000 miles. The giant satellites of Jupiter and Saturn, some of which measure more than 3,000 miles across, are larger in actual dimension, but they are dwarfed by their gargantuan parent planets--89,000-mile Jupiter and 75,000-mile Saturn.

Charon is so big, relative to Pluto’s size, that the two bodies constitute a double planet, says Alan Stern, a planetary astronomer at the Southwest Research Institute in San Antonio and head of an advisory group to the Pluto Fast Flyby team. There’s no other pair quite like them in the solar system, he adds. But we think when we get to study them close up, we’ll find parallels to binary stars. Like paired stars, Pluto and Charon appear to exchange material and to affect each other’s orbital motions. Careful analysis of their orbits shows that Pluto and Charon, roughly 12,000 miles apart, do a spiral dance around a common center of gravity, located between the two bodies at a point some 930 miles above Pluto’s surface. (In the Earth-moon system, by contrast, the common center of gravity is well below Earth’s surface.) Pluto and Charon hold each other in thrall, like tango dancers, rotating and revolving at identical paces--once every six and one-third days--so that each keeps the same face turned toward the other at all times. (Earth would have to rotate once every 28 days, instead of once every 24 hours, to keep a similar lockstep rhythm with the moon. But the moon isn’t big enough or, at 240,000 miles away, close enough to exert the gravitational pull that could make this happen.)

Within a week of Charon’s discovery, astronomers realized that Pluto and its moon, as viewed from Earth, were soon to engage each other in a series of mutual eclipses. Charon’s orbit would face Earth edge-on, so that the moon would pass in front of and then behind the planet. Thus executing their do-si-dos in space, Pluto and Charon would drop clues about themselves that would otherwise be impossible to detect at such a distance. Astronomers could, for example, arrive at more accurate measurements of the diameters of both bodies by carefully tracking and timing the beginning and ending of each eclipse. They already knew the total mass of the Pluto- Charon system from observing its orbital mechanics. If they had accurate measurements of the diameters, they could estimate the relative densities of both bodies. Density, in turn, would suggest composition--whether the bodies were mostly rock, ice, or gas. Additional chemical details on the composition of Pluto and Charon would come, during mutual eclipses, from the opportunity to view each body individually when the other was hidden behind it.

The right configuration for these mutual events occurs only twice during Pluto’s 248-year orbit, or once every 124 years, notes Stern, still marveling at this stroke of good fortune. The last time it happened we were fighting the Civil War.

Since Pluto and Charon huddled too close to be resolved by any Earth-based telescope, an observer couldn’t look at one body without looking at both. Any analysis of the light reflected by Pluto actually revealed the chemical composition of the pair. But during the mutual eclipses, which lasted from 1985 to 1990, when Charon periodically disappeared behind Pluto, a solo portrait of Pluto could be made. Subtracting this Pluto-only spectrum from the spectrum of the pair left an accurate picture of Charon. (The moon is not quite large enough to fully eclipse Pluto and pose for its own portrait.)

Pluto enthusiasts at observatories around the world, communicating with one another via an impromptu network called Ninth Planet News, milked these Pluto-Charon, Charon-Pluto eclipses for all they were worth over the five-year period they lasted. Researchers learned, for example, that Charon is covered primarily with water ice, which is as rigid as rock at the ambient temperature of approximately -360 degrees. Pluto, in comparison, counts methane ice, nitrogen ice, and carbon monoxide ice among its primary surface constituents. These are comparatively mushy materials, too structurally weak to create enduring surface features such as cliffs and scarps. The moon and the planet might thus look quite different from each other on the surface.

Even now researchers continue to extract new data from those mutual events. In a recent grand synthesis of these long-term joint efforts, Richard Binzel and Eliot Young of MIT released a rough map they created, which delineates the dark and bright areas on Pluto. (Binzel, now 34, is a longtime Pluto enthusiast--he was present at the U.S. Naval Observatory 15 years ago when Charon was discovered.)

Our map shows the south pole of Pluto to be dazzlingly bright, observes Binzel. It’s almost a perfect reflector, and that tells us the surface there is covered with some sort of frost. Binzel explains that since nothing can be expected to stay very bright for very long in dirty space, where dust particles seem to coat everything in a short time, the frost must be fresh. And that fresh frost, he thinks, reflects planetary weather patterns that change with the seasons. In his scenario, temperatures on Pluto rise as the planet reaches perihelion (its closest approach to the sun), which happened most recently in 1989. Along this sun- warmed stretch in Pluto’s orbit, some of the surface ices evaporate, forming the atmosphere now detectable. Then, as the planet recedes from the sun, some or all of the atmosphere freezes and falls back down to the surface.

If you’re a Plutonian, Binzel surmises, you get snowed on just once a Pluto year, which lasts for 248 of our years.

The existence of Pluto’s rarefied atmosphere was suspected in the mid-1970s but only proved in 1988. The discovery did not come from the Pluto-Charon mutual eclipses but from Pluto’s previously predicted passage in front of a faint star in the constellation Virgo. Astronomers, sniffing out another rare opportunity to learn something about the distant planet, observed this stellar event from eight sites in the Southern Hemisphere. As they watched, they saw the star grow distorted and gradually dim before it disappeared behind Pluto, as though blurred under a thin blanket of air.

Around Pluto, air consists of a mixture of nitrogen compounds and gaseous methane that produces a surface pressure less than one-hundred- thousandth that of Earth’s air. The atmosphere apparently billows out around the tiny planet, whose gravity is too weak to hold it close. In fact, Pluto’s atmosphere may reach all the way out to, and may even envelop, the closely orbiting Charon.

New assaults on Pluto’s unknowns continue to be made with ever- improving instruments. Just last May, Tobias Owen, of the University of Hawaii, and French astronomer Catherine de Bergh, working together at the United Kingdom Infrared Telescope on Mauna Kea, confirmed the presence of nitrogen ice on Pluto.

More recently the Hubble Space Telescope gave planetary scientists a clear view of Pluto, without the interference of Earth’s atmosphere, that allowed them to judge the planet’s density more precisely. They photographed the planet and its moon, fixing their exact position relative to background stars. By tracking the bodies’ motions as they orbited a common center of gravity, the researchers gathered the data necessary to calculate their masses. Pluto, at 1.3 x 1025 grams, is one- sixth the mass of Earth’s moon and 12 times the mass of Charon. Its density is about a third that of Earth, a finding that gives added weight to estimates that the planet’s composition is roughly half rock, half ice. Charon, barely more than half that dense, is likely to be made almost entirely of water ice.

No doubt additional information will be painstakingly wrested from Pluto by other Earth-based or space-based telescopes as the Fast Flyby takes shape, but the new mission’s close encounter will whip the pace of discovery from a slow walk to a full gallop. Finally Pluto watchers will be able to question their far-flung friend up close and personal.

What will Pluto be like? Will its surface erupt in slush volcanoes of nitrogen or methane ice? (Triton showed volcanic action when Voyager flew by it in 1989.) Will the planet’s surface features be uniform? (On close inspection, Mars showed itself to have a split personality--its moonlike southern highlands strewn with ancient craters, and its Earth- like, tumultuous north cut by ancient riverbeds and crowned with volcanoes grown to extravagant heights.) Do obscure moons the size of Manhattan lie in Pluto’s environs awaiting discovery? (All the outer planets were found to own several more moons than Earth-based telescopes could count.) Will it have a ring around it? (Every other far planet, from Jupiter to Neptune, has been found to possess several rings.)

From my experience working on the previous planetary missions, says Richard Terrile, study scientist for the Fast Flyby, there is only one thing of which I’m certain about the Pluto encounter--and that is that we’ll be surprised by what we find.

Terrile loves the dual nature of the Pluto Fast Flyby. If everything goes right, two spacecraft arriving a year apart will make it possible to watch Plutonian natural history in the making. In the worst- case scenario--say, if the first craft meets with disaster--the second stands ready in the wings as a built-in backup.

The first one will conduct the initial reconnaissance and raise new questions--questions we hope we can answer when the second spacecraft arrives and provides more information, Terrile says. The two craft will allow for one-year-interval time-lapse photography of Pluto and Charon. They will approach the planet and moon close enough to provide high- resolution views of both sides of both bodies, probably seeing details as small as a half-mile in diameter. The exact nearness of each craft’s approach has not been determined yet but will probably be in the vicinity of 6,000 miles. (The second craft may be cleared for a closer approach, depending on the findings of the first.) Even though both craft will fly by in about one hour’s time, their cameras will begin taking better-than- Hubble images at about six months out.

The instruments proposed for the spacecraft include a visible- light camera, a mapping infrared spectrometer, an ultraviolet spectrometer, and a radio transmitter to beam the findings home. The spectrometers will assess the makeup of the planet’s atmosphere and surface by looking for the fingerprints of molecules in the spectrum of sunlight passing through the Plutonian air and reflecting off the planet’s crust. The temperature and pressure of the atmosphere will be recorded by means of an experiment that requires no heavy equipment: Radio signals will be broadcast toward the spacecraft from Earth during the encounter. As the spacecraft begins to dip behind Pluto, the signal from Earth will pass right though the atmosphere, and it will continue to do so until the tiny visitor disappears into the planet’s shadow. Distortions in the signal’s phase and amplitude caused by the atmosphere can be translated into information about temperature and pressure.

The finished spacecraft will incorporate the latest microtechnology borrowed from military and commercial electronics. The star trackers that will hold it on course, for example, hail from defense satellites, while the compact, high-power computers are similar to those now nearly ubiquitous for desktop use. Design diagrams depict the craft as a small satellite dish antenna, about five feet across, sitting on top of a rack of stereo equipment with attached rocket fins (really radiator fins). It will use only about as much energy as an ordinary 60-watt light bulb, drawing its power from onboard radioisotope thermoelectric generators. These are plutonium-powered heat and electricity sources of the type used aboard Voyager and other missions to the outer planets, where solar panels prove useless in the dim sunlight.

At launch the spacecraft should weigh less than 242 pounds fully loaded so it can be propelled all the way out to Pluto with one big push from a Titan IV/Centaur rocket. We considered taking a gravity boost from Jupiter, explains Stacy Weinstein, the mission’s trajectory specialist, but we’d have to hold the launch at least until 2001 for Earth and Jupiter to line up right. If we whipped in a couple of times around Venus, where it’s really warm, and then out to Pluto, where it’s really cold, we would put other constraints on the spacecraft. The simpler the trajectory, the simpler the mission. The current Galileo mission to Jupiter, for example, increased its travel time by four years to take one boost from Venus and two from Earth. Galileo also needed fancy sunshades added to its design for heat protection in the inner solar system. The Pluto Fast Flyby, with no time to spare and strict limits on its launch weight, will aim directly for Pluto as though hurled from a slingshot.

Early, grandiose ideas called for Pluto orbiters bedecked with instruments, but these were quickly abandoned in favor of the more practical flyby, with its compact payload and direct trajectory. Even a small orbiter, Weinstein says, would greatly lengthen the trip, from a hoped-for 6 or 8 years to at least 18 years. (A fast traveler couldn’t slow down enough at Pluto to drop into orbit.) And the craft must reach Pluto soon, scientists insist, if it is to study the atmosphere while the atmosphere still exists.

For roughly the first five years of the mission, each flyby will just put miles between itself and Earth; no en route scientific activity is planned, since neither craft is expected to pass near any known objects of interest. The spacecraft cameras will first open their eyes and begin to gather images of Pluto about 12 to 18 months before reaching their destination. Once they get there, they will spend only a few hours of very- close-encounter time in the vicinity of Pluto and Charon. Then, having assailed the sun’s ninth circle, the tiny craft will keep right on going indefinitely, staying their course dead ahead.

It’s a short visit, given how long we’ve waited to see our distant neighbor. But it should be enough to accomplish the information- gathering objectives. Envision it as an act of espionage in which images are gathered quickly, then analyzed slowly after the encounter ends. Indeed, eager astronomers will wait some six months for all the data collected and stored aboard the spacecraft to trickle down to Earth at the painfully slow pace of the most energy-efficient means. Messages home will consist of about one picture a day because of the great distance between Earth and Pluto, the small size of the spacecraft’s antennas, and the necessarily low level of electric power (governed by the small scale of the equipment) available for data transmission. But just the thought of one of those pictures is enough to get Pluto enthusiasts excited.

After decades of slow moving in the wrong direction, reflects Terrile, NASA’s finally got a chance to move toward the frontier again. Right now, that frontier is 4 billion miles away, at the orbit of Pluto, and that’s where we’re going.
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