The Outer Solar System On about $25 million a Year: Pluto Express Sets its Sights on the Smallest Known Planet

by Eli Lehrer

    On a warm day in October 1991, scientists at NASA's Jet Propulsion Laboratory crowded into the Von Karmen Auditorium to witness the unveiling of a new line of postage stamps. Nine stamps, depicting eight planets and the earth's moon looked the same: a spacecraft in the foreground, a planet in the background and a spacecraft's name under the stamp's picture. The Pluto stamp looked different. Underneath a drawing of the planet anyone who cared to look could make out nineteen capital letters: "PLUTO NOT YET EXPLORED."

    Robert Staehle took the stamp's legend as a taunt.

    Talking with other scientists after leaving the auditorium, Staehle--today the project manager for the mission now known as Pluto Express--came to a conclusion: the U.S. could launch a Pluto mission quickly and cheaply.

    Just about six years later, Staehle's idea seems close to realization: President Bill Clinton included funding for Pluto Express in his 1998 budget proposal and Congress seems likely to approve funding.

    "Even with this Republican Congress, the mission looks like a go-ahead," said one senior Washington lobbyist familiar with space-policy issues. "The cost is federal pocket change and a lot of people at NASA like the idea of the mission. Anyway, a lot of scientists believe it's a place we have to go."

    Scientists guess that Pluto has about two-thirds the moon's diameter. At this size, it lies on the cusp of being counted as a particularly large asteroid. Pluto orbits anywhere from 30 to 50 times the earth's distance from the sun. Indeed, to an observer standing on its surface Pluto's moon Charon would be the largest object in the sky. This far out, the sun appears as nothing more than another star in the perpetual night sky. Indeed, Pluto is so far from Earth that astronomers find it difficult to learn much about it. While observations have yielded crude maps of one of Pluto's hemispheres and a little about its temporary atmosphere (see sidebar), Pluto's great distance makes it impossible to know much for sure. Even the planet's diameter remains one of astronomers’ many unanswered questions.

    On its way to answer these questions, Pluto Express went through a long and tortuous evolution. At first, designers called the mission Pluto Fast Flyby. Then, it consisted of an attempt to use a big rocket and special launch window to reach Pluto in only a few years. Cost considerations knocked out the idea of the launch vehicle and forced a new stage in the mission's evolution.

    Although some elements might change, any Pluto Express mission would launch some time between 2001 and 2004 and reach Pluto between 2013 and 2017.  The mission, in other words, will be anything but short.  Even if scientists use the 2001 launch window, a child born when formal planning for the mission began in 1992 would have graduated from college by the time data begins to trickle back from Pluto.

    While some missions, such as NASA's recently ended Pioneer 10, will have lasted longer than Pluto Express, the proposed mission will take far longer than any previous voyage of exploration before it reaches its primary objective.

Partly to make sure that the mission returns some data and partly for the inherent abilities of two probes to go more places than one, current mission plans call for two more-or-less identical craft.

    The crafts’ design represents a new stage in thinking about the design of space exploration. Indeed, the overall design proves so different that scientists call them sciencecraft rather than spacecraft.

    "The idea was this: you would design the science objectives first and then add whatever was needed to make them survive," said Jonathan Lunine, a professor at the University of Arizona who chaired the Pluto Express science definition team. "We would build the science first, not the spacecraft bus."

    The proposed sciencecraft weigh only a hair under 575 kilograms, small enough to fly on inexpensive rockets. Part of the weight reduction comes in an ordinary way: lightweight materials and design philosophy that doesn't add so much as a superfluous screw.

    The weight reduction also comes from a move many designers consider risky: the use of lots of high-tech components. Recently, most NASA missions have used technology years behind the times. Up until the Apollo moon landings, many missions did use the best technology: the space program virtually originated modern computers. As budgets fell, development cycles became longer and existing technology proved adequate to accomplish many objectives; missions no longer looked high-tech. The space age didn't turn very high tech at all as NASA sent out spacecraft after spacecraft powered by computers systems with one tenth the power or one one-hundredth the power of the desk top computers found in nearly every office.

    While technological advances will probably render the mission a little less high tech by the time it actually soars spaceward and hopelessly outdated by the time it reaches Pluto, the mission looks technologically sophisticated by today's standards. Largely in order to reduce weight and save money, current mission plans call for integrating nearly all major mission control and science components into tightly engineered multi-chip modules. Some of the systems involved with the mission will receive a tryout under NASA's New Millennium Program and designers will select a number of off-the-shelf components with extensive use in commercial and industrial environments. Testing on Earth or even in on missions of shorter duration will not, however, necessarily prove adequate preparation for a long trip fraught with all sorts of dangers.

    As the mission involves two separate sciencecraft with nearly identical systems, one might see its design as the ultimate in redundancy. The mission would still accomplish many of its objectives if one sciencecraft failed entirely during the mission.

    While the individual sciencecraft do have some fault correction ability, the overall level of backup and redundancy systems seems much smaller than designers have typically built into spacecraft. Redundancy has saved many missions including Voyager and the lack of redundancy concerns some.  "I'd like to see this work but personally, I think that these guys are nuts," said one Cornell professor speaking about the Pluto Express. "Still, I'd love to see some science come back."

    Scientists working on the project speak about it as an effort to do "Voyager-type science." To this end, the mission includes an instrument package similar to the Voyager probes’: an imaging device (camera), an infrared spectrometer (to detect minerals and hydrocarbons), an ultraviolet spectrometer (to study the atmosphere), radio science experiments and, perhaps, a not-yet-defined series of particles and fields experiments.

    Although the science definition team came up with sixteen objectives for the mission's Pluto flyby, one can crudely divide these into five categories: mapping, atmospheric exploration, mineralogical/geological surveys, study of Pluto's interactions with the sun and general surveys of the system surrounding Pluto and its moon Charon.

    The mapping missions are just what they sound like. Although scientists certainly would not come up with detailed maps during the proposed 15km-per-second flybys, the sciencecraft will return images far better than anything available from Earth. Looking at the infrared spectra of surface elements can also teach scientists a great deal about the planet's surface composition.

Scientists consider Pluto's atmosphere a temporary phenomenon (see sidebar). Although Lunine says that "Pluto Express isn't a race to get to Pluto before the atmosphere collapses," the planet's wispy atmosphere has drawn a lot of attention from scientists. Though some aspects, such as learning about surface temperature, may prove difficult during Pluto Express’ brief flyby, designers hope to learn as much as they can about the atmosphere.

    Scientists also want to study Pluto's ionosphere, its interaction with the solar wind, and the fraction of the solar radiation the planet's surface reflects back into space.

    Finally, the mission seeks to learn what it can about Pluto and the area of space around it. Astronomers see dozens of possibilities: among other things they want to look for new satellites around Pluto and see if Charon has an atmosphere of its own.

     The mission, in other words, will teach scientists roughly the same amount that the Voyager and Pioneer flybys taught them about the rest of the outer solar system. It will provide broad outlines of the Pluto system and lay the groundwork for the design of  future missions. Stahle has said that the craft might do some other investigations before reaching Pluto. Instruments used for these investigations might include a gamma burst detector to study the enigmatic pulses of radiation which seem to emanate from outside of the Milky Way. Scientists might also use instruments designed for the Pluto flyby to gather data during the Jupiter flyby. Depending on its course, Pluto Express might even fly by some asteroids on its way to Pluto. Decisions on science during the mission's cruise to Pluto, however, will depend mostly on the mission's level of funding.

    Although neither nation has committed to funding, Germany and Russia may participate in Pluto Express. The mission might include a German built instrument to explore the Jovian moon of Io and a Russian built module to enter Pluto's atmosphere.

    The proposed German module would, in effect, hitch a ride on board Pluto Express. During a Jupiter flyby, it would enter Io's atmosphere in order to study the moon's interactions with Jupiter's magnetic field. The proposed Russian probe would enter Pluto's atmosphere carrying an instrument package designed to make atmospheric and chemical measurements impossible during a brief flyby.

    Following its Pluto flyby, scientists would aim the craft at an object in the Kuiper belt--a flattened belt of objects surrounding the solar system that contains undisturbed comets and planetary debris. Scientists believe that the Kuiper belt represents a collection of material from the solar system's initial  formation. "We'd mostly be interested in studying the surface ices on a Kuiper belt object," said Lunine. "While we're designing this mission for Pluto, that would still be an important part of the mission."

    Upon a careful examination, Pluto Express hardly seems a modest mission. If it proves successful, it will return a treasure trove of information about the outer solar system and, in time, raise dozens of new questions. It will do all this for well under $500 million and use lots of high-tech parts in the process. The total cost will be only about twice that of NASA's low-cost Discovery missions. In its ambitions, indeed, the mission only seems a few steps away from the super-ambitious missions like the Cassini exploration of Saturn.

    "In this funding environment, there are two choices," said Lunine. "You can cut back the science and try to launch a mission anyway, or you can think about how to do lots of really good science on the cheap. That's what we're trying to do."

Eli Lehrer, SciTech Magazine's literary editor, is a medieval studies major in the College of Arts and Sciences. He does not believe that a radish the size of Kansas is coming to destroy Earth.

The Pluto We Know
    On February 18, 1930, just a few months into his search for a planet beyond the orbit of Neptune, astronomer Clyde Tombaugh found Pluto.

    At first, astronomers thought Pluto a huge world: the major contributor to irregularities in gas giants’ orbits. Calculations and observations over the past 57 years have greatly reduced the estimated size of the planet. Indeed, some astronomers joked that the rate of decrease in estimates of Pluto's size would result in the planet's disappearance by 1980. While Pluto didn't blink out of existence during 1980 clever observation has resulted in tons of new information about the planet. Following a long period during which observation yielded few major finds, James Christy discovered Pluto's moon Charon in 1978.  Charon is the largest known moon in proportion to the size of the planet it orbits. Indeed, Richard Binzel, an astronomer at MIT, has suggested that it might be proper to think of Pluto and its moon Charon as a two-planet system. Over time scientists also have charted the planet's highly irregular orbit. Pluto is now reasonably close to the sun having reached the closest approach of its 247-year circuit in 1989. During a twenty-year period that will end in 1999, Pluto has been the eighth planet out from the sun.

    In the early 1980s, astronomers had an opportunity to produce a crude map of Pluto. At that time, Charon's orbit appeared edge-on with relation to earth. These occultation/transit sequences--as eclipses of planets are called--allowed scientists to create crude maps of the planetary bodies as Charon's shadow covered features on Pluto's surface. The variation in the two bodies’ combined brightness also allowed for some crude spectral measurements, giving scientists an idea of what materials compose Pluto and Charon. Based on these observations and other previous calculations, scientists believe that Pluto is made of frozen methane and water warped around a rocky core. The surface may also contain small amounts of other frozen gases.

    Astronomers’ 1988 observations of a bright star crossing over the planet's image as seen from Earth led to the discovery of Pluto's atmosphere. While some dispute exists, most scientists believe that the atmosphere is temporary, a result of increasing heat as Pluto moves closer to the sun. Because of Pluto's low gravity, scientists believe its atmosphere is very lightweight--mostly methane with, perhaps, a few molecules of nitrogen, carbon monoxide and oxygen. Most believe that the atmosphere will snow down onto the planet's surface during the next 40 years as Pluto moves away from the sun.

    Pluto still provides plenty of mysteries. At such a great distance from the earth, it is easily the least known planet in the solar system. Although maps technically exist, they are little more than collections of light and dark bars that reveal very little about the planet's surface features.  Pluto's atmosphere represents a source of interest: scientists want to know what it is made of and what sort of weather it produces. Indeed, scientists don't necessarily know all of the interesting questions yet. Pluto Express, however, promises to raise them.