Pluto flyby may define 'planet'

Mission set for Jan. 17 launch may enlighten argument over remote body's cosmic status

Jim Erickson, Rocky Mountain News

Published January 2, 2006 at midnight

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Streaking through the solar system nearly 100 times faster than a jetliner, NASA's New Horizons spacecraft will still need more than nine years to complete its epic 3.1 billion-mile trek to Pluto.

Set to blast off from a Florida launch pad on Jan. 17, the $700 million probe will travel farther to reach its destination than any previous spacecraft and is expected to send back pictures 10,000 times sharper than the best Pluto images now available.

The first mission to the solar system's last unexplored planet, New Horizons also will be the fastest spacecraft ever launched. It will zip past the moon nine hours after launch - a trip that took Apollo astronauts about three days.

"This is, in a very real sense, the capstone of the initial reconnaissance of the planets that the United States has led for the world since the 1960s," said Boulder planetary scientist Alan Stern, the New Horizons principal investigator.

"This is a little bit about leadership. This is a little bit about rewriting the textbooks about the outer planets," he said.

"It's also about inspiring the next generation of scientists and explorers that we hope will take us to even greater heights."

And it's hoped that long before the piano-size probe arrives in 2015, scientists will have resolved the protracted, polarizing debate over the planetary status of Pluto and the other large ice-and-rock bodies littering the solar system's fringes in a vast sun-circling band called the Kuiper Belt.

Is Pluto a planet or not? What about those other large, recently discovered Kuiper Belt residents?

To Stern, who works at the Southwest Research Institute, the answer is clear: The largest Kuiper Belt objects constitute "a whole new class of planets" called ice dwarfs, he said at a recent news conference.

"We expect that there are hundreds, if not thousands, of these ice dwarfs," Stern said. "So this class of planet, which we have not yet reconnoitered, is in reality the most populous class of planetary body in the solar system."

But others, like University of Colorado planetary scientist Larry Esposito, say puny Pluto and its Kuiper Belt cohorts don't deserve "planethood."

"I think Alan is both overstating the significance of these objects and the unanimity in the planetary community about how to handle this issue," Esposito said.

"These Kuiper Belt objects are to my mind more leftovers than planets, more debris or detritus," he said.

And that includes Pluto.

Esposito argues that the faint, far-off iceball discovered by American astronomer Clyde Tombaugh in 1930 should be stripped of its planetary stripes.

Demoted. Drummed out of the familiar nine-member family of sun-orbiting orbs.

So the solar system either has eight planets, if you agree with Esposito, or perhaps hundreds, if you side with Stern.

The arbiter of such cosmic disputes is the International Astronomical Union. The IAU's General Assembly may vote in August on three suggested planet definitions submitted recently by a panel of experts, said IAU Vice President Robert Williams, an astronomer at the Space Telescope Science Institute.

"Alan Stern is free at this point to call a planet whatever he wishes to," Williams said.

"There are strong feelings about this on all sides," he said. "And the fact that three recommendations came up to us means that there isn't a consensus at all, even among the experts."

Colorado connections

This is the intellectual backdrop for the New Horizons launch from Florida's Cape Canaveral. The probe will fly past Pluto and its moon Charon, then head for one or two other Kuiper Belt objects.

The launch is currently set for 11:24 a.m. MST on Jan. 17. The compact, 1,054-pound spacecraft will ride atop NASA's most powerful launch vehicle, the Atlas V-551, built in Jefferson County by Lockheed Martin Space Systems.

The spacecraft itself was built at Johns Hopkins University and will be operated from its Applied Physics Laboratory in Baltimore. Science operations will be based at the Southwest Research Institute in Boulder.

The spacecraft carries seven science instruments, including a camera built at Ball Aerospace & Technologies in Boulder and a dust counter designed and built by University of Colorado students.

About a dozen CU students plan to attend the launch, said Beth Grogan, the project's software integration and test lead.

"It's a little overwhelming to have this kind of responsibility at my age, but I think I've handled it pretty well," said Grogan, 23, who graduated in May 2004 with a computer science degree.

A couple of hurdles still have to be overcome before the 196-foot-tall Atlas is cleared for launch.

First, NASA and Lockheed Martin must finish troubleshooting a possible problem with the Atlas V's fuel tank.

A precautionary inspection of the tank will be performed just after the New Year's holiday, said company spokeswoman Julie Andrews. If New Horizons fails to get off the pad by Feb. 14, the next launch opportunity is in February 2007.

Second, President Bush's science adviser, John Marburger, must declare that the nuclear-powered mission is safe for launch. That "nuclear safety launch approval" is expected sometime this week, said NASA spokesman Dwayne Brown.

Nuclear concerns

New Horizons' electrical power comes from a single radioisotope thermoelectric generator, or RTG. The energy-efficient spacecraft uses less power than a pair of 100-watt household light bulbs.

An RTG is not a nuclear reactor and does not rely on nuclear fission or fusion. Instead, it provides power through the natural radioactive decay of plutonium - mainly plutonium-238, a non-weapons-grade form.

Provided by the Department of Energy, the New Horizons RTG contains 24 pounds of plutonium dioxide in a fire-resistant ceramic form. If fractured, the ceramic tends to break into relatively large chunks that pose fewer hazards than microscopic particles, according to the space agency.

RTGs have been used on 24 U.S. missions over the past 40 years, including the Cassini spacecraft currently orbiting Saturn. All have performed as expected.

Even so, some anti-nuclear activists say the devices are unsafe and that a launch accident could lead to the release of cancer-causing plutonium.

A launch protest is planned for Jan. 7 at Cape Canaveral, said Bruce Gagnon, coordinator of the Maine-based Global Network Against Weapons and Nuclear Power in Space. Similar protests occurred before Cassini's 1997 launch.

"Space technology can and does fail on occasions," Gagnon said. "Our feeling is that as you escalate the number of these launches, sooner or later there's gonna be a tragic, tragic accident with this stuff."

NASA says the probability of a New Horizons launch-area accident that releases plutonium is about 1 in 350.

But even if such a release occurred, the risk to workers and the public is low, according to the space agency. The most likely launch-area accidents would result either in no radiation exposure or very low doses that are significantly below the natural background levels of radiation we encounter every day, according to NASA.

"No additional cancer fatalities in the launch area would be expected from doses at this level, even over a time span of 50 years," NASA's New Horizons press kit states.

However, some more rare types of launch-area accidents "could lead to potential latent cancer fatalities among the group of exposed individuals following an accident," according to the environmental impact statement prepared for the New Horizons project.

"Any such cancer fatalities would not occur promptly upon exposure, but could occur over the long term," according to the document.

The probabilities of those rare but more severe launch-area accidents range from 1 in 100,000 to 1 in 1 million, said Alice Caponiti, nuclear material and safety manager at the Department of Energy's Office of Space and Defense Power Systems.

The New Horizons environmental impact statement also concluded that a plutonium-releasing launch accident could result in environmental contamination costing $241 million to $1.3 billion per square mile to clean up.

The long cruise to Pluto

If the launch goes as planned, New Horizons will speed away from Earth at 36,000 mph, faster than any previous spacecraft. About a year later, the spacecraft will pass Jupiter, gaining a "gravity boost" that will increase its speed by nearly 9,000 mph.

New Horizons will spend most of the remaining cruise to Pluto in hibernation mode, broadcasting a weekly beacon to update ground engineers on its health.

Three months before arrival, the spacecraft will wake up and start taking pictures of Pluto and its largest moon, Charon. Earlier this year, two much smaller Pluto moons were discovered, and the New Horizons team will search for others.

If New Horizons launches by Jan. 27, it will swoop past Pluto at a distance of about 6,200 miles on July 14, 2015. At that distance, the probe's highest-resolution camera will be able to distinguish surface features about 80 feet across, said William Gibson, the mission's science payload manager.

New Horizons will study the global geology of Pluto and Charon, map their surface compositions and temperatures, and examine the makeup and structure of Pluto's thin atmosphere.

If the spacecraft remains healthy after the Pluto flyby, and if extra funding is available from NASA, the probe will fire its thrusters and head off to explore one or two other Kuiper Belt objects.

Discovered in 1992, the Kuiper Belt is a vast region of ancient iceballs more than a billion miles beyond Neptune's orbit.

Over the past 13 years, astronomers have discovered more than 1,000 Kuiper Belt objects, or KBOs. Researchers estimate the belt contains more than 100,000 bodies larger than 60 miles across.

One Pluto-size KBO - nicknamed Xena - already has been found, and others may be lurking in the belt's far reaches.

Harboring deep-frozen leftovers from the solar system's creation 4.6 billion years ago, the Kuiper Belt likely holds clues to the birth and evolution of the outer solar system.

"Exploring Pluto and the Kuiper Belt is like conducting an archaeological dig into the history of the outer solar system, a place where we can peek into the ancient era of planetary formation," Stern said.

Pluto at a glance

• Diameter: 1,466 miles, about two-thirds the diameter of Earth's moon.

• Surface composition: Includes ices of nitrogen, carbon monoxide, methane and water.

• Surface temperature: Estimated at minus 387 degrees Fahrenheit.

• Average distance from the sun: 3.7 billion miles, about 40 times farther from the sun than Earth.

• Orbit: Loops around the sun once every 248 years.

• Density: About twice that of water, indicating it is composed of a rock-and-ice mix.

• Moons: Charon, the largest, is 745 miles across. Two much smaller satellites were discovered in 2005.

• Light level on Pluto: 1,000 times fainter than daylight on Earth.

• Brightness and apparent size of Pluto as viewed from Earth: 10,000 times fainter than the naked eye can see; 100 times smaller than Mars.

Source: NASA

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