Pluto Mission Takes Aim at Last Unvisited Planet

Brian Handwerk
National Geographic News
February 15, 2005
Amateur astronomer Clyde Tombaugh discovered Pluto 75 years ago this week. The last planet found in our solar system remains the only one never visited by an Earth probe—but NASA's New Horizons mission hopes that distinction won't last much longer.

"We're planning on a launch in January of 2006," said program executive Kurt Lindstrom from NASA's Washington, D.C., headquarters. "This year we are assembling [the spacecraft] and beginning testing."

If the mission launches on time, the craft would reach Pluto in the summer of 2015. Jupiter is in position to give the passing spacecraft a "gravity boost," speeding its travel time. A one-year launch delay would actually mean a three-year push in arrival time, as Jupiter will have orbited away from this position.

Distances vary because of changing planetary orbits, but Pluto is currently about 31 times farther from the sun than Earth is. A signal at the speed of light would take about four hours to travel from Earth to Pluto.

Sometimes called "the first mission to the last planet," New Horizons is similar in many respects to NASA's successful Cassini or Voyager missions.

"It's a pretty straightforward trajectory out to Pluto," Lindstrom noted. "Obviously we've got to have a good-size rocket to get off the Earth and get out there in a reasonable time, but survivability comes down to the parts. We really have to worry about parts being able to last that long and about having enough power to survive that long."

Earthbound Observations Add to Pluto Knowledge

Pluto is a faint point of light in the sky, making telescope study a challenge. But advances over the last decade, including images from the Hubble Space Telescope, have allowed scientists to learn more about the planet.

Pluto's surface is covered by numerous ices including methane, nitrogen, and carbon monoxide. The different ices are revealed by spectroscopy, the study of different light wavelengths. The technique allows astronomers, like Will Grundy, to "map" ice locations and how they change. Grundy is based at Lowell Observatory in Flagstaff, Arizona, the same facility where Tombaugh discovered Pluto.

"The ices are moving about seasonally, kind of like the hydrological [water-circulation] cycle on Earth," Grundy said.

Another observation technique is the use of stellar occultations, when one celestial body passes in front of another. When Pluto passes between Earth and a bright star, its atmosphere becomes backlighted. These events allow earthbound observers to study the planet's thin nitrogen, methane, and carbon monoxide atmosphere by measuring light refraction.

"Pluto is now getting into the thick of the Milky Way, so there should be lots of opportunities to do that in the next few years," Grundy explained.

Still the vast distances involved limit observation from our neighborhood of the solar system, and sometimes just finding Pluto in the sky can be a challenge. That's why data from the New Horizons mission is so important.

"The whole point is to get close enough to look at the [features] of the surface and analyze the atmosphere and its composition," Lindstrom said. "And that just can't be done with current ground or space-based techniques."

Grundy agrees, which is one reason he's also taken a role as part of the NASA mission. "I've been studying Pluto as an astronomical object, a tiny point of light in a telescope," he explained. "The transition to a real world where you know the names of surface features, [where you can see] canyons and volcanoes, is incredible. Each time we explore a new world for the first time, it's exciting, because we know we're going to discover new things that completely take us by surprise. Nature is remarkably good at creating things that scientists didn't expect."

Pluto is a planet type in short supply within our solar system. The rocky planets (Earth, Venus, Mercury, and Mars) and the gas giants (Jupiter, Saturn, Uranus, and Neptune) are more familiar. Pluto is an ice dwarf, solid but composed of materials like frozen water, carbon dioxide, molecular nitrogen, methane, and carbon monoxide. That composition increases the mission's element of the unknown.

"We really don't know what to expect," Grundy said. "We haven't had a close look at one of those before."

Pluto Facts

• On Pluto you would tip the scales at only 1/15th of your Earth weight. Astronauts weighed much more on the moon—1/6th of their Earth weight.

• Pluto's estimated surface temperature is a brutally cold, at -378 to -396 degrees F (-228 to -238 degrees Celsius)—the coldest place in the solar system.

• Light conditions on Pluto are dim because the sun provides only one-thousandth the illumination that it does on Earth. The sun would look like a bright star from Pluto.

• Pluto's 248-year orbit is elliptical in shape, so that the planet is occasionally closer to the sun than Neptune.

• Pluto is named after the Roman god of the underworld, a moniker suggested by 11-year-old Venetia Burney of Oxford, England. The name is especially apt for the faint planet, because the Roman Pluto was able to disappear at will.

• Pluto's moon, Charon, was discovered in 1978 by James Christy of the U.S. Naval Observatory's Flagstaff Station. Charon is named for the mythical ferryman who transported souls across the river Styx to Pluto's underworld. It's possible that Pluto has other, smaller, undiscovered moons.

• Charon is about half the size of Pluto—comparatively larger than most planetary moons. In this respect Pluto and Charon form a kind of binary planet—they orbit each other around an invisible center of mass (pivotal point) that lies between them. This differs from the more typical Earth-moon orbit, where the smaller moon orbits around a largely stationary larger planet.

There are thought to be many binary planets and stars in the galaxy, but none have ever been visited by spacecraft.

• Pluto and Charon are "locked" in orbit. If you lived on the Charon-facing side of Pluto the moon would always appear in about the same place. If you lived on the opposite side of the planet you'd never see the moon.

• Measuring some 1,470 miles (2,370 km) across, Pluto is about half the size of Earth's moon. That's about the distance between Washington, D.C., and Denver, Colorado.

• The Kuiper belt is a large, distant belt of icy objects beyond Neptune, including Pluto. For this reason, some astronomers prefer to think of Pluto not as the smallest planet but as the largest asteroid in the Kuiper belt. Many of the comets that visit Earth's vicinity come from the Kuiper belt.

• There is no firm definition of a planet, but the International Astronomical Union has ruled that Pluto meets the criteria. Generally, Pluto is considered a planet because it orbits the sun and is large enough that its own gravity gives it a circular shape.

• The total New Horizons mission cost is estimated at about 650 million U.S. dollars. If the 8-foot (2.5-meter), 1,025-pound (465-kilogram) spacecraft is still functioning after its Pluto visit, it will continue its mission for another three years through the Kuiper belt.

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