Pluto Mission Takes Aim at Last Unvisited Planet

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.

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