By Guy Gugliotta
Washington Post Staff Writer
Monday, December 19, 2005
NASA is making final preparations to launch the fastest spaceship ever made on a 10-year odyssey to Pluto, the scarlet-colored "ice dwarf" that shines brightly in the chill wilderness of deep space nearly 4 billion miles away.
Pluto is the only planet that has never yet had a human-engineered visitor, but if all goes well, New Horizons, a piano-size spacecraft wrapped in thermal blankets, will spend five months in close flyby, taking pictures and gathering data on features such as the planet's atmosphere, its surface geology and its temperature.
"We really expect the mission to be transformational," said New Horizons lead scientist Alan Stern of the Southwest Research Institute in Colorado. "This is the capstone of the original visits to the planets. It takes us 4 billion miles away and 4 billion years back in time."
The $700 million mission is the first space expedition aimed specifically at a celestial body beyond Neptune in the Kuiper Belt, a remote region filled with debris from the creation of the solar system. Pluto is also the solar system's only known "binary planet," orbiting the sun in tandem with a moon, Charon, that is more than half as big as Pluto itself. Two other tiny moons were discovered earlier this year.
NASA plans to launch New Horizons from Cape Canaveral Air Force Station in Florida as early as possible during a 29-day window that opens at 1:24 p.m. on Jan. 17. If liftoff occurs during the first 11 days, the spacecraft will reach Pluto in the summer of 2015. Launching later will result in substantial delays; starting the voyage on Valentine's Day would mean arrival as late as 2020.
"We want to study the atmosphere, but Pluto is moving further away from the sun," said New Horizons mission systems engineer David Kusnierkiewicz of Johns Hopkins University's Applied Physics Laboratory. "Around 2020 the atmosphere is going to freeze and fall as snow, so we need to get there by then."
Kusnierkiewicz said the launch team has had two rehearsals for what will be the fastest space trip ever taken. It opens with liftoff aboard an Atlas V-551 rocket, NASA's biggest, with five solid rocket boosters strapped to it.
The first stage drops away four minutes and 33 seconds into the flight, and the second stage is gone after 42 minutes. When an added third stage falls away five minutes later, New Horizons will be traveling between 28,000 and 30,000 mph, passing Earth's moon in nine hours. The Apollo astronauts needed three days for the trip.
And unlike space probes that reach their destinations in ever-widening solar orbits, New Horizons is a simple rocket to Pluto, "just taking aim and shooting the gun," Kusnierkiewicz said. If launch occurs before Feb. 3, the probe will get a gravity assist as it flies by Jupiter, kicking the speed up to 47,000 mph.
For most of the 10-year flight, New Horizons will "hibernate," Kusnierkiewicz said, with only enough electronics lit to keep its insides at room temperature under the exterior thermal blankets.
Electricity will be provided by a thermoelectric generator powered by the heat from radioactive decay of a non-weapons-grade plutonium isotope. With the 200 watts supplied by the generator, New Horizons will operate seven different scientific instruments.
About 12 weeks before New Horizons's arrival at Pluto, the spacecraft's cameras will be able to get pictures better than those taken by the Hubble Space Telescope, and the data will improve steadily as it approaches the planet.
The spacecraft has no moving parts, so engineers will program it to use 16 hydrazine fuel thrusters to turn the spacecraft so it can point individual instruments at targets, a process Kusnierkiewicz described as "a fairly involved ballet." Communications between Earth and New Horizons will take four hours and 25 minutes.
"We're going to see things we've never even dreamed of," said astronomer Marc Buie, a deep space specialist from Arizona's Lowell Observatory. "Over the years I've used every tool at my disposal to learn about Pluto, but there's absolutely no way from Earth to understand the geologic context -- craters, impact basins and whatever else has happened."
In fact, Pluto remains largely an enigma 75 years after it was discovered. It is an "ice dwarf" composed of water, carbon monoxide, nitrogen, methane ices and rocky material. It is yellowish red in color and bright, probably from the ice, but with darker regions. Pluto is the smallest planet in the solar system, with a diameter two-thirds that of Earth's moon.
When Pluto was discovered, "astronomers thought it was a solar system misfit," Stern said, but since then scientists have estimated there are 500,000 objects with a diameter of at least 60 miles in the Kuiper Belt, making the belt the "largest structure in the solar system," he added. "It turns out that we and Mars and Jupiter are the misfits. Pluto is typical."
But what does that mean? "There are very clearly surface features of some sort, and we have no idea at all what those features are," said astronomer Michael Brown, a Kuiper Belt expert from the California Institute of Technology. "We don't understand this body at all."
Besides taking pictures of Pluto and Charon with three imagers, New Horizons will also analyze Pluto's atmosphere and geology, measure temperatures and space dust, and document the effects of the solar wind as it peels ions from the upper atmosphere.
Brown said Pluto undergoes tremendous temperature changes during a 248-year elliptical orbit that takes it as close as 2.8 billion miles from the sun and as far as 4.6 billion miles. When the temperature gets low enough, scientists suspect, the atmosphere "collapses," with the gas freezing and falling back to the surface. Charon is too small to hold on to an atmosphere, and has none.
Buie said bright surfaces usually mean "there's been some activity down there," perhaps from an atmospheric collapse, while darkness usually means that meteors and other "space weathering" phenomena have pocked the surface enough to dissipate light. Charon is considerably less bright than Pluto.
After New Horizons passes Pluto and Charon, it will almost certainly have enough power left for both the thrusters and the generator to search out and explore at least one other Kuiper Belt object, maybe two.
"We're not even looking yet," Stern said. "Anything I pick now will seem quaint in 10 years. It would be like planning a trip to Paris in 2015 and making a restaurant reservation today."