NASA's New Horizons spacecraft is being readied for a January liftoff to the outer reaches of the solar system. It will be humanity's first mission to Pluto and the Kuiper Belt--a vast and distant repository of the solar system's leftover building materials--and is expected to reap rich scientific rewards.
"The New Horizons mission to Pluto completes our initial survey of the nine planets we believed comprised the Solar System when we began the space age," said NASA Administrator Michael Griffin. "In that sense, this mission truly marks the end of the beginning, and all of us look forward to receiving those first pictures of Pluto and Charon when they finally arrive."
Those first pictures could arrive as early as 2015 assuming New Horizons launches on time. When the data does start flowing back from the ninth planet it promises to increase dramatically scientific understanding of the Pluto.
"Everything we know about Pluto could basically fit on a postcard," said Colleen Hartman, NASA's deputy associate administrator for science. "We are truly going to a new frontier."
The 5-billion kilometer journey is expected to take as little as nine and a half years provided New Horizons launches in time to take advantage of a gravity assist during a planned flyby of Jupiter that is designed to use the gravitational power of the solar system's largest planet to give the already speeding spacecraft an additional boost of speed.
New Horizons' window opens Jan. 11 and remains open until Feb. 14, but the spacecraft must launch no later than Feb. 2 to take advantage of the time saving Jupiter gravity assist. Otherwise, New Horizons will have to be launched on a direct trajectory that would delay its encounter with Pluto perhaps as much as three years.
If NASA misses the upcoming window entirely, it will have a second shot in February 2007. However, a launch during that two-week window would not put New Horizons in the vicinity of Pluto until perhaps as late as 2020.
Under either scenario, the New Horizons spacecraft, which is about the size of a grand piano, will be lifted into orbit atop a Lockheed Martin Atlas 5 rocket equipped with five solid-rocket boosters, a Centaur upper stage and a special STAR 48B solid propellant-fueled third stage that will propel the spacecraft out of low Earth orbit and toward its destination.
About a year after launch, assuming the spacecraft gets off in time for the gravity assist, New Horizons would encounter Jupiter, snapping pictures, making measurements and picking up speed as it slingshots past the gas giant on an eight-year cruise.
The New Horizons spacecraft was built by the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., which also is managing the mission for NASA. The total price tag for the New Horizons mission, including launch and more than 10 years of operations, has grown to $675 million. According to NASA, the cost growth is mostly due to additional work that had to be done preparing the spacecraft's compact nuclear power source and higher than expected launch costs. NASA spokesman Dwayne Brown said the launch alone is costing about $205 million.
A Long Wait
Scientists captivated by Pluto and the mysterious Kuiper Belt are no strangers to long waits. Since 1990, several missions to Pluto have been proposed only to be quickly abandoned for largely budgetary reasons. The most recent false start was Pluto Kuiper Belt Express, which NASA canceled in 2000 after its estimated price tag more than doubled. In 2001, under pressure from the U.S. Congress, NASA held an open competition to find a cheaper way to do a Pluto flyby and ultimately selected the $500 million New Horizons proposal but did not request any funding for the mission. Congress funded the New Horizons program as a budget earmark for two years before NASA finally got the message and started requesting money for the mission.
New Horizons' principal investigator, planetary scientist Alan Stern of the Boulder, Colo.-based Southwest Research Institute, has been waiting about as long as anybody to send a probe to Pluto. While Pluto has the distinction of being the only one of the solar system's nine long-discovered planets not to have been visited by a spacecraft, Stern said the reasons for going are science driven.
"I don't know anybody who thinks that checking it off the list is a good reason for going," he said. "We are going to Pluto to study the origins of the outer solar system, to better understand the formation of binary planets including the Earth-Moon system, and to explore for the first time this different kind of planet - not a terrestrial planet and not a gas giant, but an ice dwarf."
Ice dwarves like Pluto, Stern said, almost certainly outnumber the terrestrial planet and gas giants--at least in this solar system.
"It used to be said that Pluto is a misfit. But now we know Earth is the misfit," Stern said. "This is the most populous class of planet in our solar system and we have never sent a mission to this class."
Scientists believe that as Pluto continues its 248-yearlong orbit around the sun, its tenuous atmosphere eventually will freeze and collapse to the surface. Pluto has been racing away from the sun since its closest approach in 1989 and scientists do not know how much time remains before Pluto's atmosphere collapses. Once that happens its atmosphere is not expected to re-emerge for about 200 years.
"Some people think its 20 years off and some people think its five years off," said Stern. "No one really knows when Pluto's atmosphere will snow out and collapse."
As frigid Pluto grows even colder as it travels further from the sun, scientists believe that more and more of its surface will be cloaked in nitrogen-based snow, accelerating the freezing process that causes the atmosphere to collapse. Stern said the New Horizons team cannot be sure that there will still be an atmosphere to study until less than a year from encounter. That's because scientists suspect that once the process starts, it progresses fairly rapidly. "Some models say the collapse can be very sudden," Stern said. "It can happen in a matter of months."
The New Horizons spacecraft is about 2.5 meters across and weighs roughly 465 kilograms fully fueled. The spacecraft's onboard computers and suite of seven scientific instruments will be powered by a nuclear-fueled battery supplied by the U.S. Department of Energy. The single Radioisotope Thermal Generator (RTG) aboard New Horizons works by transforming heat from decaying Plutonium-238 into electrical power.
RTG's weaken over time, but New Horizons' RTG has enough Plutonium-238 on board to still be pumping out about 200 watts of power when the spacecraft encounters Pluto.
New Horizons' instruments include a high-resolution optical telescope; an ultraviolet spectrometer for measuring gas composition; a combination infrared spectrometer and color camera for mapping Pluto's surface; a radio experiment for measuring atmospheric composition and temperature; a plasma-sensing instrument for measuring properties in the solar wind at Pluto; and a student-built instrument that will record how much dust and debris hit the New Horizons spacecraft during its lengthy journey.
Richard Binzel, a Massachusetts Institute of Technology professor and member of the New Horizons science team, said the probe will produce the best images ever seen of Pluto. New Horizons' cameras are capable of recording raw images detailed enough to depict surface erosion and cratering, important clues to the tiny planet's past.
"We think the polar ice caps come and go seasonally," Binzel said. "But is it just a few millimeters or is it tens of meters of ice that comes and goes? The way we will see that is by seeing how much erosion is on the surface. Is it heavily cratered, indicating that the surface is very old or does the season process of ice movement erase the craters?"
In contrast, the best set of images available today were taken by the Hubble Space Telescope in the mid-1990s and had to be substantially enhanced just to show dark regions at Pluto's equator and brighter regions believed to be ice and snow at the poles.
New Horizons will begin making its observations of Pluto starting about four months out from its closest encounter. The busy part of the mission will last one day as the probe passes within 10,000 kilometers of Pluto and within 27,000 kilometers of its moon Charon. Stern said it is expected to take about nine months after closest encounter to transmit all the collected data back to Earth through the Deep Space Network.
After New Horizons completes its primary mission, assuming the spacecraft is still in good health, NASA expects to send the spacecraft off to take a closer look at one or two yet to be selected Kuiper Belt objects, perhaps another three to five years travel time from Pluto.
The first leg of the long journey begins at Cape Canaveral Air Force Station, Fla., where New Horizons and its Atlas 5 rocket are being prepared for launch.
The Atlas 5 was successfully fueled and tested during a so-called wet dress rehearsal Dec. 5. The rocket's fifth and final solid-rocket booster was strapped to the launcher about a week before. The spacecraft itself was filled with a full load of hydrazine fuel the day before the launch rehearsal and at press time was on track to be installed inside the Atlas 5's payload fairing Dec. 12.
A special Boeing-supplied STAR 48B solid-propellant-fueled third stage that will boost New Horizons spacecraft out of low-Earth orbit was delivered to the cape Dec. 1 as scheduled despite an ongoing Boeing machinist strike that has put three other U.S. government launches on hold.
Hartman said in a Dec. 8 interview she was confident that New Horizons' third stage was being handled properly by the five replacement workers assigned to the Lockheed Martin-provided launch.