Billed as the first mission to the last planet, NASA's New Horizons spacecraft is in the final stages of being readied for departure to Pluto.

The spacecraft and its booster, an Atlas V, are now mated. Yet to be attached is the probe's nuclear power source, followed by hardware electrical connection checks.

The compact and nuclear-powered New Horizons probe is outfitted with seven scientific instruments, built to take an unprecedented up-close look at Pluto and its moon Charon, map their surface compositions and temperatures, and examine Pluto's complex atmosphere.

Given a liftoff during its first launch window, the spacecraft will arrive at Pluto in the summer of 2015.

After the Pluto/Charon flyby, the New Horizons probe--in extended mission mode--is to shoot by still-to-be picked Kuiper Belt objects, ancient, icy and rocky mini-worlds that are leftovers from the formation of the solar system.

The cost of the mission, including the launch vehicle and operations through the Pluto-Charon encounter, will be roughly $650 million.

Booster inspection

Late last week it was announced that New Horizons was rescheduled for liftoff no earlier than January 17, 2006. That six day slip was called to support additional inspection of the booster for the Pluto-bound spacecraft, a Lockheed Martin Atlas launch vehicle. The booster-for-hire company experienced problems in September on an updated Atlas propellant tank similar to the one being flown on the New Horizons mission.

The spacecraft has a 35-day launch window. Last Saturday, New Horizons was moved to Launch Complex 41 at the Cape Canaveral Air Force Station, Florida. An integrated test of the booster now topped by the probe is slated for this Wednesday.

New Horizons is the first mission in NASA's New Frontiers program of medium-class planetary missions. The spacecraft was designed and built for NASA by the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.

Pluto is a "treasure trove" of discoveries just waiting to be uncovered, said Andrew Dantzler, Director, Solar System Division at NASA Headquarters in Washington, D.C. during a New Horizons press briefing held today.

The four billion mile voyage of New Horizons is also a four billion year trek back in time, said Alan Stern, New Horizons Principal Investigator from the Southwest Research Institute (SwRI) in Boulder, Colorado.

"The success of this mission is getting the goods at Pluto and the Kuiper Belt, Stern said. "Our objective is to get off to a good launch and make sure that the spacecraft, the entire system, is ready to go. We'll fly it when we're ready...and then we'll make some history."

The message from Stern is to expect surprises at Pluto. "We've been wrong again and again by underestimated nature," he said.

Final phases

Inspection of the Atlas V booster is underway, Dantzler said, stressing that the 35 day launch window for New Horizons runs until February 14. "We're being prudent and taking all measures to make sure it [the booster] is ready. We won't launch until it is."

"We're in the final phases of the test campaign preparing for launch," said David Kusnierkiewicz, New Horizons Mission Systems Engineer at APL.

Assuming liftoff during the primary launch window in January 2006, the first 13 months of New Horizons mission includes spacecraft and instrument checkouts, science sensor calibrations, trajectory correction maneuvers, and rehearsals for a Jupiter encounter.

New Horizons will depart Earth's vicinity at a blistering 8 miles per second, passing the orbit of the Moon in just eight hours.

"Remember that the Apollo astronauts took some three days to cover that distance," advised Glen Fountain, New Horizons Project Manager at APL. At that velocity, the spacecraft will encounter Jupiter in February 2007, picking up added speed as it zips by the massive planet.

Practice shot

The plan is to use the Jupiter encounter to test out hardware and procedures for the later Pluto flyby, 8.5 years later. "It'll be our practice shot for Pluto," Fountain said.

Stern of SwRI said New Horizons will be the eighth mission to Jupiter, filling in gaps of knowledge about the planet's dynamic atmosphere and aurora, magnetosphere, and faint ring. "So it's going to be a busy time."

New Horizons is hauling seven key instruments, said William Gibson, New Horizons Science Payload Manager for SwRI in San Antonio, Texas. The probe's camera system will begin relaying detailed images of Pluto about three months prior to closest approach, he said.

"The New Horizons payload is the most compact, low-power, high-performance payload yet to fly on U.S. planetary mission on a first reconnaissance flyby," Gibson noted.

The spacecraft is roughly 8 feet (2.5 meters) across and weighs roughly 1,025 pounds (465 kilograms)--about half a ton--when loaded with fuel.

Beacon mode

APL's Kusnierkiewicz said New Horizons is loaded with redundancy, except for the craft's single radioisotope thermoelectric generator, or RTG. "These are highly reliable devices which have many years of experience in space flight," he added.

Once past Jupiter, en route to Pluto, New Horizons will be put to sleep, Kusnierkiewicz explained. Very few systems onboard are to be active after completing the gravity assist at Jupiter, he said, to conserve the useful operating lifetime of electronics.

On the lengthy trail to Pluto, New Horizons is in "beacon mode"--the first operational use of this concept. The spacecraft will broadcast to Earth a once-a-week overall health report.

"If any remedial action is needed, the beacon will indicate the necessity for that and we will respond accordingly," Kusnierkiewicz said.

Kids in a candy shop

New Horizons is to churn out imagery of Pluto at closest approach that will be 10,000 times better than what can be seen by the Hubble Space Telescope.

Speeding past Pluto and Charon, as well as two newly found moons circling the planet--with more likely to be found--New Horizons heads for the Kuiper Belt objects. That's an extended mission, SwRI's Stern said, depending upon spacecraft health and available funds.

"It has been a long road to get here," Stern said, with launch now just a handful of days away.

"It will be like kids in a candy shop" when New Horizons reaches Pluto. "So hold onto your hats."