New Horizons is small compared to the school bus-sized Cassini currently in orbit around Saturn, measuring just 27-by-83-by-108 inches and weighing 1,054 pounds at launch, of which 170 pounds is hydrazine maneuvering fuel. It's roughly triangular and about the size of a piano.

The probe is equipped with a single RTG for electrical power and seven state-of-the-art science instruments, including a radio system that will be used to both communicate with Earth and to collect valuable data about Pluto's tenuous atmosphere. The compact instruments typically operate on less power than a night light - two to 10 watts each. Recalling "The Honeymooners" TV series and Ralph Kramden's frequent outbursts of "to the moon, Alice," two of the instruments are named Ralph and Alice.

Operating more than 30 times farther from the sun than Earth, New Horizons is built like a thermos bottle and insulated to retain the heat generated by its electrical systems. Internal temperatures between 50 and 85 degrees Fahrenheit are expected throughout the mission, but small heaters are available just in case it gets colder than expected.

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

The instruments are:

• Alice: A 10-pound ultraviolet imaging spectrometer built by the Southwest Research Institute to study the structure and composition of Pluto's atmosphere. It also will be used to look for signs of a charged ionosphere around Pluto and any traces of an atmosphere around Charon.

• Ralph: A 23-pound telescope/camera system developed by Ball Aerospace Corp., NASA's Goddard Space Flight center and the Southwest Research Institute that includes a multi-spectral visible-light camera with seven black-and-white and color CCD detectors and a single infrared channel for spectroscopic studies. Operating in light 1,000 times dimmer than on Earth, Ralph will photograph the sunlit surfaces of Pluto and Charon, providing stereoscopic views, measuring the temperature and mapping abundances of nitrogen, methane, carbon monoxide and water ice. In black-and-white mode, the camera will be able to discern surface features three-tenths of a mile across (0.9 miles across in color mode and 4.3 miles in infrared mode).

• Radio Science Experiment (REX): This is a 3.5-ounce circuit board incorporated in the spacecraft's radio system that was developed by Johns Hopkins and Stanford University. As the spacecraft flies behind Pluto and Charon, radio waves from Earth will be bent slightly as they pass through atmospheric gases. By characterizing those subtle changes, scientists will be able to gain insights into atmospheric temperature and pressure.

• LORRI (long-range reconnaissance imager): A 19-pound digital camera equipped with an 8.2-inch telescope serving as a telephoto lens that will be used for optical navigation on the way to Pluto. At close approach, LORRI, developed by Johns Hopkins, should be capable of detecting surface features as small as 82 feet across. Ninety days out from Pluto, LORRI's pictures will be 10,000 times sharper than current images taken by the Hubble Space Telescope.

• SWAP (Solar Wind at Pluto): Developed by the Southwest Research Institute, SAP will study how Pluto interacts with the solar wind. Scientists believe Pluto loses about 165 pounds of its atmosphere every second. That material is then ionized by sunlight and carried away on the solar wind.

• PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation): A 3.3-pound spectrometer developed by Johns Hopkins to study material escaping from Pluto's atmosphere as well as the atmosphere itself.

• SDC (Student Dust Counter): The only instrument that will remain on for the duration of the new Horizons Mission, the SDC was developed by students at the University of Colorado at Boulder to measure microscopic dust grains in interplanetary space.

"We're going to change that. We'll map everything that is sunlit in the system at one kilometer resolution. And with our long focal length narrow angle camera, called LORRI, we'll be doing Landsat-class resolution as good as 25-to-50 meters per pixel in selected areas. That accounts for the satellites as well as Pluto itself."

Spin stabilized on the way to Pluto, New Horizons will use small hydrazine thrusters to change its orientation as required for scientific observations based on data provided by redundant gyroscopes, accelerometers and star trackers.

The computational horsepower needed to operate the spacecraft and carry out programmed commands from Earth is provided by a radiation-hardened Mongoose V processor running at 12 megahertz.

Data will be stored on two redundant eight-gigabyte solid-state recorders and later beamed back to Earth. New Horizons is equipped with two low-gain antennas for communications when the spacecraft is near Earth, a 12-inch medium-gain dish antenna and an 83-inch-wide fixed dish antenna for use with a high-speed X-band communications system.

But at Pluto's enormous distance from Earth - so far radio signals will take four hours and 25 minutes for a one-way trip - data transmission will occur at a glacial 700 bits per second. As a result, it will take a full nine months to beam back all of the data collected during the Pluto flyby.

PREVIEW REPORT PART 5 --->