A direct airplane flight might be the quickest way across the country, but the fastest route to Pluto requires a trip past Jupiter. The giant planet's gravity can help slingshot a spacecraft into the outer solar system.

There are two reasons why the New Horizons science team wants to reach Pluto and Charon as soon as possible. The first has to do with Pluto's atmosphere: Since 1989, Pluto has been moving farther from the Sun, getting less heat every year. As Pluto gets colder scientists expect its atmosphere will "freeze out," so the team wants to arrive while there is a chance to "see" a thicker atmosphere.

The second reason is to map as much of Pluto and Charon as possible. On Earth, the North Pole and other areas above the Arctic Circle have half a year of night and half a year of daylight. In the same way, parts of Pluto or Charon never see the Sun. The longer we wait, the more of Pluto and Charon are shadowed in the "arctic night," impeding the spacecraft's ability to take pictures in reflected sunlight.

Closing In

The cameras on New Horizons will start taking data on Pluto and Charon months before the spacecraft arrives. Pluto and Charon will first appear as unresolved bright dots, but the planet and its moon appear larger as the encounter date approaches. Three months from the closest approach - when Pluto and Charon are about 65 million miles (105 million kilometers) away - the cameras on the spacecraft can make the first maps. For those three months, the mission team will take pictures and spectra measurements.

Pluto and Charon each rotate once every 6.4 Earth days. For the last two Pluto days before encounter (11 to 12 Earth days), the team will compile maps and gather spectra measurements of Pluto and Charon every half-day. The team can then compare these maps to check changes over a Pluto day, at a scale of about 30 miles (48 kilometers), as might be caused by new snows or other weather.


The Encounter

The busiest part of the Pluto-Charon flyby will last a full Earth day, from a half-day before closest approach to a half-day after. On the way in, the spacecraft will look for ultraviolet emissions from Pluto's atmosphere and make the best global maps of Pluto and Charon in green, blue, red and a special wavelength that is sensitive to methane frost on the surface. It will also take spectral maps in the near infrared, telling the science team about Pluto's and Charon's surface compositions and locations and temperatures of these materials.

In current mission designs, the spacecraft comes as close as about 6,000 miles (9,600 kilometers) from Pluto and about 17,000 miles (27,000 kilometers) from Charon. During the half-hour when the spacecraft is closest to Pluto or its largest moon, it will take close-up pictures in both visible and near-infrared wavelengths. The best pictures of Pluto will depict surface features as small as 200 feet (about 60 meters) across.

Even after the spacecraft passes Pluto and its moons, its work is far from done. Looking back at the mostly dark side of Pluto or Charon is the best way to spot haze in the atmosphere, to look for rings, and to figure out whether their surfaces are smooth or rough. Also, the spacecraft will fly through the shadows cast by Pluto and Charon. It can look back at the Sun and Earth, and watch the light from the Sun or the radio waves from transmitters on Earth. The best time to measure the atmosphere happens as the spacecraft watches the Sun and Earth set behind Pluto and Charon.

Beyond Pluto

After passing Pluto and Charon, pending NASA approval of an extended mission, the spacecraft can retarget itself for an encounter with a KBO. The KBO target would not be selected until later in the mission, but scientists expect to find one or more the spacecraft can reach that are 30-60 miles (about 50-100 kilometers) across. This encounter would be similar to the Pluto-Charon encounter; the spacecraft would map the KBO, get its composition from infrared spectroscopy and four-color maps, and look for an atmosphere and moons.