After nearly a decade speeding through the vast emptiness of the solar system, NASA’s New Horizons spacecraft is on Pluto’s doorstep. On 14 July, the mission will swoop past the dwarf planet, in the first-ever visit to the frigid region of icy bodies beyond Neptune called the Kuiper belt. Flying within 12,500 kilometers of its surface—about 1/33 as far as the moon is from Earth—New Horizons will be close enough to pick out features as small as 70 meters across.
Already, as it homes in on its target, the spacecraft is returning rich scientific data. “It’s been getting better and better, day by day,” says Andy Rivkin, a planetary scientist collaborating with the New Horizons team at Johns Hopkins University Applied Physics Laboratory (APL), which manages the mission for NASA from Laurel, Maryland.
On 8 July, NASA released an image of Pluto that shows three regions of varying brightness on its mottled, peach-colored surface. The science team suspects that these variations can be explained by the different ices—nitrogen, methane, carbon monoxide, and water—that blanket the surface. The brightest region is thought to contain nitrogen ice, while the medium-bright, peach-colored area would be rich in methane. The dark region could be dominated by tholins, organic compounds created by reaction of methane with ultraviolet light and cosmic rays.
Rivkin says there could be a feedback loop in which the dark tholin-rich region absorbs a bit more sunlight and remains warmer, encouraging the loss of the ices. Another factor in the distribution of these terrains is Pluto’s wispy atmosphere; ices can evaporate into gases and fall again somewhere else as frosts. This effect is important because Pluto undergoes extreme seasonal changes, caused by a rotational pole that’s tilted on its side and its remarkably elongated orbit. However, crater impacts and other geological features could also be responsible for some of the surface variations that scientists are seeing—and these topographic features are just now coming into view. “This image is at the cusp of geology,” says Leslie Young, the deputy project scientist for the mission at the Southwest Research Institute in Boulder, Colorado.
The image is also the face of Pluto that will be seen at closest approach. Young is pleased that there are three distinct regions to explore. The chosen trajectory also allows New Horizons to fly through the shadows of both Pluto and its largest moon, Charon, after closest approach. By turning around and viewing the sunlight passing around the two bodies’ rims, New Horizons can study their atmospheres. “This flyby face is hands down the best for [studying] atmospheres,” Young says. Intriguing features have already started to emerge on the other face of Pluto, including “teeth”—a region of dark splotches alternating regularly with light-colored areas—and a bright, ringlike “doughnut” structure. Rivkin says it’s too early to say what they are. “Human beings make patterns and see patterns.”
As New Horizons gets closer, the spacecraft will swivel between Pluto and Charon, another target of keen interest. About half the size of Pluto, Charon orbits so close to Pluto that the pair is considered a binary system. From Pluto’s surface, Charon would appear seven times as wide in the sky as the moon does from Earth. Rivkin says it will be important to study not just the origin of the two bodies—likely from a giant impact—but also their influence on each other. There could be evidence that the two are swapping material even today, he says. Pictures released in recent days are already displaying the marked color contrast between the duo. Rivkin says Charon’s duller gray could be explained by its smaller mass—with gravity strong enough to retain water ice but weak enough to let the more volatile ices of methane escape into space.
The mission is going relatively smoothly, aside from a glitch on 4 July, when the spacecraft unexpectedly went into a “safe mode” when its computer crashed while attempting two processing-intensive tasks at once: uploading new instructions and compressing scientific data. The mission team lost a few days of science. But on 7 July, the “core” sequence began: the thousands of computer instructions needed to operate the spacecraft from 7 days before closest approach to 2 days after. This script, run in its entirety in 2013, has been under lock and key as a pristine set of instructions, so the mission team can now breathe a sigh of relief.
The moment of closest approach will happen at about 8 a.m. U.S. Eastern Standard Time (EST) on the morning of 14 July. Because the spacecraft’s radio dish is not articulated, New Horizons cannot communicate with Earth while its instruments are trained on Pluto. So it will stay focused on the job throughout the day. Moreover, it takes 4.5 hours for data to travel back to Earth. The mission team will be biting its nails until about 9 p.m. EST that night, when a simple message with engineering data is supposed to arrive. “The very first link is New Horizons turning back after encounter to say ‘Hello, everything’s fine,’” Young says. Then the data from the encounter will trickle back to Earth for 16 months. Only select images and scientific data will be available on 15 July and in the subsequent days. “We’re picking the cream of the crop to come down in the next week,” Young says.
*See Science’s full coverage of Pluto, including regular updates on the New Horizons flyby.