*Update, 14 July, 9:22 p.m.: The message was short but sweet: The Pluto mission lives on. On Tuesday night, NASA received a “phone home” message from New Horizons, the signal that the spacecraft survived its passage past the dwarf planet. Earlier today, NASA scientists celebrated the probe’s closest approach past Pluto, but because the spacecraft was unable to communicate with Earth for nearly 24 hours while it performed its heavy load of scientific tasks, the mission team could only assume that all was going to plan. The probability of a catastrophic collision with debris in the Pluto system was extremely small—less than 1 in 10,000—but there was still an anxious wait for the relief of resumed contact with the beloved spacecraft.
At 8:53 p.m. EST it came, when one of NASA’s Deep Space Network radio telescopes in Spain locked on the spacecraft’s signal. The message, sent 4.5 hours earlier by New Horizons, contained mostly spacecraft health information that was quickly evaluated by mission managers at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. By 8:58 p.m. EST, Alice Bowman, the mission operations manager, had seen enough. “We have a healthy spacecraft,” she said. “We’re outbound from Pluto.”
The latest images of Pluto won’t arrive until Wednesday. NASA is planning to release the new images at a press conference at 3 p.m. EST on Wednesday.
Humankind has consummated an initial reconnaissance of the solar system. This morning, a small spacecraft called New Horizons barreled past Pluto after a 9.5-year journey, marking the first exploration of a world within the solar system’s “third zone”: a region of thousands of icy worlds called the Kuiper belt. In 2006, Pluto was demoted to the status of a “dwarf planet,” but as the largest of the Kuiper belt objects, scientific interest in the world has remained untrammeled.
Hundreds of scientists, friends, and family whooped and hollered as a clock ticked down to the moment of closest approach, at 7:49 a.m. EST, at a celebration at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, which operates the New Horizons mission for NASA. About 4.7 billion kilometers away, New Horizons was flying within 12,500 kilometers of Pluto’s surface.
Yet for all the jubilation and flag-waving, the moment was artificial: That knowledge of the spacecraft’s position was theoretical. New Horizons, focused on a preprogrammed regimen of scientific observations and unable to communicate with Earth, was speeding along on autopilot; a video shot of an APL janitor vacuuming the empty mission control room gave truth to this idea. Scientists will not know that the craft survived its encounter until 8:52 p.m. EST Tuesday night, when a basic radio message containing its health information arrives on Earth after its 4.5-hour journey at the speed of light.
What the mission team did have in hand Tuesday morning was a new image of Pluto’s peach-colored face, taken the night before, at a resolution of 4 kilometers per pixel, 1000 times better than images from the Hubble Space Telescope. Alan Stern, the mission’s principal investigator, unveiled the image to an auditorium of journalists, after it made its debut minutes earlier on NASA’s Instagram account. Smooth, bright plains of ice sat in puddles next to darker regions with cliffs, craters, and chasms. “How about a round of applause for that beautiful new planet,” he said.
Stern says Pluto’s surface appears to be much less heavily cratered than its large moon, Charon—which implies that the surface has been paved over in some way. That smoothing process could come from internal heat that keeps rock and ice soft, or by the frosts that snow down and blanket the surface in fresh ice. “Either [Pluto’s] internal engine continues to run, and there are active processes that are taking place,” Stern says, “or those atmospheric processes are themselves covering up the geology, and covering up the craters.”
Jonathan Lunine, a planetary scientist at Cornell University, not on the New Horizons team, favors the latter explanation. His initial impression is that Pluto is far more cratered than Triton, the moon of Neptune that is about the same size as Pluto and is thought to be a captured Kuiper belt object. Triton has a famous “cantaloupe” terrain, thought to have formed as heat, driven by Neptune's tidal pull, allowed molten blobs of ice to rise and overturn. Parts of Pluto’s rugged surface seem to be far more carved and cratered, Lunine says. “It’s telling me that [Pluto’s] crust was not heated and modified to the extent that it was on Triton.”
But still, Pluto’s face contains broad regions of smooth, bright ice. Going into the encounter, scientists speculated that the brightest regions would contain nitrogen ice, whereas medium-bright regions would contain more methane ice. The dark regions could contain organic molecules, created from the interaction of ices, ultraviolet light, and cosmic rays. Lunine says it’s a bit puzzling that the darkest and brightest regions girdle Pluto’s equator; he would have expected bright nitrogen ice—the most unstable at warmer temperatures—to have gathered at the poles. He says it could have something to do with Pluto’s odd rotational tilt and elongated orbit, which is suspected to drive Pluto’s thin atmosphere in strange ways.
Casey Lisse, a New Horizons team member at APL, says that the icy regions may be controlled by elevation, not latitude. The brightest region of all, the “left ventricle” of the heart-shaped bright region, appears to be a topographic depression. And the dark region just to the west appears to be a topographic high, maybe 5 to 15 kilometers higher. Within the depression, atmospheric pressures will be higher—perhaps high enough for nitrogen ice to remain stable, he says. “What we’re seeing looks to be elevation controlled,” he says.
But the early image was not yet detailed enough for the science team to offer more than speculation. More images, along with compositional chemistry information, will come down once New Horizons passes through the Pluto system and begins sending back data from the flyby. The downlink rate, at such vast distances, is a trickle—between 1000 and 4000 bits per second. But over the course of a 16-month download of the captured data, Stern says the trickle will turn into a “data waterfall.” “Stay tuned,” Stern keeps repeating.
Lunine says the emerging diversity on Pluto—of both active atmospheric and geological processes—makes him disagree with the decision of International Astronomical Union to call Pluto a dwarf. “This really is a planet,” he says. He notes that electrons have a double definition as a particle and a wave. “Why can’t we call Pluto a planet and a Kuiper belt object?” he asks. “I think we have to think of it as both.”
*See Science’s full coverage of Pluto, including regular updates on the New Horizons flyby.