APPLIED PHYSICS LABORATORY, Laurel, Md.—At 8:52 P.M. Eastern time, the New Horizons control center here received radio confirmation that the space probe had made—and survived—its closest approach to Pluto, passing 12,500 kilometers above the planet’s sunlit hemisphere.* The transmission was the first since 11:30 P.M. ET last night, ending a radio silence imposed so that the spacecraft could avoid interrupting its observations.

The flyby may well break a record for the number of historic milestones set by a single space mission: last of the first visits to the textbook planets in our solar system; most distant object ever visited; first trip to the Kuiper Belt of icy worlds that ring the outer solar system; fastest spacecraft launched; and probably most budgetary deaths and resurrections. A mission to Pluto had languished for decades in NASA’s developmental doldrums; the fact the spacecraft flew at all owes much to public support. Pluto might be fairly called the “People’s Planet.”

Even when Pluto was just a point of light in a telescope, scientists had known it would be a fascinating place. Observations in the 1950s had shown that its surface varies widely in brightness, and in the mid-1990s the Hubble Space Telescope created a very coarse map, with a resolution of 500 kilometers, revealing a patchwork of light and dark regions—a heterogeneity that suggested some degree of geologic activity. New Horizons is now turning that crude quilt into a fine filigree.

All day here at the Johns Hopkins Applied Physics Laboratory, about halfway between Washington, D.C, and Baltimore, scientists have been marveling at the last image the spacecraft sent before going into radio silence. With a resolution of four kilometers, it merges a gray-scale image with a lower-resolution color one taken earlier in the day (north is toward the top). It shows the hemisphere of Pluto that faces away from its main moon, Charon. The most prominent feature is the “heart,” below the center, a smooth, bright, presumably icy region. The heart’s two lobes actually have very distinct colors, which are hard to tell apart in this image, but jump out when the colors are exaggerated. These colors indicate distinct chemical compositions. During today's flyby, the spacecraft's point of closest approach is above the eastern lobe of the heart.

Credit: NASA


The dark, reddish regions on either side of the heart lie along the planet’s equator. The western one (on the left), tentatively named Cthulhu based on crowdsourced suggestions, forms the head of the “whale” spotted in approach images; within it is a crater dubbed the “whale’s blowhole.” Perhaps 100 kilometers in diameter, the crater has both light and dark regions, as though the impact punctured a dark hydrocarbon layer to reveal brighter ices underneath. Lines—perhaps scarps or faults—appear to cut across the dark region as well as other parts of planet. East of the heart appears to be hummocky terrain and half-buried craters, suggesting some kind of erosion.

Tomorrow, New Horizons is scheduled to beam back an even higher-resolution, stereo image of a strip along the heart, along with images of three of Pluto’s five moons.

Other discoveries made so far:

  • The mission revised the estimate of Pluto’s diameter slightly upward by about 3 percent, making the dwarf planet almost certainly larger than the next-largest trans-Neptunian object by perhaps 30 kilometers. This new size estimate also implies that the lowest layer of atmosphere (the troposphere) is much thinner than thought—a kilometer or two, at most.
  • The particle detectors picked up ionized nitrogen molecules—presumably escapees from Pluto’s tenuous atmosphere—much farther from the planet than had been predicted. Why they had strayed so far afield is unknown but the explanation might have to do with variations in the solar wind washing over the outer atmosphere.
  • Images have showed no moons beyond the five already known—Charon, Hydra, Nix, Styx and Kerberos. Nix and Hydra are 30 to 50 kilometers in diameter—the first size measurements of these bodies.
  • Images confirm the dichotomy between the more varied and seemingly more dynamic surface of Pluto and the darker, grayer, cratered terrain of its largest moon Charon. “Now we see how dramatically different they really are…. These images show a much younger surface on Pluto and an older, more battered surface on Charon,” principal investigator Alan Stern said at a press briefing today.

Pluto is proving to be very different from Neptune’s moon Triton, which researchers have long considered a model of what Pluto might be. “It’s unbelievable how alien this place is,” Stern remarked yesterday. For one thing, it is much redder than Triton, so much so that its surface evokes that of Mars. “The closer we look at Pluto, the less it looks like Triton… It looks very Mars-like,” John Spencer of Southwest Research Institute said today. Charon’s polar cap is also oddly reddish. These surfaces presumably take their color from hydrocarbons that have been baked by solar radiation, creating dark tars known as “tholins.”

Indeed, the flyby is barely the start of New Horizon’s mission. Over the coming days and weeks the probe will study Pluto’s moonlit night side and search for planetary rings backlit by the sun. Beginning on September 14 the spacecraft will download all the encounter data to radio telescopes on Earth. At a rate of about two kilobits per second—equivalent to a 1980s-era 300-baud dial-up modem—that will take 10 weeks, even in a compressed low-fidelity format. (A side note: The reason the spacecraft has been radio silent is not computer limitations per se. Rather, it cannot transmit and take data at the same time for two reasons. A short version is that (1) the spacecraft has insufficient power both to navigate and to transmit, and (2) the instruments are fixed to the body of the spacecraft rather than mounted on a rotating boom, so when the antenna is pointing toward Earth, the instruments are pointing away from Pluto. A slightly longer reason is that, to power the transmitter at the full data rate, the guidance computer has to be shut down, and to be able to do that, the spacecraft needs to be pointed at Earth and put into a stabilizing spin so that it can be recovered later; and when it's spinning, the instruments aren't pointing at Pluto anymore.) When that is done, New Horizons will spend a year resending a full-fidelity copy. All the while it will be speeding toward an encounter with another Kuiper belt object, probably early in 2019. In fact, thanks to its plutonium power source (an element named for the planet, of course), New Horizons should have enough juice to operate though the 2030s, by which point it will be about twice as far from sun as it is now. Eventually it will escape the solar system entirely, joining NASA’s four other spacecraft that are also headed to or already in interstellar space: Pioneers 10 and 11, and Voyagers 1 and 2.

*Editor's note (7/14/15): This sentence was edited after posting to update the actual time of signal acquisition.