Rendezvous with Pluto

NASA’s New Horizons spacecraft will soon introduce us to this tiny, distant world

illustration of New Horizons journey to Pluto

PLUTO IN SIGHT  The New Horizons spacecraft launched in 2006 on a 9.5-year, 5-billion-kilometer trek across the solar system to spend a few hours with Pluto and its five known moons. The probe will tear past the dwarf planet on July 14 and provide humankind with its first intimate look at the remote world in orbit around the sun. 

Adapted from NASA, JHUAPL, SWRI; photo illustration by M. Telfer; background image: NASA; Earth: NASA; other planets: see below

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Tiny, far-flung Pluto is about to have a visitor — at least for a few hours.

On July 14, NASA’s New Horizons spacecraft will reach the dwarf planet and try to learn all it can about Pluto and its five known moons. Then the probe will leave Pluto behind, vanishing into the frigid darkness beyond the planets.

In its wake, New Horizons will introduce Earth to the last of the “classical planets.” Probes have flown past, orbited, crashed into or landed on every other world that orbits the sun. Now Pluto is getting its turn.

“This is the last picture show,” says Alan Stern, the mission’s leader. “It’s the capstone moment to the reconnaissance of the planets.”

Pluto is the doorway to the solar system’s “third zone,” the Kuiper belt, an icy junkyard beyond Neptune. Far from the meddlesome heat of the sun, Pluto swims in a sea of frozen fossils that are mostly unchanged since the birth of the solar system, 4.6 billion years ago.

Pluto in 1930 and 2015
An image taken 75 million kilometers from Pluto (bottom) shows hints of details not visible in 1930. Lowell Observatory (top), NASA, JHUAPL, SwRI (bottom)

New Horizons has been traveling for 9.5 years across nearly 5 billion kilometers to take a hard, if quick, look at Pluto and its icy neighbors. Cameras will chart the landscape on a world where the atmosphere may freeze for nearly 200 years at a stretch. They may find nitrogen volcanoes or hints of a bygone subsurface ocean. The probe will also explore why Pluto’s tenuous atmosphere is leaking into space. Pluto’s tiny moons may even provide a peek at what the building blocks of the planets look like.

Anticipation is building as the encounter to reveal Pluto’s secrets draws near. “For a lot of people, this is something completely new — to see a point of light become a real place overnight,” says Stern, of the Southwest Research Institute in Boulder, Colo.

An odd little world

Flagstaff, Ariz., has a touch of Pluto fever.

Stroll downtown to a local coffee shop and you can order a Pluto Mocha. From there it’s a five-minute walk to a sushi place for Pluto Rolls. A boutique around the corner sells handcrafted Pluto ornaments — one batch made before New Horizons’ arrival and one planned for after.

Flagstaff is where Pluto’s story began to be told. Near the center of town, on a mesa peppered with ponderosa pines, sits Lowell Observatory, where Clyde Tombaugh discovered the tiny world in 1930. About seven kilometers across town lies the U.S. Naval Observatory Flagstaff Station, where in 1978 astronomer James Christy noticed a smudge of light cozied up to Pluto, which turned out to be its largest moon, Charon.

Our view of Pluto hasn’t changed much over the last 85 years, despite debates about its worthiness of the title “planet.” From Tombaugh’s vantage point in Flagstaff, Pluto was a speck of light, slowly wandering against a backdrop of stars. And for the generations of astronomers with bigger and better telescopes that followed, Pluto has remained a mostly featureless spot on the sky.

Plans for visiting Pluto began in earnest in 1989. Several ideas for a mission came and went, but it was tough to justify flying to so remote a place. “I wasn’t entirely certain what to think about the New Horizons mission,” says Mike Brown, a planetary scientist at Caltech. “You learn by studying examples of things. If there was nothing else like Pluto, why go to this oddball?”

Pluto has always been an outlier, a diminutive ice-coated body on an orbit that carried it far above and well below the plane of the solar system. It even has the audacity to cross another planet’s (Neptune’s) orbit.

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Pluto: 85 years of discovery


A fortuitous finding in 1992 kicked off a decade of discovery that finally made Pluto worth visiting. “What really put this over the top,” Stern says, “was the discovery of the Kuiper belt.” Planetary scientists David Jewitt and Jane Luu spied a small body orbiting beyond Neptune. It was the first confirmed body in the Kuiper belt, a long-hypothesized ring of frozen debris encircling the sun.

Pluto was no longer an oddball. It and Charon (Pluto’s only moon known at the time) were emissaries from an uncharted realm of the solar system. Researchers have since cataloged more than 1,300 icy boulders tumbling about the Kuiper belt, a small sample of the trillion or so suspected.

With the discovery of the Kuiper belt, a National Academy of Sciences report in 2003 concluded that a mission to Pluto and Charon “should be NASA’s highest priority for medium-size missions in the decade 2003–2013.” Three years later — about eight months before the International Astronomical Union kicked Pluto out of the planet club (SN: 9/2/06, p. 149) — New Horizons was on its way.

Probe in a hurry

New Horizons launched on January 19, 2006, traveling about 58,000 kilometers per hour. It was the fastest spacecraft ever to leave Earth (at that speed a run from New York to Los Angeles would take about four minutes). New Horizons crossed the orbit of the moon nine hours after launch — a journey that took the Apollo 11 astronauts about three days.

The probe was in a hurry. Pluto has been moving farther from the sun along its orbit since 1989. The already frigid temperatures — a warm day maxes out at −223 degrees Celsius — are dropping. Pluto gets so cold that its atmosphere might freeze for most of its 248-year trip around the sun. New Horizons needed to get there before 2020, when the atmosphere could become a giant ice shell, blocking the probe from studying the dwarf planet’s surface and the dynamics of its atmosphere.

Unfortunately, it’s impossible for the spacecraft to slow down and take its time once it gets to Pluto. It will cross the face of Pluto in just under three minutes but will be close enough to map the surface for a few days. The mission is nearly a decade of boredom capped with hours of terror.

The vessel has spent most of the trek to Pluto asleep. Once a year, mission engineers checked its instruments and electronics. In early 2007, New Horizons got its first in-space practice run as it sidled up to Jupiter for a speed boost. The planet’s gravity grabbed the spacecraft and flung it into the outer solar system. At Jupiter, mission scientists did a little sightseeing, testing out the cameras and instruments on the giant planet and four of its moons.

After Jupiter, the spacecraft had almost eight years of cruising to go, silently crossing the orbits of Saturn, Uranus and Neptune. Finally, last December, engineers woke the probe at about 260 million kilometers from its destination (SN Online: 12/8/14).

“The spacecraft’s in good health, on final approach,” Stern says. “We’re just on the cusp of where it gets interesting.”

New Horizons moves more than 1 million kilometers closer to Pluto every day. The team is busy navigating the spacecraft, tweaking and testing the final sequence of commands, looking for hazards (see sidebar “Dodging debris”), finalizing the 150-plus software tools needed to analyze the data, and examining the first, still-blurry images of the fast-approaching world.

“It’s all just ramping up together into this exciting frenzy,” says Joel Parker, a planetary scientist also in Boulder, at the Southwest Research Institute. “We are starting to get data now. It’s just going to be more and better from here on out.”

On close examination

In January, early pictures began trickling in, showing a gradually brightening white smudge enveloped by darkness (SN Online: 2/4/15). Engineers use images snapped by New Horizons’ telephoto camera, the Long Range Reconnaissance Imager, or LORRI, to keep the spacecraft pointed in the right direction, just inside the orbit of the moon Charon. The pictures also let scientists get a peek at their quarry. In July 2013, LORRI got its first look at Charon. Two other moons, Nix and Hydra, came into view in late January (SN Online: 2/19/15). The tiniest moons, Kerberos and Styx, finally revealed themselves in late April (SN Online: 5/13/15).

A sequence of images taken in early April showed the first hints of surface markings on Pluto itself, dark and light regions rotating in and out of view. A bright spot at the dwarf planet’s north pole hints at a polar ice cap (SN Online: 4/29/15). About 60 days before the encounter, the LORRI images surpassed the resolution of the most detailed pictures of Pluto, captured by the Hubble Space Telescope. From mid-May to July 14, every subsequent image will be the best one acquired of the Pluto system.

As New Horizons closes in, its other instruments will start analyzing Pluto from afar. An ultraviolet spectrometer nicknamed Alice will study Pluto’s atmosphere, examining its chemical makeup, density and temperature. Alice will also measure how quickly the atmosphere is leaking into space. The main camera, dubbed Ralph, will map the surface of Pluto and its moons (SN Online: 5/13/15), while LORRI zooms in for a more intimate look.

New Horizons’ 2.1-meter-wide radio dish will serve double duty as a link to Earth and as another probe of Pluto’s atmosphere. Two other instruments will monitor charged particles flying away from both Pluto and the sun. And a college student–built detector — an array of plastic films the size of a cake pan — is recording impacts with interplanetary dust for most of the long trek.

On July 12 and 13, New Horizons will send highlights of its most recent investigations as a backup.

And then, silence.

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For almost 24 hours, the spacecraft will stop communicating with mission control at the Johns Hopkins University Applied Physics Lab in Laurel, Md. It’s a matter of priorities. To send back data, New Horizons needs to point its radio dish at Earth. But if the camera wants to aim at Charon, for example, the whole probe needs to turn. Time spent beaming information to Earth is time not spent studying Pluto and its moons.

As New Horizons slips through Pluto’s and Charon’s shadows, the main camera will look for sunlight scattering off haze in the atmosphere or perhaps even faint rings of ice chips knocked off of Pluto’s moons from collisions with space rocks. To take pictures of Pluto’s nightside, the probe will use Charon as a mirror to bounce sunlight off Pluto’s surface. Powerful radio antennas on Earth will blast a signal aimed at New Horizons, timed to skim Pluto’s atmosphere. The degree to which the radio signal bends around the planet will tell researchers about the atmosphere’s temperature and composition. The duration for loss of radio contact with Earth will provide a precise measurement of Pluto’s (and Charon’s) size.

If all goes well, New Horizons will phone home around 9 p.m. Eastern time on the 14th. It will send its first image on the following day, encoded in radio waves taking about 4.5 hours to reach Earth. Humankind will get its first up-close glimpse of one of the most remote worlds in the solar system.

Neptune's moon Triton
Pluto may look like Neptune’s moon Triton (shown), possibly plucked from Pluto’s neighborhood. JPL-Caltech/NASA, USGS
That first glimpse might look a lot like Neptune’s largest moon, Triton, which was probably snatched from the Kuiper belt long ago, when it got a little too close to Neptune. Triton has very few craters on a surface sculpted by ice volcanoes. Geysers belch nitrogen into the atmosphere, and expanses of terrain resemble the skin of a cantaloupe.

The Voyager 2 spacecraft, which launched in 1977 on a tour of the outer solar system, found Triton to be a changing world. Pluto may be similar. “I think we’re all hoping to find some signs of active geology,” says William McKinnon, a planetary scientist at Washington University in St. Louis. The large bodies in the Kuiper belt are planets in all but name, he notes. “They have histories, and geologies, and active atmospheres, and weather and all sorts of interesting things going on.” The shape of Pluto, he says, may even betray the presence of a core or a hidden ocean: The more round it is, the more likely it’s pliable, with an interior that’s at least a little slushy.

If Pluto’s atmosphere freezes, craters fill with ice, erasing any memory of past impacts. Charon, however, doesn’t appear to have an atmosphere. So it might preserve a record of everything that’s slammed into it over the last 4.6 billion years, which could provide clues to what else is flying around out there. “Everybody’s excited for Pluto and overlooks poor Charon,” Brown points out. “I think it’s going to be the breakout star of the encounter.”

In the end though, planetary scientists both on and off the Pluto mission’s team just want to see what the place looks like. “I’m most interested in unwrapping the Christmas present and seeing the visual image,” Stern says.

While the team is itching to see what Pluto is all about, there is a twinge of melancholy. After working on New Horizons for 26 years, says Stern, it will be strange to not be looking forward to its arrival. “For most of us,” he says, “it’s the only time we will be on a first mission of exploration where we go from knowing almost nothing to just a completely different universe, scientifically.”

The mission doesn’t end in July, however. The data won’t finish downloading until late 2016 and will undoubtedly take years more to unravel. Plus, Stern and colleagues have plans for New Horizons to visit more distant shores.

Using the Hubble Space Telescope, scientists have identified two other potential stops in the Kuiper belt, each roughly 2 billion kilometers beyond Pluto. They lie in different directions, so it has to be one or the other. Stern says his team will make a decision with NASA in August, then fire up New Horizons’ engines for the next phase of the journey.

Clyde Tombaugh
Clyde Tombaugh (shown) discovered Pluto in 1930 while searching for a planet hypothesized to be tugging on Uranus and Neptune. The New Horizons probe is carrying some of his ashes to Pluto and beyond. Dale Wittner/AP
Eventually the spacecraft will sail through the Kuiper belt and join four other spacecraft — Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 — as Earth’s ambassadors to interstellar space.

The Pioneer and Voyager probes carried with them mementos that are sure to befuddle alien salvage teams. New Horizons is no different. It’s hauling two U.S. flags; state quarters of Maryland (where the mission was built) and Florida (site of its launch); a fragment of SpaceShipOne, the first privately funded, crewed spacecraft; two CD-ROMs, one carrying 434,738 names submitted by fans over the Internet and another loaded with pictures of New Horizons and its team; and a 1991 U.S. postage stamp emblazoned with the slogan, “Pluto: Not yet explored.”

The ninth memento, attached to the spacecraft’s interior wall, is a small container carrying a special passenger. One ounce of Clyde Tombaugh’s ashes is along for the ride, honoring the man who spent countless hours comparing photographs of the night sky, looking for one little white dot moving among the stars. Its label reads: “Interned herein are remains of American Clyde W. Tombaugh, discoverer of Pluto and the solar system’s ‘third zone.’ Adelle and Muron’s boy, Patricia’s husband, Annette and Alden’s father, astronomer, teacher, punster, and friend: Clyde W. Tombaugh (1906–1997).”

The Flagstaff astronomer is finally getting to visit the world he discovered 85 years ago.


Dodging debris

New Horizons can learn something from the Titanic’s epic collision: Watch out for chunks of ice. Mission control is keeping an eye out for such hazards and has a couple of backup plans, just in case. If controllers see debris ahead, and they have time to change the probe’s trajectory, they’ll go for Plan A: dive in close to Pluto and skim its atmosphere. Flying so close will make imaging the surface more difficult, but getting in closer should steer the probe clear of obstacles.

If it’s too late to change course, then Plan B is a go: Use the probe’s radio dish as a shield. Unlike the delicate instruments hiding behind it, the antenna can take some pounding. Controllers can turn the dish to face the debris and let it take the brunt of the impacts.

The odds of having to fall back on either plan are small, says Cathy Olkin, a planetary scientist at the Southwest Research Institute in Boulder, Colo. Mission planners chose a path that takes New Horizons close to Charon’s orbit. The moon’s gravity should sweep clear the road ahead. “But we have plans because it’s responsible,” she says. — Christopher Crockett


This article appears in the June 27, 2015, Science News with the headline, “Pluto: Explored.”

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