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Prime Time for Pluto: New Horizons Probe Closes In for Historic Flyby

After more than nine years and 3 billion miles of traveling, NASA's New Horizons probe closes in for its history-making close encounter with Pluto.

LAUREL, Md. — After more than nine years and 3 billion miles of traveling, NASA's New Horizons spacecraft is just hours away from its history-making close encounter with Pluto and its moons — and back on Earth, all of us humans are just leaving it alone.

"We'll be waiting with the rest of the world," said Alice Bowman, New Horizons' mission operations manager here at Johns Hopkins University Applied Physics Laboratory.

More than 1,000 VIPs and journalists are converging on APL for the climax of the $728 million mission. This marks the first up-close examination of the icy worlds beyond the orbit of Neptune — and arguably the last mission of its kind for the foreseeable future.

For the first time since the Voyager missions of the 1970s and '80s, humanity is opening up a completely new frontier of the solar system, said the mission's principal investigator, Alan Stern of the Southwest Research Institute.

"I imagine it was like this on Voyager," he told reporters during a pre-encounter briefing. "It's like stepping off an escalator and onto a supersonic transport ... so it's quite a rush."

The mission's aim is to map Pluto and its largest moon, Charon; learn more about their composition and the whisper-thin atmosphere they may share; and take pictures of at least a couple of Pluto's four smaller moons as well. Those readings could help fill out scientists' understanding of how planets were formed billions of years ago — and reveal weird wonders on the edge of the present-day solar system.

The images sent back during New Horizons' approach already have revealed curious dark splotches and what appears to be a bright heart-shaped region on Pluto, as well as chasms deeper than the Grand Canyon and a mysterious "dark pole" on Charon.

Image: Pluto
An image of Pluto captured on Saturday by NASA's New Horizons probe reveals linear features that may be cliffs, as well as a circular feature that could be an impact crater. Rotating into view is the bright heart-shaped feature that will be seen in more detail during New Horizons’ closest approach on Tuesday. The annotated version includes a diagram indicating Pluto’s north pole, equator and central meridian.NASA / JHUAPL / SwRI

Stern said the pictures to come from the close encounter should be 100 times sharper than the best that's come before. Such observations should help scientists figure out whether Pluto has ice geysers, or rivers of liquid nitrogen, or a liquid subsurface ocean, or clouds in its thin atmosphere.

"It really is a mission of raw exploration," Stern said.

Delayed gratification

The climax rolls around at 7:49 a.m. ET Tuesday. That's when New Horizons will come within 7,800 miles of Pluto, taking close-up pictures and gathering data as it zooms past at more than 30,000 mph. The best pictures should be sharp enough to show features that are 230 feet (70 meters) wide — the equivalent of New York's Central Park on Earth, Stern said.

But because of the mission's budgetary and design constraints, the spacecraft can't store up all those observations and communicate with Earth at the same time. What's more, it takes 4.5 hours for signals to travel at the speed of light back to Earth, and another 4.5 hours for commands to be sent back.

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As a result, New Horizons has to be left to its own devices during the climactic part of the encounter. If anything goes wrong, the spacecraft will have to fix itself by rebooting its computer and then picking up where it left off.

A little more than a week ago, the New Horizons team had to cope with a potentially crippling computer glitch — but Bowman and her colleagues have now determined that the spacecraft is in excellent health and right on course for its flyby. At this point, making a needless tweak would run the risk of tweaking things the wrong way, said APL's Chris Hersman, mission system engineer for New Horizons.

"The less we interact with it, the better," Hersman explained.

Cheers and flag-waving

The last data before the close approach will be received from the spacecraft at 11:17 p.m. ET Monday, and then there'll be almost 22 hours of radio silence, Bowman said. The signals confirming that the spacecraft survived the encounter in good shape won't come in to APL's Mission Operations Center until sometime between 8:53 and 9:09 p.m. ET Tuesday.

If the ground controllers at APL receive those signals, that's when you can expect the room to erupt in cheers and flag-waving.

And then the spacecraft goes right back to work. "On Wednesday, it starts raining data," Stern said.

The data will come from seven instruments — including cameras that capture imagery in black-and-white, color, infrared and ultraviolet, spectrometers capable of analyzing the surface composition of Pluto and Charon, particle detectors to determine what's happening to Pluto's atmosphere, a radio-sensing experiment and a student-built dust counter.

Ninety-nine percent of all the data from the encounter will be stored on solid-state recorders for transmission back to Earth over the course of 16 months, Stern said. The transmission rate is expected to be 2,000 bits per second, which is slower than the modems that hooked up to the Internet 20 years ago. That rate is so slow because the data stream has to travel so far, from a plutonium-powered spacecraft that has only 200 watts of electricity at its disposal.

The next chapter

After the Pluto encounter, New Horizons' team members plan to ask NASA to extend their mission so they can study at least one more icy world in the broad ring of icy material that lies beyond Neptune's orbit — a region known as the Kuiper Belt. But that encounter wouldn't take place until New Horizons has traveled another three and a half years and another billion miles.

Past revelations about such far-flung regions of the solar system — including the discovery of a world more massive than Pluto, now known as Eris — led the International Astronomical Union to reclassify Pluto as a dwarf planet in 2006, just months after New Horizons was launched.

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At the time, the IAU said dwarf planets didn't count as as true planets, a decision that's still controversial in some circles. Will New Horizons' observations revive the debate over Pluto's planetary status? Some of the Plutophiles celebrating this week's flyby no doubt hope so — but Jim Green, director of NASA's Planetary Science Division, said he's staying out of that controversy.

"NASA's position is really quite simple," he told reporters. "We don't care what we call this. At all."

Whether you call Pluto a planet, or a dwarf planet, or a Kuiper Belt object, or a plutoid, "it's an object well worth observing," Green said.