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With each passing day, mankind gets a better look at Pluto. And each day, Pluto is showing mankind it has a lot to learn. First, Pluto revealed itself in a mix of beige and orange, while Charon, its largest moon, appeared gray when NASA’s New Horizons spacecraft captured color images of the dwarf planet in early June. Then, scientists at the Johns Hopkins Applied Physics Laboratory in Laurel, Md., spotted unusual dark poles on Charon. In late June, they saw mysterious dark streaks spaced evenly along half of Pluto’s equator.

New Horizons will pass within 7,800 miles of Pluto on Tuesday after a nearly decadelong, 3 billion-mile journey. “We are running the anchor leg in a 50-year exploration of the planets,” said Alan Stern, the principal investigator — the leader — of the $700 million mission. “I tell people, this is it, it’s the last picture show, it’s the last train to Clarksville. Better watch!”

As the spacecraft — which is bearing down on the dwarf planet at 32,000 miles per hour — nears that close encounter, the scientists are getting never-before-seen images of Pluto.

“A few weeks ago, the faucet hadn’t turned on,” Stern said. “Now it’s dripping a little every day. Soon it’ll be a rush.”

The scientists behind New Horizons say the mission will help us understand the origin and evolution of the solar system. Already, the little spaceship’s long-range camera has detected intriguing patterns on the surface of Pluto that scientists cannot easily explain. The tiny world may have mountains and valleys, possibly frozen methane lakes or even a liquid water ocean far beneath the frozen surface. Or maybe Pluto has no topography at all and is just a smooth ball covered in a deep layer of nitrogen slush.

Stern said he began pushing for a Pluto mission in 1988. NASA considered numerous proposals before deciding in 2001 to go with the New Horizons mission, which called for a relatively inexpensive, no-frills spacecraft using off-the-shelf technology.

The probe rode into space one afternoon in 2006 atop an Atlas V rocket, reaching record-breaking velocity. Before the day was out, it had already flown past the orbit of the moon.

A gravity boost from Jupiter shortened the trip to Pluto by three years. “You had to hit a little keyhole in space near Jupiter,” said project scientist Hal Weaver.

Because of the way the instruments and the antennae are configured, New Horizons cannot observe Pluto and simultaneously transmit data to Earth. Normally, it does one or the other. But the flyby is so fast, and such a precious opportunity, that the spacecraft will focus entirely on Pluto during Tuesday’s encounter, at the cost of leaving scientists back home in suspense.

“We have to trust that it’s doing what it needs to do,” said Alice Bowman, the mission operations manager.

Scientists and prominent officials won’t know until late that evening whether the spacecraft survived the encounter.

A very bad scenario would involve red alarms going off. “The worst case is we don’t hear at all,” Stern said.

His team came up with 249 “contingencies,” or problems, ranging from power brownouts on the spacecraft to a catastrophic fire at mission control. The biggest concern has been debris around Pluto.

While planning the commands that guide the spacecraft’s operations typically takes a couple of months, the scientists at the Hopkins lab — which designed, built and operates the spacecraft — have worked since 2009 on the plan for the final days of the mission’s journey to Pluto.

When New Horizons launched in 2006, exploration of Pluto and the Kuiper belt in which it resides was among planetary scientists’ top priorities. The relatively small, icy bodies there contrast with the gaseous giants of the outer solar system and the terrestrial planets closer to the sun.

Since then, scientists have learned more about Pluto. They knew about Charon but found four more moons between 2005 and 2012. They also reclassified Pluto — for the Roman god of the underworld — from its status as the solar system’s ninth planet to dwarf planet in 2006.

Seen in better detail, the faces of both Pluto and Charon suggest a diversity of landscapes, including what may be frost left over from a polar cap on Pluto that has evaporated in summer sun.

And color images from one of New Horizons’ two cameras showed two contrasting faces of Pluto, with one hemisphere marked by 300-mile-wide dark streaks distributed along its equator. And Charon’s poles also are marked by mysterious dark terrain, something not seen anywhere in the solar system except for Jupiter’s moon Io, Stern said.

“They’re puzzling,” said Stern, who is a planetary scientist at the Southwest Research Institute in Boulder, Colo., but will spend the next month at the Hopkins lab overseeing the mission.

New Horizons is the first spacecraft to explore Pluto, and the first NASA space probe to any “new” planetary frontier since the Voyager missions launched in 1977 to explore Jupiter, Saturn, Uranus and Neptune.

Already, the mission has pleasantly surprised scientists. “Pluto is turning out to be the belle of the ball,” Stern said.

And New Horizons’ exploration might not stop with Pluto. If NASA approves a budget for it and the spacecraft holds up, an extended mission could venture beyond the solar system to three bodies in the Kuiper belt — only another billion miles away.

“This” — Stern said — “is a moment. People should watch it. They should sit their freakin’ kids down and say, think about this technology. Think about people who worked on this for 25 years to bring this knowledge. … It’s a long way to go to the outer edge, the very edge of the solar system.”

The Washington Post contributed to this report.

“We are running the anchor leg in a 50-year exploration of the planets. I tell people, this is it, it’s the last picture show, it’s the last train to Clarksville. Better watch!” Alan Stern, the principal investigator of the New Horizons mission