Historic Milestone: Pluto Mission Reaches Neptune's Orbit

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Neptune as imaged by NASA's Voyager 2 mission in 1989.
NASA

As we wait in anticipation for NASA’s New Horizons probe to make its historic Pluto flyby on July 14, 2015, the mission is about to cross another milestone — it will zoom past Neptune’s orbit at 7:04 p.m. PDT (10:04 p.m. EDT) today (Monday), and it has already snapped a pixelated view of the outer solar system’s ice giant from 2.5 billion miles away (around 27 times the Earth-sun distance).

Today’s milestone is also a grand coincidence for solar system exploration. Precisely 25 years ago, on Aug. 25, 1989, NASA’s Voyager 2 probe reached Neptune as it made its way to the outermost reaches of the sun’s sphere of influence — a magnetic bubble called the heliosphere. Voyager 2 was the first spacecraft to encounter the majestic blue planet.

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NASA New Horizons' view of distant Neptune and Triton.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

“It’s a cosmic coincidence that connects one of NASA’s iconic past outer solar system explorers, with our next outer solar system explorer,” said Jim Green, director of NASA’s Planetary Science Division, NASA Headquarters in Washington, D.C. “Exactly 25 years ago at Neptune, Voyager 2 delivered our ‘first’ look at an unexplored planet. Now it will be New Horizons’ turn to reveal the unexplored Pluto and its moons in stunning detail next summer on its way into the vast outer reaches of the solar system.”

“NASA’s Voyager 1 and 2 explored the entire middle zone of the solar system where the giant planets orbit,” said Alan Stern, New Horizons principal investigator at the Southwest Research Institute in Boulder, Colo. “Now we stand on Voyager’s broad shoulders to explore the even more distant and mysterious Pluto system.”

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Voyager 2 was the first to gain dramatically clear views of Neptune and its moons in 1989, the last time a “new planet” has been shown to us in beautiful clarity. Next up is dwarf planet Pluto that will be visited by New Horizons, using 21st century camera technology to image the little world — and its largest moon (or “binary partner”?) Charon — in crystal clarity.

As New Horizons approached Neptune’s orbit, it was able to grab a distant photo of Neptune and largest moon Triton. Through information gathered from Voyager 2 observations a quarter of a century ago, planetary scientists believe that the strange moon may actually be a “captured” Kuiper Belt object.

Rather than originating from the planetary material that accumulated to form the other Neptunian moons, weird Triton — which has a retrograde orbit around Neptune — seems to have formed in the same outer solar system region as Pluto but somehow migrated toward Neptune and became gravitationally bound to the ice giant. Triton’s origin has therefore sparked theories that the moon may have similar features to Pluto, theories that will be tested in a few months time.

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“There is a lot of speculation over whether Pluto will look like Triton, and how well they’ll match up,” said Ralph McNutt of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. “That’s the great thing about first-time encounters like this — we don’t know exactly what we’ll see, but we know from decades of experience in first-time exploration of new planets that we will be very surprised.”

According to Stern at a special press conference on Monday, we can expect to see a better-than-Hubble image of Pluto as New Horizons approaches the Kuiper Belt from January 2015 onwards — and that’s when many questions about Pluto’s mysterious nature will start to be answered.

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