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Pluto probe closes in

An artist's conception shows New Horizons at Pluto.

NASA's New Horizons probe passed a key milestone today on its nine-year journey and is now closer to Pluto, its primary target, than it is to Earth. But it still has more than five years and more than 1.5 billion miles to go.

The 1,054-pound (480-kilogram) piano-sized spacecraft blasted off for the solar system's most controversial dwarf planet almost four years ago. New Horizons was the fastest spacecraft ever launched from Earth, and thanks to a gravitational boost from Jupiter, it's closing in on Pluto at the rate of 750,000 miles (1.2 million kilometers) per day. The probe is due to zoom past Pluto and its three moons on July 14, 2015.

As of today, New Horizons is between the orbits of Saturn and Uranus - a little more than 1.527 billion miles (2.463 billion kilometers) from Earth and 1.526 billion miles (nearly 2.462 billion kilometers) from Pluto, according to today's status report from mission control at Johns Hopkins University's Applied Physics Laboratory in Maryland. (APL is managing the mission on NASA's behalf.)

So this is the halfway point, right? Well, that all depends on what your definition of "halfway" is. "This is the first of several milestones over the next 10 months that mark the halfway points in our journey to the solar system's frontier, where Pluto lies," said Alan Stern, the mission's principal investigator and a planetary scientist at the Southwest Research Institute.

In an e-mail, Stern explained why there's more than one halfway point:

"We are not yet halfway. That comes in late February. We are closer to Pluto than Earth as of today. It's not the same! Why? Because Earth is on the far side of the sun from Pluto. Make sense? ... We get halfway in miles before halfway in days because we are slowing down, owing to the sun's gravity, and therefore the second half of the miles takes a little over half the time. Halfway [in mileage] is February 25; half the days is October 17, both 2010."

Another complication is that Pluto is getting farther and farther away from the sun each day, and that means the halfway point between the sun and the place where Pluto will be for the 2015 encounter is a different day altogether: April 20, to be precise.

This graphic shows the position of the New Horizons probe with relation to Pluto, Earth and other planets.

As far as New Horizons' camera is concerned, Pluto is still just a speck in a black sky. In fact, the spacecraft snoozed through today's milestone and won't wake up until next month, for a quick 10-day tuneup. But when 2015 comes around, New Horizons will send back our first-ever close-up of a world that has a thin atmosphere and icy clouds, a mottled surface, and possibly ice volcanoes as well.

The view just might be cool enough to put Pluto in an unprecedented spotlight, almost a decade after its demotion from the nine-planet pantheon. Will its current dwarf-planet designation be reconsidered? Will that designation really matter? "Today's pedantic fuss over planetary semantics will seem naive and irrelevant," the Space Telescope Science Institute's Ray Villard predicted today in a Discovery News blog posting.

It might have been different a decade ago, when it wasn't clear whether the $700 million mission would ever get off the ground. As it was, Stern and his colleagues had to struggle for 17 years to get New Horizons launched. At the time, Pluto was sometimes called the "last unexplored planet," even though the discovery of other icy objects on the solar system's ring was already making it clear that Pluto wasn't the last planet after all.

Suppose the International Astronomical Union had reclassified Pluto while New Horizons was in doubt. Would there have been any space mission to the solar system's icy edge?  "I am convinced ... if the IAU had acted prior to 2003, we would probably not be en route today," Stern told me earlier this year.

As it is, New Horizons will continue its journey through the Kuiper Belt long after the 2015 encounter with Pluto, sending back data about other ice worlds as well. For how long? Who knows? Villard points out that the solar system's frontier stretches for a mind-boggling distance - so far that it would take New Horizons 80,000 years to get to the nearest star.

To learn more about New Horizons, Pluto and the planetary frontier, check out my book, "The Case for Pluto" - and stay tuned for more Plutonian milestones in the months ahead.

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