New Horizons spacecraft speeds toward Pluto rendezvous

When I was a young boy, NASA began its outreach to our planetary neighbors by launching primitive probes to Venus and Mars.  We didn't learn much due to the limitations of technology back then, but it was exciting nonetheless to actually get a close-up view of those planets.

Then, in the 1970s, we landed on Mars and launched spacecraft to fly by Jupiter, Saturn, Uranus, and Neptune, adding immeasurably to our knowledge of the solar system and our place in it. 

But our explorations were incomplete.  Lying beyond the orbit of Neptune is tiny Pluto, and given its distance from the Earth, it didn't seem possible that we'd get out there in my lifetime.  Now, in less than 36 days, NASA's most ambitious planetary probe in a generation will achieve its closest approach to the former planet.

When the New Horizons spacecraft left the earth in 2006, Pluto was still considered a planet.  But a re-examination of the definition of "planet" ended up in the demotion of Pluto to the classification of a "dwarf planet."  This was necessary, because observers have discovered a half-dozen icy bodies beyond Neptune as big as or bigger than Pluto.  Rather than expanding the number of planets to accommodate what are surely many thousands of bodies like Pluto, scientists opted for clarity and simplicity in classification.

It hardly matters.  Pluto is a strange, weird place, and the pictures and data that we've already been able to get from New Horizons promise to upend our theories of Pluto and its bizarre moons.


Pluto was once considered the oddest of the solar system's nine planets, but over the past decade, more objects like Pluto have been discovered on the solar system's icy edge, more than 3 billion miles (5 billion kilometers) from the sun. Now Pluto is considered one of the largest objects in a category known as dwarf planets.

"'Dwarf' is an honorific. It's like saying 'Sir Pluto,'" joked John Spencer, a space scientist at the Southwest Research Institute who was not involved in the Nature study but is part of the science team for NASA's New Horizons mission to Pluto.

Related: Pluto Debate Is About More Than One Little World

After a cruise of more than nine years, the piano-sized New Horizons spacecraftwill capture unprecedented views of Pluto during a flyby on July 14. The findings outlined in Nature are based on Hubble observations made between 2005 and 2012, primarily to make sure New Horizons won't slam into something as it zooms past.

The observations led to the discovery of the moons Nix and Hydra in 2005,Kerberos in 2011, and Styx in 2012. In contrast to Charon, which is about half as wide as Pluto itself, these four smaller moons are thought to be no wider than 36 miles (60 kilometers) — and Styx's diameter could be less than 5 miles (8 kilometers).

Showalter said that he and the study's co-author — Douglas Hamilton, an astronomer at the University of Maryland — started out intending merely to tie up the loose ends in the Hubble observational campaign. But as they looked more closely at the readings, the moons "started getting weirder and weirder and weirder," he said.

Pluto's six moons dance, tumble, spin, and wobble in their orbits – unlike anything else seen in the solar system.  Scientists hope to get an idea of how these small bodies were captured by Pluto in the first place, which might tell us something about the region beyond the icy dwarfs in Pluto's neighborhood.

After its flyby of Pluto, New Horizons will continue its journey out to the Kuiper Belt – a mysterious region of our solar system where more icy rocks containing the oldest matter in the solar system reside.  The probe should reach one of three of those Kuiper Belt Objects between 2016 and 2020.

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