Hunting Pluto's Dangerous Rings

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Pluto: Cold and dark; surrounded by a small system of four moons. That just about sums up the neighborhood of the dwarf planet, right? Actually, it may be a little more exciting — and dangerous — than that.

The outer solar system planets all have rings. Saturn is the most famous ringed planet, but Jupiter, Uranus and Neptune have rings too. However, until now, the dinky Pluto that orbits the solar system’s hinterland hasn’t shown any signs of a ring system.

Could that be because the world isn’t a gas giant like its larger cousins? Or, due to its great distance, we just haven’t seen its rings yet?

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If the latter is true, we may have a problem, and scientists from NASA’s New Horizons Pluto flyby mission are beginning to sweat. New Horizons is due to blast past Pluto at 31,300 miles per hour in 2015, and if Pluto has a ring system, it could spell doom for the robotic explorer.

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Concerns for the mission arose when the Hubble Space Telescope discovered a fourth moon (designated “P4″) orbiting Pluto in July 2011. In November, New Horizons principal investigator Alan Stern went on the record to air his team’s spacecraft safety concerns.

“When we discovered P4 this summer, along with possible evidence of a couple of still-fainter moons (something we need more study to confirm or reject), we began to worry about just how many tiny moons Pluto might have and whether we might have to dodge them,” Stern said in his Nov. 7 mission update.

Fortunately, help is at hand. Using the four-meter Anglo-Australian Telescope in Australia, Planetary Science Institute (PSI) Senior Scientist Henry Throop and his team have been studying Pluto, watching for any signs of a ring system. However, due to its distance, Pluto’s rings cannot be directly observed.

“From the ground, Pluto’s rings would be too faint and too small to see directly. But occasionally, Pluto passes in front of a distant star, and that lets us study it in exquisite detail,” said Throop. “As Pluto passes in front of the star, the star’s light blinks out, like a moth blocking out the beam from a flashlight. We searched through the observations to try to find any hint that the star light was being blocked by rings of Pluto.”

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As it turns out, should there be a cloud of errant debris surrounding Pluto, the New Horizons team may decide to aim the probe through the orbital path of Charon, Pluto’s largest moon. The gravity of Charon would clear a “safe zone” through the hypothetical ring, thus reducing the risk of debris impact as the spacecraft glides by.

Although Throop’s team have yet to spot any sign of a Plutonian ring system, these observations will continue until New Horizons takes a look for itself in just three years time.

Image: The Pluto system as seen by Hubble in May 2005 — Pluto (bright white), Charon (blueish), Nix and Hydra (fainter dots). This observation contributed to the discovery of Nix and Hydra. Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team

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