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September 26, 2014

Rings and Other Solar System Surprises

Mark ShowalterMark Showalter
New Horizons Co-Investigator

I am known as the "rings guy" on the New Horizons team. I've spent most of my career studying the ring systems of the four giant planets. This unofficial title contains a bit of irony because, to the best of our knowledge, Pluto has no rings. I've received plenty of good-natured jests from my colleagues about being the resident expert on something that doesn't exist. (In my defense, I also observe and study the dynamics of small moons, and Pluto has not disappointed us in that regard.)

Nevertheless, as a rings guy, I had a truly jaw-dropping moment earlier this year when Nature magazine announced the discovery of rings orbiting a 250-kilometer asteroid designated 10199 Chariklo. South American astronomers detected two narrow, dense rings while watching Chariklo pass in front of a star last year.

I can come up with all sorts of reasons why Chariklo's rings should not exist. Asteroids have very weak gravity fields. It is hard for anything to stay on a tidy circular orbit around an object with such a lumpy gravity field. Collisions between ring particles should make the system dissipate. The list goes on...

Nevertheless, the observations are completely unambiguous. The Chariklo rings are real. For me, as a scientist, it has driven home an important message - apparently, I don't understand rings quite as well as I thought I did.

rings around asteroid Chariklo

The discovery of rings around asteroid Chariklo last year - depicted in this artist’s impression - drove home the important message that the solar system is still full of plenty of surprises. (Image credit: ESO/L. Calçada/M. Kornmesser/Nick Risinger)

Which brings me back to New Horizons. We are about to fly a spacecraft past Pluto at nearly 14 kilometers per second - about 31,000 miles per hour. A millimeter-sized ring particle could do serious damage, possibly jeopardizing the mission and all of the science data we plan to collect. We have designed alternative trajectories to use if we see something hazardous in our path. I will be part of the team scouring the images as we approach Pluto to look out for any unwelcome surprises and recommend which of those trajectories to use, if necessary.

Overall, however, we remain highly confident the flyby next July will come off as planned. We will be flying through the Pluto system at a place where we expect orbits to be unstable, so the risk of encountering an errant grain of sand is very, very small. So far, nothing we have seen has given us any reason to worry. Still, the message I take away from the Chariklo rings discovery is that a bit of humility is in order. There are still plenty of things out there in the universe that scientists haven't thought of yet...

Mark Showalter, a New Horizons science team co-investigator and senior research scientist at the SETI Institute, is the discoverer of six moons (including Pluto's moons Styx and Kerberos) and three planetary rings. In addition to searching for faint rings and additional small moons, Showalter will be investigating the dynamics, evolution and origin of the Pluto system.