New Horizons: Encounter Planning Accelerates
Back in 2005 and 2006, when Pluto’s second and third moons (Nix and Hydra) were discovered, searches by astronomers for still more moons didn’t reveal any. So the accidental discovery of Pluto’s fourth moon by the Hubble Space Telescope in mid-2011 (during a search for Plutonian rings) raised the possibility that the hazards in the Pluto system might be greater than previously anticipated. Those concerns were amplified when Hubble discovered a fifth moon in 2012. As a result of those discoveries, the New Horizons science and operations teams began to more carefully scrutinize the true level of hazards to our spacecraft at closest approach and devise mitigation strategies to make sure the encounter with Pluto would be successful.
We’ve now largely completed that work and presented the results to both an independent, NASA-appointed technical review team, led by the Jet Propulsion Laboratory’s Keyur Patel, and then to senior executives at NASA Headquarters. Both groups have concurred with our findings, which I can summarize as follows:
New Horizons benefits from its approach trajectory because that trajectory is steeply inclined to Pluto’s satellite plane and associated debris hazards that models show should lie close to the satellite plane. As a result, most of the risk New Horizons faces occurs only at closest approach, when the spacecraft is very near the satellite plane.
The Pluto system appears to be far safer than early fears and initial calculations indicated when the new moons began popping up. In fact, the best current models predict a 0.3% (1-in-300) chance of a mission-ending impact near closest approach on the nominal trajectory. Much of the reason for this lowered risk assessment is that more sophisticated dust-impact models revealed a decrease (by about a factor of 100) in lethal impact probability for trajectories that fly into the region where New Horizons is aimed now – a region where the gravitational effects of Pluto’s largest moon Charon clear debris. Another important factor is that when we tested spacecraft components against high-velocity impacts using gun ranges in New Mexico and Ohio, we found the spacecraft shielding is considerably “harder”– that is, more resistant to impacts – than preflight estimates indicated.
http://www.planetary.org/blogs/guest-blogs/2013/20130520-new-horizons-encounter-planning-accelerates.html
