“Not finding new moons or rings present is a bit of a scientific surprise to most of us,” Stern said. “But as a result, no engine burn is needed to steer clear of potential hazards. We presented these data to NASA for review and received approval to proceed on course and plan. We are ‘go’ for the best of our planned Pluto encounter trajectories.”
New Horizons formed a hazard analysis team in 2011 after the discovery of Pluto’s fourth moon, Kerberos, raised concerns the cratering of these moons by small debris from the outer area of the solar system known as the Kuiper Belt could spread additional hazardous debris into New Horizons’ path. Mission engineers retested spare spacecraft blanketing and parts back on Earth to determine how well they would stand up to particle impacts, and scientists modeled the likely formation and locations of rings and debris in the Pluto system. By the time New Horizons’ cameras were close enough to Pluto to start the search last month, the team had already estimated the chances of a catastrophic incident at far less than 1 percent.
The images used in the latest searches that cleared the mission to stay on its current course were taken June 22, 23 and 26. Pluto and all five of its known moons are visible in the images, but scientists saw no rings, new moons, or hazards of any kind. The hazards team determined that satellites as faint as about 15 times dimmer than Pluto’s faintest known moon, Styx, would have been seen if they existed beyond the orbit of Pluto’s largest and closest moon, Charon.
If any rings do exist, the hazard team determined they must be extremely faint, reflecting less than one 5-millionth of the incoming sunlight.
“The suspense — at least most of it — is behind us,” says John Spencer of SwRI, who leads the New Horizons hazard analysis team. “As a scientist, I’m a bit disappointed that we didn’t spot additional moons to study, but as a New Horizons team member I am much more relieved that we didn’t find something that could harm the spacecraft. New Horizons already has six amazing objects to analyze in this incredible system.”
While the lack of new moons was unexpected, finding frozen methane on Pluto's surface was not. Earth-based astronomers first observed the chemical compound on the distant planet in 1976.
“We already knew there was methane on Pluto, but these are our first detections,” said Will Grundy, the New Horizons Surface Composition team leader with the Lowell Observatory in Flagstaff, Arizona. “Soon we will know if there are differences in the presence of methane ice from one part of Pluto to another.”
Methane is an odorless, colorless gas that is present underground and in the atmosphere on Earth. On Pluto, methane may be primordial, inherited from the solar nebula from which the solar system formed 4.5 billion years ago. Methane was originally detected on Pluto’s surface by a team of ground-based astronomers led by New Horizons team member Dale Cruikshank, of NASA’s Ames Research Center, Mountain View, California.
Approaching Pluto