Tiny, Gentle Pluto May In Fact Be A Killer

Somewhere between Saturn and Uranus, a 1,050 pound spacecraft is on its way to Pluto. It's been on its cosmic journey for over five years – it launched on January 19, 2006 – and it won't reach its target until 2015. But there's a kink in the plan. As astronomers discover new moons and small bodies in orbit around Pluto, New Horizons might actually be racing towards its death.

Pluto was discovered in 1930. Initial observations and measures of its apparent brightness led astronomers to believe that the planet the size of Earth. But advances in technology revealed that a moon was in Pluto's system, making the planet appear larger and brighter than it was. In 1978, astronomers realized that Pluto is quite small and it had a relatively large moon, Charon.

For decades Pluto and Charon lived in apparent harmony. As far as astronomers on Earth knew, there were no other bodies anywhere near the tidally locked planet and its moon. But in the early 1990s, astronomers began finding bodies in the vicinity of Pluto's system. In the early 2000s, bodies very near Pluto started popping up, so close that there was a chance they were satellites of the small planet.

In 2002, Pluto became a target. Alan Stern – best known to some as the fiercest proponent of Pluto regaining full planetary status – proposed the New Horizons mission to NASA. The spacecraft would fly by Pluto and explore the nearby Kuiper Belt objects. The proposal was accepted. While the spacecraft was being developed and built, Stern as the Principle Investigator began lobbying for time on the Hubble Space Telescope to look for and confirm bodies around Pluto that might affect his mission.

After initial rejections, the New Horizons team finally granted an opportunity to use Hubble in 2004. Their chance came in May 2005. For two nights, Hubble took long exposure photographs on such sensitive settings that images would reveal objects up to 100,000 fainter than Pluto. The team had hoped to find a few objects in the general vicinity of the Kuiper Belt. What they found was two faint objects that appeared to be quite close to Pluto.

Stern and one of his post doctoral students Andrew Steffl studied the two objects and determined that they were likely in orbit around Pluto. As confirmation of their conclusion, other team members Hal Weaver and Max Muchler had also found the same objects. Both sets of scientists agreed that the two objects were apparently in orbit around Pluto and not just nearby Kuiper Belt objects.

Discovery of the new moons was announced in September 2005, but further study to confirm the findings was easier said than done. Maintenance on Hubble to extend its usable life meant Pluto would be unobservable during the early part of 2006 when the team needed its observational capabilities. Instead, they turned to the largets ground based telescopes available. But even the mammoth Keck and Gemini telescopes in Hawaii couldn't resolve details around Pluto, let alone tiny possible satellites.

It wasn't until a year after Hubble first found the possible satellites that they were formally accepted by the International Astronomical Union. In May 2006, Nix and Hydra joined Pluto's system. The problem was, Nix and Hydra weren't the only possible satellites out there.

Further study of Pluto revealed more objects in its vicinity, but only one is a potential satellite: S/2011 P1 or P4 – the fourth moon of Pluto. Discovered in July 2011, P4 orbits between Nix and Hydra. All three orbit outside of Charon.

But P4 wasn't the only body around Pluto discovered this year. Two more candidate satellites, smaller than P4, were also found. But they aren't satellites yet. The team needs to do more research and make more observations before the new bodies can be confirmed or rejected as Plutonian satellites.

So what does this mean for New Horizons? As new objects and possible moons keep popping up, there's some concern that there may be more undiscovered objects around Pluto too small to discern from Earth. If there are, Stern and the New Horizon team only have four more years to find them and adjust New Horizons' path to dodge them.

Another, greater concern is that these small moons will have generated rings or a cloud of debris around Pluto, creating multiple and much smaller impact hazards for New Horizons. This could be a serious problem. New Horizons is set to fly by Pluto in the space between the planet and Charon at a little over 8 miles per second. At that speed, even particles less than three ounces could penetrate the spacecraft's micrometeoroid blankets and do serious damage to its electronics, fuel lines, and sensors.

To mitigate the possible death of their mission, the New Horizons is continuing to work on assessing the dangers associated with the Pluto system. Their principle tools are the Hubble Space Telescope, some very large ground-based telescopes, and radio telescopes telescopes. The hope is to resolve object in the space between Pluto and Charon since that's where New Horizons is heading.

If it turns out that there is a significant amount of debris in New Horizons' planned path, the team will have to adjust the mission. Preliminary studies have determined that a safe alternate trajectory would have the spacecraft flyby the Pluto much closer to Charon – the small moon has enough gravitational influence to have cleared its immediate orbit of debris, creating a safe zone.

The question of whether the Pluto system could be a death trap for New Horizons is still open. The team will continue to study the potential dangers over the next year with everything from computer models to big ground-based telescopes to the Hubble. But we all might have to wait until 2015 to see what's really out there.