Pluto may have lost its status as a planet, but right now it's the hottest thing in the solar system besides the sun. With the recent discovery of a fifth moon orbiting Pluto, scientists are rethinking the New Horizons spacecraft's approach to the area, which is scheduled to take place in 2015. As astronomers train their eyes on the region around Pluto and the Kuiper Belt looking for debris that could harm the spacecraft, it is becoming increasingly clear there may be more obstacles than first thought.
The as-yet-unnamed fifth moon, called simply P5 for now, is about 6 to 15 miles wide. This comes after the discovery last year of a fourth moon, called P4, which is only slightly larger—about 8 to 21 miles wide. And there may be more.
AAAS member Alan Stern, Associate Vice President of Research and Development in the Space Science and Engineering Division at the Southwest Research Institute in Boulder, Colorado, spoke with AAASMC about the discovery and about the New Horizons mission.
AAASMC: The new moon was discovered as scientists were looking for objects in the Kuiper Belt that may pose a threat to the New Horizons spacecraft when it eventually gets to that point. Was the discovery of another moon completely unexpected, or did we suspect there may have been more than we already knew about? What are the chances there are even more moons to be discovered?
Alan Stern: We suspected in the last year that there was a significant possibility of more moons, so it was not unexpected. I expect there is a good chance that we will find more in the coming months and years.
AAASMC: What does this mean for the New Horizons mission?
Stern: It means that our concerns about hazards to the spacecraft during the flyby are increasing.
AAASMC: What are the chances the New Horizons craft will collide with an object in the Kuiper Belt when it gets to that point? How "nimble" is it in terms of our changing its trajectory if needed?
Stern: It's not likely that we would have to do that in the Kuiper Belt but it is possible, and it may be even probable, that we would have to do that at Pluto. The spacecraft is capable of changing its trajectory even days before we arrive.
AAASMC: I understand the New Horizons craft is headed toward Neptune's orbit and will reach Pluto's in 2015. What have we learned from the spacecraft so far?
Stern: We had a Jupiter flyby in 2007 and there have been several dozen scientific publications out of that flyby, including the cover of Science. That flyby was not a science flyby, it was specifically to accelerate our trip to Pluto by about three and a half years by using the planet's gravity. But we used the flyby to conduct scientific experiments since we were there.
In addition, we have been studying the interplanetary medium with more scientific instruments—the kinds of things that Voyager was doing but with much more modern instrumentation and some new capability like a dust detector, as we traverse the region from Jupiter out to Pluto. There have been publications resulting from those studies as well, telling us about the particle and plasma populations, about the dust environment in the deep middle solar system and about the variation in heliosphere helium with distance from the Sun.
AAASMC: What do you expect to find, or hope to find, when New Horizons reaches Pluto in three years?
Stern: As you know, our spacecraft is the first to reach this new class of planet called the "ice dwarfs." We have been surprised again and again in planetary science: the first missions to Mars discovered river valleys, and no one expected that; Venus was discovered by the first missions fifty years ago to have a thick, hot atmosphere, and that was not expected; we found volcanoes in the outer solar system the first time we looked at the moons of Jupiter; all of this is to say, we're going to a completely new class of world, and we don't know what we will find.
It's the job of New Horizons to make a basic reconnaissance of Pluto and its satellites, using cameras and spectrometers and other instruments, from which questions will emerge and of course much new knowledge. We don't have a list of specific scientific questions; we're going there purely to explore.
For later missions, if we go back to Pluto, we'll probably go to answer specific questions. But this is first-time reconnaissance like in the early days of the space program. We're not there to fill in the blanks; we're there to write the textbook to begin with.
We're fifty years into planetary exploration; it's a completely unprecedented opportunity in my generation to go to a new class of planet for the first time to uncover what is essentially completely unknown.
AAASMC: After studying Pluto and its moons, what will be the goal of the New Horizons craft? Will it follow Pioneer 10 out into space? What is the end of its mission?
Stern: We hope NASA will extend the mission to make flybys of small Kuiper belt objects and we are currently searching for targets to fly by. After we leave the Kuiper Belt, in the mid 2020's, there is a second extended mission possible, to study the heliosphere as Voyager and Pioneer have done but which we would study with this very modern instrumentation that is so much more capable and sensitive.
There will be an end, and the end will come either due to some malfunction—hopefully not—in some distant year, or we will run out of fuel, or we will run out of power. When the first of those things happens, the mission will end. From the standpoint of planning, the most likely scenario is that if nothing breaks, we will lose power before fuel, and that would occur in the late 2030s, when the spacecraft is about a hundred times as far from the sun as the Earth is and three times as far as Pluto.