A Wake-up Call for Solar System Studies
The wake-up call to the New Horizons spacecraft this week marks a major milestone in our study of the solar system — particularly the outer solar system. No other spacecraft has been sent specifically to study Pluto and the worlds that lay out in the Kuiper Belt (which stretches out beyond the orbit of Neptune) and beyond. New Horizons is, for now, the best hope we have of finding out what this region is really like.
New Horizons is well equipped to study this part of the solar system. It has instruments designed to image the worlds it visits, sense the plasma environment (due largely to the solar wind), take the temperature of space and the surfaces of its targets, probe the chemical composition of those surfaces, capture information about high-energy particles, and study dust particles in the Kuiper Belt environment. It is on track to fly past Pluto and its retinue of moons on July 14, 2015.
After that, its trajectory will take it past one or two other worlds recently discovered by Hubble Space Telescope, before heading out on a long trek to interstellar space. If all goes well, they will be encountered within the next decade.
The Pluto Adventure Begins
Pluto and its five moons are, of course, the first and most anxiously awaited port of call for New Horizons. Once the spacecraft is fully awake and all its systems tested out, it will start taking “approach” data and observations as it nears the Pluto system. The high-resolution images will be spectacular, finally giving us a really clear look at this distant place.
Pluto is an interesting world in its own right. It’s a dwarf planet, meaning that it’s smaller than all the other ones (although not by much) and it hasn’t yet (according to the goofy definition adopted by the IAU) cleared its orbit of other objects. It has a retinue of moons, the largest of which is Charon. It appears that Charon may have some active cryo-geysers (as seen by observations made at Gemini Observatory) spouting water to the surface. Charon itself is largely an icy world.
Pluto, on the other hand, is more rocky and has a nitrogen ice surface that varies in appearance from dark black to orange-ish and even some white spots. It has an atmosphere made of nitrogen, methane, and carbon monoxide. It will freeze out and fall to the surface in a decade or so as Pluto gets farther from the Sun in its orbit. New Horizons will arrive in time to study that atmosphere. Pluto is definitely an interesting and challenging dwarf planet to understand!
What Lies Out There?
Recent discoveries have uncovered the existence of 11 KNOWN dwarf planets (of which Pluto is one). Five of them have been confirmed as dwarfs: Pluto, Ceres, Haumea, Makemake, and Eris. The others are still being observed and await confirmation. AND, there are other objects in the outer reaches of our solar system that could well turn out to be worlds in their own rights, too. The trick is to find them and do long-term studies of them.
Look at it this way: we have four rocky worlds (Mercury, Venus, Earth, Mars); two gas giants (Jupiter and Saturn), and two gas/ice giants (Uranus and Neptune). That’s eight “regular” planets and a whole lot of dwarf planets to be explored.
Beyond the much-maligned definition of “planet” that will likely change as we better understand the worlds of the solar system, Pluto presents an interesting challenge to us: what do we do when we find out that the outer solar system has many MORE of the Pluto-type of planet than all the other types of planets? At that point, our solar system will be so much more complex than we ever imagined, richer in worlds than we expected, and beckoning to us to send MORE explorers out there to study the most primordial bits of solar system real estate in existence.
By primordial, I mean that these objects, plus the countless cometary nuclei that exist in the Kuiper Belt and the Oort Cloud (a shell of icy nuclei that envelopes our solar system) date back to the earliest formation of the Sun and planets. Analyze THESE bits of solar system history in their home environment and you know a great deal about what it was like back 4.5 billion years ago!
So, New Horizons’ mission is a long shot, both historically and figuratively. It will tell us more about the conditions in the part of the solar system it has now entered. Temperatures are vastly colder than in regions closer to the Sun, which means that the worlds out there are much more likely to be frozen over.
They may have solid rocky cores (or not), but they will be coated with frozen volatiles — the ices made when water, methane, nitrogen and other gases freeze out and stick to their surfaces. We know from simple chemistry what temperatures are required for these and other compounds to freeze out. We also know that the reddish surfaces of some worlds “out there” come from complex photochemical reactions between surface compounds and solar (and other) radiation that pounds their surfaces. Far from being shimmering worlds of ice, these places are going to show us their true colors, plus the effects of whatever bombardments and collisions they have suffered since they formed in the early epochs of solar system history. By the time New Horizons finishes its mission, our view of the outer solar system will be forever changed.
Practically speaking, the end of the New Horizons mission will be whenever it stops functioning. It is built well enough that it could possibly last for several decades. The mission timeline puts it at 100 AU from the Sun in 2038, and if it is still functioning, it will tell us about the space in the outer parts of the Sun’s heliosphere. While it may soar in silence for centuries, the lessons it teaches us in the next few years will significantly enlarge our understanding of our corner of the galaxy.