Early this morning, if all has gone well, the first golden age of interplanetary exploration will have come to a close. At 7:49 Eastern time, NASA’s New Horizons spacecraft was slated to reach its primary target, Pluto and its moons, concluding what some call the preliminary reconnaissance of the known solar system.
Though it was conceived in the late 1980s, New Horizons wasn’t launched until 2006, after long years of delays, redesigns, and even near-death cancellations. Its unlikely five-billion-kilometer voyage to Pluto has been the work of decades. And yet today, at the climax of its mission, the spacecraft was expected to traverse the expanse of Pluto in less than three minutes, whizzing 12,500 kilometers above the surface at nearly 50,000 kilometers per hour. From the start, the spacecraft was custom-built for speed. Carrying enough fuel to crash into orbit at Pluto would have made New Horizons too bulky, expensive, and slow to even launch in the first place, so instead it will flyby and continue outward, on an endless journey into interstellar night.
During its brief close encounter, New Horizons will be too busy gathering data to immediately phone home, instead using those precious moments to scrutinize the planet with a suite of seven instruments all running on a nightlight’s share of electricity. Mission planners will only learn of the flyby’s success later tonight, via a radio signal. Or, they could learn of its failure by hearing nothing – the spacecraft has an estimated 1 in 10,000 chance of suffering a destructive high-speed collision with debris while passing through the Plutonian system.
Presuming New Horizons’ flyby is successful, its confirmatory signal traveling sunward at the speed of light will reach Earth some 4.5 hours after being transmitted, and is expected to arrive at 8:53 pm. Though the primary encounter is best measured in minutes and hours, the slow data-transmission rate imposed by such vast distances ensures that New Horizons will be beaming its archived images home well into 2017.
For a feat so audacious and triumphant, much of the coverage for New Horizons’ imminent encounter has taken on an elegiac, somewhat pessimistic tone. Among most knowledgeable observers, it seems, the prevailing view is that this mission marks the end of an era in more ways than one: New Horizons marks the end not only of our interplanetary reconnaissance, but also of any possibility of approaching or surpassing its exploratory grandeur ever again. In the New York Times, the science writer Dennis Overbye went as far as predicting that “none of us alive today will see a new planet up close for the first time again,” noting that New Horizons’ lead scientist Alan Stern had called his mission “the last picture show.”
Perhaps they are right, and New Horizons’ success may bring the curtain down for exploring the far outer reaches of the solar system, though I know Overbye, Stern, and most others with similar opinions would love to be proved wrong. There is as yet neither a compelling motive nor a pathway for the private sector to undertake such voyages, and little reason to believe this will change anytime soon. For governments, ongoing flat (or downward) trends in science budgets paired with the rising costs of epoch-making missions offer a dismal forecast for further ambitious wanderings out in the final frontier. And, yes, Pluto is indeed the last unexplored world of the solar system – but only if you take a rather restrictive view of what a “world” is, and what constitutes exploring it. Sometimes, new views can make you see even a familiar thing as if for the first time. New Horizons’ voyage to Pluto is not really the end; instead, it’s just the end of a beginning.
The astronomer Clyde Tombaugh discovered Pluto in 1930, spying it as a small dot moving against background stars in a series of photographic plates. It would remain scarcely more than a dot for most of the intervening 85 years. Yet it could be said that low-resolution images from ground-based observatories and the Hubble Space Telescope have explored Pluto and its largest moon, Charon, for more than a decade now. How deeply do you have to look to call something “explored?” Consider that even after spacecraft flybys, we still scarcely know much at all about the planets Uranus and Neptune, or, for that matter, the nature of what lurks within the subsurface oceans of Europa, Enceladus, and other icy moons. The outer solar system remains almost entirely unexplored – for now – despite our pretensions otherwise.
For Pluto, respectable pictures of surface features on both bodies really only emerged in recent weeks as New Horizons drew closer and the alien worlds swam into view. Those newer images have already revealed details begging further study. Pluto bears a bright polar cap of methane and nitrogen ice, and mottled regions at its equator that signal strange and complex geology. Charon, by contrast, harbors a mysteriously dark polar region apparently bereft of bright ice, and an impact-generated chasm deeper and longer than Earth’s own Grand Canyon. More and better images will soon stream down from New Horizons’ far-distant memory banks, no doubt filled with even greater wonders – perhaps signs of ice volcanoes, or of ancient frozen seas, or of things so strange and unexpected they cannot yet be imagined. The only thing unimaginable is that they will contain nothing tantalizing enough to someday call us back.
After Pluto, New Horizons’ mission will not be over. Its trajectory will take it deep into the unexplored frontier of the Kuiper belt, a sprawling realm sparsely populated with millions of icy leftovers left in deep freeze ever since our solar system’s infancy. Studying those leftovers, scientists hope to unlock the secrets of how the Sun and its retinue of worlds formed in the first place. In coming years, New Horizons will visit one or more small Kuiper belt objects, each tens of kilometers in size, though some Kuiper belt objects are at least as big as Pluto, which is three-quarters the size of Earth’s moon.
The profusion of Pluto-sized bodies tumbling in the outer dark has been both a blessing and a curse for New Horizons: shortly after the mission launched, all those newfound worlds spurred astronomers to reclassify Pluto, demoting it from full planethood to its current “dwarf planet” status. One object in particular, dubbed “Eris” by its discoverer, the Caltech astronomer Mike Brown, was crucial to the case against Pluto. Eris orbits three times further out than Pluto does, and was thought to be even larger. If Pluto was a planet, why not Eris and all its scattered cousins, too? Rather than admit dozens—hundreds, even—of new planets to the solar system, astronomers chose to trim the classical count by one, giving us eight planets and many, many also-rans. Even so, those also-rans still remain full-fledged worlds, no matter what they’re called, and we know hardly anything about any of them.
Surprisingly, New Horizons’ latest, best measurements of Pluto’s size, announced at a press conference yesterday, reveal that Pluto is in fact a bit larger than previously believed. The new measurements confirm it as the reigning king of the Kuiper belt, outsizing Eris by about 30 kilometers. But even though it is larger than Eris, other measurements have shown Eris to be much more massive – the two bodies are similar in size, but very different in composition, for reasons unknown.
How could we solve the mystery of Eris, and learn more about the deep origins of our solar system? “Maybe we should go,” Brown says.
Even further out, far beyond Pluto and Eris, a greater mystery awaits. Before Brown discovered Eris, he found a weirder world, called Sedna, in a wild orbit past the outer reaches of the Kuiper belt, stretching at its farthest extent more than 30 times further from the Sun than Neptune. How did Sedna get all the way out there? No one yet knows for certain, but the most probable answer seems to be that gravitational nudges from something very big and very far out had something to do with it.
Sedna and other new-discovered objects like it make Brown and many other astronomers suspect at least one more bona fide planet, several times the size of Earth, lurks undiscovered in the Sun’s dark hinterlands. New Horizons lacks the instrumentation or the fuel to detect or visit such a faint, far-off world, but soon-to-debut ground- and space-based observatories should be up to the task of finding it. And if, in fact, there proves to be a large, mysterious planet far beyond Neptune, it’s hard to imagine we wouldn’t someday at least try to pay it a visit.
Is that something you and I could live to see? Maybe not. But stranger things have happened – like the New Horizons mission to Pluto itself, the end to a beginning.