When the New Horizons spacecraft launched toward Pluto in 2006, no one knew that the tagline “the first mission to the last planet” would be short-lived. Except for me. Two weeks earlier I had discovered Eris, an icy body 30 percent more massive than Pluto. It was clear my discovery would end Pluto’s then 76-year reign as the most distant planet in our solar system. Either Eris would join its ranks, or Pluto would lose its planetary status.
One thing everyone agreed on: how very little we knew about Pluto. It’s too tiny and far away to see in more than the fuzziest of detail from here on Earth. New Horizons was designed to bring clarity. For the past nine years it’s been speeding across the 3 billion miles between Earth and Pluto, and later this month it will finally reach its target. The spacecraft carries infrared and ultraviolet cameras and high-energy particle collectors, among other tools. As it sails past at 31,000 mph, it will capture images of Pluto and its moon, Charon, in more detail than anyone has ever seen.
For the guy whose discoveries led to Pluto’s demotion, the flyby might not seem so exciting anymore. But that couldn’t be further from the truth. New Horizons is now about way more than just Pluto. It is poised to help us understand a whole new region of the solar system.
Here’s why. When the mission was first conceived, we astronomers thought of Pluto as a bit of an anomaly. It is tiny, for one, with an icy, rocky surface—a stark contrast to the giant, gaseous planets that orbit nearer to the sun. But since the launch we’ve learned much more about Pluto and its environs. And rather than the exception, it seems to be the rule.
Pluto lies at the inner edge of the Kuiper belt, a vast region beyond Neptune. The belt contains hundreds of thousands of 100-kilometer-across bodies, and many more that are smaller. Each has its own quirks. For example, Eris—still the most massive known body in the belt—has an atmosphere that alternately freezes onto the surface and re-evaporates over the course of the 558-year orbit. Football-shaped Haumea spins faster than any other large body in the solar system. Its whirling is the result of an ancient collision that flung icy debris across the solar system’s outermost reaches. Makemake has a surface like a chemical factory: It slowly changes slabs of methane ice into a stew of exotic ices, some of which had never been seen anywhere outside of laboratories on Earth. And there are more, with names like Quaoar and Orcus and Sedna and Snow White (don’t ask).
I spotted each of these oddballs. And each added to the body of evidence that Pluto shouldn’t count as a planet but rather should be dubbed a dwarf planet. It shares an orbit with its Kuiper belt neighbors, unlike the other things we call planets, which orbit solo. Classifying objects properly is a critical first step to understanding how they work and how they formed in the first place. But not everyone agreed with Pluto’s new status, especially in the broader public. To this day I get mail from people asking if Pluto could be grandfathered in to the planet pantheon. (Answer: no.) There’s definitely a lot of nostalgia out there.
I get it. I’ve had a picture in my head since I was in grade school of what the surface of Pluto looks like. On my Pluto, icy spires cast ominous shadows across a dimly lit landscape. You could topple one with a single touch. The sun is so far away, you could blot it out with the tiny cracked shards littering the ground. But after this year, my Pluto will be gone, replaced by images of the real thing. And I’m OK with that. Because New Horizons doesn’t signal an end but rather a beginning.
The Pluto of my childhood imagination will be gone, replaced by images of the real thing. And I’m OK with that.
As the spacecraft approaches, early images show Pluto and Charon coming into focus as little spheres. We can see the first hints of surface features. Four tiny moons, apart from Charon, have emerged from the planet’s glare. As the craft gets even closer, will we discover previously unknown moons? I’d guess yes, but it might take a while to know: Though the New Horizons flyby will be over in an instant, the spacecraft speeding on toward interstellar space, the images will dribble back over many months.
For the most part, those images should confirm what we’ve deduced from the fuzzy pictures taken by ground-based and Earth-orbiting telescopes. Pluto should be relatively smooth, with bright concentrations of frozen nitrogen at the poles and darker regions along the equator. There, methane will have frozen onto the surface and begun decomposing into a deep-red tarlike substance. Charon should look completely different: old and heavily cratered. The ices that move across Pluto’s surface, smoothing it out, should have evaporated to space on Charon, thanks to the moon’s small size and weak gravity. I suspect we will also see hints of something more going on at Charon’s surface, perhaps some regions where ancient volcanic-water eruptions flowed across the moon long after the rest of the surface had been battered.
As a scientist, it’s great to have findings confirmed. But it’s much more exciting when new questions arise. And New Horizons will raise a host. Do frosty geysers dot the surface of an object like Pluto? Can such a tiny atmosphere have weather? Did Charon really have ice volcanoes in the past, and if so, what exactly is an ice volcano, and how does it work?
How many meteors have hit these bodies in the 4.5 billion years since they formed? There are many questions to answer, but the most interesting ones will be those we have not yet thought to ask.
Ultimately, New Horizons will help us learn how our solar system came to be. That’s partly because the ancient surfaces of the Kuiper belt bear the scars of their history far longer than gas planets such as Neptune do. So I’d like to propose a new tagline: “The first mission to the last frontier.”
Mike Brown is a planetary astronomer at the California Institute of Technology and the author of the book How I Killed Pluto and Why It Had It Coming.
This article originally appeared in the July 2015 issue of Popular Science.
