Five years ago today, we started to appreciate just how remarkable Pluto really is.
The distant dwarf planet had been a frigid enigma since its 1930 discovery, remaining a fuzzy blob even in photos captured by the powerful Hubble Space Telescope. But everything changed on July 14, 2015, when NASA's New Horizons spacecraft zoomed within 7,800 miles (12,550 kilometers) of Pluto's icy surface.
The historic flyby completed the initial reconnaissance of the solar system's nine traditionally recognized planets and revealed a stunning complexity and diversity of terrain, from nitrogen glaciers to towering mountains of rock-hard water ice. ("Traditionally recognized" is a required qualifier here, because the International Astronomical Union "demoted" Pluto to dwarf-planet status in 2006, a decision that remains controversial to this day.)
"It is one amazing world," New Horizons principal investigator Alan Stern, of the Southwest Research Institute (SwRI) in Boulder, Colorado, told Space.com. "It even has a heart on it! Hollywood couldn't have planned it better."
Pluto flyby photos: New Horizons leader Alan Stern reveals 10 favorite views
High drama in the home stretch
The $720 million New Horizons mission launched in January 2006, speeding away from Earth at a record-breaking 36,400 mph (58,580 km/h).
Even at that blistering pace, it still took the probe 9.5 years to reach Pluto, which was about 3 billion miles (5 billion km) from Earth on the day of the flyby. And in the home stretch of that deep-space trek, New Horizons suffered a glitch that threatened to scuttle the epic encounter entirely.
The spacecraft went dark for 90 minutes on July 4, 2015, sending mission team members scrambling. But they were up to the challenge, in short order diagnosing and fixing the problem — an overloaded main computer that was trying to do two big things at once.
This high-pressure troubleshooting was far from routine, Stern stressed, praising the talent, preparation and dedication of the mission-operations team.
"We nearly lost this thing on July 4," he said. If the same glitch had cropped up just two days later, he added, it probably would have been too late to salvage the flyby.
A shockingly complex and active world
A mere three days after the glitch, New Horizons photographed a stunning sight: a huge, heart-shaped feature on Pluto's reddish surface. Pluto's now-iconic heart came into sharper and sharper focus over the ensuing days, as did the rest of the dwarf planet's "encounter hemisphere" (the side that New Horizons flew over).
And then came closest approach. On July 14, New Horizons skimmed just over Pluto, photographing and studying a staggering diversity of terrain.
For example, the heart — now known as Tombaugh Regio, after Pluto discoverer Clyde Tombaugh — is bordered in places by 2-mile-high (3 km) mountains made not of rock but of water ice. In another part of the dwarf planet, methane ice has eroded into bizarre and unique "bladed terrain." New Horizons also saw huge structures that appear to be cryovolcanoes, the largest of which is about 4.3 miles tall and 155 miles wide (7 by 250 kilometers).
All of these dramatic landscapes and more are rubbing shoulders on a world just 1,477 miles (2,377 km) wide.
"Pluto's kind of like if you took a whole bunch of national parks … and you crammed them all in a small space right next to each other," mission science team member Kelsi Singer, also of SwRI, told Space.com.
But the pretty pictures just scratch the surface of Pluto's story. For example, the left lobe of Tombaugh Regio, a 600-mile-wide (1,000 km) plain of nitrogen ice called Sputnik Planitia, sports no detectable craters. That means the region has been resurfaced very recently, which in turn shows that Pluto is geologically active.
That came as a big surprise to many scientists, who had assumed the dwarf planet is dead. Pluto is incredibly far from the sun, after all, orbiting 39.5 astronomical units (AU) from our star on average. (One AU is the average Earth-sun distance, about 93 million miles, or 150 million km.) And there's no giant planet nearby to heat the dwarf planet's innards via tidal stretching and flexing, as happens with the active moons of Jupiter and Saturn.
Indeed, the power source for Pluto's activity remains mysterious and the subject of considerable debate. For example, some researchers think heat from the radioactive decay of material in Pluto's core may be responsible. But others, including Stern, suspect the activity is driven by latent heat released by the slow, ongoing freezing of Pluto's subsurface ocean.
That's right: New Horizons' observations suggest that the dwarf planet has an ocean of salty liquid water sloshing beneath its surface. Mission data also indicate that two other ingredients crucial for life as we know it — carbon-containing organic molecules and an energy source — may be abundant on Pluto as well.
"With a straight face, you can say in 2020 that New Horizons put Pluto on the map as a world with astrobiological potential," Stern said.
The flyby led to many other discoveries as well, far too many to recount in one story. For example, New Horizons photographed gorgeously blue skies as it sped away from Pluto after the close encounter. And the probe's observations support the theory that Charon and Pluto's other four moons were formed by a long-ago giant impact in the system.
Related: Photos of Pluto and its moons
Not done yet
Scientists around the world are still analyzing data from the Pluto flyby, and they'll continue to do so for years to come.
"We were surprised by how much we were surprised by," Singer said. "There's tons of stuff left to be done."
Researchers are also still poring over information from New Horizons' second close encounter — a flyby of the 22-mile-long (35 km) object Arrokoth, conducted during the probe's ongoing extended mission.
The Arrokoth encounter occurred on Jan. 1, 2019, when New Horizons was about 1 billion miles (1.6 billion km) beyond the Pluto flyby location. The spacecraft's observations revealed that Arrokoth looks like a flattened, reddish snowman, and that the odd object formed via the very gentle merger of two primordial bodies.
The mission has therefore given us up-close looks at two very different objects in the Kuiper Belt, the ring of frigid bodies beyond Neptune's orbit. And New Horizons' flyby days may not be over just yet.
The probe remains in good health, and it has one-eighth of a tank of fuel left — the same amount that was required for the Arrokoth flyby, Stern said. So the probe might be able to squeeze in one more close encounter, provided a suitable target can be found along its flight path. The mission team recently began hunting in earnest for such a target using a variety of powerful telescopes.
"The numerical odds are long, because of the amount of fuel that's left," Stern said. "If we get lucky, then we'll have another flyby. And if we don't, we won't."
New Horizons' legacy is assured either way. The mission has pioneered the exploration of the far outer solar system, revealing just how interesting this cold, dark realm is. And it showed Pluto to be deserving of more than a mere fleeting look, stressed Stern and Singer. They've been working, with other researchers, on a concept for a mission that would orbit the dwarf planet and potentially explore other Kuiper Belt objects up close as well.
"Pluto really is such a complex world, and system of worlds, that this push to get an orbiter is really gaining traction," Stern said.
Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.