Five years ago today, we began to appreciate how remarkable Pluto really is.
The distant dwarf planet had been a frigid puzzle since its discovery in 1930, and remained a blurry drop even in photos captured by the powerful Hubble Space Telescope. But everything changed on July 14, 2015, when NASA New Horizons spaceship approached 7,800 miles (12,550 km) from Pluto’s icy surface.
The historic flyby completed the initial survey of the nine traditionally recognized planets in the solar system and revealed surprising complexity and terrain diversity, from nitrogen glaciers to towering mountains of rock-hard water ice. (“Traditionally recognized” is a qualifier required here, because the International Astronomical Union “demoted” Pluto to the dwarf planet state in 2006, a decision that remains controversial to this day.)
“It’s an amazing world,” New Horizons principal investigator Alan Stern of the Southwest Research Institute (SwRI) in Boulder, Colorado told Space.com. “He even has a heart! Hollywood couldn’t have planned it better.”
Pluto flyby photos: New Horizons leader Alan Stern reveals 10 favorite views
Great drama in the final stretch
The $ 720 million New Horizons mission launched in January 2006, moving away from Earth at a record speed of 36,400 mph (58,580 km / h).
Even at that breakneck pace, it took 9.5 years for the probe to reach Pluto, which was about 3 billion miles (5 billion km) from Earth on the day of the flyby. And in the final stretch of that deep space walk, New Horizons suffered a failure that threatened to sink the epic encounter entirely.
The spacecraft darkened for 90 minutes on July 4, 2015, sending mission team members to fight. But they were up to the challenge, before long they diagnosed and fixed the problem: an overloaded mainframe computer trying to do two great things at once.
This high-pressure problem solving was far from routine, Stern emphasized, praising the mission operations team’s talent, preparation and dedication.
“We almost lost this on July 4,” he said. If the same problem had come up just two days later, he added, it probably would have been too late to save the flyby.
Related: Destiny Pluto: NASA’s New Horizons mission in pictures
A surprisingly complex and active world.
Just three days after the failure, New Horizons photographed a dazzling sight – a huge heart-shaped feature on Pluto’s reddish surface. The now iconic heart of Pluto it became increasingly sharp in focus in the days that followed, as did the rest of the dwarf planet’s “meeting hemisphere” (the side New Horizons flew over).
And then came the closest approach. On July 14, New Horizons passed Pluto, photographing and studying an amazing diversity of terrain.
For example, the heart, now known as Tombaugh Regius, after the discoverer of Pluto. Clyde Tombaugh – It is bordered in places by mountains 2 miles high (3 km) made not of rock but of water ice. Elsewhere on the dwarf planet, methane ice has eroded into strange and unique “ground with blades“New Horizons also saw huge structures that appear to be cryovolcanoes, the largest of which is approximately 4.3 miles high and 155 miles wide (7 by 250 kilometers).
All of these dramatic landscapes and more are rubbing shoulders in a world only 1,477 miles (2,377 km) wide.
“Pluto is like you take a bunch of national parks … and you huddle them in a little space next to each other,” Kelsi Singer, a member of the mission science team, also from SwRI, told Space.com.
But the beautiful images only scratch the surface of Pluto’s history. For example, the left lobe of Tombaugh Regio, a 600 mile wide (1,000 km) nitrogen ice plain called Sputnik Planitia, has no detectable craters. That means the region has recently resurfaced, which in turn shows that Pluto is geologically active.
Related: Pluto is beautiful, complex, and completely baffling.
That was a big surprise to many scientists, who had assumed that 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. (An AU is the average distance from Earth to the Sun, about 93 million miles, or 150 million km.) And there is no giant planet nearby to heat the bowels of the dwarf planet by stretching and flexing the tides. , as it happens with the active moons of Jupiter and Saturn. .
In fact, the energy source for Pluto’s activity remains mysterious and the subject of considerable debate. For example, some researchers think that the heat from the radioactive decay of the material in Pluto’s core may be responsible. But others, including Stern, suspect that the activity is fueled by the latent heat released by the slow, continuous freezing of Pluto’s subsurface ocean.
That’s right: New Horizons observations suggest the dwarf planet has an ocean of salty liquid water splashing below its surface. The mission data also indicates that two other crucial ingredients for life as we know it, organic molecules that contain carbon and an energy source, may also be abundant on Pluto.
“With a straight face, it can be said in 2020 that New Horizons put Pluto on the map as a world with astrobiological potential,” Stern said.
The flyby also led to many other discoveries, too many to tell in one story. For example, New Horizons photographed wonderfully blue skies as it quickly moved 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 giant impact on the system a long time ago.
Related: Photos of Pluto and its moons
Not done yet
Scientists around the world are still analyzing data from the Pluto flyby, and will continue to do so for years to come.
“We were surprised how much we were surprised,” said Singer. “There are tons of things to do.”
Investigators are also continuing to carefully study information from New Horizons’ second close encounter – a 22-mile (35 km) flyby of the Arrokoth object, conducted during the probe’s ongoing extended mission.
The Arrokoth encounter occurred on January 1, 2019, when New Horizons was approximately 1 trillion miles (1.6 trillion km) beyond Pluto’s orbit. Observations from the spacecraft revealed that Arrokoth looks like a flattened, reddish snowman, and that the strange object formed through the very smooth fusion of two primordial bodies.
Therefore, the mission has given us a close look at two very different objects in the kuiper belt, the ring of cold bodies beyond Neptune’s orbit. And the New Horizons flyby days may not be over yet.
The probe remains in good health and has an eighth of fuel tank left, the same amount required for the Arrokoth flyby, Stern said. Then the probe might be able to squeeze in a closer encounter, provided that a suitable target can be found along its flight path. The mission team recently began to seriously pursue that target using a variety of powerful telescopes.
“The numerical probabilities are long, due to the amount of fuel left,” Stern said. “If we’re lucky, we’ll have another flyby. And if we don’t, we won’t.”
The New Horizons legacy is assured in any way. The mission has pioneered the exploration of the distant outer solar system, revealing just how interesting this cold and dark realm is. And it showed that Pluto deserves more than just a quick glance, Stern and Singer stressed. They have been working, with other researchers, on a concept for a mission that would orbit the dwarf planet and possibly also explore other Kuiper Belt objects up close.
“Pluto really is such a complex world and world system that this push to get an orbiter it’s really gaining traction, “Stern said.
Mike Wall is the author of “Out There” (Grand Central Publishing, 2018; illustrated by Karl Tate), a book on the search for extraterrestrial life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.
