We began to understand now, five years ago, just how unbelievable Pluto really is.

Since its discovery in 1930, the distant dwarf planet had been a frigid enigma, remaining a flippant blob even in images captured by the mighty Hubble Space Telescope. But all changed when NASA's New Horizons spacecraft zoomed within 7,800 miles (12,550 kilometers) of Pluto's icy surface on July 14, 2015.

The historic flyby completed the initial identification of the nine historically recognized planets of the solar system. It unveiled incredible complexity and terrain variety, from nitrogen glaciers to rock-hard water ice towering mountains.

"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."

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High drama in the home stretch

The $720 million New Horizons spacecraft launched in January 2006, heading off Earth at a 36,400 mph (58,580 km / h) record-breaking point.

But at that blistering speed, reaching Pluto, which was about 3 billion miles (5 billion km) from Earth on the day of the flyby, took the probe 9.5 years. And New Horizons suffered a glitch in the home stretch of that deep-space trek which threatened to scuttle the epic encounter entirely.

On July 4, 2015, the spacecraft went dark for 90 minutes, sending out scrambling task team members. But they were up to the challenge, diagnosing and solving the problem in short order - an overloaded main machine that tried to do two major things at once.

This high-pressure troubleshooting was far from routine, stressed Stern. He also commended the mission-operations team's skill, planning, and commitment.

"We nearly lost this thing on July 4," he said. Stern said it would probably have been too late to save the flyby unless the same glitch had cropped up just two days later.

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Not done yet

The Pluto flyby data are still being studied by scientists worldwide, and they will continue to do so for years to come.

"We were surprised by how much we were surprised by," Singer said.

Researchers are also still poring over information from the second close encounter of New Horizons - a flyby of the Arrokoth object, 22-mile-long (35 km), conducted during the ongoing extended mission of the probe.

The Arrokoth encounter occurred on January 1, 2019, when New Horizons was about 1 billion miles (1.6 billion km) outside the orbit of Pluto. Observations of the spacecraft revealed that Arrokoth looks like a flattened, reddish snowman. The odd object was formed through the very gentle fusion of two primordial bodies.

New Horizon's mission pioneered discovering the far outer solar system, showing just how fascinating this deep, cold world is. And it proved Pluto worthy of more than a mere brief glance, Stern and Singer stressed. Researchers collaborated with other researchers on a project design that would orbit the dwarf planet and possibly explore other bodies in the Kuiper Belt up close.

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