NASA's New Horizons Pluto spacecraft is still exploring, 50 AU from the sun

New Horizons is about to reach some very rarefied space, but don't expect the NASA probe to rest on its considerable laurels.

On Saturday night (April 17), New Horizons will reach 50 astronomical units (AU) from the sun, a distance achieved by just four other operational probes in the history of spaceflight. (One AU is the average Earth-sun distance — about 93 million miles, or 150 million kilometers.)

The milestone is an occasion to celebrate and appreciate New Horizons' epic mission, which gave humanity its first up-close looks at Pluto in July 2015 and followed that up with a flyby of Arrokoth, an even more distant world, three and a half years later.

Destination Pluto: NASA's New Horizons mission in pictures

"When I stop from all the day-to-day hubbub of planning and managing and data analysis and budgets and all those things, just stop and think what we've accomplished as a team, it's really inspiring," New Horizons principal investigator Alan Stern told Space.com. "Sometimes I want to pinch myself."

But there's plenty of reason to look ahead as well as back, because New Horizons is far from done. Though it's been streaking through space for 15 years, the probe remains in perfect health, Stern said, and it could continue to study its exotic environs for many years to come.

"We have power and fuel to go on into the late 2030s," said Stern, who's based at the Southwest Research Institute in Boulder, Colorado. "So, we're kind of halfway into this mission, in terms of what's possible from an engineering standpoint."

New Horizons is powered by a radioisotope thermoelectric generator (RTG), which produces electricity from the heat emitted by the radioactive decay of plutonium-238. RTGs have powered most other NASA deep-space probes as well, including the four that crossed the 50-AU threshold before New Horizons did — Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2.

Pioneer 10 and 11 ceased operations years ago, but the two Voyagers remain active today, more than 40 years after launch. Both are exploring interstellar space: Voyager 1 is currently about 152 AU from Earth, and Voyager 2 is nearly 127 AU from us.

A very long journey

New Horizons' journey has been three decades in the making, and it's full of twists and turns. Stern and his colleagues began developing a Pluto project in the late 1980s, but the $720 million mission didn't gain official approval until the early 2000s. (For much more about the mission's tortuous history, read Stern and planetary scientist David Grinspoon's 2018 book, "Chasing New Horizons.")

New Horizons launched in January 2006, tasked with performing the first-ever flyby of Pluto. The distant dwarf planet had been mysterious since Clyde Tombaugh discovered it in 1930, appearing as but a fuzzy blob in even the best photos NASA's Hubble Space Telescope could muster.

New Horizons aced its highly anticipated flyby on July 14, 2015, zooming within 7,800 miles (12,550 km) of Pluto's frigid surface. The observations made by the probe during this close encounter transformed Pluto from that fuzzy blob into a real place — and a stunningly diverse and interesting place at that, featuring towering water-ice mountains, bizarre "bladed terrain" and a giant nitrogen-ice plain that makes up one lobe of a now-famous "heart."

After the flyby, New Horizons continued to collect data about its surroundings, the ring of widely spaced, frigid bodies beyond Neptune's orbit known as the Kuiper Belt. The probe studied its local environment, observed a number of Kuiper Belt objects (KBOs) from a distance and, on Jan. 1, 2019, performed its second close flyby, this time of a small KBO.

During that New Year's Day encounter, New Horizons zoomed within a mere 2,200 miles (3,540 km) of Arrokoth, which was about 1 billion miles (1.6 billion km) beyond Pluto's orbit at the time. The returns of this close encounter, the centerpiece of the probe's ongoing extended mission, were perhaps even more surprising than the Pluto data: The 22-mile-wide (36 km) Arrokoth looks like a flattened, reddish space snowman, with two distinct lobes.

New Horizons' observations show that Arrokoth is a pristine and primordial object, a planetary building block left over from the solar system's very early days. And its two lobes were likely once distinct objects, which came together in a gentle merger, mission team members have said.

"Both of our main targets turned out to be scientific wonderlands — beyond our wildest expectations in both cases," Stern said.

Related: New Horizons' Arrokoth flyby in pictures

Looking for flyby target number three

The New Horizons team has already begun searching for another KBO along the spacecraft's path, using photos captured by powerful instruments such as the Subaru Telescope in Hawaii. Stern stressed that a third flyby is a longshot, given how thinly populated the Kuiper Belt is, but he and his colleagues are doing all they can to boost their odds.

For example, mission team members J.J. Kavelaars and Wes Patrick recently began applying machine-learning techniques to the hunt for KBOs to study, both from a distance and up close.

When the duo "reran the 2020 search data through their new software tools, it not only worked 100 times faster, but it turned up dozens of new KBOs that human searchers had not found in the search images!" Stern wrote in a mission update last month. "We'll be taking advantage of this important new tool again later this year, and next year and after that as well."

Even if no suitable flyby target turns up, New Horizons will have plenty to do over the coming months and years. The probe has already eyed nearly 30 KBOs to date from afar, Stern said, and will study three more next month if all goes according to plan.

The May campaign will be "another brick in the wall of building up a statistically relevant collection of KBOs that we have studied in ways that you cannot do except by being in the Kuiper Belt, either by dint of the close range or as a result of the different angles that we see things at," Stern said. "We're building up this database. It's a legacy."

New Horizons will do other work as well. It will continue gathering data about Uranus and Neptune, for example, and keep characterizing its Kuiper Belt environment, a realm that very few probes have explored to date. And, provided it stays healthy and NASA continues to approve mission extensions, New Horizons will teach scientists about the realms beyond the Kuiper Belt, the outer edge of which is thought to lie about 70 AU from the sun.

New Horizons will reach that boundary in the late 2020s, Stern said. The spacecraft will likely be able to get to 100 AU or so by the time its power runs out in the late 2030s, further cementing its spot in exploration history. 

"We said we would build a spacecraft that could fly across the solar system and explore new worlds," Stern said. "And we did that, and we're still doing that. But when I utter the words — they sound like science fiction, but they're not."

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.