Space

NASA spacecraft completes farthest flyby in history. What's next?

Scientists at last have their first good look at a primordial piece of the solar system orbiting more than four billion miles away.Wednesday, January 2, 2019

By Michael Greshko
NASA's New Horizons spacecraft encounters 2014 MU69, a small world also nicknamed Ultima Thule, in an illustration of the historic flyby.

On the final evening of 2018, the biggest New Year's Eve party in the solar system unfolded across four billion miles of space. At 12:33 a.m. ET on January 1, NASA's New Horizons probe flew by a small lump of rock and ice called 2014 MU69. Also nicknamed Ultima Thule (UL-tee-ma TOO-le), it's now the most distant, and most primitive, place ever visited by humans.

Due to the vast distance messages must travel between Earth and the spacecraft, members of the New Horizons team didn't immediately know for sure whether the flyby was successful. But just after 10:30 a.m., as a plush of the Disney dog Pluto watched over them, the confirmation signal came in, and the teams in charge of each New Horizons subsystem started reporting back.

In the crowd gathered outside mission control, cheers went out when one scientist announced “the SSR pointers are right where we predicted”—meaning that New Horizons had collected the data it was meant to.

“We have a healthy spacecraft. We've just accomplished the most distant flyby,” mission operations manager Alice Bowman said with a smile.

Ten hours before, New Horizons had rung in the new year by taking as many pictures of MU69 as it could as it zoomed by at ten miles a second. Back on Earth, hundreds of enthusiasts, journalists, academics, and engineers descended on the Kossiakoff Centre at the Johns Hopkins University Applied Physics Laboratory in Maryland to celebrate the distant feat—and, nearly as an afterthought, the arrival of 2019.

“This has to be the nerdiest New Year’s Eve I could imagine,” one attendee quipped as the crowd filed in that afternoon.

It was a raucous scene. Children in astronaut costumes bounded through the conference centre, while others wore “Happy New Year” hats emblazoned with the New Horizons logo. The throngs included New Horizons contributing scientist and Queen guitarist Brian May, who debuted a song he wrote about the mission. Forbidden from speaking in their official capacities because of an ongoing government shutdown, leading NASA officials also attended on their own time.

“I'm just a planetary scientist, and in a planetary event, wild horses can't drag you away,” says Jim Green, the head of NASA's planetary science division. “It's all about discovery here; it's all about something that's never been seen before.”

As attendees filed back into the conference centre the next morning, anticipation filled the air. Had New Horizons pulled it off? Once Bowman gave word that the spacecraft was healthy, the crowd erupted and, one by one, rose to a standing ovation.

“It was this feeling like, gosh, we already celebrated, but we really haven't gotten that signal back yet,” Bowman said at a press briefing after the flyby. “I was probably a little more nervous this time—but we did it again.”

We will rock you

New Horizons launched toward the solar system's outskirts on January 19, 2006, building up enough speed to soar past Jupiter in just 13 months. Even with a boost from the mighty planet's gravity, it took more than two billion more miles for the craft to get to Pluto. But at last, on July 14, 2015, New Horizons revealed the king of the Kuiper belt and its retinue of moons in spectacular fashion. (Mysterious Planet X detected beyond Pluto.)

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Now, after making history at the Pluto system, New Horizons has become the first spacecraft ever to visit an object discovered after said spacecraft launched. As the probe sailed toward its main target in 2014, astronomers used the Hubble Space Telescope to find 2014 MU69, a dim object that orbits the sun every 297 Earth-years on a near-circular orbit.

“We didn’t know about this thing, [and] that makes it so crazy,” says Marc Buie, the astronomer who discovered MU69.

At more than four billion miles from the sun, MU69 is now the most distant object humans have ever explored. And because it has probably been in its current orbit since it formed 4.5 billion years ago, it is an ideal time capsule for understanding the raw material that formed the planets in our solar system.

However, its scientific appeal made MU69 a far trickier destination than Pluto to visit. It's so far from the sun, it's extremely difficult to see: noon on MU69 would be dimmer than Earth's twilight, and its surface is about as reflective as asphalt. Its great distance also means that it takes a message from home more than six hours to reach the spacecraft. To top it off, MU69 is a small world that's probably only between 15 and 20 miles across.

“I can't promise you success; we are straining the capabilities of this spacecraft,” New Horizons principal investigator Alan Stern warned at a press briefing on Monday. “There are no second chances for New Horizons.”

Don't stop me now

Before the flyby, researchers knew very little about MU69. Was it two objects orbiting each other, two objects in close contact, or one object shaped like a potato? Why is it reddish in colour? And how much had impact craters pockmarked MU69 over time?

For the flyby of Pluto, the New Horizons team had six months of teaser images to pore over, as the dwarf planet blossomed from a single pixel into a geologically active wonderland of ice. Not so with MU69. To oohs and ahs in a Monday press conference, the New Horizons team revealed their best pre-flyby image of the object: an elongated blob two pixels wide.

“When that picture first came down, I'd never seen so many people so excited about two pixels,” Stern says.

Once Bowman's team confirmed the spacecraft's health, she and her colleagues unveiled the latest information on MU69 based on the final pre-flyby images.

Now, researchers can make out that the object is shaped like a bowling pin, elongated with two lobes. The team has also pegged MU69's rotation rate as either 15 or 30 hours, and its axis of rotation falls along our sightline from Earth, as if it were a propeller facing us. As a result, we always see the same sunlit half—explaining why MU69 wasn't seen dimming and brightening with each rotation, as researchers had originally expected. Higher-resolution images are incoming in the days ahead.

“Even though it's a pixelated blob still, it's a better pixelated blob,” says New Horizons project scientist Hal Weaver. “This will all be revealed tomorrow or the next day; Ultima Thule will be a real world.”

Had you been a passenger on New Horizons as it flew by MU69, you wouldn't have been able to see the tiny world with your naked eye until a few hours before closest approach. Suddenly, a ruddy pinprick would have appeared in the distance. It would have rapidly brightened and ballooned to the size of the full moon as seen from Earth.

Then, just as soon as it appeared, it would have vanished into darkness as you whizzed by at more than nine miles a second. From your imaginary perch, it might seem like MU69 hadn't existed at all—until you took stock of the spacecraft's freshly filled data banks.

“It's nerve-wracking, and exciting, and butterflies—it's overwhelming how quickly it's happening,” said Southwest Research Institute scientist John Spencer, the planning scientist for New Horizons's extended mission, hours before the flyby. “You're going from knowing nothing to knowing everything in a couple of days ... Wow, we're here already?”

The full set of measurements New Horizons gathered will take 20 months to make their way back to Earth, streaming in bytes at a time over a radio transmitter a quarter as powerful as an incandescent light bulb. The first data back will help with mapping MU69's surface and searching for any small satellites of the object. The highest-resolution images of MU69 won't come back to Earth until February 2019.

“This mission's always been about delayed gratification,” Stern quipped.

Another one bites the dust

Now that New Horizons has raced by MU69, there's no way for it to slam on the brakes. It will keep careening outward through the solar system, pedal to the metal at more than 32,000 miles an hour.

Along the way, the spacecraft will continue doing science. Its dust counter will keep track of the outer solar system's dust density, adding to the record from the Voyager probes. New Horizons will also use LORRI, its onboard telescope, to image MU69's siblings from a unique vantage point: from within the Kuiper belt itself. Because it will see MU69's siblings from the side, or even from behind, New Horizons can let scientists infer the objects' surface properties in unprecedented detail.

But the mission's ultimate goal is to fly by yet another Kuiper belt object—one that hasn't even been discovered yet. Team scientists are actively using the Cerro Tololo Inter-American Observatory, located in Chile, to scan the skies for objects like MU69 that the spacecraft could visit. The team may also try to use instruments on board New Horizons to survey for new targets, a task they were not technically designed for. If successful, New Horizons could become the first spacecraft ever to discover its own target.

“It's a pretty low chance, but it's worth a look, and we're going to try,” says New Horizons team member Kelsi Singer, a postdoctoral researcher at the Southwest Research Institute.

The sooner they can find a target, the better. Each passing week or month narrows the area that New Horizons can visit within MU69's neighbourhood. Its radioisotope generator, essentially a Thermos bottle full of hot, decaying plutonium, gets less powerful over time as more plutonium breaks down. When New Horizons flew by Pluto, it could provide 200 total watts of power; now it's down to 190 watts.

At some point in the 2030s, enough plutonium will have decayed that New Horizons will no longer be able to keep its sensitive electronics warm. Until that frigid end, the spacecraft's radio transmitter will be beaming data back to Earth, across time and billions of miles from the edge of our cosmic neighbourhood.

“It'll be a cold death, but at least we'll hear about it,” says New Horizons team member Alex Parker, a planetary astronomer with the Southwest Research Institute.

Princes of the universe

Luckily, New Horizons is one of the many missions that space agencies around the world are starting to send to small solar system bodies. From 2014 to 2016, the European Space Agency's Rosetta probe gave humans a stunning glimpse of Comet 67P. The Japanese mission Hayabusa2 recently arrived at the asteroid Ryugu and dropped two puck-size landers onto its surface.

Mere hours before New Horizons made its flyby, NASA's OSIRIS-REx mission successfully entered orbit around the asteroid Bennu. In 2020, it will collect samples from Bennu's surface with the goal of returning them to Earth in 2023.

And more such missions are on the way. Stern's colleague Hal Levison, a planetary scientist at the Southwest Research Institute, is leading the Lucy mission to the Trojans, asteroid swarms that orbit the sun just behind and ahead of Jupiter. Some models hold that the Trojans formed in the cold, outer disk that fringed the early solar system, making them cousins of MU69.

At the same time, Arizona State University scientist Lindy Elkins-Tanton is leading NASA's Psyche mission, which will visit its namesake asteroid: a massive object that's at least 95 percent metal. It's thought to have been the core of a proto-planet, stripped of its outer shell by a cataclysmic impact in the early solar system.

For Elkins-Tanton, New Horizons and NASA's other small-world missions represent a big leap.

“This is the way that we as humans do exploration now; we do it in the boldest and most astonishing way,” she says. “We use technological miracles, where we send spacecraft four billion miles away, and they still work.”