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12 years after launch, New Horizons probe zeroes in on mysterious Ultima Thule

New Horizons at Ultima Thule
Artwork shows NASA’s New Horizons spacecraft silhouetted by the sun during its New Year’s Day encounter with an icy object, or objects, known as Ultima Thule. (NASA / JHUAPL / SwRI Illustration / Steve Gribben)

Act Two of the 12-year-old New Horizons mission to Pluto and the solar system’s icy Kuiper Belt is heating up, with less than a month to go before NASA’s piano-sized spacecraft makes history’s farthest-out close encounter with a celestial object.

The New Year’s flyby of a mysterious Kuiper Belt object (or objects) known as Ultima Thule (UL-ti-ma THOO-lee) follows up on the mission’s first act, which hit a climax three years ago with a history-making flyby of Pluto.

Launched in 2006, New Horizons was never meant to be a one-shot deal. Even before the Pluto flyby, mission managers used the Hubble Space Telescope to identify its next quarry, a billion miles farther out in the Kuiper Belt. Now it’s crunch time for New Horizons principal investigator Alan Stern and his team.


“This flyby is a lot harder than Pluto,” Stern said. “Ultima is tiny, and faint, much harder to navigate on. … Another difficulty, or challenge, really, is that we’re farther away, and that means communication times are longer. Bit rates are lower.”

Today the team beamed out commands to fine-tune New Horizons’ trajectory using the spacecraft’s navigational thrusters (which, by the way, were built at Aerojet Rocketdyne’s facility in Redmond, Wash.). It took more than six hours for the commands to reach the probe at the speed of light, at a rate of 1,000 bits per second.

By the time mission managers get confirmation that their commands have been executed (or not), New Horizons will have traveled hundreds of thousands of miles farther on its path (or off its path).

Alan Stern
New Horizons principal investigator Alan Stern is getting set for the Ultima Thule flyby. (Alan Stern via Twitter)

“It’s a one-shot, ‘get it right or go home’ deal, because there’s no U-turn to go back and have a re-do. … You have to plan every chess move with the spacecraft more carefully,” said Stern, a planetary scientist at the Southwest Research Institute.

Dozens of scientists and engineers are due to converge on Johns Hopkins University’s Applied Physics Laboratory in Maryland to get set for the flyby, which is scheduled to come closest to Ultima Thule at 12:33 a.m. ET Jan. 1 (9:33 p.m. PT Dec. 31).

If all goes according to plan, New Horizons will pass by Ultima at a distance of 2,200 miles, or less than a third of the distance used for the Pluto flyby, But the mission team is on the watch for any mini-moons that would force a shift to a safer, more distant trajectory.

During a recent rehearsal, the team had to cope with a worst-case scenario in which New Horizons spotted 11 satellites in Ultima’s vicinity. “It was just flying into a hornet’s nest,” Stern recalled.

Neither Stern nor anyone else knows exactly what New Horizons will actually see.

“We don’t know what a primordial, ancient, perfectly preserved object like Ultima is, because no one’s ever been to something like this,” Stern explained. “It’s terra incognita. It is pure exploration. We’ll just see what it’s all about — if it’s got rings, if it’s got a swarm of satellites.”

The Hubble imagery suggests that Ultima Thule (also known as 2014 MU69) measures roughly 20 miles wide — and might consist of two or more mutually orbiting objects. The dearth of knowledge leaves plenty of room for surprises.

“Considering how much we knew about Pluto, and how much it astounded us, here we’re starting from complete scratch,” Stern said. “We barely know its size and its color. We can’t tell you anything about its composition or its atmosphere, or satellites, any of that. But we’re going to find out.”

To find out, the plutonium-powered New Horizons probe will employ the same suite of scientific instruments that worked so well to study Pluto and its moons more than three years earlier.

New Horizons’ long-range camera, known as LORRI, currently sees Ultima as a mere speck, but it should be able to make out its shape starting a few days before the flyby. During the closest phase of the encounter, LORRI could detect features as small as the boats floating on the lake in New York’s Central Park, Stern said.

New Horizons will make use of an ultraviolet imager called Alice and an infrared and visible-light imaging spectrometer called Ralph to characterize Ultima’s composition. A radio science experiment will take its temperature, and other instruments will analyze particles in Ultima’s cosmic neighborhood.

It’s likely to take months to send back all the data from the Ultima flyby, just as it did in the wake of 2015’s Pluto flyby. But eventually, Act Two of the New Horizons mission is expected to add to Act One’s already-substantial record of revelations about the icy worlds on the solar system’s edge.

Will there be an Act Three? Stern said he and his colleagues fully intend to ask NASA for another mission extension once Ultima is behind them.

He noted that at its current speed, New Horizons will be flying through the Kuiper Belt for almost a decade longer.

“We’re going to look for another flyby target, and we’re going to continue to observe Kuiper Belt objects with the telescopes on board,” he said. “If NASA approves that, there will be a third act.”

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