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Cosmic Space

5 years after flyby, the case for Pluto still holds up

It was exactly five years ago today that NASA’s New Horizons probe made a history-making flyby past Pluto — and since then, the mission’s scientific discoveries and newly raised mysteries continue to pile up.

“I think the solar system literally saved the best for last with Pluto,” New Horizons’ principal investigator, Alan Stern of the Southwest Research Institute, said in his anniversary blog post. “Of course, I’m a little biased — as we all are on New Horizons — but I can’t think of a more beautiful and scientifically richer way to have completed the first era of the reconnaissance of the planets.”

This year marks another, more personal anniversary: It’s been 10 years since the publication of “The Case for Pluto,” my book about the put-upon planet. Back then, the big question was whether Pluto deserved the planet label — and although I argued the case that it does, the clash over classification really isn’t that big of a deal anymore.

You can call Pluto a dwarf planet (my preferred term), a Kuiper Belt object or a “bloog.” (That last term is the one Caltech astronomer and self-described Pluto-killer Mike Brown came up with to make fun of the tiff over terminology.)

But in light of New Horizons’ discoveries, you can never call Pluto uninteresting.

It’s interesting to leaf through the pages of “The Case for Pluto” and size up how the speculation from 2010 matches up with the science as we know it in 2020. In honor of the fifth anniversary of the flyby and the 10th anniversary of the book, here are updates on five of the big questions about Pluto:

Is there liquid water on Pluto? Looks like it. New Horizons’ pictures of tectonic structures and mountains made of water ice, plus an analysis of the dwarf planet’s mass distribution, suggest that there are bodies of liquid water hidden beneath the surface layer of nitrogen ice. What’s more, shifts in the state of that water due to freezing may be what’s driving the creation of new faults in the surface ice.

“If Pluto is an active ocean world, that suggests that the Kuiper Belt may be filled with other ocean worlds among its dwarf planets, dramatically expanding the number of potentially habitable places in the solar system,” New Horizons team member James Tuttle Keane, a planetary scientist at NASA’s Jet Propulsion Laboratory, said today in a mission recap.

Does Pluto have plains of methane? Sort of. One of Pluto’s best-known features, a light-colored, heart-shaped zone that was named Tombaugh Regio in honor of the dwarf planet’s discoverer, is dominated by a plain known as Sputnik Planitia. The plain is made up of patchy nitrogen-ice glaciers, but New Horizons also detected the presence of frozen methane, carbon monoxide and water (in the form of icebergs).

A close analysis of the imagery led scientists to conclude that grains of methane sand have risen to the surface of Sputnik Planitia and are being blown around into icy dunes, probably by gentle winds in Pluto’s ultra-thin, nitrogen-rich atmosphere.

Will Pluto’s atmosphere freeze out? The latest evidence suggests a freezing trend. Pluto cycles through seasons in the course of its 248-Earth-year orbit, the dwarf planet’s elliptical orbit is currently taking it farther away from the sun. That means the already-chilly planet and its atmosphere will be getting even colder.

At the time of the New Horizons flyby, scientists saw signs that the atmosphere was still holding steady rather than freezing into flecks of ice, probably due to thermal inertia. But this year, a Japanese team reported that the pressure has apparently dropped by more than 20% since 2016. That’s a much more dramatic collapse than expected, and will need to be confirmed (or discounted) by follow-up observations.

Are there ice volcanoes? You bet … not only on Pluto but on its largest moon, Charon. The pictures from New Horizons suggest that slushy “cryolava” has blurped out onto the surfaces of the two worlds through fissures in the surface ice.

Although the flyby went by too quickly to see the actual blurping, scientists spotted large central pits on two Plutonian mountains known as Wright Mons and Piccard Mons that they believe serve as the mouths of ice volcanoes. And in a region on Charon called Vulcan Planitia, the New Horizons team saw signs of a huge flow of ammonia-rich water ice.

Is there another Planet X out there? Ask again later. Even when “The Case for Pluto” was written, there was plenty of speculation over whether an undetected planet much bigger than Pluto lurked on the solar system’s edge. Caltech’s Mike Brown and other researchers said anomalies in the orbits of objects in the Kuiper Belt hinted at the presence of what they called Planet Nine.

Years of searching through telescope data haven’t yet turned up hard evidence for Planet Nine’s presence, and some astronomers now speculate that the anomalies associated with the hypothetical planet are due instead to the gravitational influence of a grapefruit-sized black hole. Others suggest it’s just a glitch in the data.

Even if Planet X is crossed out. there’s much more to be discovered on the solar system’s last frontier.

The New Horizons team is still sorting through the data sent back from the last year’s follow-up flyby of a double-lobed Kuiper Belt object known as Arrokoth. A huge compendium of Pluto research, running to more than 1,000 pages in length, is being prepared for publication.

Stern and his colleagues are already working to identify a potential target for New Horizons’ third Kuiper Belt flyby. And they’re talking about sending out an orbiter with powerful sensors to conduct a longer-lasting survey of Pluto and the Kuiper Belt.

“By the time we mark the 10th anniversary of the Pluto flyby in July 2025, such a mission could even be under construction,” Stern wrote today.

Who knows? Maybe a 15th-anniversary edition of “The Case for Pluto” will be in the works as well.

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Historical study revisits debate over Pluto

Pluto and Charon
A composite image from NASA’s New Horizons spacecraft shows enhanced-color views of Pluto at lower right and Charon, its largest moon, at upper left. (NASA / JHUAPL / SwRI Photo)

Twelve years after the International Astronomical Union voted in a definition of planethood that reclassified Pluto, the debate goes on.

A newly published study uses the historical record to take aim at the definition’s most controversial clause: the idea that a planet in the solar system has to “clear the neighborhood of its orbit,” so that no other worlds are at a similar orbital distance.

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New Horizons probe wakes up for post-Pluto flyby

Ultima Thule and New Horizons
An artist’s conception shows Ultima Thule with the New Horizons probe silhouetted by the sun. (NASA / JHUAPL / SwRI Illustration)

The mission operations team for NASA’s New Horizons probe has awakened the spacecraft from its robotic hibernation, and now it’ll stay awake for its scheduled Jan. 1 flyby of a mysterious object on the solar system’s edge, known as 2014 MU69 or Ultima Thule.

New Horizons has been traveling toward Ultima Thule since its history-making Pluto flyby in 2015. To save on resources, the piano-sized probe has been in hibernation mode since last Dec. 21.

The radio signals confirming New Horizons’ latest wakeup call took more than five and a half hours to flash at the speed of light from the solar system’s frontier to NASA’s Deep Space Network, and onward to mission control at Johns Hopkins University’s Applied Physics Laboratory in Maryland. The good news finally arrived at 2:12 a.m. ET today (11:12 p.m. PT June 4).

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Scientists say Pluto was made from a billion comets

Pluto composition
These maps, assembled using data from the Ralph spectral imager on NASA’s New Horizons probe, shows the relative concentration of four chemicals on Pluto’s surface. Methane is shown in purple, nitrogen in yellow, carbon monoxide in green, and water ice in blue. (NASA / JHUAPL / SwRI Images)

Did Pluto form like its closer-in brethren in the solar system, or is it the result of an agglomeration of comets from the edge of the solar system? A study published in the journal Icarus makes the case for comets.

To reach that conclusion, Christopher Glein and J. Hunter Waite Jr. of the Southwest Research Institute compared chemical analyses from NASA’s New Horizons mission to Pluto with readings from the European Space Agency’s Rosetta mission to Comet 67P/Churyumov-Gerasimenko.

The result is what’s known as the “giant comet” cosmochemical model of Pluto formation.

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Pluto’s champions tell the tale of an epic mission

Image: Pluto stamp
The 1991 stamp that served as the rallying cry for the New Horizons Mission to Pluto is “updated” by members of the New Horizons science team on July 14, 2015, the day the spacecraft flew past Pluto. Principal investigator Alan Stern is at far left. (Credit: Bill Ingalls / NASA)

It took nine years for NASA’s New Horizons spacecraft to get to Pluto, and laying the groundwork for that history-making space mission here on Earth took nearly twice as long.

The drama and intrigue surrounding New Horizons during those decades, as chronicled in a new book titled “Chasing New Horizons: Inside the First Mission to Pluto,” might be enough for any planetary scientist. But Alan Stern — the book’s co-author, the mission’s principal investigator and arguably Pluto’s most ardent defender — is ready to do it all again.

Stern doesn’t expect his campaign to send an orbiter to Pluto to face quite as many challenges, now that the world knows so much more about the dwarf planet with a giant heart.

“I hope it’s a more straightforward process,” Stern told GeekWire. “First of all, there are now a lot more people who are interested in going back to Pluto. … Now that we’ve done the flyby, there isn’t a planetary scientist in the world that isn’t impressed.”

Last month, Stern and other New Horizons scientists signed onto a white papercalling for NASA to fund an in-depth study of potential Pluto orbiter missions. That grass-roots approach mirrors how the “Pluto Underground” campaign for New Horizons got started around a restaurant table in Baltimore,  back in 1989.

“Chasing New Horizons,” written by Stern and astrobiologist David Grinspoon, traces the twists and turns that led from there to the piano-sized spacecraft’s launch in 2006 and its Pluto flyby in 2015.

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Pluto’s first place names win official blessing

Pluto map
Pluto’s first official surface-feature names are marked on this map, compiled from images and data gathered by NASA’s New Horizons spacecraft during its flight past Pluto in 2015. (IAU Map)

Some of the best-known places on Pluto, including the heart-shaped Tombaugh Regio, have finally gone legit. But lots of other places, such as Cthulhu Regio, are still in the dark.

That’s the upshot of today’s announcement from the International Astronomical Union, confirming that 14 features on Pluto’s surface have been approved for official use.

The decision from the IAU’s Working Group for Planetary System Nomenclature comes more than two years after NASA’s New Horizons probe flew past Pluto and gave scientists their first-ever detailed look at the dwarf planet.

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Peace pact reached over Pluto’s map

160714-pluto6
A composite image shows Pluto (lower right) and Charon (upper left). Credit: NASA / JHUAPL / SwRI

It’s taken a year and a half, but the International Astronomical Union and the science team behind NASA’s New Horizons mission have finally struck a deal for naming the features on Pluto and its moons.

The agreement, announced today, will open the way for the already well-known “informal” names for places on Pluto, such as Tombaugh Regio and Sputnik Planum, to become formal.

It also allows for features on Charon, Pluto’s biggest moon, to be officially associated with fictional characters and locales – including Mordor from “Lord of the Rings,” Mr. Spock from “Star Trek” and Princess Leia from “Star Wars.”

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How a heavy heart made Pluto flip out

Pluto's flip
Sputnik Planitia, the left lobe of Pluto’s “heart,” is thought to have formed as the result of a giant impact early in the dwarf planet’s history. This graphic shows how the icy world reoriented itself as the impact basin filled with volatile ices. (Credit: James Keane / University of Arizona)

Pluto’s famous heart-shaped feature may well have migrated over the course of millions of years as the dwarf planet spun, and that would add to the evidence for a slushy ocean hidden beneath the ice, two groups of scientists say.

In separate reports published today by the journal Nature, the scientists say a reorientation of the faraway world’s most famous feature would provide the best explanation for phenomena observed during last year’s flyby of NASA’s New Horizons probe – including patterns of cracks in the ice.

Most tellingly, it would explain why the heart-shaped feature, known informally as Tombaugh Regio, lines up almost precisely opposite Pluto’s biggest moon, Charon.

“We asked, ‘What’s the chance of that randomly happening?’ And it’s less than 5 percent that it would be so perfectly opposite,” MIT Professor Richard Binzel, a co-author of one of the reports, said in a news release. “And then the question becomes, what was it that caused the alignment?”

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New Horizons probe sends its last Pluto data

New Horizons probe
An artist’s conception shows the New Horizons spacecraft beaming data back to Earth. (Credit: NASA / JHUAPL / SwRI)

More than 15 months after NASA’s New Horizons spacecraft flew past Pluto, the last bit of data captured during the flyby has finally been transmitted back to Earth.

NASA said the final transmission was a digital segment of an observation sequence captured by New Horizons’ Ralph/LEISA imager, focusing on Pluto and its biggest moon, Charon. The readings were stored up on July 14, 2015, as the piano-sized probe zoomed past Pluto about 3 billion miles from Earth. Since then, New Horizons’ distance from Earth has grown to 3.4 billion miles.

The downlink took place at 2:48 a.m. PT Oct. 25 via NASA’s Deep Space Network station in Canberra, Australia. In all, more than 50 gigabits of data relating to Pluto and its moons have been transmitted back to Earth.

Why did it take so long to send back an amount of data that would basically fit on a thumb drive?

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Pluto’s ocean may go deeper than Earth’s

Sputnik Planum
The left side of Pluto’s bright “heart” is known informally as Sputnik Planum. (Credit: NASA / JHUAPL / SwRI)

Scientists have been saying for months that Pluto could have a salty, sloshing ocean beneath its icy surface, but now they’ve worked out just how deep it could go. The answer? As deep as 60 miles, or nine times the depth of Earth’s deepest seas.

The estimate, based on computer modeling of the impact dynamics behind a heart-shaped region of Pluto, was published this month in Geophysical Research Letters.

The bright-colored “heart,” first seen last year by NASA’s New Horizons probe, is arguably Pluto’s best-known surface feature. But it’s actually two features. Scientists say the left lobe of the heart, known informally as Sputnik Planum, was created in the aftermath of an ancient impact. The object that made that impact is thought to have been about 120 miles (200 kilometers) wide.

In June, researchers reported that the geological features mapped on Pluto’s surface would be consistent with the presence of a liquid water ocean far below, perhaps heated by the decay of radioactive materials in Pluto’s rocky core. But they didn’t estimate the size of the ocean.

This month’s findings address that question.

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