It sounds like science fiction, but it's not: NASA's New Horizons mission explored the Pluto system this summer!

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Exactly 50 years to the day after Mariner 4 became the first mission to explore Mars, New Horizons completed the first era of planetary reconnaissance by flying past Pluto on July 14, 2015. In my final "insider blog" for, I want to give you a recap of the main findings that came from the initial data returned from the spacecraft.

Pluto's heart, July 13th
NASA’s New Horizons spacecraft took this image of Pluto with its Long Range Reconnaissance Imager (LORRI) on July 13, 2015. The color image has been combined with lower-resolution color information from the Ralph instrument acquired earlier. This view is dominated by the large, bright feature informally named Tombaugh Regio, roughly 1,000 miles (1,600 km) across. This is the last and most detailed image sent to Earth before the spacecraft’s closest approach on July 14th.

Regarding Pluto, we found a wonderland of diverse geological expression, with both old and young surfaces, mountain ranges, polygon-subdivided ice plains, flowing glaciers, and possibly even evidence for subsurface liquids. Pluto's mountains require strong materials to survive (and not slump) over time, indicating Pluto's crust is likely to be composed of water ice, rather than a deep layer of frozen nitrogen, which is soft and malleable to form long-lived mountains.

We also found that Pluto was bigger — 2,374 km in diameter — than most past estimates. This larger true size, combined with Pluto's already well-known mass, means its true density is lower than we thought. So the ice fraction is higher (35% or 40%) and its rock fraction lower (60% or perhaps 65%).

Meanwhile, its tenuous atmosphere has a base pressure of less than 10 microbars (about half what ground-based measurements had predicted), and it contains widespread hazes, several new molecular species (including acetylene and ethylene).

Charon close-up from July 13
Pluto's moon Charon, as seen by New Horizons on July 13, 2015, shows an array of landforms that has stunned mission scientists.

Regarding Charon, we found no evidence for an atmosphere — though the final verdict depends on data not yet back on Earth. We also found a more complex geological story than many had anticipated.

Most of us expected Charon to be little more than a battered ball of water ice and craters. Instead, we found tectonic ridges, chasms, and mountains, along with a strangely dark red stain covering its north polar region.

Next Steps

The New Horizons science team is now at work mapping both bodies and preparing to formally submit names for specific surface features to the International Astronomical Union. We've been naming features informally, drawing from the "OurPluto" name banks that New Horizons and NASA conducted with the public's help. Preliminary maps of both Pluto and Charon are below.

Informal names on Pluto
Although informal for now, these feature names follow an approved scheme and have been submitted to the IAU for approval.
Informal names on Charon
Similarly, the initial, informal names used by the New Horizons team for the features on Charon, Pluto's largest moon, were selected based on input received from the OurPluto naming campaign.

Regarding Pluto's small satellites, we've learned the sizes of Nix (35 km in diameter) and Hydra (41 km), and our first looks reveal brightness and color variegation across their surfaces. We found their albedos (reflectivities) are higher than expected — so high in fact that both are likely ice covered. (Resolved images of Styx and Kerberos have not yet been returned as of this writing.)

Nix and Hydra
New Horizons recorded these images of two of Pluto's four small moons. Nix has an irregular shape (seen here end-on in false color), about 42 km (26 miles) long and 36 km (22 miles wide. Hydra appears roughly spherical and is 55 km (34 miles) across.

Most surprising to me about Pluto's satellites, however, is that, despite searching with about 15 times more sensitivity than even the Hubble Space Telescope, we didn't find any more —not even one. Few on our science team would have predicted this, including myself.

The flyby of Pluto and its system of moons by New Horizons is complete, but over 95% of the data from that reconnaissance is still aboard the spacecraft, awaiting downlink to Earth. Getting all those observations back will take some 16 months and won't complete until the fall of 2016. So expect many more images and spectra and, from those, many more discoveries in the months ahead. New Horizons is a gift that will keep on giving.

Check out Alan Stern's first, second, and third previous blogs for


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August 3, 2015 at 2:30 am

The course isn't just set for Pluto, it's set for you.

Nothing is more important than the strive of science to mankind.

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August 7, 2015 at 5:29 pm

I was aghast to see the name "Cthulhu" proposed for a major feature of Pluto. This is a creature from the fiction of H.P. Lovecraft, who was an unrepentant racist and virulent anti-Semite. The following is a letter written by him in 1915:

“A Jew is capable of infinite nastiness when he seeks revenge, & I believe I shall have ample grounds for making [Isaacson] the hero of a spirited Dunciad. … Isaacson belongs to a stock wholly broken & emasculated by two thousand years of cringing at the feet of Aryan masters. But I, thank the Gods, am an Aryan, & can rejoice in the glorious victory of T. Flavius Vespasianus, under whose legions the Jewish race & their capital were trodden out of national existence! I am an anti-Semitic by nature, but thought I had concealed my prejudice in my remarks concerning Isaacson. I showed him every consideration in my article, carefully saying that I attacked not the man, but the ideas. However, if Jerusalem wishes to start trouble, he will find in me a new Titus, eager to inscribe on my eagles the triumphant legend IVDAEA CAPTA! I might here remark that my anti-Semitism is not entirely due to blind prejudice. The Jews are fundamentally Orientals, whilst the rising civilization of the world is Western—Teutonic—Anglo-Saxon. The struggle between the East & the West dates back to Marathon & Salamis, & it is the West which has ever represented progress & superior culture. The Jew is an adverse influence, since he insidiously degrades or Orientalizes our robust Aryan civilization. The intellect of the race is indisputably great, but its nature is not such that it may be safely employed in forming Western political & social ideas. Oppressive as it seems, the Jew must be muzzled.”

Verification available:

I am not Jewish, and am generally opposed to "political correctness", but this man, among other things, favored worldwide laws that prohibited Anglo-Saxons from "interbreeding" with "darker races". He is the sort of sorry throwback that does not deserve any honors attached to any discovery in the solar system..

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August 7, 2015 at 5:34 pm

Before you fasten the name "Cthulhu" forever on Pluto, please read this letter from the racist, anti-Semitic author (H.P. Lovecraft) who created the creature:

I am not Jewish, but I am appalled that H.P. Lovecraft's work would be even remotely considered.

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August 9, 2015 at 12:49 pm

Lovecraft was a writer of Gothic-horror fiction. The character Cthulhu was a fictional character that inflamed fans of the generation that read Lovecraft, readers that, in turn, popularized endless spin-offs of that character. It is in memory of those fans and their favored mythical monster that the name was given to a far off astronomical body geological feature. It is not a tribute to the racist and ant--Semitic author. Not in any way to make a qualitative comparison with Lovecraft's narrow and simplistic work, but Richard Wagner who worked a half-century earlier also had a Jewish heritage and sported the anti-Semitic, anti-miscegenation racial attitudes prevailing in his era. Wagner's music rises far above his personal, mid-19th century social ideals and is widely enjoyed today separated from any knowledge or opinion of them. So it is, if on a narrower scale, with Lovecraft. There is horrible racism and discrimination abounding and all to clearly visible in 2015's real society. Preach against that before dredging faint, obtuse allegations against a fictional name applied to a distant body invisible to the unaided eye.

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August 7, 2015 at 7:04 pm

So Jim-Baugman, just because you don't like someone, their name should be banned from use? Well similarly, If I don't like your postings, so you too should be banned. Everyone has a God-given right to have their own beliefs and express them as they wish. Our 1st Amendment ensures this in the USA. If you don't like it, you have the ability and right to ignore it. Propagating your distortion of it on the internet serves no positive purpose, but gives what you don't like greater power to survive and prosper.

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August 8, 2015 at 4:16 am

Everyone does NOT have a God-given right to express their beliefs as they wish. If that is enshrined in the U.S Constitution, I am extremely glad to be British, where anti-Semitic and other racist outpourings are illegal and offenders are liable to prosecution.
Now, let's move on and stick to Astronomy shall we?

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August 7, 2015 at 7:25 pm

I wonder if Nix and Hydra are captured comets...

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August 8, 2015 at 7:52 am

I think we found a geologically active small planet with 5 moons.

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August 8, 2015 at 8:25 am

Jim - We could cull through the names of dozens, perhaps hundreds of named bodies and find the names of many racists and characters created by racists. Racism has been a favoured mode of thought for billions of people, providing entertainment, a sense of superiority and/or security to these people. If your wish is granted, I'm afraid there will be a call to rename these hundreds of bodies. I'm just too old to want to be bothered to learn new names.
Granted, if we name a body Hitler, we should make sure it will not be orbit crossing with any bodies named Czechoslovakia or Poland, for example.

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August 10, 2015 at 10:42 am

If all our heroes have to be sinless, then we'll have no heroes!
We don't have the same liberties we once had, but we have more than most other places in the world, and for that I am grateful to be an American.
It's wonderful to see it proposed that a region on Pluto be named for Percival Lowell, the man ultimately responsible for its discovery.

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August 10, 2015 at 1:46 pm

The problem is that too many of these names are being proposed by gentlemen from the basement-dwelling tech-geek species. Tombaugh and Lowell Regios? Wonderful. Kirk, Spock, Sulu, Skywalker and Vader Craters? Grow up, guys.

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