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The Three Discoveries of Pan


Recent closeup of Pan, imaged by the Cassini spacecraft

This morning, NASA’s Cassini spacecraft obtained the first closeup images of Saturn’s innermost moon, Pan. The images show a peculiar body shaped like a “flying saucer”. Pan occupies a unique position in the rings, at the center of the 300-km wide Encke Gap. As best we can tell, Pan probably started its life as a more spherical moon, but it subsequently swept up a thick equatorial belt of ring-dust. A smattering of crevasses and craters across the surface add to our view of a moon that has endured a long and dynamic history. Yesterday, Pan was just a “tiny ring-moon”; today, it has been revealed as a world in its own right.

Seeing these images has brought back vivid memories of the day way back in June, 1990 when I became the first person to see Pan. That story is worth retelling, because Pan’s discovery story has almost as many twists and turns as Pan’s battered and bruised geology. For example, most planetary bodies have just a single discovery paper; Pan has three.

The story starts when the twin Voyager spacecraft flew by Saturn in 1980 and 1981. My colleague and mentor Jeff Cuzzi, of NASA’s Ames Research Center, was a member of the Imaging Team at the time, studying some of the newly revealed properties of the ring system. In those days, “image analysis” most often meant staring at photographic prints. Jeff had a stack of prints with him one day while waiting at the Albuquerque airport. By bending one particular print and looking along it lengthwise, he noticed something very strange–the Encke Gap had a wavy edge.


A sideways look at a Voyager image of the Encke Gap. By taking a print and sighting along the gap as shown, the wavy pattern at the inner (right) edge becomes visible.

Jeff realized that a small moon must be orbiting within this gap. As each particle at the gap edge passes the moon, it receives a gravitational tug that sets up this pattern. Once Jeff got home, he and his colleague Jeff Scargle set about examining all the other Voyager images of the Encke Gap. By assembling all of that data, they pinned down the moon–for now, let’s just call it “TBD”–into a 30-degree sector of longitude where no high-resolution images were available. They published their work in 1985 under the title, Wavy Edges Suggest Moonlet in Encke’s Gap. This is “discovery” paper #1.

Meanwhile, scientists were examining other ring data from the Voyager flyby. Two instruments had obtained “occultation profiles”–measurements of ring opacity at fine resolution across the rings. Both of the occultations showed periodic bright-dark variations near the Encke Gap, but at different locations and with different wavelengths. Jeff and I realized that these were slices through spiral patterns, which could be understood as another aspect of TBD’s influence on the ring. We called the phenomenon a “moonlet wake”. These patterns provided enough information that we were then able to pin down the precise orbit of still-unseen TBD. We published that paper in 1986, Satellite “wakes” and the orbit of the Encke Gap moonlet. “Discovery” paper #2.

After that, our interest in the topic subsided because it seemed, due to bad luck, that the Voyagers simply had not imaged the moon. (Also, meanwhile, we had the Voyager flybys of Uranus and Neptune to deal with.)

This brings us to June 1990, and one fundamental change. The Voyager images were finally available on that brand new, high-capacity storage medium, the CD-ROM. One morning it dawned on me that I had everything I needed to perform a truly comprehensive search for TBD. I had all 30,000 Voyager images of Saturn at my fingertips. I knew when they were taken and where they were pointed. Also, from paper #2, I knew exactly where TBD ought to be along its orbital path at each image time. I left for work that morning telling my husband that my plans for the day were to discover a moon of Saturn.


The discovery image of Pan. It occupies just a single bright pixel in the middle of the dark gap.

By early afternoon my program had printed out a table of every Voyager image that ought to contain TBD, along with where in the image to look. Scrolling down that list, I quickly identified the most promising image, loaded up the CDROM, and checked out the predicted location. Just there, I saw a single bright pixel in the middle of the Encke Gap! A single pixel could be a glitch, but I quickly examined the half-dozen next-best images, and they all showed a bright pixel at the predicted location. Nailed it!

“Eureka” moments don’t come along very often in a research career. That was mine.

Shortly thereafter, I published Visual Detection of 1981S13, Saturn’s Eighteenth Satellite, and its Role in the Encke Gap, discovery paper #3. However, in astronomy as in life, it’s seeing the thing that makes it real.

The Author

The Author, a very long time ago.

Two postscripts. First I am often asked how I chose the name. It was a no-brainer. The name of the process whereby a moon can open up a gap in rings is known as “shepherding.” Moons of Saturn are named after Greek gods. The god of shepherds in Greek mythology was that flute-playing satyr, Pan. Happily, the International Astronomical Union liked my reasoning and approved the name. Conveniently, the name is also short enough to fit on my license plate.

Second postscript. I proudly submitted my manuscript to the journal Nature. It was nearly rejected, however because the reviewer said, “we already knew the moon was there, so seeing it for the first time is no big deal.” Luckily for me, the editor was of the “seeing is believing” school of thought, and Nature published the paper anyway.



Our Predictions about Pluto: Getting it Right/Getting it Wrong

Family portrait of Pluto's four small moons

Family portrait of Pluto’s four small moons

Back in June, my research colleage Doug Hamilton and I put out a paper in Nature magazine about the four small moons of Pluto. The timing was not accidental. Although the paper was the culmination of years of work with the Hubble Telescope, we knew that a lot of our predictions would be tested barely a month later, when the New Horizons spacecraft passed Pluto. Making predictions that might be proven wrong is part of the fun, and also part of the danger, of scientific research.

As we all know, the flyby was a great success and we are now waiting, patiently, for the slow trickle of images and other data to come back from the spacecraft. Today, NASA has released the first “family portrait” of Pluto’s four small moons. As someone who has spent years studying these objects as nothing but faint dots, I find it is enormously gratifying to see them as resolved bodies, with shapes, colors and surface features.

So what did we get right? Well, based on years of studying how the brightness of each body varies, we were able to determine the rough shapes of the two larger moons, Nix and Hydra. On that point, we nailed it! Also, by making assumptions about how bright the surfaces are, we could make estimates of their sizes. We have learned that the moons are a bit brighter than we expected, and therefore a bit smaller, but overall our predictions have held up well.

…with one big exception–Kerberos. Our paper predicted that Kerberos would be big and dark, whereas the image clearly shows an object that is small and bright. What went wrong? Well, it all comes down to the moons’ masses—how much they would weigh if you could put them on a scale. The only way to determine the masses of these moons is to study how each one subtly alters the courses of the others. Based on years of precise measurements, our colleagues believed that they had weighed Kerberos, and its mass was surprisingly high–about a third that of Nix and Hydra. On the other hand, from our measurements, it only reflected about 5% as much sunlight. The only way to make an object that faint and that massive is to make it very very dark. That darkness, comparable to that of a charcoal briquette, is not out of line with other bodies in the outer Solar System, but it certainly made it different from its Plutonian siblings.

We know now that Kerberos is small and bright. If the mass determination is right, then Kerberos is absurdly dense–many times denser than lead. That seems unlikely. We therefore conclude that we probably had a broken scale. Because weighing the moons of Pluto is such a tough job, this is not out of the question. Now that we have new and more precise measurements of the orbits of the moons, I boldly predict that we will soon learn that the mass of Kerberos is much lower than we had previously thought.

One word of caution, however: my bold predictions don’t always turn out to be right.

The Slumbering Dwarf Awakens: Pluto is about to come back into the limelight


The best images so far of Nix and Hydra. Much better images will be sent down from the New Horizons spacecraft in early September

After the busiest July of our lives, the New Horizons team members have finally caught up on sleep. A few of us have even had a chance to take vacations. It’s good that we’re rested up, because an onslaught of new data from Pluto is about to begin.

Right now, 95% of the data obtained during the July 14 flyby is still stored on the spacecraft. After a quiet August, new images will start flowing down to Earth again on Saturday, We will get a few images almost every day. Why just a few? Well, Pluto is very far away, and the New Horizons transmitter is not very powerful. The downlink data rate is 125 bytes per second. For those who are old enough to remember dialup modems, this is slow even by that standard. It takes more than an hour for a single image to come down from the spacecraft. It will take almost a year before we have seen every image. That seems like a long time, but compared to the decade it took to plan, build and launch the spacecraft, and the second decade that it took to travel three billion miles to Pluto, waiting another year doesn’t seem so bad.

I am particularly looking forward to seeing some of our first closeup images of Hydra and Nix, two of Pluto’s small outer moons. Expect them to have weirdly irregular shapes, pockmarked by craters. The small moons of Pluto a particularly odd bunch. Whereas most of the moons in the Solar System rotate in a simple way, keeping one face toward the central planet at all times, we believe that Pluto’s moons might be tumbling chaotically. In a few days, maybe we will finally know for sure. Stay tuned.

By the way, every Friday the latest New Horizons images will be released to the public here:

Telling Pluto’s Story, One Fragment at a Time

The Pluto System

Pluto and Charon are in the middle, orbiting around their common center of mass. The four smaller moons orbit around them.

When you begin a new research project, you usually have expectations about where it will lead. Most projects take you or less to the expected destination. Some go nowhere. However, every now and then a project picks you up and makes you feel like you’re just coming along for the ride.

Today, in the journal Nature, we have published the results of a research project that fits solidly into the third category.

Our original plan was straightforward. We had recently discovered two small moons of Pluto, now known as Kerberos and Styx. We wanted to publish a short discovery paper that would just cover the basics: How did we find the moons? What are their orbits? How big are they? What are the implications of the discovery?

The Pluto system had other ideas. (more…)

New Explorers on the OurPluto Ballot, April 2

We have added three new historic explorers to the OurPluto ballot today. Keep those great ideas coming in! I have been learning so much.

Map of Europe, Asia and North Africa by Al-IdrisiSeveral people nominated the renowned 10th-century explorer and cartographer Muhammad al-Idrisi. He traveled as far west as Ireland and as far east as China, mapping much of the known world in the process. His exquisite maps were still being used centuries later. The title of his compendium of geographic information roughly translates from the Arabic as The Pleasure of Him Who Longs to Cross the Horizons. How appropriate it will be for a spacecraft named New Horizons to memorialize his work!

We also learned about for Hyecho, a Korean Buddhist monk who lived in the 8th century. The nomination from East Asia reads as follows: Hyecho was the first man to travel across the Asian continent, from far east to far west, by sea and land and to record his journey. He wrote a travelogue, consisting of originally 11,300 characters, called Memoir of the pilgrimage to the five kingdoms of India during his journey. The work of Hyecho offers a full account of a long journey that lasted four years spanning 9,000 kilometers in distance by ship, and 11,000 kilometers by land. To this day, It is praised as a valuable archeological and anthropological reference for its unprecedentedly comprehensive scope and depth.

Looking back to the way home under the reflection of moonlight
I see only the clouds floating
Though a letter was sent on a cloud
a gusty wind blows away its answer
-Hyecho (704-787 CE)

Isabella BirdFinally, this is the nomination for a remarkable woman named Isabella Bird: Born in Yorkshire England in 1831 she holds a special place in history. Isabella was the first woman to be elected Fellow of the Royal Geographical Society. Isabella battled with ill health all her life. However this did not stop her traveling the world and writing many incredible books about her travels. She visited Australia, Hawaii, America (where she traveled over 800 miles on horseback and met some very interesting characters including one-eyed outlaw Jim Nugent “Rocky Mountain Jim”). Battling ill health she went traveling to Asia: Japan, China, Vietnam, Singapore and Malaysia. At nearly 60 years of age she set off for India covering Ladakh on the borders of Tibet, and then travelled in Persia, Kurdistan and Turkey. In India, she worked with Fanny Jane Butler to found the John Bishop Memorial Hospital in memory of her recently deceased husband. The following year she joined a group of British soldiers traveling between Baghdad and Tehran. She remained with the unit’s commanding officer during his survey work in the region, armed with her revolver and a medicine chest supplied – in possibly an early example of corporate sponsorship – by Henry Wellcome’s company in London. By now Isabella was a household name in the Royal Geographical Society. Her final great journey took place in 1897 where she travelled up the Yangtze and Han rivers which are in China and Korea, respectively. Later still, she went to Morocco, where she travelled among the Berbers and had to use a ladder to mount her black stallion, a gift from the Sultan. She died in Edinburgh within a few months of her return in 1904, just shy of her seventy-third birthday. She was still planning another trip to China. What an amazing achievement for a person who battled with severe illness her entire life. Horizons exploration of Pluto should give credit to this amazing explorer by having a piece of Pluto named after her.

We agree.

More Nominations for OurPluto

We continue to receive a fascinating collection of nominees for the OurPluto campaign. Here are a few that we recently added to the ballot.

doreSeveral wrote to remind us of the iconic prints by Gustav Doré, a French artist from the 19th century who illustrated an 1861 edition of Dante’s Inferno.




In the category of historical explorers, we learned about Michał Boym: “He is notable as one of the first westerners to travel within the Chinese mainland and his Flora Sinensis was the first description of an ecosystem of the Far East published in Europe. Boym also published the first dictionary to translate between Chinese and European languages.

1024px-AltamiraBisonAnother historic explorer reminds of the roles that serendipity and patience play in all of science: Marcelino Sanz de Sautuola, who discovered the first prehistoric cave paintings known to the modern world. “He was a Spanish amateur archaeologist and explorer of the Cave of Altamira. These caves provide a window into human development, much as Pluto provides a window into Solar System development. Although Sautuola’s discoveries were discredited by experts early on, he was later completely vindicated and his discoveries appreciated, albeit after his death.”

Finally, a New Jersey professor provided a very good reason for his endorsement of Krun, a monster of the darkness. “I’m trying to raise awareness of the Mandaean community of Iraq and Iran. They are one of the few communities from the Middle East that still preserve the ancient Babylonian tradition of divination by the stars and heavenly bodies (astrology), directly from the source (they even retain the traditional Akkadian names for the stars and the visible planets). [….] Unfortunately, with the Second Gulf War, their community (a minority faith in both Iraq and Iran) has become progressively endangered, and much of it has gone into a global diaspora. The lives of those that remain and their ancient culture are threatened by religious extremists, such as ISIS, who seek to eliminate anything pre-Islamic in the Middle East. I hope that OurPluto can establish a monument to them in the heavens, where these extremists cannot reach them.”

OurPluto Campaign, Day 6

The OurPluto naming campaign has been an exhilarating experience for me so far. The amount of thought that our site visitors have been putting into their nominations astounds me. You can visit the Site News page for regular updates. Today I would just like to highlight a few of the new names that we added to the ballot last night.

640px-Lowell_blink_comparatorOn the list of scientists and engineers, we have added Carl Pulfrich, 1858-1927. Although he died before the 1930 discovery of Pluto, he contributed in a critical way—he invented the blink comparator. This is a device that lets you switch back and forth between two sky plates, looking for subtle changes. Clyde Tombaugh discovered Pluto using a blink comparator. The discovery would not have been possible without the device.



From Europe, we were reminded of the importance of the Soviet The Luna Program: “Luna was a series of robotic spacecraft missions sent to the Moon by the Soviet Union between 1959 and 1976. Fifteen were successful, each designed as either an orbiter or lander, and accomplished many firsts in space exploration. They also performed many experiments, studying the Moon’s chemical composition, gravity, temperature, and radiation. Twenty-four spacecraft were formally given the Luna designation, although more were launched.”

Two prominent women explorers were also nominated. For Alexandrine Tinné, who explored the Nile and the Sahara in the 1860s, the nomination reads, “It is rare that we take the opportunity to praise some of the women who have contributed to the exploration of our world. Ms. Tinné was courageous, dedicated, and passed on a legacy of adventure for today’s women to aspire to.”

Jeanne_BarreWe also learned about Jeanne Baré who, a century earlier, became the first woman to circumnavigate the globe. She was naturalist studying the world’s plants. However, times being what they were, she had to impersonate a man for the journey.



Our fictional explorers now include several popular nominees. Among others, we include a pair of the great travelers in literature, Don Quijote and his patient squire Sancho Panza.


In the category of Underworld beings, we learned that the King of the Underworld in Vietnam has a Pluto connection. “Diem Vuong is the King of Hades. Demons obey and serve him. He is the ruler of the underworld and the judge of the bad souls after death. Diem Vuong once upon a time has been added to form “Diem Vuong Tinh” to name for Pluto in Vietnamese.”

In Inuit mythology, we learned that the realm of the dead has an astronomical connection. The souls of the dead first spend time under ground in Adlivun, but later ascend to a permanent home in Quidlivun, on the Moon.

The Mapping of Pluto Begins Today

Pluto and Charon, before and after

Pluto is just 3.5 pixels across in the latest images from the New Horizons spacecraft. That’s nine square pixels. You can’t do much with nine pixels. You might be able to see crude patterns of light and dark, but you probably wouldn’t call it a map. Still, it’s a start.

In a few months, this will all change. Craters, mountains and other landforms will take shape before our eyes. When New Horizons flies past Pluto in July, we will see a new, alien landscape in stark detail. At that point, we will have a lot to talk about. The only way we can talk about it is if those features, whatever they turn out to be, have names.

Today we are beginning a campaign called “Our Pluto”. The goal is to gather together the names that we will eventually use to label the maps of Pluto and its large moon, Charon. After discussions with the International Astronomical Union (IAU), we have defined a set of broad themes for these names, related to mythology, literature and history.

The New Horizons science team is doing something unprecedented. Naming campaigns have been held before, but on a different scale. Today, the entire landscapes of Pluto and Charon is open to the public. We have called the campaign “Our Pluto” because we think that everyone should have a say in the names we use on those strange and distant worlds. At, you can vote for your favorite names, talk about them, and nominate names that we might have overlooked.

After the campaign ends, the New Horizons science team will select your best ideas and pitch them to the IAU. The IAU will have final say over the names on the maps of Pluto and Charon.

Let the conversation begin!

Thanks, Anna!

Eshoo Letter

How to Photograph a Racehorse …and how this relates to a tiny moon of Neptune

Muybridge studies a galloping horse

The name Eadweard James Muybridge does not roll off the tongue, but we have all seen his images. In the earliest days of photography, he pioneered the use of the newly-invented camera to study animals and people in motion. In his classic studies of horses, for example, he demonstrated for the first time that there exists a moment in the cadence of a trotting horse when it is airborn–all four feet are off the ground. The technical setup required to obtain one of these sequences was substantial. Each image was taken by a different camera, which was triggered by a separate thread as the horse passed by.

Needless to say, the cameras of the 19th century lacked “burst mode”, or even auto-advance, or even film. These are the features that have made it trivial for 20th- and 21st-century photographers to capture a similar sequence of images. Today, I can just set my iPhone on video mode and let it do all the work.

However, one truth about action photography has never changed–you need to follow the motion. Otherwise, you will get nothing but a blur. Save your tripod for subjects that are willing to sit still. Had Muybridge attempted the same photo sequence without all those separate triggers, his image would have looked something more like this. All background, no horse!


So it is, strangely enough, when trying to take pictures of distant moons. Like fidgety children, moons are unwilling to sit still for the camera. This is the challenge we dealt with in finding a tiny moon of Neptune whose discovery was announced today. It has a special distinction–it never sits still long enough to have its picture taken. Neptune is a big planet with a strong gravity field, and moons whip around it very fast. This one circles the entire planet, following a path of over 600,000 km, in less than a day.

The moon is also very small and dark. We estimate that it is no more than 20 km across and as dark as if it were paved with dirty asphalt. Naturally, taking its picture requires long exposures. But there’s the rub. If you expose it for too long, the moon vanishes in a blur, much like the disappearing racehorse above.

The images in which we found tiny “S/2004 N 1″ (don’t worry, it will get a better name soon!) were taken by the Hubble Space Telescope between 2004 and 2009. In fact, these images have been freely available to anyone in the world, some for almost a decade. However, what they have in common is that they were all taken by keeping the camera fixed on Neptune. In the past, we have always discovered moons just by stacking up images like this, or perhaps animating them into a movie, and seeing what turns up. This actually works fine for Pluto, a dwarf planet with a weak gravity field. The small moons of Pluto, including the recently-named Kerberos and Styx, take weeks to circle Pluto, so summing up a few hours of data works just fine. Not so at Neptune.

Once the images are on the ground, it is too late to track the motion of a tiny moon. Besides, if we don’t know where a moon is, how could we know how to track it? The solution we found builds on the fact that moons are a bit more predictable than racehorses and fidgety children. We might not know where the moons are, but we do know how they move. As a result, we were able to write software to do all the motion-tracking, by letting a computer shift and add up the images after the fact. The procedure I devised predicts where any given moon ought to move from one image to the next, and then combines the images with a “twist” that compensates for the expected motion. I developed this procedure (and I am certainly not the first) to study some peculiar arcs in the rings of Neptune. However, I soon realized it could be useful for moons as well. It was only when I expanded my analysis out to regions well beyond Neptune’s ring system that an extra little dot turned up, over and over again. In less than a week, we went from our first detection to ten.

The discovery of one little moon might not be Earth-shaking, but somehow it is has to fit into the big picture of how the Neptune system formed. Astronomers had long noted that the moons of Neptune get progressively smaller as you go inward. However, this little rock, orbiting between 200-km Larissa and 400-km Proteus, throws a bit of a wrench into that simple story.

What we do know is that Triton, a very large moon, got captured into orbit around Neptune eons ago. Triton’s distinction is that it orbits backwards, circling the planet opposite to the planet’s rotation and opposite to the direction of the other moons. We can be pretty sure that when Triton arrived, it disrupted whatever system of moons was originally circling Neptune. The moons that we see today somehow re-formed after that event. With the discovery of S/2004 N 1, the key points of this story have not changed, but we now have one more piece of the puzzle to fit into place.