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A Full Day at Pluto/Charon

Have a look at the latest imagery from the New Horizons spacecraft to get an idea of how center of mass — barycenter — works in astronomy. When two objects orbit each other, the barycenter is the point where they are in balance. A planet orbiting a star may look as if it orbits without influencing the much larger object, but in actuality both bodies orbit around a point that is offset from the center of the larger body. A good thing, too, because this is one of the ways we can spot exoplanets, by the observed ‘wobble’ in the stars they orbit.

The phenomenon is really evident in what the New Horizons team describes as the ‘Pluto-Charon dance.’ Here we have a case where the two objects are close enough in size — unlike planet and star, or the Moon and the Earth — so that the barycenter actually falls outside both of them. The time-lapse frames in the movie below show Pluto and Charon orbiting a barycenter above Pluto’s surface, where Pluto and Charon’s gravity effectively cancel each other. Each frame here has an exposure time of one-tenth of a second.


Charon is about one-eighth as massive as Pluto. The images in play here come from New Horizons’ Long-Range Reconnaissance Imager (LORRI), being made between January 25th and 31st of this year. The New Horizons team is in the midst of an optical navigation (Opnav) campaign to nail down the locations of Pluto and Charon as preparations continue for the July 14th flyby. None of the other four moons of Pluto are visible here because of the short exposure times, but focus in on Charon. We’re looking at an object about the size of Texas.

Now take a look at Pluto/Charon back in 1978 when James Christy, an astronomer at the U.S. Naval Observatory, could see it using the 1.55-m (61-inch) Kaj Strand Astrometric Reflector at the USNO Flagstaff Station in Arizona. Christy was studying what was then considered to be a solitary ‘planet’ (since demoted) when he noticed that in a number of the images, Pluto seemed to be elongated, a distortion in shape that varied with respect to background stars over time. The discovery of a moon was formally announced in early July of that year by the International Astronomical Union. Charon received its official name in 1985.


Image: What Pluto/Charon looked like to James Christy in 1978. Credit: U.S. Naval Observatory.

The New Horizons time-lapse movie shows an entire rotation of each body, the first of the images being taken when the spacecraft was 203 million kilometers from Pluto. The last frame, six and a half days later, was taken when New Horizons was 8 million kilometers closer. Alan Stern (Southwest Research Institute), principal investigator for New Horizons, notes the significance of the latest imagery:

“These images allow the New Horizons navigators to refine the positions of Pluto and Charon, and they have the additional benefit of allowing the mission scientists to study the variations in brightness of Pluto and Charon as they rotate, providing a preview of what to expect during the close encounter in July.”

That’s an encounter that will close an early chapter in space exploration — all nine of the objects formerly designated planets will have had close-up examination — but of course it opens up yet another, as New Horizons looks toward an encounter with a Kuiper Belt object as it moves ever outward. Just as our Voyagers are still communicating long after Voyager 2 left Neptune, New Horizons gives us much to look forward to.


Comments on this entry are closed.

  • Cherei February 13, 2015, 11:18

    The size of Texas.. Well, I’ll be! (a Texan too)

  • Leigh Kimmel February 13, 2015, 12:56

    Fascinating. One of the things I noted in my review of Ursula K LeGuin’s The Dispossessed is how little attention she paid to the astrophysics of a double-planet system. Anarres seems to be something like Mars was imagined to be in the 1940’s and 1950’s, while Urras is pretty much an Earth analog, with no consideration to the tidal effects of their relationship. Most obviously, it appears that each world can be seen at some time on any point on the surface of the other, whereas with realistic astrophysics, both worlds would probably become tidally locked to each other, and thus visible on only one hemisphere of the other world.

  • sglover February 13, 2015, 14:18

    Is Charon’s plane of revolution at a really high angle to the plane of Pluto’s orbit? It looks like New Horizons has an almost “top down” view of the system.

  • Ashley Baldwin February 13, 2015, 16:28

    A lot to learn about KBOs and the early solar system.Did you know that LORRI is one of only two optical instruments , the other is NIRSpec on JWST ,that NASA have that are made from silicon carbide composite. Incredibly versatile , crucially lightweight ,strong and very cheap . Perfect for space in cost conscious times. Mirror and structural material.Watch that space . This is a material with a big future and lots to give .

  • Astronist February 14, 2015, 6:50

    No need to be ungracious about what to call Pluto, or to talk about its having been “demoted”. What has changed is that planets used to be a single category, but may now be grouped into three: giants, terrestrial planets, and dwarfs. As Alan Boyle wrote in his book on the subject: “Even before Pluto was discovered, the solar system was divided into two classes of planets: the rocky worlds like Earth, and the gas giants beyond. Pluto has pointed the way to the solar system’s third great class of planets, no less important than the other two.”

    So far as I know, I am the first to propose precise, practical definitions which classify bodies orbiting a star into giant, terran or dwarf groups: http://www.astronist.co.uk/astro_ev/ae109.html

    Stephen A.

  • Christopher L. Bennett February 14, 2015, 10:27

    “That’s an encounter that will close an early chapter in space exploration — all nine of the objects formerly designated planets will have had close-up examination…”

    Well, not quite, since Ceres, Vesta, and a lot of the larger Main Belt asteroids were initially regarded as planets (or “minor planets”) for decades before being downgraded to asteroids. Ceres was counted as a planet in some texts for nearly as long as Pluto was. So by the end of this year, eleven, but not all, of the objects formerly designated as planets will have had close-up examination.

    You could say, though, that all nine objects that have been considered Solar planets during our lifetimes will have been visited.

  • Joy February 14, 2015, 19:43

    “silicon carbide composite. Incredibly versatile , crucially lightweight ,strong and very cheap . Perfect for space in cost conscious times. Mirror and structural material.Watch that space . This is a material with a big future and lots to give .”

    @ Ashley,
    Interesting. How cheap is this material? Would a silicon carbide composite 8 metre mirror be cheaper than the ones Roger Angel makes? I am thinking that a large part of the cost overruns on JWST were related to having to make a folding mirror to fit inside our existing payload fairings – that and the beryllium issues. Hopefully, if SpaceX keeps moving forward, we will see the FalconXX built which could launch telescopes with one piece 8 metre mirrors at a cost comparable to Hubble.

  • BillO February 15, 2015, 0:32

    Great images, but does it look a bit out of scale to anyone else? Shouldn’t Pluto and Charon be smaller in diameter, or the orbit much larger?


  • Rob Henry February 15, 2015, 2:43

    @Astronist, I think this year will retrospectively determine whether Pluto was demoted. We have visited Vesta, and it proved only moderately interesting, and now we come to Ceres and Pluto. If they prove as or more interesting as Io and Europa (and the pre-encounter known colour changes on Pluto are even more extreme than those for Io, with Ceres having potential for more water activity close to its surface than Europa), then your Alan Boyle quote could be spot on.

  • Michael Spencer February 15, 2015, 8:08

    @ sglover: I wondered about that as well. Pluto’s orbit is quite inclined to the plane, yes, but the images almost look ‘top down’. It’s possible they are much more oblique and that will become apparent soon, one supposes.

  • AABAR February 15, 2015, 10:02

    @Michael Spencer @sglover: Lurker here: Pluto’s axial tilt is almost 120° – so that could be said to be just over 90° to the ecliptic…

  • Ron S February 15, 2015, 12:46

    BillO, look at the pixellation of the animation images. These are taken near the resolution limit of the instrument. Smearing or bleeding into adjacent CCD cells is normal. The actual disks might in reality be no more than 1 pixel wide.

  • Brian Altmeyer February 15, 2015, 21:34

    @Astronist: “What has changed is that planets used to be a single category, but may now be grouped into three: giants, terrestrial planets, and dwarfs.”

    I would add the birth of a fourth category: Current exoplanet research into planets in the Earth-Neptune/Uranus gap is yielding what heretofore have been called “super-Earths” or “mini-Neptunes” to be consolidated into a class likely to be called Gas Dwarfs.

  • Keith Cooper February 16, 2015, 10:41

    What a great animation, incredible to see. If anyone is ever confused over how a planet can influence the motion of a star in radial velocity measurements, just ask them to imagine Pluto here as the star and Charon the planet.

    I concur with Stephen about categorising planets, but I’d take it further. It’s not just about labelling them, but about what they can tell us about how they formed and the region of the Solar System they formed in. We’ve been lumbered with this term ‘planets’ from antiquity, but now we find that this catch-all label is not appropriate, because for example, Jupiter is a completely different kind of world to, say, Mars. They formed differently, and those formation histories are fascinating in what they can tell us about the birth of the Solar System. Think of all the different types of worlds we have in the Solar System alone: we’ve got the terrestrial planets, the protoplanets (Ceres and Vesta), the gas giant planets (and the ice giants if you want to split them into their own category) and then the dwarf planets of the Kuiper Belt. You might even want to find a way to include the larger moons too. Each category has its own story to tell about how it formed. Maybe we need a classification system similar to the Hubble tuning fork or de Vaucouleurs sequence for galaxies.

    Pluto wasn’t demoted, just reclassified. But at the end of the day, Pluto is what it is, whatever we call it. Can’t wait for New Horizons to get there!

  • AABAR February 17, 2015, 1:21

    @Keith Cooper: You consider Ceres not a dwarf planet like Pluto? I don’t know much about planet formation, but what if Ceres was in the Kuiper belt (it will be another large round KBO/TNO?) & Pluto was in the main belt (that would be interesting, though it probably won’t have moons?) ? – we’ll see what Dawn & New Horizons will uncover very soon…

  • Keith Cooper February 18, 2015, 12:17

    @AABAR –

    Christopher Russell, PI of the Dawn mission, calls both Ceres and Vesta protoplanets. Under the IAU definition Ceres is a dwarf planet, but I’m talking about their formation histories, which defines their properties, rather than location or size (although size obviously comes into it). If Ceres was in the Kuiper Belt, it would have different characteristics to what it has in the Asteroid Belt, because it would have formed in a different part of the Solar System from different abundances of materials and in a different environment. Hence it would not be the same object. Because our current planetary definitions tell us nothing at all about the history of these worlds, I’m arguing that we need to base our definitions on what makes these worlds unique, and that can all be traced back to how and where they formed.

  • ljk February 20, 2015, 11:35

    Getting Closer! New Horizons Sees Two of Pluto’s Smaller Moons for First Time

    By Paul Scott Anderson

    The New Horizons spacecraft, on course for a historic encounter with Pluto this summer, is now close enough to see two of its smaller moons for the first time. The new views also come 85 years after the discovery of Pluto by astronomer Clyde Tombaugh on Feb. 18, 1930.

    The images were taken between Jan. 27 and Feb. 8, 2015, at distances ranging from about 125 million to 115 million miles (201 million to 186 million kilometers), and for the first time show the tiny moons Nix and Hydra, which are much smaller than Pluto’s largest moon, Charon. They are the first of a series of long-exposure images that will be taken until early March, which will allow the New Horizons team to refine the moons’ orbits ahead of the encounter on July 14.

    “Professor Tombaugh’s discovery of Pluto was far ahead its time, heralding the discovery of the Kuiper Belt and a new class of planet,” said Alan Stern, the principal investigator for New Horizons from Southwest Research Institute, Boulder, Colorado. “The New Horizons team salutes his historic accomplishment.”

    As New Horizons science team member John Spencer, also from Southwest Research Institute, noted, “It’s thrilling to watch the details of the Pluto system emerge as we close the distance to the spacecraft’s July 14 encounter. This first good view of Nix and Hydra marks another major milestone, and a perfect way to celebrate the anniversary of Pluto’s discovery.”

    Full article here:


  • ljk February 20, 2015, 16:14

    What if Pluto is actually a gift from the Galactic Federation:


  • ljk February 27, 2015, 11:05

    NASA Probe Bound for Pluto Carries Piece of Pioneering SpaceShipOne

    by Leonard David, Space.com’s Space Insider Columnist | February 27, 2015 07:00 am ET

    A small piece of spaceflight history will zoom through the Pluto system this July aboard NASA’s New Horizons probe.

    Inside New Horizons is a piece of SpaceShipOne, the first privately owned and operated manned spacecraft to reach space — which begins at an altitude of 62 miles (100 kilometers) above Earth — twice within a 14-day span. That 2004 feat bagged the SpaceShipOne team the $10 million Ansari X Prize, which was designed to spur the development of the private spaceflight industry.

    “It is a beautiful, curved, autoclaved carbon-fiber part about 3 inches [7.6 centimeters] long, removed from the SpaceShipOne’s pilot seat before it went to the National Air and Space Museum” in October 2005, said Burt Rutan, the lead designer of the famed vehicle and founder of the California-based aerospace firm Scaled Composites.

    “Yes, I was very proud to have a piece of SpaceShipOne at the edge of our solar system,” Rutan told Space.com. “In fact, it has brought me to tears talking about it.”

    Full article here:


    To quote:

    Two-sided inscription

    New Horizons was built at, and is being operated by, the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.

    According to APL spokesman Michael Buckley, the piece of SpaceShipOne is bonded to the aft deck of the spacecraft, or what would be considered the lower deck if the probe’s dish antenna were deemed to be at the top.

    Buckley told Space.com that the item is located opposite the panel that includes New Horizons’ Long Range Reconnaissance Imager (LORRI) and star-tracking cameras. Stern said that the small piece of SpaceShipOne is installed on New Horizons’ lower inside deck and sports a two-sided inscription.

    The front side of the inscription reads, “To commemorate its historic role in the advancement of spaceflight, this piece of SpaceShipOne is being flown on another historic spacecraft: New Horizons. New Horizons is Earth’s first mission to Pluto, the farthest known planet in our solar system.”

    The inscription on the back reads, “SpaceShipOne was Earth’s first privately funded manned spacecraft. SpaceShipOne flew from the United States of America in 2004.”

  • ljk March 2, 2015, 11:01

    Pluto Science, on the Surface

    Posted by Alan Stern

    2015/02/27 20:39 UTC

    New Horizons remains healthy and on course for its prime Pluto system science in July!

    As I write these words I am flying to a dress-rehearsal simulation (called a “sim”) of the closest approach portion of the Pluto flyby. About 40 science team members are involved—atmospheric scientists, geologists, geochemists, planetary astronomers and space physics experts. This is the second of three such multiday sims that the science team is doing to prepare for the flyby in July.

    Full article here:


  • Mark Zambelli March 2, 2015, 13:17

    @ Ron S
    Yep, both Pluto and Charon are much less than 1 pixel wide in those images. As an astrophotographer I see this all the time with any image of the starry sky… all stars are effectively point sources but of different brightnesses so as the exposure builds, the faint stars stay as small tight discs whereas the brighter stars become larger discs. This has a lot to do with atmospheric scintillation (causing the point star’s light to spread out over a small area of pixels) but also a lot to do with the inherrant dispersion in any optical train of any instrument due to the properties of the materials used.
    NH used 6 second exposures at the limit of it’s capabilities, as mentioned, so the very faint point sources appear several pixels wide once zoomed-in.

  • ljk March 11, 2015, 12:44

    What to expect when you’re expecting a flyby: Planning your July around New Horizons’ Pluto pictures

    Posted By Emily Lakdawalla

    2015/03/11 00:06 UTC

    New Horizons is now closer to Pluto than the Sun is to Earth: 1 astronomical unit, or 150 million kilometers. A tiny course correction burn steered them closer to their goal today. As New Horizons begins to approach Pluto, there’s a single question everybody keeps asking me: when will we get the first pictures? The trivial answer to that question is that we already have them.

    But as exciting as it is to know that New Horizons has sighted Pluto, a teeny dot wobbling among stars is not what people mean when they ask about New Horizons’ pictures. What people want, of course, is the portrait photo, the one we’re going to be seeing on magazine covers and in textbooks for years to come.

    As New Horizons approaches, every image of Pluto and Charon that each instrument returns will be the best it has ever taken. They will be thrilling to see. But until mid-July, all the images will still be pretty small. In the days leading up to the July 14 flyby, with only one exception, all of the images that we will receive of Pluto and Charon will be lower-resolution than the best Ceres pictures we have received from Dawn to date.

    Full article here:


  • ljk March 19, 2015, 8:58

    Pluto made an appearance at the beginning of the classic 1953 science fiction film The War of the Worlds produced by George Pal as shown in the YouTube link below. The artist was none other than Chesley Bonestell.


    It will be interesting to see how accurate he was come this July.

  • ljk April 2, 2015, 12:06

    Planetary Pioneer

    Tom Krimigis, exploring the solar system since 1965.

    By Heather Goss


    March 24, 2015

    Dr. Stamatios M. (Tom) Krimigis has his fingerprint on many of the unmanned space missions that have left this planet, from the Mariner spacecraft that did the first flyby of Mars in 1965 to the Cassini spacecraft that is still sending back data from Saturn today. He designed a particle detector that continues to operate on both Voyager 1 and 2 as they travel out beyond our solar system.

    To honor his long career at the Johns Hopkins University Applied Physics Laboratory, he was awarded with the 2015 National Air and Space Trophy for lifetime achievement. Dr. Krimigis spoke with departments editor Heather Goss in January.

    Full interview here:


    To quote from the interview (where the New Horizons connection comes in):

    But I have to say that now that we know what we know from Voyager, I think the country is missing a big opportunity to really further explore where our solar system is headed into the galaxy. We have been studying for years—by “we” I mean the whole scientific community—what’s called an interstellar probe that would go much faster than Voyager and get into the galaxy and provide data for the next 30-40 years, but unfortunately, the funding isn’t there to do such pioneering missions anymore.

    Do you think New Horizons will provide some of that data?

    Not really, New Horizons of course will do this exploratory mission of Pluto, and after that we’re going to try to head it to a Kuiper belt object, but it is not as fast as Voyager, simply because in addition to going by Pluto we will also do this KBO flyby.

    So New Horizons will not have either the communications capability or the speed, even if it lives as long as Voyager. We’re going to have to come up with a brand new mission with new technology and ways to speed it up a lot, maybe by getting the gravitational assist from the sun itself to really move it out at a speed at least three to four times that of Voyager. And I don’t see that happening, unfortunately.

    My dream for years had been to do a solar probe, and finally we are doing that here at this laboratory. It’s called Solar Probe Plus. It will get within six million kilometers of the sun after we launch in 2018. So we’re making progress—we have a mission to a star!

  • ljk April 3, 2015, 13:26

    New Horizons: Navigating to Pluto

    By: Alan Stern | April 3, 2015

    In the first of a series of installments written exclusively for Sky & Telescope, New Horizons principal investigator Alan Stern offers his behind-the-scenes perspective on the navigational effort needed to get the spacecraft to Pluto.

    It sounds like science fiction, but it’s not: New Horizons is on final approach to the Pluto system! After 111 months in flight, the fastest spacecraft ever launched is now less than 100 days from its destination.

    Closest approach of the flyby occurs on July 14th at 11:49:58 Universal Time, at a carefully selected miss distance just 7,800 miles (12,500 km) from Pluto’s surface.

    In this first “insider” blog about the mission for Sky & Telescope, I want to give you a look at an under-appreciated but crucial part of the approach to Pluto: navigating and homing in on the target point of the flyby. This activity involves the New Horizons team at the Johns Hopkins Applied Physics Laboratory in Maryland; our primary navigation team at KinetX Aerospace in California; and our independent navigation team at NASA’s Jet Propulsion Laboratory, also in California.

    Full article here:


  • ljk April 6, 2015, 9:54

    Another piece about how Clyde Tombaugh is on his way, in a sense, to become the first human being to leave the Sol system:


  • ljk April 10, 2015, 10:05

    NASA’s New Horizons Begins Pluto Approach Phase-2 for July Encounter

    By Leonidas Papadopoulos

    If one were to characterise NASA’s New Horizons mission, it would be fitting to call it the ultimate exercise in patience. Having spent almost a decade in the conceptual phase, the Pluto-bound spacecraft was eventually launched in January 2006 on a speedy trajectory toward the Solar System’s distant and unexplored region of small icy bodies beyond the orbit of Neptune, known as the Kuiper Belt. Having broken the record for the fastest man-made object to ever leave our planet—with an Earth-relative speed of 58,536 km/h—New Horizons nevertheless spent the better part of nine years quietly traversing the interplanetary void in order to reach its ultimate destination.

    Finally, following a journey of more than 4.5 billion km, the spacecraft emerged from its electronic hibernation one last time on Dec. 6 of last year, just in time to begin preparing for the final leg of its approach toward Pluto in mid-January, while marking the official start of the mission’s first Pluto encounter stage, called Approach Phase 1. Now, with less than 100 days remaining before New Horizons flies through the Pluto system on July 14, the mission has marked another milestone on its way to the distant dwarf planet by advancing to Approach Phase 2, during which Pluto science observations will start to swing into high gear.

    Full article here: