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The Angle on Pluto

The progress of New Horizons through the outer Solar System has me thinking back to Voyager’s great encounters. In 1986, when Voyager 2 whisked past Uranus, I was about to head off for a weekend of intensive work as a flight instructor — a client we had contracted with had a large number of pilots in need of recurrent training, and I knew I would be in the cockpit well into each night, as indeed I was.


Those long days and the memory of Voyager at Uranus are, of course, tinged with the explosion of Challenger, which took place a scant four days after Voyager’s closest approach to the planet. We were all riveted by the coverage of the event but could only catch it in between training flights, but I remember trying to keep my mind off the fallen Shuttle as we dealt with constantly challenging weather over Maryland and West Virginia. And it was only later that I was able to really sit down and go over the images from Uranus, whose system of moons had always intrigued me — it has continued to do so, especially after my first glimpse of Miranda.

Image: Launch of New Horizons atop an Atlas V rocket. Credit: NASA/KSC.

Triton had much the same effect on me later. Neptune was in the summer of 1989, twelve years after launch, and I remember taping the encounter on a couple of VCR tapes that I still have around here someplace. New Horizons, we can hope, will offer up equal wonders, for it seemed that everything that Voyager — and for that matter, Cassini — saw has re-written our knowledge and assumptions about outer system objects. We’re assuming, of course, that New Horizons can stay healthy through the encounter, which is the subject of principal investigator Alan Stern’s most recent update from the Applied Physics Laboratory at Johns Hopkins University.

Parameters of the Encounter

There are ways of maximizing science, something we definitely want to do, and ways of minimizing danger, and the New Horizons team has to balance between the two. After all, with the discovery of a fifth moon in the system in 2012, rising concern that passage through Pluto/Charon space might be hazardous was inevitable. Stern says a NASA-appointed technical review team and a separate group of senior executives at NASA headquarters has signed off on the findings of the New Horizon team. He adds:

The Pluto system appears to be far safer than early fears and initial calculations indicated when the new moons began popping up. In fact, the best current models predict a 0.3% (1-in-300) chance of a mission-ending impact near closest approach on the nominal trajectory. Much of the reason for this lowered risk assessment is that more sophisticated dust-impact models revealed a decrease (by about a factor of 100) in lethal impact probability for trajectories that fly into the region where New Horizons is aimed now – a region where the gravitational effects of Pluto’s largest moon Charon clear debris. Another important factor is that when we tested spacecraft components against high-velocity impacts using gun ranges in New Mexico and Ohio, we found the spacecraft shielding is considerably “harder”– that is, more resistant to impacts – than preflight estimates indicated.

How will New Horizons maximize its chances? For one thing, the spacecraft’s approach trajectory is steeply inclined to the plane of Pluto’s satellites and any debris that may accompany them. The highest risk would be during closest approach. And that closest approach is in the region where Charon effectively cleanses the area of debris. The encounter plan, then, seems sound despite the recent satellite discoveries. Add to that the fact that New Horizons will itself search for hazards during the final weeks of its approach in the summer of 2015.


Image: The New Horizons trajectory (red line) is steeply inclined to Pluto’s satellite plane, thereby restricting satellite debris hazards – which lie near the satellite plane – to the short time near closest approach. Credit: JHU/APL/Alan Stern.

Stern adds:

…we’ve also added “fail safe” data downlinks just two days and one day before the encounter to send home the best images and spectra stored on the spacecraft’s recorders, just in case our current estimates are wrong and we do lose New Horizons at closest approach.

Moreover, New Horizons has two alternate encounter sequences that can be uploaded to the spacecraft in the final weeks if needed. Stern’s post gives an explanation of these SHBOT’s (Safe Haven by Other Trajectory), in which the spacecraft can be repointed to protect its dish antenna, or the spacecraft itself directed toward a closer encounter with Pluto just inside 3000 kilometers from the surface (the planned encounter is at 12,500 kilometers). Going closer in allows more ‘drag clearing’ of debris particles, caused by Pluto’s tenuous upper atmosphere.

Let’s assume that prudent planning like this will give us a survivable encounter in 2015. New Horizons’ first image of Pluto/Charon is scheduled to be made this July, which happens to be the 35th anniversary of the discovery of Charon. The craft recently emerged from hibernation for summer encounter rehearsals and systems checks, and green onboard beacons continue to tell us that all is well. Ahead of us we have not just Pluto but Charon, a moon the size of Texas (Nix and Hydra are about the size of Rhode Island, according to the New Horizons team, while P4 and P5 are just the size of counties). Interestingly enough, Nix and Hydra are also about the size of the Kuiper Belt objects the team is hoping to flyby years after the Pluto/Charon encounter.


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  • ljk May 29, 2013, 18:03

    Lesser-known views of Uranus and Neptune

    Posted By Emily Lakdawalla

    2013/05/28 10:58 CDT

    Topics: pretty pictures, amateur image processing, Uranus, Neptune, Voyager 1 and 2, Hubble Space Telescope

    Today I needed to come up with a list of great and unusual Neptune and Uranus photos to recommend to another space writer, and I figured that the best way to go about that was to write a blog entry!

    I claim that despite the fact that Voyager 2 returned relatively few high-resolution images from either of those worlds, there are many more photos in the archives than regularly make it to public view. You can visit JPL’s Planetary Photojournal to see all of these — 48 press-released photos for Uranus and its moons, and 74 for Neptune and its moons. These contain classics such as this ultramarine-blue Neptune with its great dark spot and this much paler Uranus and its freakishly exaggerated “bull’s eye” version. Together, those two images have resulted in the common depiction of Uranus as much lighter or paler (and often, as in this version by Calvin Hamilton, greener) than Neptune.

    In fact, though, the two ice giants are near-twins, which you can see if you compare these two versions by Icelandic amateur image processor Björn Jónsson. At the time that Voyager 2 flew past, Uranus was grayer and less feature-rich than Neptune, but both shared the same serene methane-blue color.

    Full article and images here:


    Ironic that New Horizons will be flying through the Pluto system in a similar path as Voyager 2 did for Uranus in 1986, as both worlds are pretty much tipped on their sides, so to speak.

  • Dave Moore May 29, 2013, 19:55

    I watched Voyager’s first encounter with Triton at the Planetary Society’s Planetfest in Pasadena. The closest approach was at 5 a.m. Pacific Time, and I and a friend decided to stay up all night to watch it live.

    While the Planetfest was still open, by 3 a.m. it was almost deserted. We had the big screen and the screening room to ourselves, and we watched, uninterrupted, as every 5 minutes or so a new, more detailed image would scroll down the screen (or we did after we bullied the Planetfest video technician into stopping playing the endless Planetary Society advertisements).

    We watched the moon in growing detail speculating and arguing about the features as they manifest themselves: “Those black things are craters!”
    “No, they’ve got to be volcanoes–too zit like.”
    The moon was fascinating melange. We spotted bits of the Moon, Mars, Mercury, and Io, and we had no idea what to make of the cantaloupe terrain.

    The scientists and dignitaries at JPL were tucked up in their beds. I don’t think JPL had a live feed out except to Planetfest. Except for the crew monitoring the incoming feed, we were the first people to see Triton close up, to visit the place. Knowing this and and watching the moon growing larger on the big screen, I can say that this was a close as I will come “to boldly go, to visit strange new worlds.”

  • David Stafford May 29, 2013, 22:18

    I’ve watched every one of these since the Ranger missions smacked the moon, live on TV. To think – to realize – to understand – that I’ve actually lived to see the Grand Tour… It’s been a wild ride. I’ve got another 20 years left in me (if Seattle rush hour doesn’t get me), so it’s time for the people at breakthrough propulsion physics to pull a rabbit out of their hat. I AIN’T DONE!!!

  • Laura May 30, 2013, 4:02

    What a wonderful time the grand tour was! After growing up fascinated by the blurry images of the planets that were the best available at the time, I found myself being treated to that incredible ten year series of encounters that brought those planets and their moons (and rings) so vividly to life. I felt lucky to be living at such a wonderful time.

    Although the Jupiter and Saturn encounters were the richest in terms of science and imagery, my memories of the Neptune encounter include a personal aspect that has always made it special. NASA had arranged for the
    live images of the closest approach portion of the flyby to be distributed in real-time to a number of centers around the country, one of them being the Hayden Planetarium at the Museum of Natural History in New York. At the time, I was co-founder of a company that developed imaging software. A few weeks before the encounter I got a call from one of our marketing people, who explained that one of our imaging products was going to be used to acquire, store, and display the close encounter images as they came in to the planetarium in real time, and asked if I would be willing to be on site at the museum to oversee the process.

    Willing indeed! My daughter was 8 years old at the time, and we knew the museum and planetarium well, so although it would be considerably past her bedtime (the closest approach would be around 10pm Eastern Time), it seemed like the perfect opportunity to take her along for an experience she would enjoy and probably long remember.

    The night arrived, and after a quick cab ride and a brief exchange with a security guard, we headed into the museum toward the planetarium, our footsteps echoing in the familiar but darkened hallways. When we entered the planetarium, we encountered more of a media event than I had expected. In the middle of the room was a projection screen displaying the blue disk of Neptune in beautiful detail, while TV and print reporters were busy setting up cameras and interviewing the experts from the planetarium. I remember a TV news person doing a rehearsal of her upcoming live segment. Each time she said Neptune she pointed to the tiny disk of one of the moons instead of the huge image of Neptune. During a break, I mentioned to her that the big blue disk was Neptune. She got it right in the actual broadcast. My contribution to journalistic accuracy.

    A photographer for one of the major newspapers asked me if he could take a picture with my daughter looking up at the image of Neptune on the projection screen. I was later able to show her the picture as it appeared in magazines and newspapers. She had mixed feelings about the picture when she saw it in print, since the kick of seeing herself as a witness to history was offset by the fact that she was at that awkward stage where with her braces and thick glasses she did look rather like a classic geek. She’s long past that stage today, and working on her research PhD (psychology, not astronomy, but hey, I’m interested in the neurosciences too). Apples, trees, I guess.

    I can’t wait for Pluto.

  • Michael Spencer May 30, 2013, 7:11

    The trajectory is a bit confusing. I suppose that I thought all bodies in the solar system were in the same plane–not exactly, but close, and I do understand that Pluto is inclined somewhat (17°, to be exact). But the diagram Paul provides show NH approaching at something like 90°.

    The other part of the puzzlement: frequently I read that changing planes is very fuel intensive–in this case the plane has changed by around 73°.

  • Eric May 30, 2013, 13:43


    Great observation! Until your question prompted a google search, I didn’t know that, like Uranus, the Pluto system is “tipped over”. Visit this website:
    and scroll down to the Pluto section. There is a blue diagram which explains all.

  • Daniel May 30, 2013, 22:12

    Thank you to ljk and Eric for the two good and informative links and to Paul for another interesting and educational article. Keep ’em coming, please!

  • Sedjak May 31, 2013, 21:55

    So Pluto has not cleared its orbit and that is why it is not considered a full-blooded planet. But it is so inclined to the ecliptic at 17 degrees that from certain web images of that part of the solar system, it looks like its relatively isolated in that orbit. Super large zoomable 3-D models would help here. Anyone care to elaborate?

  • ljk June 18, 2013, 9:26

    18 June 2013

    ** Contacts are listed below. **

    Text, image, and video:



    ** Synopsis: Astrophysicists from the Complutense University of Madrid have confirmed that Crantor, a large asteroid with a diameter of 70 km, has an orbit similar to that of Uranus and takes the same amount of time to orbit the Sun. Researchers have demonstrated for the first time that this and a further two objects of the group of the Centaurs are co-orbital with Uranus. **

    Uruguayan astronomer Tabarï¿œ Gallardo suggested in 2006 that the asteroids Crantor and 2000 SN331 complete their orbits of the Sun in the same time period as Uranus — an orbit of approximately 84 Earth years. Now two researchers at the Complutense University of Madrid (UCM, Spain) have confirmed that in the case of Crantor this is true.

    “The simulations we have carried out in the Data Processing Center of the UCM indicate that 2000 SN331 does not have 1:1 commensurability with Uranus, but Crantor does, which means it orbits the Sun in exactly the same time period as the planet,” Carlos de la Fuente Marcos, one of the authors of the study, explains to SINC.

    In addition, Crantor’s orbit has a very similar semi-major axis to that of Uranus, although its eccentricity and inclination vary. The trajectories, figures and animations are published in the journal Astronomy & Astrophysics.

    “This 70 km-wide asteroid’s orbit is controlled by the Sun and Uranus but is unstable due to disturbances from nearby Saturn,” states De la Fuente Marcos.

    The researcher also reveals that they found another object, which has been named 2010 EU65 and moves in a similar orbit to Crantor’s, “although much more stable because its trajectory is less eccentric.”

    Similarly, the latest data of a third asteroid, 2011 QF99 — the discovery of which was made public only a few weeks ago — also indicate that its orbit is in line with that of Uranus.

    According to the Minor Planet Center, the regulating organization for the naming of asteroids and comets, the three objects that “follow” Uranus belong to the group of the Centaurs. These icy planetoids endowed with a mythological name orbit the Sun between Jupiter and Neptune.

    “Crantor, 2010 EU65 and 2011 QF99 are the first bodies to be documented as co-orbiting with Uranus,” affirms De la Fuente Marcos, “although with distinct movements and trajectories.”

    Horseshoe and Tadpole Orbits

    From the point of view of an observer rotating along with Uranus, both Crantor and 2010 EU65 have “horseshoe” orbits, since they acquire this form as they move towards and away from the planet. In fact, these two centaurs periodically have close encounters with Uranus.

    However, 2011 QF99 maintains a more stable, Trojan or “tadpole” orbit, which means that it moves 60 degrees in front of Uranus. This asteroid always maintains a relatively large distance from the planet.

    The scientists calculate that the orbits of these three objects associated with Uranus could remain stable for a few million years. In astronomical terms this is not very long. With the Data Processing Center’s simulations, the same team has identified three new Mars Trojan asteroids with stable orbits of up to 10,000 million years.

    Media Contact:

    Enrique Sacristï¿œn Lï¿œpez
    Agencia SINC
    +34 91 425 09 09 x251

    Science Contact:

    Carlos de la Fuente Marcos
    Universidad Complutense de Madrid


    * Carlos de la Fuente Marcos y Raï¿œl de la Fuente Marcos. “Crantor, a short-lived horseshoe companion to Uranus.” Astronomy & Astrophysics 551: A114, March 2013.


    * Carlos de la Fuente Marcos y Raï¿œl de la Fuente Marcos. “Three new stable L5 Mars Trojans.” Monthly Notices of the Royal Astronomical Society Letters 432: 31-35, May 2013.




  • ljk June 18, 2013, 9:29

    Great News: New Horizons to “stay the course” at Pluto

    Posted By Emily Lakdawalla

    2013/06/17 02:47 CDT

    Topics: mission status, New Horizons

    This is extremely good news: after more than a year of analysis, the New Horizons mission and NASA have concluded and agreed that New Horizons’ originally-planned trajectory past Pluto is likely safe from dust. Dust has been a worry for New Horizons ever since a team led by Mark Showalter discovered Pluto’s fourth moon two years ago. Further intense searches of the Pluto system yielded the announcement of a fifth moon and the intensification of the worry a year later.

    The problem with all those little moons is that little impacts onto little moons produce lots of dust; the region near Pluto in the neighborhood of the moons is likely thick with it. Well, not thick, actually the amount of dust is very small in terms of mass, but just one particle of the right size in the wrong place at the wrong time could kill New Horizons before it finished taking data at Pluto. This would be bad for any space mission, but especially bad for New Horizons. Other flyby missions, like Voyager, returned a lot of their data in real-time; a mission-ending disaster would at least have seen data back to Earth from up to the moment of the end.

    New Horizons will be recording tons more data than the Voyagers, but returning very little of that haul to Earth before the close-approach phase of the mission. There’s only so fast it can transmit data, even with a 2.7-meter-diameter dish, from all the way out at Pluto; and time pointed at Earth is time taken away from the precious moments close to Pluto when it would rather be doing science observations. So the nominal plan has New Horizons taking a year or so after passing Pluto to get all the precious data back. Which means it had better survive its passage through Pluto’s potentially dusty system.

    Full article here: