Helene of Saturnian Troy

By Phil Plait | March 11, 2010 7:24 am

The Cassini spacecraft recently passed very near the tiny moon Helene and returned amazing pictures of it.

cassini_heleneHelene is a dinky iceball, only about 36x32x30 km (22x19x18 miles) in size (this picture has an incredible resolution of about 113 meters (123 yards) per pixel). It circles Saturn in the same orbit as the much larger Dione, and is in fact in the larger moon’s leading Trojan point: a peculiar artifact of gravity when an object orbits another. It’s a gravitational stable point, like a valley between two mountains.

Clearly battered, Helene has an oddly smooth appearance, which may be due to the feeble gravity of the moon collecting dust also trapped in the Trojan point. At The Planetary Society Blog, Emily has more info on Helene and speculates about its appearance. She also has a good description of how the Trojan points work.

Credit: NASA/JPL/Space Science Institute

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures
MORE ABOUT: Cassini, Dione, Helene, Saturn

Comments (28)

  1. Yoweigh

    So this “Trojan” point is really a Lagrangian point? I’ve never heard it called that before.

  2. IVAN3MAN AT LARGE
  3. Oli

    Should this really be called a moon? It’s so tiny, and it doesn’t even really have its own orbit. I propose a new class of dwarf-moons, consisting of all the irregular moons (including big Proteus).

  4. Peter Davey

    IS THIS THE FACE THAT LAUNCHED A THOUSAND SPACESHIPS? ONE CAN ONLY HOPE.

  5. A meter is more than a yard (39.37 inches as opposed to 36), so something is wrong.

  6. Messier Tidy Upper

    @ 1. Yoweigh – Look up Lissajous orbit for an even better word!

    See : http://en.wikipedia.org/wiki/Lissajous_orbit

    @3. Oli – Moonlet maybe?

    @ 4. Peter Davey – I dunno, Helene’s face looks pretty blank & frankly a bit horror movie or the gas mask kid from an old Eccleston Dr Who episode to me! ;-)

    @ 2. IVAN3MAN AT LARGE : Sorry to leave you out but I can’t think of anything to say about your one. ;-)

    PS. No black hole in this one as was theorised for Promethus on the most recent carnival of space site then? ;-)

  7. Seconded to Phillip Helbig.

    Don’t worry, it’s a depressingly common mistake not to know whether to multiply or divide by a conversion factor.

  8. you got the meters to yards mixed up. Yards are shorter than meters so there should be more yards, the actual number 124 yards.

  9. Bandsaw

    These pics are great! You can see the dust has slid down inside the craters! I love seeing this level of detail.

  10. oldamateurastronomer

    Or it could be that the numbers were reversed since 103 meters is a little less than 133 yards!?!

  11. 103 meters=113.3 yards so the numbers could be reversed too.

  12. IVAN3MAN AT LARGE

    At the CICLOPS website — Helene "Rev 127" Flyby Raw Preview #2 — it actually states: “Image scale is 113 meters (370 feet) per pixel.”

    Therefore, 113 metres (370 feet) = 123.6 yards.

  13. Bubba

    What is important about knowing anything about such a small object in such a large planet’s orbit?

  14. Dale

    Here’s an only slightly offtopic question for y’all… would placing a similarly sized asteroid in the same place in Earth’s orbit help to clear any of the debris that threatens our space craft?

  15. Gary Ansorge

    12. Bubba:

    Important? What’s important about an infant?

    ALL knowledge is potentially important. It just takes time to see how it applies to our very human problems. NO human can see all the implications up front. We’re at our best when breaking reality into bite size.

    13. Dale:

    Probably not. The average distance between bits of debris in earth orbit is large and at varying angles/orbits, so it would likely take several million years to clear earths orbital debris.
    ,,,but if we wait THAT long, atmospheric drag by itself should result in all that debris falling to earth,,,eventually,,,

    Gary 7

  16. MoonShark

    I’d always heard them called Lagrangian points too, or L1, L2, etc. Seems more common and it gives credit to a mathematician. Not sure why Phil prefers Trojans here, but hey, I’m not the astronomer.

  17. Chip

    I love the mostly shaded image. Fantastic. Looks like a Bonestell painting.

  18. I believe Trojan is the imaginative Greek-themed name for the moons that orbit WITHIN the Lagrange points, not the term for the points themselves.

    For example, Tethys, a 662-mile-wide moon, has two “Trojans”, a trailing one (Calypso) and a leading one (Telesto). These twentysomething-mile-wide moons each orbit Saturn within the same path as Tethys, 60º before and behind.

    Where they came up with the term, I dunno. Another IAU edict, I guess.

  19. Bubba

    @14:
    Infant implies young. Is this moon young and in the process of gathering material and becoming larger, or has it been around and in it’s current form for eons? It sure looks smooth, which, as I’ve learned, means that there have been no impacts for a long time. Which means that it is no longer growing. Which means it’s old.
    As for the philosophical bent of your answer, not helpful. What I want to know is, since we’ve known about shepherd moons for c. 30 years now, what do we know that they *do* or are for? Or is it irrelevant because they’re just cute and small. Like an infant.

  20. Asteroids in Jupiter’s L4 and L5 points were the first bodies discovered in that relationship to a planet. Asteroids in both groups were named for heroes from the Iliad. One group was named mostly for the Trojans, and one group mostly for the Greeks (I believe there was an odd one out in each group: a “spy in the enemy camp.”)

    For some reason (maybe because the Trojans were the leading group, although I don’t know that for sure) “Trojan asteroids” became the generic term for bodies in that arrangement.

    Although there are 5 Lagrangian points, “Trojans” applies only to the L4 and L5 points, so it is a more specific term than “Lagrangian.”

  21. jcm

    … leading Trojan point: a peculiar artifact of gravity when an object orbits another. It’s a gravitational stable point, like a valley between two mountains.

    Is this the same as Lagrangian points?


    BTW, there is a show on PBS about the demotion of pluto as a planet hosted and narrated by none other than Neil DeGrass Tyson himself.

    http://www.pbs.org/wgbh/nova/pluto/
    The program can be watched online at:
    http://video.pbs.org/video/1425502261

  22. Wayne on the plains

    What Don said, although I’ve always just used the term for the Jupiter-associated asteroids specifically, but it’s a logical extrapolation to use it as a generic term for bodies in any L4/L5 points. Does anyone know if this is the first body in a “Trojan Point” other than Jupiter’s Trojans?

    Nevermind, two seconds on Wikipedia answered my question. Mars and Neptune also have “Trojan” asteroids.

  23. Is there any chance that this could be a captured comet?

  24. tresmal

    I don’t know if this is a dumb question or not, but here goes. Let’s say that somehow we get our hands on 5 more Earth sized masses, (Where? Maybe Ebay?) and distribute them at 60° intervals in Earth’s orbit so that there are 6 Earth masses orbiting the sun at 1 AU. Would this be a stable configuration?

  25. Plutonium being from Pluto

    @ 23. Colm Says:

    Is there any chance that this could be a captured comet?

    I’d say there’s certainly a chance of that – although its quite a way inside the Saturnian system* & in a fairly regular orbit so equally or more likely to have formed there or be a fragment split from Dione or another moon.

    * Saturn’s moons or moonlets going outwards from the planet : Atlas (within A ring), Pan (in Encke division), Prometheus & Pandora (within F ring), Janus, Epimetheus, Mimas, Enceladus, Tethys & Telesto & Calypso (co-orbital), Dione & Helene (co-orbital), Rhea, Titan, Hyperion, Iapetus, Phoebe & more.

    That’s not an exhaustive list but the main ones. As Astronomica (Fred Watson – Chief Consultant, Millennium House, 2007, page 86) notes :

    Saturn has 57 known moons and a few suspected ones, not counting all the individual particles and chunks within the rings which technically can all be considered satellites of the beautiful ringed planet. … A large bunch of tiny moons were added to the list in 204 and 2006; many of them are in retrograde orbits – they orbit the planet in the opposite direction to its rotation and are almost certainly objects captured into orbit by Saturn’s gravity. Only 35 of the 57 moons discovered around Saturn have, so far, been given names …

    Whether an object such as Helene was ever a comet or not also depends on how close it got to the Sun – if it was too far out it remains a tail-ess -coma-less cometary nuclei or Trans-Neptunian object or Centaur type body.

    @24. tresmal Says:

    I don’t know if this is a dumb question or not, but here goes. Let’s say that somehow we get our hands on 5 more Earth sized masses, (Where? Maybe Ebay?) and distribute them at 60° intervals in Earth’s orbit so that there are 6 Earth masses orbiting the sun at 1 AU. Would this be a stable configuration?

    Not a dumb question but no probably not – Earth’s Moon was probably formed when a Mars-sized object formed in a Lagrangian (or Lissajous) type orbit and then became unstable and drifted into earth causing the Big Splash. I don’t think a system with more than one Earth-mass planet in Earth’s orbit would be stable.

    @22. Wayne on the plains Says:

    … Does anyone know if this is the first body in a “Trojan Point” other than Jupiter’s Trojans? Nevermind, two seconds on Wikipedia answered my question. Mars and Neptune also have “Trojan” asteroids.

    In fact, Neptune may turn out to have many more “trojans” than Jupiter – up to twenty times as many Trojan asteroids sharing its orbit as Jupiter according to a study by Scott Sheppard of the Carnegie Institution of Washington and Chadwick Trujillo of the Gemini Observatory in Hawaii using the 6.5-meter Magellan telescope in Chile.

    See : http://www.space.com/scienceastronomy/070130_st_neptune_trojans.html

  26. IVAN3MAN AT LARGE

    @ Messier Tidy Upper (#6),

    Dude, what’s your problem with the link to the “Trojan (astronomy)” article in Wikipedia, that I have provided at my post (#2) above?

    I would suggest that you check that page again, and then you will see that there is a link to the “Trojan moon” article which refers to the moon Helene.

    Whereas your link to the “Lissajous orbit” article is irrelevant in this case because it applies to artificial satellites only.

    So there! :P

  27. Messier Tidy Upper

    @ 26. IVAN3MAN AT LARGE Says:

    @ Messier Tidy Upper (#6), Dude, what’s your problem with the link to the “Trojan (astronomy)” article in Wikipedia, that I have provided at my post (#2) above?

    No problem with your link at all. I like it too – I just like using & introducing to others the word Lissajous as well! Is that so bad? ;-)

    Whereas your link to the “Lissajous orbit” article is irrelevant in this case because it applies to artificial satellites only.

    Hey, just because it mainly applies to artificial satellites doesn’t mean it can’t also be used for natural satellites in the same type of orbit does it?

  28. IVAN3MAN AT LARGE

    Messier Tidy Upper (# 27):

    [J]ust because it mainly applies to artificial satellites doesn’t mean it can’t also be used for natural satellites in the same type of orbit does it?

    Au contraire, mon ami. The orbits at Lagrangian points L4 and L5 are fairly stable, so it’s not a problem for natural satellites to maintain an indefinite orbit at those points; whereas a Lissajous orbit around an L1/L2 Lagrangian point is unstable and, therefore, artificial satellites must use their propulsion systems to perform orbital station-keeping.

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