Cassini eavesdrops on orbit-swapping moons

By Phil Plait | April 8, 2010 2:00 pm

The Cassini spacecraft just had a few close encounters with some of the odder moons in the Saturn system… and given how weird Saturn is, that’s saying something. I was particularly enthralled with these two small worlds:

cassini_janus_epimetheus

On the left is the moon Janus and on the right is Epimetheus. The scales are not quite the same; Janus is roughly half again as big as Epimetheus’ size of 135 x 110 x 105 km (81 x 66 x 63 miles). Cassini was a little over 100,000 km from Epimetheus and 75,000 km from Janus when these images a were taken.

These are raw images, so they haven’t been processed yet to remove cosmic ray hits, brightness variations, and so on. But they are still fascinating. Epimetheus looks to me exactly how I picture a big asteroid; beaten, battered, looming. The low angle of sunlight on the side accentuates the craters there, making this almost a caricature of what an asteroid looks like. Technically it’s not an asteroid; it’s a moon. And even if it weren’t orbiting Saturn we might not call it an asteroid; it has a high reflectivity indicating a lot of ice on the surface (and a low density consistent with that too). If it orbited the Sun on an elliptical path, we might very well call it a comet!

But there’s more to these moons. Amazingly, Janus and Epimetheus are on almost — but not quite — the same orbit around Saturn! Currently, Janus is a bit closer to Saturn than Epimetheus.

I say "currently", because every four years these moons swap orbits! Since Janus has an orbit slightly closer to Saturn, it is moving faster around the planet than Epimetheus. It slowly but eventually catches up to the outer moon. As they approach, Janus pulls back slightly on Epimetheus, and Epimetheus pulls Janus forward. In other words, Janus steals orbital energy Epimetheus! This means Epimetheus drops into a slightly lower orbit, and Janus gets boosted into a slightly higher one, effectively swapping the orbits of the two moons. Although the two orbital paths are separated by only about 50 km (30 miles) — smaller than the radii of either moon — they never collide. The swap takes place when the moons are still more than 10,000 km apart, so they never get a chance to bump uglies.

How did this weird situation arise? Perhaps, in the distant past, there was one bigger moon orbiting Saturn, and it got whacked by an interloper. The moon disrupted, breaking into two big pieces and lots of littler ones. The debris got cleaned up by the gravity of the two big pieces and other gravitational effects, leaving these two square-dancing satellites on slightly different but still interacting paths.

However, the actual cause of this still isn’t known for sure. Cassini observations like this one may help astronomers figure out how it is these two little moons came to be, and why it is that although they can always approach each other, they can never actually touch.

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (29)

  1. Chip

    This has probably been asked before but if you were an astronaut standing on Janus with one of those mini strap-on pulse rocket packs that actual astronauts don’t have yet but do have in old SciFi movies, and you jumped off Janus as it approached Epimetheus, would you make it over to touch down on Epimetheus?

  2. Brian

    That’s very weird. If you had asked me I would have guessed that such a balanced setup was impossible, at least in practice.

  3. The Other Ian

    It is very weird. It doesn’t seem to be all that unusual, though, since there are several known asteroids that have similar relationships with the Earth (although of course none of those have the distinction of being nearly the same size, as Janus and Epimetheus are). See http://en.wikipedia.org/wiki/Horseshoe_orbit

  4. Joey Joe Joe

    Orbit-swapping moons? Is that some kind of nerd porn?
    ;)

  5. qbsmd

    Is there any evidence that they are related beyond the orbits (same composition, etc)? If so wouldn’t this situation be close to what you’d expect for a captured comet that wasn’t cohesive enough to survive the tidal forces in the orbit where it ended up?

  6. IVAN3MAN AT LARGE

    Chip (#1):

    [I]f you were an astronaut standing on Janus with one of those mini strap-on pulse rocket packs…, and you jumped off Janus as it approached Epimetheus, would you make it over to touch down on Epimetheus?

    Err… no, because the timing and magnitude of the momentum exchange is such that as the moons "trade" orbits, they never approach closer than about 10,000 km.*


    *Source: Wikipedia — Epimetheus.

  7. qbsmd

    IVAN3MAN AT LARGE #5

    The delta V needed to get from one to the other isn’t really related to distance. The same Wikipedia page gives the escape velocity as 32m/s. Assuming a rocket with an exhaust velocity of 666m/s (see Wikipedia Cold_gas_thruster), you’d need about 5kg of compressed gas for every 100kg of stuff you want to move off of Epimetheus. If you picked the right direction, you’d hit Janus eventually. Hopefully you brought enough oxygen for the trip, and have enough compressed gas left to give yourself a soft landing.

  8. IVAN3MAN AT LARGE

    ADDENDUM: After the orbital exchange, Epimetheus would then, as the inner moon, start to accelerate away from Janus.

  9. The Other Ian

    Yeah, it would be a 4-day trip, so you’d want to launch a couple days before the time of closest approach in order to arrive a couple days after, for maximum efficiency.

  10. PG

    Would it be accurate to say that these two moons make up a binary moon system whose center of mass is in orbit around Saturn? (i.e., they are orbiting each other) It seems to me that if the Moon took a several years to orbit the Earth, the Earth-Moon system might appear to do the same thing while it orbited the Sun. Just curious.

  11. The Other Ian

    PG,

    No, they’re not gravitationally bound to one another. If they were, you would never find them on opposite sides of the planet. The Epimetheus – Janus system is analogous to the Earth – Cruithne system, not to the Earth – Luna system.

  12. IVAN3MAN AT LARGE

    ADDENDUM II: After rechecking my math, I find that Epimetheus, after the orbital exchange, would only be accelerating away from Janus by just 2.227 m/s-1. Hmm… not much.

  13. Chip

    Thanks folks for the answers to my question. Now I think it probably would be better to launch in a nice little shuttle craft with plenty of sandwiches on board. The view of Saturn would be terrific.

  14. amphiox

    In a sense #1′s question can be answered in the affirmative for any two objects in the universe, given that an astronaut’s spacesuit is for all intents and purposes a small, man-shaped spacecraft.

    Arriving on destination alive, of course, is a question of logistics. . . . .

  15. Argus

    Some “back of the envelope” calculations, taking into account that Janus has a larger escape velocity, suggest that a delta-V adequate for a gentle takeoff and landing is about enough to cover such a distance in three days (assuming you want to stop at the end). If your rocket pack or shuttle or whatever can knock that travel time down, it shouldn’t have any trouble with the gravity of the moons.

  16. Chip

    Here is an animation of a succession of images of the actual objects exchanging orbital positions. The faint star was identified as HD 217167.
    http://www.greuti.ch/cassini/fb_epimetheus_n00043503-36.gif

    Here are the successive images overlaid on a selected star field background with star HD 217167 and galaxies in the distance.
    http://www.greuti.ch/cassini/epimetheus_janus_starfield.gif

    Source:
    http://www.unmannedspaceflight.com/index.php?showtopic=1842

  17. John Baxter

    I don’t think we can say the Epimetheus and Janus can never touch. We haven’t see never yet, and something might come along and perturb the system.

    None of which detracts from the fascinating situation.

  18. How wonderfully weird! I adore the fact that out the more we know, the more we don’t know. :-)

  19. On a completely unrelated note, square dancing moons is only one letter away from square dancing morons:

    http://www.youtube.com/watch?v=OnJRFRd8lEo

  20. Messier Tidy Upper

    Great images & post here – Thanks Cassini & Dr Plait. :-)

    @14. amphiox Says:

    In a sense #1’s question can be answered in the affirmative for any two objects in the universe, given that an astronaut’s spacesuit is for all intents and purposes a small, man-shaped spacecraft. Arriving on destination alive, of course, is a question of logistics. . .

    Except for propulsion and the ability to escape certain gravity wells & the fact that “objects in the universe” covers a *very* wide range of things from moons and comets to stars and Hot Jupiter’s and galaxies and gas clouds .. yeah! ;-)

    @17. John Baxter Says:

    I don’t think we can say the Epimetheus and Janus can never touch. We haven’t see never yet, and something might come along and perturb the system. None of which detracts from the fascinating situation.

    True & this makes me wonder about the long term stability of this co-orbital pairing.

    What about the gravitational influence of the other moons in the Saturnian system over time? Could this really be stable for millennia or is it just a temporary situation that will eventually end in collision or ejection? If so, over what time-scale?

    Plus could Cassini or another human spacecraft flying past and using a gravitational slingshot technique – enough times – exchange enough momentum to alter this neat set-up?

  21. jcm

    They look like potatoes.

  22. fernly

    Although the image is small, Janus’s surface looks a LOT like the Phobos pic you published just last week (http://blogs.discovermagazine.com/badastronomy/2010/03/31/more-incredible-phobos-imagery/). In particular I think I see the same kind of grooves or striations that are “thought to be from boulders rolling around in the low gravity” — a weird idea I’d like to see explained better.

  23. Brian:

    That’s very weird. If you had asked me I would have guessed that such a balanced setup was impossible, at least in practice.

    Do you know how long it took FSM to get that “just right”? That’s where the rings came from — all those “oops, they collided again” mistakes add up.

  24. Faultystring

    This is really cool, but will they both eventually lose enough energy and fall into Saturn’s atmosphere? There must be some energy lost after each change in orbit.

  25. Faultystring:

    There must be some energy lost after each change in orbit.

    Why? Where would the “lost” energy go?

    I suppose it’s possible that there might be some tidal forces generating heat. But the exchanging of orbits itself doesn’t sound like any energy would be “lost” there, any more than energy is “gained” by gravitational boosts.

  26. Faultystring

    I’m way out of my league, it just seems to me that the energy transfer can not be 100% each time, can it?

  27. qbsmd

    Faultystring,

    Energy is conserved, so when you say energy loss, I assume you mean irrevocable transfer to heat. That does happen; some of the energy deforms the shape of the object. That phenomenon causes some moons to be volcanically active, and keeps Europa’s interior liquid. There is also a conservation of angular momentum which my intuition says would probably result in the moons just getting less and less close to each other at each pass until they eventually end up permanently on opposite sides of the planet. Someone would have to do a calculation to check that to be sure.

    The interaction between a moon and planet, however, could theoretically bring down a moon’s orbit. Earth is actually boosting it’s moon’s orbit: the gravity of the moon causes the earth to deform toward it. Since the earth is rotating faster than the moon is orbiting, that bulge leads the moon, and exerts a gravitational force accelerating the moon. The moon simultaneously exerts an equal force decelerating the Earth’s rotation. The situation only becomes stable when the moon is in a geosynchronous orbit (only one side of earth is visible from the moon in the same way that only one side of the moon is visible from earth). If a moon of Saturn where in a lower orbit than geosynchronous, that effect would be revered, and the planet would speed up while the moon would decelerate into lower and lower orbits.

  28. Faultystring

    That’s makes more sense than what I was thinking, thanks for the explanation.

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