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Thread: Interstellar Comet

  1. #61
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    Quote Originally Posted by chornedsnorkack View Post
    The gaps in our coverage also include:

    The conspicuously weird lightcurve may be expected to change as the directions Oumuamua to Sun and to Earth relative to rotation axis change. However, we cannot analyze the part of lightcurve before discovery - too few observations

    Also, we know that Oumuamua has had no coma since discovery. But if it followed a ballistic trajectory, it passed within 0,25 AU of Earth. Could Oumuamua have emitted a small puff of coma which was gone by the time it was discovered at 1 AU and already cooling?
    My bold. It's within the realm of possibility. If so, it would not be the first time a comet went "poof" on the way in or near perihelion.

  2. #62
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    Looking at the MPC data on Oumuamua, I found that the ascending node of its orbit seemed to change in a substantial, largely systematic way. Between 25 Oct. and 20 Nov. The difference was apparently about 1 arc min., which built up gradually, with the exception of a small retreat between 29 Oct. and 01 Nov. As this was mainly a systematic change, it's not clear that it was solely due to refined calculations of the orbit.

  3. #63
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    Quote Originally Posted by Ross 54 View Post
    Looking at the MPC data on Oumuamua, I found that the ascending node of its orbit seemed to change in a substantial, largely systematic way. Between 25 Oct. and 20 Nov. The difference was apparently about 1 arc min., which built up gradually, with the exception of a small retreat between 29 Oct. and 01 Nov. As this was mainly a systematic change, it's not clear that it was solely due to refined calculations of the orbit.
    What, in appropriate mathematical detail, do you mean by "mainly a systematic change"?

    It should be no surprise that the elements of the best fit hyperbola will change a bit as more observations come in. The object is being perturbed by the planets, so its path is not a pure hyperbola. From its motion we can calculate an osculating orbit for a particular point in time. That is the path it would follow if the gravitational effects from everything but the Sun magically went away at that instant, and it will change continuously with the passage of time.

  4. #64
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    By 'systematic' I meant largely in one direction, rather than points converging from either side toward a particular value, as one might see as the orbit was refined. I don't know that these figures have any significance, beyond what you suggest, I'm merely wondering. The figures are as follows:

    Date/ Asc. Node (degrees)
    25 Oct.-- 24.61531
    26 Oct.-- 24.60870
    27 Oct. -- 24.60195
    29 Oct. -- 24.60261
    01 Nov. -- 24.60235
    10 Nov. -- 24.59939
    13 Nov. -- 24.59963
    13 Nov. -- 24.59969
    20 Nov. -- 24.59939

  5. #65
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    Is the albedo of Oumuamua directly measured? As in, is the thermal infrared into which the nonreflected visible sunlight may have been converted into detected, measured and accounted for?

  6. #66
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    Quote Originally Posted by Ross 54 View Post
    Looking at the MPC data on Oumuamua, I found that the ascending node of its orbit seemed to change in a substantial, largely systematic way. Between 25 Oct. and 20 Nov. The difference was apparently about 1 arc min., which built up gradually, with the exception of a small retreat between 29 Oct. and 01 Nov. As this was mainly a systematic change, it's not clear that it was solely due to refined calculations of the orbit.
    Yarkovsky effect, or ion thruster?
    Et tu BAUT? Quantum mutatus ab illo.

  7. #67
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    Or maybe gravitational effects due to the various orbiting bodies in our solar system. I'm sure this is a topic of ongoing research.
    Selden

  8. #68
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    Here is a simulation where you can pan around. It's similar to the one I already posted, except it includes a path for Oumuamua. The previous one didn't because the code only knew how to superimpose Keplerian ellipses, not hyperbolas. So I had to hand-code Oumuamua's path.
    http://orbitsimulator.com/gravitySim.../oumuamua.html

  9. #69
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    A sudden brightening of Oumuamua, whether due to a puff of coma, engines turned on, lights switched on, or a bright white patch coming to visibility as the aspect changes, would attract attention.

    When would a sudden unexpected dimming of Oumuamua cease to be suspicious?

  10. #70
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    Quote Originally Posted by chornedsnorkack View Post
    When would a sudden unexpected dimming of Oumuamua cease to be suspicious?
    Suspicious of what?

    Do you mean interesting?

    Unexpected things will always be interesting.
    0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 ...
    Skepticism enables us to distinguish fancy from fact, to test our speculations. --Carl Sagan

  11. #71
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    Green Bank (West Virginia) to scan Oumuamua for alien radio signals:

    https://www.forbes.com/sites/brianko...s#76e092fd6186

    I gotta say the more I look at this, the more odd it seems. The 10 to 1 aspect ratio seems physically impossible to form naturally: (a) there is no similar object in our solar system that comes close; (b) it would have to be a fragment of a solidified metallic core that got shattered, but the spectra doesn't match those sorts of objects.

    The other thing is it's orbit: let's say we identified the 3rd planet around Alpha Centauri and wanted to do a flyby mission to scan it. It would be hard to design a better orbit than the observed Oumuamua orbit--compare the way it kind of loops back around Planet Earth on its way out.

    Also, it looks like it passes about halfway between the orbit of Mercury and the Sun, about 0.255 AU at perihelion, according to the Wikipedia. Consider the odds of that being a random coincidence: cross-section area of Solar System out to Pluto is ~5000 AU^2; cross-section of circle w/ radius of 0.255 AU is 0.2 AU^2. Thus, assuming an equal chance of crossing the plane of the Solar System anywhere, the odds of randomly getting that close to the Sun is 0.004% (4 x 10^-5).

    That is a bulls eye. To put that in perspective, in an ordinary dartboard, the inner bulls comprises about 0.08% of the total area of the dart board. If Oumuamua was a dart, it hit DEAD CENTER--well, it would have been 0.02 mm from dead center, a small fraction of the diameter of a steel-tipped dart.

    Now, I'm no expert on orbital dynamics (maybe Tony can answer that better), and I know the Sun exerts gravitational pull, but it still seems to me there's nothing stopping an interstellar rock from passing through the Solar System at say, the orbit of Saturn, as opposed to inside of Mercury. Perhaps there's something I'm missing, but if not, that is a crazily improbable coincidence--IF it is indeed a coincidence.

    And again, it passed within at least 0.22 AU of Planet Earth--even closer than it went to the Sun, so that's two highly unlikely coincidences. And why Earth instead of Jupiter or Mars? Because Earth is the most interesting planet??

    We think it came from the direction of Vega, but if it were an alien spacecraft, that could be an illusion because if there was a delta v burn at perihelion, we never would have noticed it, because we have no information on it's orbit before it encountered the Sun.

    I am very curious as to where it is headed next. A prediction of the alien hypothesis is that it will be heading directly for a relatively nearby star. Then again, if it's not, that doesn't mean it wasn't a one-off probe like many we have sent.

  12. #72
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    I can think of two related counter arguments for the orbital dynamics aspects:

    There must be many "free flying" interstellar objects passing by and through the solar system at a variety of distances. This just happened to be one that we noticed.

    We're still very unlikely to notice an object with its physical characteristics doing a single-pass flyby through the outer solar system. (Hopefully the LSST will help with that situation.)
    Selden

  13. #73
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    Seems like such a shame to come all this way and to not stick around.

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    Quote Originally Posted by Spacedude View Post
    Seems like such a shame to come all this way and to not stick around.
    The delta v to stop would be too much to stop.

    @ Selden: I see your point and thought of the same thing. There might be a thousand such objects that pass through the Solar System every year that we just never see. But if there really were that many, it seems we would notice at least a few them. Oumuamua is the very first. So if it is a natural object, such creatures must be relatively rare.

    And again, it went through two keyholes: 0.255 AU by the Sun, and <0.22 AU to Planet Earth.

    And there's really a 3rd coincidence: the 10 to 1 length to width ratio. It is completely unprecedented. There are what: 100,000 asteroids that have been cataloged? And none of them come close. So if Oumuamua is a natural object, the odds of it happening to have its very peculiar length to width ratio is on the order of 1 out 100,000 or 10-5 at a maximum.

    So we have 3 coincidences, each on the order of 10-5; adding all three coincidences together, we're looking at a probability of ~10-15. In comparison, your odds of winning the Powerball Lottery are much, much higher (3.42 x 10-9).

    So while it is logically possible that Oumuamua could be a natural object, it is a practical impossibility that it is not an artificial object IMHO. YMMV.

  15. #75
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    Sorry, I have a tendency not to explicitly spell out my points. In this case, that the two coincidental "keyholes" are actually caused by our limited detection capability. I.e. we saw it going through that region of space because that region is the just about the only place we could see things like it. It's similar to the proverbial drunk looking for his car keys under a streetlight because that's the only place he would be able to see them. In this case, someone else's car keys (so to speak ) happened to be there. We really don't know (yet) how many car keys might be out in the middle of the street where we can't see them.

    I agree, though, that its structural aspects are very puzzling.

    It's too bad we don't (yet) have any way to intercept it and do a physical analysis. I wonder if that capability could be developed in the not too distant future? It'd be a lot of work, of course, and (if it happened) the intercept doubtless would wind up being far outside the solar system.
    Selden

  16. #76
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    Yes, Selden. From this paper (Interstellar Interloper 1I/2017 U1: Observations from the NOT and WIYN Telescopes):

    The steady-state population of similar, ∼100 m scale interstellar objects inside the orbit of Neptune is ∼104, each with a residence time of ∼10 years.
    I wonder if anyone ever did a study one what shapes of such objects we should expect, how rounded vs angular. ie if they are ejected from their originating solar system, perhaps there is less time and material for them to clump into more rounded shapes. After they get their initial shape, perhaps from a collision or tidal breakup.
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  17. #77
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    That's a lot!

    My guess would be that there would be no a priori reason to expect that the distribution of II shapes would differ much from those of Solar System asteroids, so the shape of 1I/2017 U1 certainly is very unexpected and hard to explain. As you say, though, they probably tend to have had less time to interact with other proto-planitesimals (probably the wrong term) which might have some bearing on it.

    ETA:

    If many of them were ejected from protoplanetary systems by their interactions with gas giants, I'd expect some of them to have experienced substantial tidal forces, which might produce an elongated shape.

    I just wish the LSST could be functional a lot sooner.
    Last edited by selden; 2017-Dec-12 at 10:35 PM.
    Selden

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    OK, if there really is a population of 104 interstellar objects within Neptune's orbit (Area = 2827 AU^2), with a residence time of 10 years, then 1,000 such objects cross the plane per year. Thus: 1000/2827 = 0.35 objects/AU^2/yr.

    So we have two keyholes: KH1 = 0.2 AU^2 (radius 0.255 AU). Thus 0.2 AU^2 * 0.35 obj/AU2/yr = 0.07 obj/yr = 1 object every 14 years. Fair enough.

    But keyhole #2: we can assume an object passing that close to the Sun would get ejected in a random direction. Meanwhile, KH2 = 0.08 AU^2 (closest approach to Earth was 0.16 AU). Area of a sphere of 1 AU is 12.56 AU^2. Thus conditional Prob#2 = 0.08 / 12.56 = 0.00636; multiplying the two probabilities = 0.00045 = 1 object every ~2300 years. A pretty remarkable coincidence, but not unbelievably so.

    Yet there is still the weird, elongated shape. You almost need to invoke strange formatory processes that do not occur in this solar system. If we still go with the 10-5 probability that an asteroid would have that shape, then we get 0.00045 * 10-5 = 4.5 x 10-9. We're still in winning the Lotto territory.
    Last edited by Warren Platts; 2017-Dec-12 at 11:32 PM.

  19. #79
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    Quote Originally Posted by Warren Platts View Post
    Yet there is still the weird, elongated shape. You almost need to invoke strange formatory processes that do not occur in this solar system.
    Yet there are not very strange at all destructive processes that do occur in this (and other) solar system(s). Destructive processes that might well add to ejected bodies, and might even influence the shape "range" such objects might have.
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    Quote Originally Posted by slang View Post
    Yet there are not very strange at all destructive processes that do occur in this (and other) solar system(s). Destructive processes that might well add to ejected bodies, and might even influence the shape "range" such objects might have.
    Well, our own solar system is rather mature, so such elongated asteroids--if they formed in the first place--might be expected to be broken down as a result of the 4 billion year old pinball machine. Thus Oumuamua might have formed in the early phase of a solar system, and been ejected into collisionless interstellar space, thus preserving its odd shape. So it could be the case that interstellar interlopers as a population might be expected to have more odd shapes than we observe in our relatively mature ecosystem....

    ETA: I don't know if you guys have seen this article by Phil Plait himself, but he kind of agrees with me that Oumuamua's trajectory is about what an interplanetary scientist would design....

    QUOTE: "
    Space is vast. Even in our solar system, the outer planets are billions of kilometers away, with smaller icy objects extending for a trillion or two past that. No doubt there is debris from other stars passing us at all kinds of distances. Most would never get within a light-year, 10 trillion km.

    From that distance, the inner solar system is a ridiculously small target. Mercury’s orbit is only about 115 million km across. For something coming from interstellar space, getting that close to the Sun is threading the eye of a very, very narrow needle.

    Yet A/2017 U1 did just that. It passed the Sun at a distance of about 45 million kilometers. That’s … weird.

    Part of this is what we call a selection effect: An object like this passing out by the orbit of Jupiter would be very faint, so it’s harder to discover. We only see the ones that happen to pass close to Earth. So even before this, I would’ve wagered the first one we discover would be passing through the inner solar system, as opposed to farther out.

    But still, getting that close to the Sun seems unusual.

    Let me be clear: I am NOT saying this is an alien spaceship. But if I were an alien race interested in exploring other systems, this is pretty much the sort of path I’d put my probe on. I’d aim it to pass deep within the alien solar system, check out the habitable planets, and use the star’s gravity to bend the orbit to aim it at the next target."


    http://www.syfy.com/syfywire/breakin...ote-not-aliens
    Last edited by Warren Platts; 2017-Dec-13 at 01:15 AM.

  21. #81
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    Quote Originally Posted by Warren Platts View Post

    And there's really a 3rd coincidence: the 10 to 1 length to width ratio. It is completely unprecedented. There are what: 100,000 asteroids that have been cataloged? And none of them come close. So if Oumuamua is a natural object, the odds of it happening to have its very peculiar length to width ratio is on the order of 1 out 100,000 or 10-5 at a maximum.
    I think it is interesting to speculate that it could be artificial. It is, yes, a trajectory that might be taken.

    But your point about the length doesn't make sense to me, or perhaps I don't understand clearly enough. The fact that it is elongated when most comets are not is significant. But the specific ratio isn't, in my opinion. I don't think there are that many artificial objects that have a 10/1 ratio. Cassini probably doesn't, Hubble probably doesn't. Why would an artificial object have that ratio? So it seems like just a coincidence, unless you're implying that the aliens know that we count in base 10 and therefore made it that size to give us a hint that it's artificial. But in that case, why not just broadcast?
    As above, so below

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    I place no significance on the 10 to 1 per se; there is a big uncertainty, it could be 6 to 1, or even 100 to 1 if it were aligned exactly right. Point merely is that the axis ratios we've seen in this solar system don't exceed 2 to 1. So it is an anomaly no matter how you slice it...

  23. #83
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    Quote Originally Posted by Warren Platts View Post
    Well, our own solar system is rather mature, so such elongated asteroids--if they formed in the first place-- [...]
    No, my points was that it need not have formed in an elongated shape. It may have been part of a much larger "parent" body. Also requiring it to be formed in that shape adds unnecessary "specialness" to it. Unless of course it can be shown that such shapes do form more often than get such shapes from collisions and such destructive processes. So if we rephrase that last part into ", so any shape of solar system body [...]"

    Quote Originally Posted by Warren Platts View Post
    [...] might be expected to be broken down as a result of the 4 billion year old pinball machine. Thus Oumuamua might have formed in the early phase of a solar system, and been ejected into collisionless interstellar space, thus preserving its odd shape. So it could be the case that interstellar interlopers as a population might be expected to have more odd shapes than we observe in our relatively mature ecosystem....
    in which case I say yes, so....

    Quote Originally Posted by Warren Platts View Post
    I place no significance on the 10 to 1 per se; there is a big uncertainty, it could be 6 to 1, or even 100 to 1 if it were aligned exactly right. Point merely is that the axis ratios we've seen in this solar system don't exceed 2 to 1. So it is an anomaly no matter how you slice it...
    ... why are you still comparing two different populations: rocks that remain in a solar system, and rocks that don't. Unless you can show that those populations have exactly the same properties you can't extrapolate the expectations from the one population exactly to the other.

    I think there is enough reason to expect that there may be differences in those populations based on some of the processes that may eject rocks. And you rightly said that most collisions probably happened a very long time ago in our solar system, thus providing lots and lots and lots of time and material for the remaining fragments to clump and become more rounded. See Itokawa.
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  24. #84
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    Quote Originally Posted by slang
    No, my points was that it need not have formed in an elongated shape. It may have been part of a much larger "parent" body.
    Sorry--perhaps my word choice wasn't very precise; I was including disruption of larger bodies as a type of "formation". Agree that it is most likely a fragment from a larger body.

    Quote Originally Posted by Slang
    you still comparing two different populations: rocks that remain in a solar system, and rocks that don't. Unless you can show that those populations have exactly the same properties you can't extrapolate the expectations from the one population exactly to the other.
    Interesting. If it is a natural object, it probably did not originate from the Vega system, because the Vega system is probably on the order of 400 million years old, and it would only take about 300,000 years to get here from there, meaning it would have been ejected late in its history. And if it is an artificial object, 400 million years is probably not enough time to evolve a technological civilization (unless Vega had been colonized by a non-Vegan civilization). So if it did come from Vega, it must have simply passed through like it passed through our Solar System. Therefore, it must be a pretty ancient object: probably on the order of a million years at a minimum.

    Which raises a new question: the rotation. "According to Jason Wright, an astronomer at Penn State University ... a long journey across the cosmos can slow an object’s tumbling, but ‘Oumuamua has shown no signs of stopping its spinning."

    https://www.theatlantic.com/science/...teroid/547985/

    So the mere fact that it is rotating as fast as it is is puzzling, and not what we would expect from a natural object.

    (And then there's still the trajectory itself: it's like it was designed to take a peek at Planet Earth...)

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    Quote Originally Posted by Warren Platts View Post
    probably on the order of a million years at a minimum.
    Billion? Perhaps, why not.

    Quote Originally Posted by Warren Platts View Post
    https://www.theatlantic.com/science/...teroid/547985/

    So the mere fact that it is rotating as fast as it is is puzzling, and not what we would expect from a natural object.
    That would depend on how fast it was spinning to begin with, and how much exactly "a long journey across the cosmos can slow an object’s tumbling", wouldn't it? But I don't even know how to begin to put that in numbers. How would it slow tumbling in interstellar space anyway? The article is pretty vague. A "rubble pile" can't rotate that fast without falling apart... ok, so it's not a "rubble pile"...
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    "The Planetary Council of Masa Four was in session. It was not a happy gathering. Scanning beams had revealed that a supposed new comet, driving in on a perfectly convincing orbit, was actually an artifact- a spaceship. It used no drive and seemed empty of life. But it had come through the gravitation field of the outermost planets - and it showed no sign of rotation. Which was impossible unless gyroscopes or some similar device were running within it." - Murray Leinster. "Propagandist", 1947.

    It seems to be tumbling randomly. One pass of the solar system at high speed is maybe not enough to affect the spin, but it must have had a very brief history in its source system as well.
    Last edited by transreality; 2017-Dec-14 at 09:02 AM.

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    The chance that a particular randomly ejected body from many lightyears away would make such a close approach to Earth is indeed tiny, but that is not grounds from first principles to rule it out. Let me give an example of a highly improbable event that really did happen, as reported by Sky and Telescope. During World War II, the firing tables for finding the range for various elevation angles of naval guns were prepared by adding a series of 15 empirically determined terms. These terms typically were rounded off to two decimal places, which usually was sufficient to avoid rounding errors in the sum. One gun was showing an error for one angle. The calculation was significantly off from the value obtained by interpolating from the ranges for the same gun at different angles. On a hunch, the supervisor tried carrying one more decimal place for each term, and the discrepancy went away. In 14 out of the 15 terms the number in the third decimal place was 4, which when dropped threw the sum off significantly. The odds against that were about 1014 to 1, but it happened. With the test firing of numerous guns during an all-out war, there were plenty of opportunities.

    Source: Luigi Jacchia, Center for Astrophysics, Harvard and Smithsonian Observatories, Some Thoughts About Randomness, Sky and Telescope, December 1975, p. 371

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    Quote Originally Posted by Hornblower View Post
    The chance that a particular randomly ejected body from many lightyears away would make such a close approach to Earth is indeed tiny, but that is not grounds from first principles to rule it out.
    Agreed. And while the odds might be low for any single object, there may be many such objects out there, which we have not detected, and so the odds of one such object having a close approach to Earth, out of the many, is not low.
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    Quote Originally Posted by slang
    But I don't even know how to begin to put that in numbers. How would it slow tumbling in interstellar space anyway?
    OK, I did a little reading up on this topic, and it turns out that damping of rotation can come about from internal energy dissipation and the so-called YORP effect. Since the latter depends on abundant solar radiation, we can exclude that possibility for Oumuamua.

    According to Burns et al. 1973 the damping time can be estimated by the following formula:

    t = 50,000 x P^3 / r^2 / K

    where t is the characteristic damping time to stop tumbling in years, P is the observed rotation time in hours, r is the average radius in km, and K is a numerical measure of how round (or not) it is; K = 0.01 for nearly spherical, and K = 0.1 for oblong asteroids.

    To be conservative, since Oumuamua is so rod shaped, I'll set K = 1 and r = 0.2 and we know P = 8 hours. In that case t = 640 million years--a long time, and that is really the very minimum bound. If we set K = 0.2 and r = 0.1 then t = 13 billion years....

    So I guess we shouldn't make a big deal out of the tumbling rate, even if the risk of collision is zero....

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    Cool, thanks for finding that.

    ETA: heh, I just got home from work, now I see the timestamps it's like I sit here hitting refresh...
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