ROB GIBSON, 10 AUGUST 1997:
My deep abiding love of the story "Island in the Sky" was and is based on the moral message of the story. Scrooge pays a huge price for landing an extra time to run the rope from the savages' little planet to the lush green planet so they won't starve (I never noticed the astronomical impossibility of this until I recently reread it) [...]I'm afraid this is going to be a very stupid question, but what is so astronomical impossible about this?
A reason I'm thinking of is that the green planet turns around just like the Earth, so that very soon the rope would be too short because of the distance getting bigger. (But maybe Scrooge reckoned with that?)
BTW. Another reason could of course be that the distance between the two objects is too close, so that - because of the gravity of the green planet - the rock would have fallen to the planet instead of floating near it, but let's say that Barks wanted to fit both objects into one scene and thus placed them near to each other. OK? ;-)
I know it's only fiction and fantasy, but I've always liked discussions like this (for example last year's discussion on the "Universal Solvent" and Christopher Barat's recent comment on "Micro Ducks from Outer Space").
Imagine a rope tied between earth and the moon that people can crawl
along to get from one to the other. What might cause a problem with such
a notion...
One, the earth rotates independently of the moon. That's why the moon
appears to move across the sky.
Two, the orbit of the moon about the earth is an ellipse, hence the
distance between the earth and the moon changes constantly.
Three, there is no atmosphere for most of the distance between them. Not
only will there be no air to breathe, but anyone crawling into a vacuum
without a space suit will experience his or her blood boiling.
Alternately, were there an atmosphere for the entire distance between
them, then the friction of that atmosphere (against the surface of the
smaller astronomical body rotating around the larger one) would cause
the orbit to decay and the smaller body to crash into the larger one.
Other problems:
birds cannot fly through empty space; even if they could, see #3.
If a bird can achieve escape velocity to "fly" between the asteroids,
then neither asteroid has anywhere near sufficient mass to retain an
atmosphere.
Of course, these astronomical limitations apply within the context of the known universe and the currently known laws of Physics.
Rob Gibson <[...]@[...]> writes:
Imagine a rope tied between earth and the moon that people can crawl along to get from one to the other. What might cause a problem with such a notion...I wish we still had astronomer William Hathaway on this List, to give the definitive answers to these issues! Since he's no longer a member, I'll try to step in, but I suspect many other members can call me to task for the errors which I shall likely commit below as I try to comment on your objections to this story.
One, the earth rotates independently of the moon. That's why the moon appears to move across the sky.But the moon itself always keeps the same face pointed towards the earth, which is half of what you want. An example of exactly what you want is the planet Pluto and its moon Charon (some would call Pluto/Charon actually a dual planet system, rather than a planet and a satellite). Pluto and Charon each keep the same face pointed towards the other. Charon thus always stays in the same place in Pluto's sky, and vice versa. Neither one rises or sets as seen from the other. This spin-orbit coupling comes about as a result of the dissapation of rotational energy through the friction induced by the mutual tides each body raises on the other. Now, let's all send a greeting to any List members living there: <in a squeaky voice> "Hiya', Pluto!"
Two, the orbit of the moon about the earth is an ellipse, hence the distance between the earth and the moon changes constantly.It isn't that much of an ellipse, and in a case of spin-orbit coupling such as noted above, it would have decayed to an even more nearly circular one. Anyway, the rope between the two planets would have a certain elasticity, due to its non-neglible value of Young's modulus. Ropes stretch and contract, as tension is applied and then relaxed.
Three, there is no atmosphere for most of the distance between them.Do we know for sure how far apart the two planets are? The earth's atmosphere is still dense enough at about 6 km altitude to be breathable. Of course, for a small planet, the atmosphere would be a lot less dense (in fact, probably non-existent). Note that the atmospheric pressure measured by the Mars probe is only about 5 or 6 mm of mercury, isn't it? So we're sort of violating physics by just agreeing that there's an atmosphere at all. It's not much more of a violation to assume that this hypothetical atmosphere would extend halfway to the other planet, so they would have an atmosphere around them for the whole distance.
Alternately, were there an atmosphere for the entire distance between them, then the friction of that atmosphere (against the surface of the smaller astronomical body rotating around the larger one) would cause the orbit to decay and the smaller body to crash into the larger one.The atmosphere would be in the same orbit about the planet's mutual barycenter as the planets themselves would be. Neither planet would be moving through the mutual atmosphere; the atmosphere would be moving along with them. But you're right about there being friction; this comes about from tides, which is how the two planets got locked into spin resonance in the first place (see # 1). Tidal friction will continue to dissipate gravitational potential energy, and the two bodies will move closer to one another, until the tidal forces are so strong that one body completely tears apart. (The distance at which this happens is called the Roche limit.) Then you will be left with one body (the larger one) and a ring system, like Saturn. Of course, this will all happen over an astronomical time frame, so everyone in the story will have been dead for millions of years before it becomes a problem!
Other problems:As noted above, the problem of an asteroid's having insufficient mass to retain an atmosphere is the real scientific inaccuracy of this story, regardless of whether we're talking about a single asteroid or two of them, tethered by a rope. We must assume that somehow an atmosphere has been created around the asteroid so recently that it has not yet escaped into space. Perhaps this was the result of a cometary impact. The main difference between a comet and an asteroid is that asteroids are rocks whereas comets are a mixture of rocks and ice. This ice is a frozen mixture of all sorts of volatile materials. Let's just postulate that a collision with a comet released enough water and oxygen recently for this story to take place.
birds cannot fly through empty space; even if they could, see #3. If a bird can achieve escape velocity to "fly" between the asteroids, then neither asteroid has anywhere near sufficient mass to retain an atmosphere.
And hey, while we're at it, let's postulate that ducks have four fingers and can talk, too! :)
Thank you very much, Wilmer, for the excellent points you made showing that the dynamics of Island in the Sky present an astronomical predicament, and not just an astronomical impossibility. I think we can assume that both asteroids in the story are tidally locked with each other, with both always appearing in the same location in the other s sky.
That there was a permanent relationship between the two planetoids is evidenced by the birds apparently flying regularly between them to feed and nest, kind of in the way birds on Earth sometimes forage for food inland from the seacoast, but which nest on small rocky islands offshore.
Personally, I don t think anything Scrooge did in the story is, strictly speaking, impossible. Much of it may have been difficult, but not impossible. For example, to successfully stretch a rope between the two anchor points, Scrooge could have used a marvelous monofilament material, a super-hard, super-thin, and super-light type of line invented by Gyro. Arthur C. Clarke once wrote of letting-out such a line into space from the Earth s surface in his science fiction book, The Fountains of Paradise. The story described a device located on a mountain top on the island of Ceylon from which monofilament was let upward into the sky, terminating at a space station in low Earth orbit. It was the basis of a system known as a space elevator, and was supposed to provide cheap and easy access to space. The book Red Mars by Kim Stanley Robinson built on the same concept, but with heavier hardware.
As far as both planetoids using the same atmosphere, I'm reminded of Larry Niven's book, "The Integral Trees," in which an entire biome, complete with floating trees,existed in weightless space in a huge torus ring of atmosphere in orbit around a star; a really fascinating book.
I ve always thought Island in the Sky was a marvelous story. But I ve also always wondered how the indians got on the small asteroid in the first place. Were they marooned there by someone? Did they accidentally maroon themselves? Did they originally come from the large asteroid, or from some other place altogether? Did they once possess high technology? I ll bet these questions sound familiar to others on this list.
Anyway, Island in the Sky seems ripe for a (good) sequel.
On a related note, I think the golden moon in The 24-Carat Moon presents a much better candidate for an astronomical impossibility. I really like this story, but I ve never been able to understand how the golden moon could have possibly remained hidden from the view of Earth behind the Earth s moon.
RICH MORRISSEY pointed out:
As one might expect of a largely self-educated man, Barks' stories became more scientifically accurate as he grew older. (One could compare him to Herge in that way: his complete fantasy views of real countries in TINTIN IN THE LAND OF THE SOVIETS and TINTIN IN THE CONGO as compared to the realistic (though with fantasy elements) TINTIN IN TIBET...or, for that matter, his very accurately anticipated linar mission in DESTINATION MOON/EXPLORERS ON THE MOON.) Barks' early Duck stories had people (well, ducks) in space without even helmets; later on they wore full space suits (as the real-life astronauts were doing by then).This is an excellent analogy! With Herge, this changeover occurred sometime in the 30's ("The Blue Lotus," "The Broken Ear"), when he began to be influenced strongly by outside events in the real world such as the Sino-Japanese war, the South American wars, etc., and made his stories more "realistic" as a result of that influence. I would imagine that the birth of the Space Age and the concurrent upswing in interest about space travel (with the accompanying articles, etc. in popular magazines) spurred Barks to more scientific accuracy. Of course, there was still a VERY strong fantasy tinge in Barks' later space stories, sometimes even to excess; however, Barks did take new knowledge about the physical realities of space travel into account.
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