posted
Set the stage: a planet with a moon in orbit around it. Any meteroids that find their way to the planet's atmosphere would either burn up in the atmosphere or, if large enough, strike the surface of the planet. They would not end up in orbit around the planet. Would that be correct?
If so, if a space wayfarer detected a large rock in orbit that was not the moon, it would be stupid for him to say there was a meteor in orbit around the planet. Would that be correct?
Or, is it possible, impossible or only highly unlikely a meteoroid would collide with the atmosphere, and a chunk of the meteoroid would jettison into orbit around the planet?
(And don't worry. This wayfarer is asymptomatic. )
quote:Set the stage: a planet with a moon in orbit around it. Any meteroids that find their way to the planet's atmosphere would either burn up in the atmosphere or, if large enough, strike the surface of the planet. They would not end up in orbit around the planet. Would that be correct?
I'm not sure that is completely correct. If the meteoroid entered the atmosphere at a sufficiently shallow angle, I believe it could "skip" off the atmosphere. This is a potential problem with reentry for spacecraft, and I see no reason why it couldn't happen with a meteoroid. Given the right velocity and gravitational interaction between the meteoroid, the planet and the moon, it is possible the meteoroid could end up in orbit around the planet.
quote:If so, if a space wayfarer detected a large rock in orbit that was not the moon, it would be stupid for him to say there was a meteor in orbit around the planet. Would that be correct?
A meteoroid could definitely end up in orbit around the planet, though it would be more likely to happen just through gravitational capture in space, without involving the atmosphere of the planet at all.
quote:Or, is it possible, impossible or only highly unlikely a meteoroid would collide with the atmosphere, and a chunk of the meteoroid would jettison into orbit around the planet?
Assuming a meteoroid was large enough, and of a composition likely to explode in the atmosphere, I think it possible that chunks of it might make it back into space.
A larger body that impacted the ground could send debris from the impact into space.
posted
Yes, I am aware of the atmosphere reentry angle necessity, but thought that would get rid of the meteoroid entirely and my problem would be moot, but seeing your response made me realize even that deflection could leave a chunk captured in orbit. Possibly. Right?
Okay, let me think about the rest of this.
The only thing I'm going for is the likelihood behind a comment and the proper response to it.
[This message has been edited by Kolona (edited May 08, 2004).]
posted
Without any other context, if someone notices a large chunk of rock in orbit around a planet, to assume it is a gravitationally captured meteoroid is reasonable.
Without any other context, to assume that it is the remains of a particular meteoroid that exploded in the atmosphere is would be irrational, because that is much more unlikely than a simple capture.
posted
By the way, we need to get some terms straight.
A meteor cannot ever be in orbit around a planet, any planet, because the term refers solely to the phenomenon visible from a planetary surface when a meteoroid burns up in the atmosphere. This is the original term.
A meteorite is a rock that has fallen from space, producing a meteor in the process, after it has landed on the ground.
A meteoroid is any rock flying about in space that could enter a planetary atmosphere and produce a meteor.
Any rock in a stable orbit cannot be any of these three things, by definition. It would be a secondary moon, by definition.
So leaving terminology aside, could it have ever been a meteoroid?
Simple answer, yes. But this seems unlikely. I would think it more plausible that it had been an asteroid previously...that is to say, that it had previously had a stable orbit around the primary of the stellar system which includes the planet it now orbits, and that it had been orbiting within the elliptic of that system. If it were in a hyperbolic trajectory or out of the planar elliptic, then the relative velocity would have been too great to allow any significant probability of capture.
Again, we are talking about terminlogy...I'm saying that if the object were captured, it would almost certainly have been an asteroid rather than a meteoroid, based on probable trajectory.
If the object is very large, then it would be extreemly unlikely that it were part of an even larger object that entered the atmosphere and exploded...when something blows up on hitting the atmosphere, the resulting pieces are usually very small. But a large object composed of ejecta from an impact is actually quite plausible--many astronomers believe that this is the origin of our own moon. But this is probably off your point.
To recap, I would think that a "space wayfarer" would regard a "large rock" which was "not the moon" (I'll parse this as meaning that it appears to be a relatively new satellite) in orbit around the planet as being most probably a captured asteroid, not a captured meteoroid.
The same reference defines an asteroid as ranging in size between a few and several hundred kilometers, so from that standpoint the difference between meteoroids and asteroids is one of size, not orbital mechanics.
Because Kolona specifically used the term meteoroid (rather than making the common mistake of calling it a meteor or meteorite), I assumed we were discussing an object smaller than an asteroid, and limited my discussion to such objects, except in referring to the possibility of orbital debris from impact of a larger object.
posted
Also, I hate to be picky (no, actually that's not true. I love to be picky), but it's ecliptic, not elliptic.
Posts: 932 | Registered: Jul 2001
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posted
I can't answer the question, but I thought the discussion was interesting, and it leads me to ask: If a significant chunk of something DID get caught in the gravitational pull of the earth, such that it could be said to be in orbit, what effect might that have on earth? Would it change the tides, weather, etc? And if it was smaller than our original moon, would it be closer to the earth or further away? Do you think we would turn it into a spacestation and/or colonize it? Or do you think we would try to blow it away? Would the moon also have a pull on it? As a new moon, how much "moonlight" would it give? If it were mineral rich, would we mine it? What effect might it have on satellites?
Only if it were big enough and close enough. And all the things big enough that I know about are in very stable orbits elsewhere.
quote:And if it was smaller than our original moon, would it be closer to the earth or further away?
It would certainly be smaller than our moon. Pluto is smaller than our moon. However, it's size has nothing to do with its distance. On the other hand, any captured object would probably have a highly elliptical orbit, unlike the Moon's fairly circular one. (All orbits are elliptical to some extent, though.)
quote:Do you think we would turn it into a spacestation and/or colonize it? Or do you think we would try to blow it away?
Maybe (to the space station) if it was big enough to tunnel in. Or maybe we'd just process its materials to build a more "normal" space station, without having to haul all the materials up from Earth orbit (although we'd first have to haul up the processing capabilities). Why would we blow it away? Unless, perhaps, the Moon was affecting its orbit in such a way that it would eventually crash into us.
quote:Would the moon also have a pull on it?
The Moon has a pull on everything in the universe (well, within about 5 billion light years, since its gravitational attraction has only had about 5 billion years to travel). But I presume you mean a significant pull, so, yes. See previous answer.
quote:As a new moon, how much "moonlight" would it give?
As a "new moon" it wouldn't give any light at all. As a "full moon," however, it would depend on size, distance, and composition. Earth actually has a huge moon in comparison to the size of the primary (based on just our own solar system, of course). People often forget this when writing about the appearance of moons in the skies of extra-solar planets. The way Mars's two moons are often described, for example, gives the impression that they would look much like our moon does, just with different markings. In fact, Phobos would appear to have about half the diameter of our moon, which means about 1/4 of the moonlight (actually about 1/10 when you take into account its greater distance from the sun). Deimos would appear as a starlike point. You could never see its phases without a telescope, except as a dimming and brightening.
quote:If it were mineral rich, would we mine it?
Why not?
quote: What effect might it have on satellites?
Depends on its orbit and size, but probably none.
Posts: 932 | Registered: Jul 2001
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Every time I read this thread, I read that as 'asymptotic', and start wondering in what way a wayfarer could be asymptotic. Presumably he never quite gets to wherever he's going, no matter how close he comes.
posted
LOL I don't know why that struck me as so funny, but thank you, Jules, for a great belly laugh. <pulls self together, smiling>
I was, of course, referring to the 'Medical Help, Please' thread, but your comment prompted me to look up 'asymptotic.' An 'asymptote,' for those of you like me who have no idea, is a mathematic term meaning "a straight line which an indefinitely extended curve continually approaches as a limit." Cool. Kind of like walking the riverwalk here.
posted
Rick, your answers are great, bravo. But I think you are jumping to a conclusion when you assume it must be smaller than the moon. Fron a physics POV size has nothing to do with it. Any object that approaches the Earth on the right vector could find itself in orbit.
I understand that your conclusion is well founded. To be captured the vector would have to be relatively close to the vector of the Earth, i.e. an Earth crossing asteroid. Something falling toward the sun from the Oort cloud would not get captured by the Earth (unless it destroyed the Earth in the process). If there were such a moon-sized or larger Eqarth crossing asteroid we would know about it.
But as long as we're talking about science fiction, why stop at moon-sized? Why not make it Earth sized, or even bigger! Suppose a rogue gas giant wandered into the inner solar system and captured the Earth. Astronomically silly? Of course. But doggone what a fun story that would be!
posted
No, that's not right. Anything that approaches the Earth on any vector whatsoever will simply leave on another vector. The only way to get something into an orbit is to either boost it from the surface or apply a braking force to it at some point.
For a small delta V x mass, a couple of brushes with our outer atmosphere would do fine. But when you start talking about something really big...it ain't gonna work. There's just nothing to slow it down enough, and it'll be off again. The same applies in reverse for a really big item, unless we braked pretty doggon hard against its atmosphere, we would be the ones taking a long trip out of a fairly tight orbit.
Yeah, the dictionary is wrong, I'm right. A meteoroid is correctly defined as an object that might become a meteor. But on another level, if the rock is large enough to be visible, then it is an asteroid. If I noticed a large rock, then that would make it an asteroid (a rock big enough to see).
Nice and precise, eh? Forget about it.
Our intrepid star wanderer would say, "look, there's a smaller satellite near the planet. Hmmm, looks like a recent capture, I wonder whence it came?"
quote:Cruithne takes 770 years to complete its horseshoe orbit. Every 385 years, it comes to its closest point to Earth, some 9.3 million miles (15 million kilometers) away. Its next close approach to Earth comes in 2285.
I have to point out that technically, this thing isn't in orbid of the Earth at all, it's in orbit around the sun, but it is disturbed by our Lagrange points and so tends to precess and then lag as it bounces off the L3, L4, and L5 points. But while the movement diagram they've got does make this point clear enough, it also looks freaking weird ("freaking" as in "tending towards freakishness" rather than "freak-dancing").
If this is off the topic forgive me, but I have a question about moons as well. I think it would be cool to have my WIP set on a moon instead of a planet, probably orbiting a gas giant. It has about 80% Earth's gravity, but it does have Earth-like atmosphere. My questions about it are thus: does making it a moon make the tides so great as to be implausible for the support of normal human life, or for fields and forests and mountains and such? Would it have to be larger or smaller than Earth to make the above parameters work out? I want my novel to be pretty hard SF, and I would hate to make erroneous assumptions, but without getting a degree in astronomy I am not sure how to find out about that sort of thing. I imagine storms on this moon would be a doozy too. Any thoughts are appreciated.
posted
Shoot. There are two resources that I want to recommend, but I don't have the links for either. One is a hard science book on world building for SF writers. The other is a webpage that does all the math for you about rotation and such for world building. Does anyone know the ones that I'm talking about?
Posts: 2022 | Registered: Jul 2003
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posted
On the subject of aptured asteroid/metroid, you can look at Mars. Phobos and deimos are captured asteroids, so it is possible for a planetary body to capture a planetesimal of inferior gravity (inferior mass).
The conditions for a capture have to be just right, otherwise the planetesimal (or asteroid) will either continue on (in a significantly altered trajectory) or crash into the larger body.
The presence of a moon the size of our moon (bigger than Pluto and Charon) complicates thigs. Because of the moon, and the tidal pull, any captured body would find itself in a very unstable orbit. Larger planets like gas giants are so massive that they can have multiple moons. the tidal force of each moon is so low compared to Jupiter's or Saturn's gravity wells, for exemple, that they are insignificant. Phobos and Deimos' mass are also insignificant compared to Mars', only as big as small mountains.
The moon's g however is one sixth that of earth. Not an insigificant proportion at all. Theorists came to the conclusion that the moon almost had to be ejecta from an impact on Earth because it would have been almost impossible for Earth to capture something that big. None of the inner Telluric planets have moons this large.
As to definition, once a planetisimal has been captured, it becomes a moon. Phobos and deimos are called moons, not asteroids, because they are in a stable orbit.
Someone mentionned life on a moon orbiting a gas giant. It would be extremely difficult. Night/day cycles, for exemple, would have no bearings on that moon, because it would spend a long time in its primary's shadow. The tidal pull of the giant planet on the moon would cause oceanic surges of nearly cataclysmic proportions, and the radiation from the Van Allen belts would be extreme. But life has a way of encroaching on even the most hostile of environments, so it could be possible. Their evolution would be extremely odd, however, and any aliens' biology VERY alien.
posted
No earthly idea on the book. As for whether or not a moon would be able to support life, that's a kinda shaky question. I would depend on how quickly the moon circled the planet of this enormous gas giant, if it was eclipsed from the sun for any length of time (read more than one or two days max) the massive temperature changes would be practically unbearable to most earth-borne species. This problem is sort of a result of having two orbits instead of one. That would however make for some interesting possibilities, climatically speaking (giant frozen deserts that are eclipsed regularly and parts of the planet that have normal climates except for the cataclysmic cold fronts.)
The second major issue would be if you wanted to have any sort of native life form. This is because the circumstances that have to exist in order for complex life to arise are pretty complex on their own. For instance, the shallow seas where life is thought to have begun would not exist if the ocean were somewhere around a thousand feet lower (continental shelves). The earth has had massive climate changes twice in its history (recent theories have the earth's surface actually cooling twice, the second one being after whatever knocked the moon off into orbit.)
As an interesting side not about the moon, it was apparently liquid at one point because a scan of the minerals places most of the heavier ones closer to use, as if they were pulled there by our gravity while it was cooling. This would also explain why the moon is round while a good number of other moons are not.
To answer the first question, I think that the addition of any sort of large body would have some effect on our climate, particularly if it messed up the orbit of the moon. (Which would be one object that might be able to slow down a large object without blowing away the planet).
Then again, astrophysics aren't exactly my specialty, so take what I say with a grain of salt. Maybe it's helped someone come up with an idea.
Jon
[This message has been edited by bladeofwords (edited August 04, 2004).]
posted
Our moon is planetoid sized so it would be round anyway, rock is only so strong, after all.
The solution for tides, eclipses, and all that stuff is to have the moon further away from the gas giant. If you're far enough away from the planet so it looks about the size of our moon (seen from the hypothesized moon and Earth respectively), then you should be fine...particularly with a moon slightly smaller than Earth.
Naturally, the moons of a gas giant tend to be a heck of a lot closer than that, but even then, the far shorter orbital period means that the midday eclipse only lasts a few hours. You can eliminate the effect of tides completely by having a gravitationally locked moon. Or you could have a slow enough rotation so that tides would rise and fall slowly. That does mean having a day that lasts nearly the full length of the orbital period, but if you're in one of those 50 hour or less orbits, then that isn't so harsh.
It on the planetward side of your moon, it wouldn't be colder at all. In fact, it would be quite a bit warmer. Gas giants radiate a pretty fair amount of heat in addition to the reflected radiation from the primary. Of course, this would definately give you some strange weather, particularly as gas giants in the habitable range of their primaries tend to have orbits much more elliptical than Earth's (at least, this is what the extrasolar planets discovered thus far seem to indicate).
The thermal radiation from the planet would buffer that variation in stellar radiation somewhat, but things would still be pretty darn interesting. Also, right now it seems at least plausible that life didn't arise on Earth originally. In fact, I would tend to regard it as a scientifically proven fact, but there are still diehards on the issue. Anyway, it has been pretty well established that there is no insuperable barrier to life arising on one planet and then colonizing another.
posted
Thanks, guys. I think the moon idea won't work for this particular WIP, but I'll keep it on the back burner. MaryRobinette, I have read that book on world building, but I also can't remember the title. The site on world building sounds intriguing. I didn't know it was out there. I'll try to Google it.
Posts: 818 | Registered: Aug 2004
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posted
As it happens, the Earth itself was originally a moon of Saturn. It was in a gravitationally locked orbit, and very close; close enough that the gravity of Saturn made things on the Saturn-ward side of the Earth weigh less. That's why the plants and animals of the dinosaur age were able to grow taller than plants and animals today.
If you believe one version of Velikovsky's catastrophe theory, that is.
posted
Survivor, I don't know why I even try. You make me look like a highschool student (then again...that's pretty accurate). :-).
However, I didn't say there was anything to keep life from being planted somewhere else, I think it would be much more difficult to start from scratch without interference.
Are you suggesting that the proverbial spark of life came from somewhere else or that it was physically transported en masse from somewhere? Either way, that lends the question of why we have a pretty thoroughly established chain of evolution in complexity. I suppose if only the most basic life forms were introduced it wouldn't be that big of a problem. What exactly are you saying there?
All this discussion of orbits and such is very interesting. It reminded me of a fact I read somewhere. Apparently, if you put an animal (including a person) in a place (underground) with no outside stimulus that would indicate time it reverts to a 26 hour cycle. This appears to show that sometime in a relatively recent past something major happened to our day/night cycle.
posted
Nah, I think it just indicates that we tend to lag rather than precess in our circadian cycle. There are some obvious benefits to being generally able to stay awake a couple of extra hours rather than tending to drop in our tracks just before the sun goes down, after all.
The probable first life on Earth would have been Archaea (sometimes still called Archae[o]bacteria). These little bugs are very different from other Earth life and they prefer to live in all kinds of extreme conditions hostile or inhospitable to most other terrestrial life. Some species could easily live on Mars, others could readily colonize the various moons of Jupiter and Saturn (a few of the species which life directly inside of high temperature rocks could probably survive on any geologically active rocky planetiod with even a small amount of water).
The existance of these life forms strongly suggests that it is possible for the Earth to have "seeded" life onto at least some other planets, and their relationship to later Earth life seems to raise the strong possibility that they are evolutionary holdouts from life that evolved elsewhere and came to Earth somehow.
Velikovsky's catastrophe theory? That's some wild stuff...but from what I'm reading he suggested that it was Venus that was supposed to have been a moon of Jupiter, not Saturn. The man apparently was originally trained as a psycholotist or something, which probably explains most of his wild ideas.
Of course his fundamental premise, that most of the catastrophic events recorded in ancient record have some basis in real catastrophes, is not so wild. A great many catastrophic events recorded in ancient record turn out to have happened. But his explanations of these events don't appear informed by any sort of understanding of the physical sciences.
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I don't know why that struck me as so funny, but thank you, Jules, for a great belly laugh. <pulls self together, smiling> 
