You'll probably need a fish-eye lens more than a wide-field. The spread is 135 degrees, which I'm not sure really counts as a small section of sky.
I'd say that it's less a chance for astrophotography and more for a chance to go outside and view the 5 nake-eye planets with your own two eyes. Not many people have seen all 5 of them, particularly Mercury.
The Sapir-Wharf hypothesis is certainly adhered to by quite a few anthropologists, and it makes quite a lot of sense. The Eskimo snow digress is, as near as I can tell, either a very odd digression on your part or a helluva strawman you knocked down. As for translating the Bible, you are assuming that it means the same thing in every language, which I think is quite a risky thing to assume.
But all of this is irrelevent to my point.
I agree that teaching kids definitions isn't science, it's rote memorization (which is easy to "teach", but isn't what we really need to be teaching kids about science). However, you seem to dismiss the idea of spending time devising precise terms. So I ask you: how would you have us (scientists) discuss our research with each other if we didn't have a common language to use? Very often I have seen two scientists (and occasionally been one of them myself) argue for a great length of time, only to discover that their terms didn't mean the same thing and that they didn't really disagree. (Of course, this happens outside of science, too...) This is at best a waste of time.
Getting the terms down isn't a scientific result, but it's a stepping stone. You're asking us to forego the groundwork and jump to the conclusion.
Yes, but if Voyager 2 were to encounter Sedna now, it wouldn't do a bit of good. It doesn't have nearly enough power to collect any useful data, let alone return it. Both Voyagers and Pioneers 10 and 11 have been essentially comatose for many years now: they're alive, but just barely.
Re:Intermingling of fact and definition
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Sedna May Have A Moon
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· Score: 3, Informative
Kuiper Belt Objects are about half and half rock and ice, actually. Beyond about 5 AU you expect icey bodies, since hydrogen compounds are way more common than metals and silicates. (The only trouble being that it has to be cool enough for them to condense.)
And, yes, there has long been hypothesized to be (based on dynamical models and, I believe, comet distributions) an Inner and Outer Oort Cloud. There is some thought that the Inner Oort Cloud should sort of merge with the Edgeworth-Kuiper Belt, although that's mostly speculation.
Since the Edgeworth-Kuiper Belt is thought to stop around 50 AU or so, this object isn't likely to be a KBO. Inner Oort Cloud seems to fit, though.
The problem is that different planet scientists need the term "planet" (as in "major planets") to mean somewhat different things. A dynamicists like myself is concerned with the probable origin and current dynamical behavior of the object. Geologists care about the surface and interior and therefore whether the has been any endogenic geological processes active in the past. Atmospheres people have an entirely different set of concerns. And so forth.
Re:Intermingling of fact and definition
on
Sedna May Have A Moon
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· Score: 4, Insightful
I am an astronomer and as far as I know, none of us is trying to get this beast classified as a (major) planet. It's most likely (by anyone's definition) an Inner Oort Cloud object. That more than adequately describes it in relation to the rest of the solar system. The whole idea of there being a controversy is more of a media/NASA press-office construction than a reality.
And while on one level you're right, names are artifical, on another you miss the point of naming entirely. What we call things affect how we consider and perceive those things. Language shapes our thoughts, after all. So classification isn't "prescience" it's a part of the scientific process. Classifying living organisms was the first step towards understanding their relation and then their evolution. Classifying celestial objects plays the same role. It may not be as scientific sounding as producing numbers and plots, but it's important none the less.
I saw one yesterday during Chad Trujillo's colloquium here at CU. Sedna is more or less opposite the Sun from Pluto. Besides, even at 76AU that's more than double the trip for New Horizons. It probably won't make it that far, not to mention the science team still being around.
Of course not every accident would prevent the shuttle from docking with ISS. That's not what I said. But, as the CAIB pointed out, many can. That's the concern.
As for HST being more prone to an accident, where are you getting *that* from? As far as tile damage goes, I'd expect more disk from ISS if anything, although I'd guess that it's about break even. (Note that at least one NASA engineer has aruged that HST trips are less likely to cause accidents than ISS ones, by the way.) Please cite your source for that claim.
The only trouble with that interpretation is that it doesn't really match the data. The Colombia Accident Investigation Board did recommend that future missions that cannot make it to ISS should carry an autonomous repair kit. (Basically, something that lets them repair the shuttle in a pinch.) They did NOT say "Don't got to HST," just that NASA needs some more safety in place. Note that while O'Keefe keeps trying to spin this as an issue of astronaut safety, it's really about the cost of that kit.
Here's where it gets really weird. (And I'm confused as to why the press doesn't seem to have bothered to check this.) Two paragraphs latter, the CAIB also recommends developping this same kit for ISS-bound missions. Why? Because, as they very intelligently point out, there's no guarantee that you can get to ISS in the case of an accident. That's the very nature of an accident after all: to reduce your functionality. They point out that the shuttle might not be able to match the ISS's orbit, that docking might be impossible, or even that the accident might occur during undocking. In that case, you still want the repair kit. (Actually, I'd want it even if I were docked at ISS. It's always better to have the tools, supplies, and equipment needed to repair the shuttle already on hand, no? And since we've been told that they need all of those shuttle flights to finish ISS, they'd better repair the shuttle.)
The result of this is that I find it almost inescapable that O'Keefe isn't that concerned with safety. He might be trying to cut costs or he might be responding to orders from above (denials not withstanding), but I find it hard to believe that he's following recommendations to the letter or just really, really safety conscious given the situation.
Um, yes, Newton does talk about rest mass. That's what he MEANS by "mass", period. Newton figured that mass was invariant. Which is why the second half of your post is, I hope, a comedy and not serious. Newton would have no idea what you were talking about, especially since you used the relativistic E=mc^2.
Where you got that 2, by the way, I have no idea. But, then, given how blithely you applied only part of the correct equation (and "proved" that photons have non-zero rest mass, oddly enough), I'm not surprised.
Only if you fudge it. Newtonian mechanics figures that since photons have zero rest mass, they aren't affected by gravity at all. (Without SR you don't get the mass-energy equivelence.)
Careful there. A simple-minded Newtonian derivation gives the correct Schwartschild radius for a black hole, despite having two deep physical flaws and relying on completely inapplicable physics. For that matter, two words: "Bode's Law."
"If they all have the same period, then wouldn't their average distance from the Sun be directly proportional to the eccentricity of their orbit?"
Mr. Kepler says a big nope. He says that the orbital semi-major axis(call it the average distance from the Sun, although it's not technically a time-averaged distance), a, relates only to the period, p, as a^2=p^3, measureing a in AU and p in years. Eccentricity don't enter into it. (It does enter into the time-averaged distance, although I'm going to go out on a limb and guess that it would be a lot of an effect for even reasonably small eccentricities.)
All "Plutinoes" orbit with the same semi-major axis (to within some fairly small range) thanks to have that same period.
Well, let me address (although not outright answer) the question you actually posted: the chances of a given KBO occulting a given star are pretty small. Particularly if the star doesn't lie close to the ecliptic...
That said, the odds of a KBO occulting a star or, perhaps better, any KBO occulting any star aren't all that tiny. Pluto occults stars on occasion. That's how we get data on the atmospheric structure, after all. Admittedly, astronomers keenly search for these occultations, but they do happen every few years or so. Now, mind you, Pluto is a big KBO so that it has a much higher probablity of passing in front of a star than most of the others. Recall that it is, in fact, one of the very few KBOs that have been resolved in our images. (I believe that Quaoar, Charon, Varuna, and perhaps one or two others have been. It's because we have not resolved many of these that we don't know the albedoes/sizes of KBOs that well.)
The next question is why should we look for occultations? Using them as a means to detect KBOs is probably a waste of time. Occultations can give you a profile of the atmosphere which might not be interesting for most KBOs, although a non-detection would at least tell us something about KBOs in general. (i.e. - That they don't have atmospheres. Not that I think people would be generally stunned by this.)
Quite possibly. The assertion made might have been confused with that Colombia was the only shuttle beefy enough to LAND with the sucker in storage. But that's what I was told.
The plan always was to bring HST back down to Earth and put it in the Smithsonian. So I was very confused about why they started talking about deorbiting it in the more destructive way about a year ago. Then a fellow astronomer told me that that is no longer possible. The only shuttle that had a large enough bay to carry it (into orbit or back down again) was Colombia. That's no longer a possibility, obviously.
So as much as I agree with your sentiment, I think I can understand the reasons for the decision not to bring it back down intact.
NASA is actually considering a new Venus mission, perhaps with a lander. It's one of the missions topics that they've suggested and are taking proposals for right now. I know someone who is actually working on proposing just such a mission. So there is hope:-)
No offense, but I'll trust my professors, who have flown multiple spacecraft for NASA, over you're estimate of the failure rates. That 1 in 20 is the number we were given, and I believe them.
And I'm not handwaving, you're just refusing to comprehend: the Apollo 1 fire occured during a simulated launch. It was NOT a test of the equipment. If they'd been testing the equipment, they wouldn't have had the crew on board in the first place. (If you're trying to see if some equipment will go wrong, you don't want extra people in the way, potentially getting hurt.)
And you're still not getting the point of where it failed. It failed on the pad, as it would have in launch had they not decided to do a dry run. We're not here to speculate about where it could have failed, we're looking at data. The data say it failed during a launch simulation. You can speculate as much as you like about where you would like to think it might have failed in another universe, but the data says, "It happened on the pad." End of discussion.
Finally: I promise you, the US government did NOT insure Cassini in ANY sense of the word. The spacecraft, like Galileo before it, costed better than a billion dollars. If NASA had flubbed it, they probably would have never gotten the money to do it again. Certainly not anytime remotely soon. Even the Mars missions, cheaper and better-funded though they are, aren't replaced as such. Even if they fly a similar instrument, it takes years for that to go up again. If they could avoid this easily and cheaply by saving the spacecraft via an ejection system, they would.
The failure (shorted insulation) could have happened during routine (normal atmosphere) testing as well.
I continue to maintain: the short would have shown up in the actual launch when that system went active. It obviously didn't cause problems in the factory (because that's not when the failure occured) and the spacecraft never would have *made* it to orbit. If a system is goign to fail, it'll fail the first time the conditions are right. For many, that's launch. Ergo, this was a launch failure.
Saying that the problem could have occured elsewhere doesn't change that it did occur in a launch situation (if only a training exercise). You can argue about when you personally think the failure might also have occured, but it doesn't change the data that are in front of you: it occured on the launch pad during a simulated launch situation.
Countless is a word that sounds awfully scary, but the reality is the failure rate is fairly stable at about 2% of launch attempts.
I'm drawing on on my gradute spacecraft instrumentation course from a few years ago, here, but the Delta IV's launch failure rate is around one in twenty. Which is more like 5%. And that's relatively good compared to some other systems.
Since an unmanned payload can be replaced by insurance, and the failure rate is so low, there is no incentive to take the payload/performance hit.
Yeah, I'm sure that Cassini is fully insured, as was the Russian Mars mission (Mars 96, was it?) that ended up in the drink. Insurance doesn't cover everything, and even if you're covered, it doesn't make up for the time you lose when you're rebuilding the spacecraft.
I do if it kills three astronuats in a situation that was only different from the real launch in that they didn't intent to actual light the thing. If they hadn't been training shortly before the intended launch, the accident would have happened during the lanuch itself. Therefore, I see no difference between the two in terms of evaulating the ease and safety of launch.
I'd point out that Apollo 1 also is a launch failure. They were doing a sort of a dry run before the actual launch, but it was the same set of conditions as launch and the failure would have cropped up in the same way.
There also countless cases of unmanned launch disasters where the payload didn't survive. The priority for saving the payload is much less in that case, but the fact that it isn't evidently deemed worthwhile to add an ejection system for a multi-million (or even a billion) dollar spacecraft is, I think, indicative that it isn't that cheap/easy to manage.
Glad you saw Apollo 13, too. Alas, it doesn't actually apply. The skipping off the atmosphere is more true of a spacecraft returning from the Moon. It's a lot easier to manage to de-orbit, since all you have to do is hit the brakes a bit to ease into the atmosphere. You have lots of time and it's pretty easy to get it right. If you don't, you generally have time to do it again, since you're not going anywhere except perhaps down. (As opposed to missing the Earth and flying off on a multi-day orbit, which Apollo would have done.)
Sure you can stop reentry once you start. You'd just have to carry enough fuel to boost yourself back out of the atmosphere. It's much more difficult to stop an explosion once it starts, though.
As for dumping KE, timescale IS relevent. Since you don't *gain* all of that potential energy all at once, you can slowly bleed off the KE that it becomes as you re-enter. Note that in many ways, Colombia was the worst-case senario: it was still moving very quickly in a relatively dense part of the atmosphere with a craft that is *not* self-stabalizing. (Unlike many capsules.)
It isn't impossible to escape an explosion, but it's definately hard. You have at best a few seconds of warning to get clear. Having a craft that can jettison its tanks only helps if there is enough *time* to let them loose and move away. The nature of explosions is to happen really fast, by definition.
I forgot to point out that there is a lot more energy in the fuel at launch than in the craft upon re-entry. Remember, the fuel has to not only loft the capsule but also the other fuel that well be burned up later in the flight and all of the later stages of the rocket.
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You'll probably need a fish-eye lens more than a wide-field. The spread is 135 degrees, which I'm not sure really counts as a small section of sky.
I'd say that it's less a chance for astrophotography and more for a chance to go outside and view the 5 nake-eye planets with your own two eyes. Not many people have seen all 5 of them, particularly Mercury.
The Sapir-Wharf hypothesis is certainly adhered to by quite a few anthropologists, and it makes quite a lot of sense. The Eskimo snow digress is, as near as I can tell, either a very odd digression on your part or a helluva strawman you knocked down. As for translating the Bible, you are assuming that it means the same thing in every language, which I think is quite a risky thing to assume.
But all of this is irrelevent to my point.
I agree that teaching kids definitions isn't science, it's rote memorization (which is easy to "teach", but isn't what we really need to be teaching kids about science). However, you seem to dismiss the idea of spending time devising precise terms. So I ask you: how would you have us (scientists) discuss our research with each other if we didn't have a common language to use? Very often I have seen two scientists (and occasionally been one of them myself) argue for a great length of time, only to discover that their terms didn't mean the same thing and that they didn't really disagree. (Of course, this happens outside of science, too...) This is at best a waste of time.
Getting the terms down isn't a scientific result, but it's a stepping stone. You're asking us to forego the groundwork and jump to the conclusion.
Yes, but if Voyager 2 were to encounter Sedna now, it wouldn't do a bit of good. It doesn't have nearly enough power to collect any useful data, let alone return it. Both Voyagers and Pioneers 10 and 11 have been essentially comatose for many years now: they're alive, but just barely.
Kuiper Belt Objects are about half and half rock and ice, actually. Beyond about 5 AU you expect icey bodies, since hydrogen compounds are way more common than metals and silicates. (The only trouble being that it has to be cool enough for them to condense.)
And, yes, there has long been hypothesized to be (based on dynamical models and, I believe, comet distributions) an Inner and Outer Oort Cloud.
There is some thought that the Inner Oort Cloud should sort of merge with the Edgeworth-Kuiper Belt, although that's mostly speculation.
Since the Edgeworth-Kuiper Belt is thought to stop around 50 AU or so, this object isn't likely to be a KBO. Inner Oort Cloud seems to fit, though.
The problem is that different planet scientists need the term "planet" (as in "major planets") to mean somewhat different things. A dynamicists like myself is concerned with the probable origin and current dynamical behavior of the object. Geologists care about the surface and interior and therefore whether the has been any endogenic geological processes active in the past. Atmospheres people have an entirely different set of concerns. And so forth.
I am an astronomer and as far as I know, none of us is trying to get this beast classified as a (major) planet. It's most likely (by anyone's definition) an Inner Oort Cloud object. That more than adequately describes it in relation to the rest of the solar system. The whole idea of there being a controversy is more of a media/NASA press-office construction than a reality.
And while on one level you're right, names are artifical, on another you miss the point of naming entirely. What we call things affect how we consider and perceive those things. Language shapes our thoughts, after all. So classification isn't "prescience" it's a part of the scientific process. Classifying living organisms was the first step towards understanding their relation and then their evolution. Classifying celestial objects plays the same role. It may not be as scientific sounding as producing numbers and plots, but it's important none the less.
I saw one yesterday during Chad Trujillo's colloquium here at CU. Sedna is more or less opposite the Sun from Pluto. Besides, even at 76AU that's more than double the trip for New Horizons. It probably won't make it that far, not to mention the science team still being around.
Of course not every accident would prevent the shuttle from docking with ISS. That's not what I said. But, as the CAIB pointed out, many can. That's the concern.
As for HST being more prone to an accident, where are you getting *that* from? As far as tile damage goes, I'd expect more disk from ISS if anything, although I'd guess that it's about break even. (Note that at least one NASA engineer has aruged that HST trips are less likely to cause accidents than ISS ones, by the way.) Please cite your source for that claim.
The only trouble with that interpretation is that it doesn't really match the data. The Colombia Accident Investigation Board did recommend that future missions that cannot make it to ISS should carry an autonomous repair kit. (Basically, something that lets them repair the shuttle in a pinch.) They did NOT say "Don't got to HST," just that NASA needs some more safety in place. Note that while O'Keefe keeps trying to spin this as an issue of astronaut safety, it's really about the cost of that kit.
Here's where it gets really weird. (And I'm confused as to why the press doesn't seem to have bothered to check this.) Two paragraphs latter, the CAIB also recommends developping this same kit for ISS-bound missions. Why? Because, as they very intelligently point out, there's no guarantee that you can get to ISS in the case of an accident. That's the very nature of an accident after all: to reduce your functionality. They point out that the shuttle might not be able to match the ISS's orbit, that docking might be impossible, or even that the accident might occur during undocking. In that case, you still want the repair kit. (Actually, I'd want it even if I were docked at ISS. It's always better to have the tools, supplies, and equipment needed to repair the shuttle already on hand, no? And since we've been told that they need all of those shuttle flights to finish ISS, they'd better repair the shuttle.)
The result of this is that I find it almost inescapable that O'Keefe isn't that concerned with safety. He might be trying to cut costs or he might be responding to orders from above (denials not withstanding), but I find it hard to believe that he's following recommendations to the letter or just really, really safety conscious given the situation.
Um, yes, Newton does talk about rest mass. That's what he MEANS by "mass", period. Newton figured that mass was invariant. Which is why the second half of your post is, I hope, a comedy and not serious. Newton would have no idea what you were talking about, especially since you used the relativistic E=mc^2.
Where you got that 2, by the way, I have no idea. But, then, given how blithely you applied only part of the correct equation (and "proved" that photons have non-zero rest mass, oddly enough), I'm not surprised.
Only if you fudge it. Newtonian mechanics figures that since photons have zero rest mass, they aren't affected by gravity at all. (Without SR you don't get the mass-energy equivelence.)
Careful there. A simple-minded Newtonian derivation gives the correct Schwartschild radius for a black hole, despite having two deep physical flaws and relying on completely inapplicable physics. For that matter, two words: "Bode's Law."
"If they all have the same period, then wouldn't their average distance from the Sun be directly proportional to the eccentricity of their orbit?"
Mr. Kepler says a big nope. He says that the orbital semi-major axis(call it the average distance from the Sun, although it's not technically a time-averaged distance), a, relates only to the period, p, as a^2=p^3, measureing a in AU and p in years. Eccentricity don't enter into it. (It does enter into the time-averaged distance, although I'm going to go out on a limb and guess that it would be a lot of an effect for even reasonably small eccentricities.)
All "Plutinoes" orbit with the same semi-major axis (to within some fairly small range) thanks to have that same period.
Well, let me address (although not outright answer) the question you actually posted: the chances of a given KBO occulting a given star are pretty small. Particularly if the star doesn't lie close to the ecliptic...
That said, the odds of a KBO occulting a star or, perhaps better, any KBO occulting any star aren't all that tiny. Pluto occults stars on occasion. That's how we get data on the atmospheric structure, after all. Admittedly, astronomers keenly search for these occultations, but they do happen every few years or so. Now, mind you, Pluto is a big KBO so that it has a much higher probablity of passing in front of a star than most of the others. Recall that it is, in fact, one of the very few KBOs that have been resolved in our images. (I believe that Quaoar, Charon, Varuna, and perhaps one or two others have been. It's because we have not resolved many of these that we don't know the albedoes/sizes of KBOs that well.)
The next question is why should we look for occultations? Using them as a means to detect KBOs is probably a waste of time. Occultations can give you a profile of the atmosphere which might not be interesting for most KBOs, although a non-detection would at least tell us something about KBOs in general. (i.e. - That they don't have atmospheres. Not that I think people would be generally stunned by this.)
Ah, thanks for the clairifying. All the peices appear to fall into place now :-)
Quite possibly. The assertion made might have been confused with that Colombia was the only shuttle beefy enough to LAND with the sucker in storage. But that's what I was told.
The plan always was to bring HST back down to Earth and put it in the Smithsonian. So I was very confused about why they started talking about deorbiting it in the more destructive way about a year ago. Then a fellow astronomer told me that that is no longer possible. The only shuttle that had a large enough bay to carry it (into orbit or back down again) was Colombia. That's no longer a possibility, obviously.
So as much as I agree with your sentiment, I think I can understand the reasons for the decision not to bring it back down intact.
NASA is actually considering a new Venus mission, perhaps with a lander. It's one of the missions topics that they've suggested and are taking proposals for right now. I know someone who is actually working on proposing just such a mission. So there is hope :-)
No offense, but I'll trust my professors, who have flown multiple spacecraft for NASA, over you're estimate of the failure rates. That 1 in 20 is the number we were given, and I believe them.
And I'm not handwaving, you're just refusing to comprehend: the Apollo 1 fire occured during a simulated launch. It was NOT a test of the equipment. If they'd been testing the equipment, they wouldn't have had the crew on board in the first place. (If you're trying to see if some equipment will go wrong, you don't want extra people in the way, potentially getting hurt.)
And you're still not getting the point of where it failed. It failed on the pad, as it would have in launch had they not decided to do a dry run. We're not here to speculate about where it could have failed, we're looking at data. The data say it failed during a launch simulation. You can speculate as much as you like about where you would like to think it might have failed in another universe, but the data says, "It happened on the pad." End of discussion.
Finally: I promise you, the US government did NOT insure Cassini in ANY sense of the word. The spacecraft, like Galileo before it, costed better than a billion dollars. If NASA had flubbed it, they probably would have never gotten the money to do it again. Certainly not anytime remotely soon. Even the Mars missions, cheaper and better-funded though they are, aren't replaced as such. Even if they fly a similar instrument, it takes years for that to go up again. If they could avoid this easily and cheaply by saving the spacecraft via an ejection system, they would.
The failure (shorted insulation) could have happened during routine (normal atmosphere) testing as well.
I continue to maintain: the short would have shown up in the actual launch when that system went active. It obviously didn't cause problems in the factory (because that's not when the failure occured) and the spacecraft never would have *made* it to orbit. If a system is goign to fail, it'll fail the first time the conditions are right. For many, that's launch. Ergo, this was a launch failure.
Saying that the problem could have occured elsewhere doesn't change that it did occur in a launch situation (if only a training exercise). You can argue about when you personally think the failure might also have occured, but it doesn't change the data that are in front of you: it occured on the launch pad during a simulated launch situation.
Countless is a word that sounds awfully scary, but the reality is the failure rate is fairly stable at about 2% of launch attempts.
I'm drawing on on my gradute spacecraft instrumentation course from a few years ago, here, but the Delta IV's launch failure rate is around one in twenty. Which is more like 5%. And that's relatively good compared to some other systems.
Since an unmanned payload can be replaced by insurance, and the failure rate is so low, there is no incentive to take the payload/performance hit.
Yeah, I'm sure that Cassini is fully insured, as was the Russian Mars mission (Mars 96, was it?) that ended up in the drink. Insurance doesn't cover everything, and even if you're covered, it doesn't make up for the time you lose when you're rebuilding the spacecraft.
I do if it kills three astronuats in a situation that was only different from the real launch in that they didn't intent to actual light the thing. If they hadn't been training shortly before the intended launch, the accident would have happened during the lanuch itself. Therefore, I see no difference between the two in terms of evaulating the ease and safety of launch.
I'd point out that Apollo 1 also is a launch failure. They were doing a sort of a dry run before the actual launch, but it was the same set of conditions as launch and the failure would have cropped up in the same way.
There also countless cases of unmanned launch disasters where the payload didn't survive. The priority for saving the payload is much less in that case, but the fact that it isn't evidently deemed worthwhile to add an ejection system for a multi-million (or even a billion) dollar spacecraft is, I think, indicative that it isn't that cheap/easy to manage.
Glad you saw Apollo 13, too. Alas, it doesn't actually apply. The skipping off the atmosphere is more true of a spacecraft returning from the Moon. It's a lot easier to manage to de-orbit, since all you have to do is hit the brakes a bit to ease into the atmosphere. You have lots of time and it's pretty easy to get it right. If you don't, you generally have time to do it again, since you're not going anywhere except perhaps down. (As opposed to missing the Earth and flying off on a multi-day orbit, which Apollo would have done.)
Sure you can stop reentry once you start. You'd just have to carry enough fuel to boost yourself back out of the atmosphere. It's much more difficult to stop an explosion once it starts, though.
As for dumping KE, timescale IS relevent. Since you don't *gain* all of that potential energy all at once, you can slowly bleed off the KE that it becomes as you re-enter. Note that in many ways, Colombia was the worst-case senario: it was still moving very quickly in a relatively dense part of the atmosphere with a craft that is *not* self-stabalizing. (Unlike many capsules.)
It isn't impossible to escape an explosion, but it's definately hard. You have at best a few seconds of warning to get clear. Having a craft that can jettison its tanks only helps if there is enough *time* to let them loose and move away. The nature of explosions is to happen really fast, by definition.
Addendum:
I forgot to point out that there is a lot more energy in the fuel at launch than in the craft upon re-entry. Remember, the fuel has to not only loft the capsule but also the other fuel that well be burned up later in the flight and all of the later stages of the rocket.