Wikipedia:Reference desk/Archives/Science/2017 February 21

= February 21 =

Wendy's Frosty
I'd like them better without trans fats, which they say they have, at least in the large size:.

I've noticed if I put one in the fridge for later, it separates into a liquid with a thin skin on top. If I remove this skin, will I also remove the majority of the trans fats ? StuRat (talk) 00:31, 21 February 2017 (UTC)
 * Both the menu you linked to and our article Frosty (frozen dairy dessert) say it's dessert made with milk and it's frozen, strongly imply there's at least some milk fat. In fact if you check it out, the description of that page you linked to explicitly says "Nothing beats this Wendy’s original that's made from fresh Grade A milk and rich cream." In other words, you should not need to look at the nutrition info to tell you it contains transfat anymore then you need to look at the nutrition info on your 100% organic all natural ingredients ice cream or Kobe beef contains transfat. P.S. I have zero interest in getting involved in the argument over whether or not naturally occuring transfat are as dangerous as industrially produced ones i.e. whether or not people are right to be concerned over such transfats. Simply pointing out that labelling in most places including I'm pretty sure the US  does not distinguish between naturally occuring and industrially produced transfats. So by definition, any product with sufficient naturally occuring transfat to exceed any nutrition labelling requirements, including many products with significant cream and some beef edit: and lamb and maybe very occasionally pork and chicken and other meat products, will have transfats so it's silly to make a big deal over the nutrition labels when simply common sense will tell you already.  Nil Einne (talk) 03:57, 21 February 2017 (UTC)
 * Wendy's either doesn't provide a full ingredient list of their site is broken. But if is accurate, and that does list the same amount of transfats plus a similar list is elsewhere, it does seem quite likely that most or all of the transfats comes from the cream, perhaps with a small amount from processing to make the dessert. Again I'm not saying whether this means you should or shouldn't be worried, simply pointing out it's silly to be using the nutritional facts info. It's a product which contains a significant amount of cream so it also contains transfats, an educated consumer should already know this. Edit: Actually found  which does work properly for me and you can get stuff like  which seems to basically be the same ingredient list as earlier.Nil Einne (talk) 04:12, 21 February 2017 (UTC)
 * I've had Frostys. They're just a milkshake. You can find a million milkshake recipes online. A Frosty is perhaps a little thicker than your average milkshake, so a little experimentation with some recipes might be necessary. --47.138.163.230 (talk) 05:31, 21 February 2017 (UTC)
 * Well, they're similar to a milkshake, but they're not identical. Most people don't use cellulose in their milkshakes. - Nunh-huh 17:16, 21 February 2017 (UTC)
 * But that's not likely to significantly change the amount of transfat unless you use very low fat milk (which tends to significantly affect the flavour and texture). You'd need to use some sort of milk fat substitute. Nil Einne (talk) 09:05, 21 February 2017 (UTC)
 * Milk doesn't have any trans fats. Trans fats are created by partial hydrogenation of polyunsaturated fats.  This is not a natural process.  A natural product like milk (or even butter, which is just concentrated milk fats) contains no trans fats.  Basically trans fats show up in the Western diet in two ingredients: margarine and vegetable shortening.  Purely natural fat sources don't have any trans fats.  -- Jayron 32 13:34, 21 February 2017 (UTC)
 * "Milk doesn't have any trans fats." -- that is in direct contradiction to (my reading of) the table included at Trans_fat. It shows that milk and butter both have small amounts of trans fats (0.0007-7% by mass), while shortening (being largely industrially hydrogenated oil) is up to 1/3 trans fat by mass. SemanticMantis (talk) 14:45, 21 February 2017 (UTC)


 * There's probably a uranium atom or two in milk also. The question is whether there is any dietarily significant quantities in milk.  0.0007% is 7 ppm.  I'm not sure that I'd call that dietarily significant.  That borders on homeopathic concentrations.  Reading up in some additional articles, it DOES appear that pure animal fats (such as tallow or lard) is about 3-8% by weight trans fats.  Which is probably why the OP noticed that the trans fats only started showing up on a large Frosty; at smaller sizes the amount of trans fats rounds down to 0 grams.  So, it does appear that natural fats do contain small concentrations of trans fats; the issue should then be do such sources contain enough trans fats to concern ourselves with.  -- Jayron 32 15:03, 21 February 2017 (UTC)


 * I said nothing of dietary significance, nor do I really care. I'm only concerned at the moment with simple clear facts. 7% trans fat in butter is a hell of a lot more than " A natural product... contains no trans fats." as you claimed. I just wanted to point out the error, so that nobody was accidentally misinformed here. It might make sense for you to strike some of your comments above for clarity, in light of reading the information contained in the article you cited. SemanticMantis (talk) 15:45, 21 February 2017 (UTC)
 * Since you're more concerned with winning than spreading correct information, let me confirm: You win. Before you even made this comment, I had already confirmed what you said and found a source to confirm what you said.  I am far more concerned with getting information correct than winning some silly pissing contest as you seem to be, but if you wish me to concede defeat and proclaim you better than me as a human so we can move on, I will do so:  I'm a worthless piece of shit, you're perfect, and we can leave my references there so people can read up more on the amount of trans fats in foods for themselves.  -- Jayron 32 15:50, 21 February 2017 (UTC)
 * Don't sulk, it's unbecoming. I posted a simple, polite correction, but you didn't seem to accept it, and instead focused on how 0.0007% is very small. So I clarified what I was correcting, because you brought up things not relevant to my post. I think we're done now, have a nice day! SemanticMantis (talk) 16:18, 21 February 2017 (UTC)
 * You still continue to have been correct, and I still continue to have already conceded that before you decided to continue to carry this farther. You aren't going to suddenly become wrong because you argue more.  I will not become all of a sudden right because you argue more.  Demanding a correction after I already provided one seems a bit beyond the pale.  -- Jayron 32 16:41, 21 February 2017 (UTC)
 * (Multiple-EC) Since I know my comments can be TL;DR the key summary is whether you want to worry about transfats and at what levels and from what sources is up to you. But if you do want to worry, you should understand where the transfats are coming from and not be surprised that your cream or beef contains transfat. Understanding this will also hopefully mean you don't think you can avoid transfats by making it at home when actually you're probably going to be using the same key source of transfats. 0.0007% is a tiny percentage but as you've now realised, the percentage in animal fats can easily be far higher, in the single digit percentage range of total fats. AFAIK, fats from cows and evidentally lamb tends to be higher then chicken or pork, and it does tend to be transfats from dairy/cream and occasionally beef or ruminants that gets the most attention. But the precise percentage of the fat that is trans depends on diet and other factors. (If you're wondering, I believe that these are mostly produced by the gut bacteria  .)  As mention, I've no desire to get into the debate over whether it makes sense to worry over the transfats in cream etc. Instead I simply want to establish, that AFAICT, it should not be a surprise that the Frosty contains transfat given it's a product with significant amounts of cream. And all evidence I've seen suggests that the transfat in Frosty comes mostly from the cream. This also means that making one at home is pointless if your only concern is the transfat, unless you intend to use a milk fat substitute.  I do agree that the amount of transfats from natural sources is far lower than that from PHO. I will however note if it's simply an issue of quantity then it also doesn't matter if the Frosty transfat is from cream or PHO. The concern over transfat is strong enough that when safe levels have been suggested they're normally very low e.g. 2 g a day on a 2000 calorie diet  a level you likely could exceed just from naturally occuring transfats. Again no comment on whether you should be concerned (or whether you're actually likely to have bigger concerns anyway with such a diet). But I suspect this is one reason why dairy promoters etc, if they talk about transfat at all, tend to talk about how there's no evidence natural transfats are harmful or they might even be good for you and they've been in the diet for 10k years etc    rather than just about quantities. (Again explicitly not commenting on such claims.)  BTW, unlike with labelling, most bans tend to concentrate on intentional industrially produced PHO and acknowledge there's still going to be some transfat from naturally occuring sources and some very small amounts accidentally produced during processing  . Notably although diet appears to be significant, at least publicly there doesn't yet seem to be a push towards reducing them. Again no comment on whether any of this is justified.  Nil Einne (talk) 16:38, 21 February 2017 (UTC)


 * Note that the ban being on artificial trans fats doesn't necessarily mean they are worse, they might just be easier to remove. StuRat (talk) 19:38, 21 February 2017 (UTC)
 * In case there is still some confusion (I thought I was fairly clear), I was explicitly not commenting on any possible reason for the ban, whether that was justified or politics or ease or anything else. P.S. One part of my reply which may have been unclear was the last part on diet. What I meant to say is that although diet seems to be a good way to significantly reduce TFA in ruminants, there doesn't seem to be any major public push to doing so. Nil Einne (talk) 21:58, 21 February 2017 (UTC)


 * This was originally part of my reply above, but since the section is collapsed, I'll leave it here as it's IMO significant enough even if completely unsourced. I've been reluctant to say this since it's not something I can reference. But meh, this discussion is already a mess and I doubt there's going to be a referenced answer. I'm fairly sure it's not that easy to simply remove transfat from cream. If it was, dairy producers would be just doing it rather than trying to convince people dairy transfats are good for you. Nil Einne (talk) 16:38, 21 February 2017 (UTC)
 * (I didn't intend to collapse your comment, I only collapsed Jayron doing his histrionic admissions of having a mistake.) Also, I think your logic here regarding easy removal of trans fats is sound. SemanticMantis (talk) 19:20, 21 February 2017 (UTC)
 * When I first noticed the collapsed section (after I was ready to post), Jayron32's reply was also part of the collapse and as my reply was partly in response to what Jayron32 said, I felt it untenable to keep it uncollapsed. Jayron32's comment was removed from the collapsed section before I actually manage to submit my reply. I didn't notice this initially but in the end, I felt it best to just leave it be, since other than this part, the stuff most related to the question was basically a repetition of what I said earlier and which remains uncollapsed. Nil Einne (talk) 21:58, 21 February 2017 (UTC)
 * I found this source which seems to indicate that trans fats in dairy products can be controlled by controlling the diet of the cows. The conclusion of that study indicates "Milk trans-C18:1 increased from 2.9 to 11.2% of the total fatty acids for cows fed the control diet and the diets supplemented with fat, respectively." Which is to say that a specific trans-fatty acid's presence in milk went up 4 fold when cows were fed diets supplemented with certain fats.  I could not find any information on removing trans fats from existing natural products, such as milk, butter, or cream, after it gets out of the cow.  But it appears, at least from that study, that one can minimize the amount of trans fats in the final product by controlling the diet of the cow herself.  -- Jayron 32 20:29, 21 February 2017 (UTC)
 * Yes I'm aware of that (I mentioned it above and I think several of my refs discuss dietary issues). I also looked for info on removing transfat afterwards (before anyone had replied actually) but didn't find anything either. This is IMO further indication that it's a far from a simple thing, although of course absence of evidence is not evidence of absence. One thing I didn't really look into is removing saturated fats from milk, it seems something that may have been studied particularly when there was greater concern over saturated fats. I did come across which talks more generally about controlling milk composition via diet, including saturated fat. It's clearly complicated stuff. Taking all this together IMO despite all the caveats I even more strongly feel the idea of some simple process to remove transfats (or saturated fats) later is a clear cut case of 'gets little consideration since it's just too difficult without major possible costs and consequences' Nil Einne (talk) 22:55, 21 February 2017 (UTC)
 * This whole discussion about whether or not certain transfats that can also be found in natural products may not be that bad misses an important point. Fat in general is bad for you, even the most healthy fats like olive oil (you only need to eat a few grams of the essential Omega-3 and Omega-6 fats). What's even worse are saturated fats, and transfats are the worst of the worsts sorts of fats you can eat. If you stop adding any fat to your diet, you'll need to make up for the many hundreds of Kcal in the form of carbs. If you then fill this gap with whole grains, brown rice, then you'll also want to add some more vegetables. Consider the dinner I ate yesterday, 1 kg of potatoes with 700 grams of spinach. This contains pretty much all of the essential fats, moreover it contains all the essential amino-acids including those that are supposedly hard to get from vegetables. The real problem is that lack of exercise causes a lack of appetite, you can then get by with eating 1500 Kcal instead of 3000 Kcal per day (in fact you have the tendency to get obese just by eating according to your appetite), if you then also fill this meager 1500 Kcal with ice cream then there is no room left for you to get your nutrients from healthy sources. Count Iblis (talk) 19:49, 21 February 2017 (UTC)
 * , not that it matters, you'll just link to some source of questionable reliability that you've cherry-picked to support your idiosyncratic diet. But thanks anyways for sharing!  -- Jayron 32 20:22, 21 February 2017 (UTC)
 * What can be sourced is the fact that the Institute of Medicine does admit that their dietary guidelines are not purely based on what's optimal for health but also if people are able to stick to the guidelines given their current eating habits. So, suppose that you would have a very solid scientific result that eating only cabbage would be optimal for health (unlikely, of course, but let's assume this for Argument's sake). Then an IoM guideline to eat only cabbage would obviously be widely ignored by the Big Mac, ice cream and pizza eating population, therefore nothing would change. Instead the IoM will adjust its advice by taking into account to what degree people are capable of making adjustments to their current diet. This is pointed out by Esselstyn who has done experiments on heart patients who were in too bad shape to have a bypass and who couldn't be cured using stents. His diet is extremely strict, but adherence was de-facto enforced by the prospect of dying within one or two years. The results look good, but it has been criticized for not being a rigorously conducted double blind study. Now, I'm not going to advocate sticking to any extreme diet here, some of the arguments used by Esselstyn are likely not correct. But a lot of the basic facts are undisputed, they have been used to motivate the research into statins, salt etc. etc.


 * The role of cholesterol in heart disease was originally uncovered by scientists in the 1950s who noted that in rural Uganda, heart disease seems to absent (when correcting for lower life expectancy, so you look at aged matched causes of death between rural Ugandans and Americans). The role of salt became apparent in studies showing that the Yanomami Indians who hardly get any salt from their diet don't get high bloodpressure as they age, in fact their bloodpressure doesn't rise with age at all and they ar free of cardiovascular disease. This sort of data is undisputed, the problem really is what you do with this knowledge. The approach of modern medicine has been to use this data to get to better treatments of cardiovascular disease primarily using drugs and operations while making some suggestions about lifestyle change. The alternative view is that we need to make more radical lifestyle changes that are not recommended by the IoM (not because they are problematic, but because of the reasons mentioned above) to drastically reduce the incidence of diseases such as heart disease and diabetes.


 * So, it's in the end not all that different to what you tell your students who you think are not studying hard enough, but who do have a talent for the subject you are teaching. They have choice between studying a bit more to pass their exams and get a good job, or they can do a lot more than that and become experts in the subject. The latter is not what everyone wants to do, but they do have choice here. The same is true for the lifestyle we stick to. Count Iblis (talk) 22:22, 21 February 2017 (UTC)
 * Look I don't think anyone is denying that dietary advice by nearly any party comes with a whole host of caveats including political involvement, the problems surrounding the science to support such advice (including funding, edit: weak correlations etc), the importance over people actually following the advice, etc. But just because this is true, and even if radically dietary change would be better, it doesn't mean some random diet you've come up with would actually work for everyone even if properly followed. Note that this doesn't mean the diet isn't working for you, perhaps it is and you should stick with it. But that's a single point of data. I could point to the fellow who smoked 20 times a day, was fairly overweight, did hardly any exercise, regularly got extremely drunk, consumed a diet rich in saturated fats, highly processed carbohydrates including sugar, salt etc and still lived to 100 without significant health concerns and say hey all dietary advice is complete nonsense. If you have the preponderance of evidence to support your diet then present it. (Of course the preponderance of evidence is a tricky business when it comes to diet anyway.) If not, then while you're free to believe whatever you want and follow whatever diet you want, please don't present it on the RD as something more than it is, your own personal belief. Nil Einne (talk) 22:36, 21 February 2017 (UTC) 23:30, 21 February 2017 (UTC)
 * See here: "The Yanomamo Indians still lead a life very similar to the last million or so years of our evolution,1 and like primitive man eat a diet that is very low in salt and saturated fat and high in fruit, vegetables and roots.2 The Yanomamo Indians are not overweight, do not smoke and are very fit. Their blood pressure does not rise with age although they spend much of their time fighting and are under great stress.3 This tribe does not develop vascular disease, although many die of infection. However, when they migrate to a Venezuelan or Brazilian town and adopt a western lifestyle, they, like native Americans, become overweight and develop diabetes and premature vascular disease. They appear therefore, to be a group which, though predisposed to vascular disease, is protected by the way they live."
 * Also note that the diet I'm eating is not suitable for most people simply because most people don't burn enough calories and won't be able to start doing so; it would take years building up their cardiovascular fitness to be able to run fast for an hour a day. Count Iblis (talk) 23:20, 21 February 2017 (UTC)
 * I made a minor edit to my reply above after you replied. Anyway so you're admitting then that you don't have significant evidence, just a single study? I'd note even in the study you linked to, there doesn't seem a good way to separate the effects of diet from other lifestyle issues particularly exercise. Nil Einne (talk) 23:30, 21 February 2017 (UTC)
 * There are many results that I can dig up later, e.g. autopsy studies done on US soldiers and Vietcong soldiers who died in Vietnam which shows a clear difference in atherosclerosis. But note that lifestyle factors that are not 100% clearly identified are still useful. If we know that people who eat a lot of vegetables and exercise a lot do much better compared to people who don't eat much vegetables and don't exercise, then you can choose to do both without first waiting for studies to appear that will point out if the difference is due to exercise or eating vegetables or perhaps both. Unless there are reasonable grounds to believe that eating vegetables or exercising a lot is actually unhealthy, doing both in the absence of more evidence looks like the optimal strategy for the time being. Also, separation may not be an option. E.g. I can eat less dairy products and still get enough calcium from vegetables, only because I exercise a lot allowing me to eat huge amounts of vegetables that contain calcium like broccoli. Today I'm going to eat 500 grams of broccoli, which contains about as much calcium as in one slice of cheese. But most people will struggle to get 500 grams of broccoli into their stomach, so they may need to eat more dairy products, they would risk nutritional deficiencies if they attempt to copy me. Count Iblis (talk) 00:04, 22 February 2017 (UTC)

Eating loads of fruit and vegetables - 10 portions a day - may give us longer lives, say researchers. Count Iblis (talk) 06:30, 23 February 2017 (UTC)


 * When looking at the nutritional quality of fats or other heterogeneous biological molecules, it is important to remember that the chemical classification may not be a relevant biological classification. For example, an early generation was urged to use products loaded with trans fats by connivers who used the logic that unsaturated fats were healthier than saturated fats, so partially hydrogenated unsaturated vegetable oils should be healthier than highly saturated lard.  But it turned out that it mattered whether an unsaturated fat was bent one way or the other, i.e. cis or trans.
 * Likewise, if you look at WebMD they say that natural trans fats are not unhealthy in the way of vegetable oil that's been chemically converted into a fake lard over a platinum catalyst. Now why is that?  I don't know, but if I had to take a wild guess, I'd say that animal digestive systems might be better equipped to break down trans fats produced by animal tissue than fats bent at random places that are not found in nature. Wnt (talk) 20:38, 21 February 2017 (UTC)
 * While I still don't want to get into this argument, I'd like to point out that great care should be taken making assumptions from single sources. Sure it's convient for the dairy (or cattle/lamb generally) lobby if true, as with so many things and also emphasised by Jayron32, you need to look at more than just one random WebMD claim  . (The third link to a single study didn't find it harmful, but also didn't find it beneficial as the dairy lobby likes to claim.) Note that this doesn't mean I'm saying it's untrue, but rather scepticism should be applied to either claim, the answer may very well be, we don't know and there is insufficient evidence to reliably conclude one way or the other (this is actually what I believe hence my reluctance to get involved although I also don't claim to have looked into this that well). All of these are duplicates of the refs I included above, but from, I'll perhaps also include  and [//www.sciencedaily.com/releases/2015/09/150922221251.htm]/[//academic.oup.com/eurheartj/article-lookup/doi/10.1093/eurheartj/ehv446]. And before you think the second/third study proves the WebMD claim, note that while this is another study which found the (low levels) of naturally occuring TFAs seemed to be beneficial, it also found low levels of industrial produced TFAs were not harmful so even it's completely true it's not the case that naturally occuring = universally good, industrially produced = universally bad. Nil Einne (talk) 23:05, 21 February 2017 (UTC)
 * That's a fair point. I haven't researched it in depth in part because I suspect that the level of natural trans fats is so low that the equivalent mixture of synthetic trans fats might not be provably harmful in a doable study.  But philosophically, I just want to emphasize that two chemicals aren't really members of some class with common properties just because they both kink the same direction - it may matter exactly where in the molecule that is.  And so I think of milk trans fats as being components of a natural product with longstanding use, whereas the others are components of a chemical product mixture known to be harmful.  There is an assumption that natural is, if not guaranteed safe, at least something tested by biological and cultural evolution over a long period -- and this is built into some regulatory regimes that consider "dietary supplements" isolated or potentially isolated from natural products normally consumed to have a greater degree of presumed safety. Wnt (talk) 03:00, 22 February 2017 (UTC)


 * Looks like there's 2 grams of natural trans fat in 100 grams of greasy hamburger meat: . Of course, your average glutton may well have 5 times as much.  That's getting to be a significant amount, nutritionally. StuRat (talk) 15:19, 22 February 2017 (UTC)

Question about the Newton's Universal Law of Gravitation
Let we have

Spherical Mass M = 5.97219 x 10^24 kilograms (= mass of earth)

Spherical Mass m = 7.34767309 × 10^22 kilograms (= mass of moon)

Radius of Mass M; R = 6371 km (= radius of earth)

Radius of Mass m; r = 1737 km (= radius of moon)

Acceleration due to gravity of mass M = g1 = GM/R^2

Acceleration due to gravity of mass m = g2 = Gm/r^2

G = 6.67408 × 10-11 m3 kg-1 s-2

O/C distance between M and m = 8608 km (center to center)

This means surface-to-surface distance between M and m = 500 km. Let X be the point on the said center-to-center distance of 8608 km where both falling bodies M and m strike each other violently due to Newton’s law of gravitation (F = GMm/d^2).

Can somebody calculate the following

1- Final velocity of M at the time of hitting m

2- Final velocity of m at the time of hitting M

3- Falling time of M and m when hit each other

4- Location of point X on aforementioned o/c distance 2001:56A:7399:1200:D12B:44DC:83EE:1060 (talk) 01:34, 21 February 2017 (UTC)EEK


 * See the top of this page: "We don't do your homework for you." Shock Brigade Harvester Boris (talk) 02:04, 21 February 2017 (UTC)


 * Indeed, especially not for the MIT MOOC on Advanced Introductory Classical Mechanics. Big clue, you know something about the system you haven't considered yet. Do the lecture again. It's not exactly a trick question but it would be tricky to bumble through to the right answer. If you are on the right track it is solvable in about 4 lines. Greglocock (talk) 06:07, 21 February 2017 (UTC)

Neither I'm not a student nor this is homework. I asked this tricky question for a reason so is there ant volunteer astronomer / physicist. Do M and m really accelarate towards each other or higher type of motion involved. — Preceding unsigned comment added by 2001:56A:7399:1200:DDBE:829F:1B04:5946 (talk) 03:02, 22 February 2017 (UTC)
 * Please do not insult our intelligence. We will gladly help you past the stuck point, but "we have foo and bar, calculate X, Y and Z", including numerical values of more than 3 significant figures, is clearly a homework question.
 * Moreover, we cannot really help you with stuff like "Do M and m really accelarate towards each other or higher type of motion involved". Making English mistakes is fine, but that sentence does not make much sense. Tigraan Click here to contact me 15:39, 22 February 2017 (UTC)


 * The answer is very closely related to how long it would take the moon to fall 500 km from a stationary Earth, due to the ratio of the masses. By my working the fall would take 7 minutes, and the earth would move 6 km.Greglocock (talk) 06:03, 23 February 2017 (UTC)

Don't they (M and m) attract each other with incomprehensible complex motion (complicated higher derivatives of motion such as gravitational acceleration, jerk, snap, jounce, crackle, pop etc) due to the continuous change (reduction) in o/c center distance from both side instead of simply gravitational accelerations — Preceding unsigned comment added by 2001:56A:7399:1200:254B:3FDA:4D0:D0CA (talk) 06:19, 24 February 2017 (UTC)


 * Yes, this is a two-body problem, which is why I (without saying anything) doubted it was a homework question. Someguy1221 (talk) 06:43, 24 February 2017 (UTC)
 * Except that it is almost identical to a question in the course I mentioned above. Sheesh. Greglocock (talk) 18:44, 24 February 2017 (UTC)
 * No. They obey F=m.a and m is known and F is easy to calculate given the assumptions in the question. It varies as they get closer but you can still calculate it, as I did The only complexity is the air resistance, which I ignored in my answer. Greglocock (talk) 18:44, 24 February 2017 (UTC)

It seems to be two-body problem theoretically but i don't think both earth and moon (or celestial bodies) can orbit each other at such a close surface to surface distance of 500 km (=distance b/t the two cities) or less. thanks all — Preceding unsigned comment added by 2001:56A:7399:1200:6809:489F:26CE:ED02 (talk) 05:28, 25 February 2017 (UTC)


 * Correct. This would be a low Earth orbit but it would place the Moon inside its Roche limit so it would be broken apart by tidal forces. Gandalf61 (talk) 08:45, 25 February 2017 (UTC)

SpaceX
Why does SpaceX choose to use rocket engines to land their reuable rockets?

Wouldn't it be much more efficient if they install an air-breathing engine on (maybe) top of the rocket so they don't need to carry so much oxygen all the way up and all the way down?

The exact weight of the spent rocket is a fixed and predetermined number. They can licence a proven jet engine design and have it optimized for the home coming trip. Maybe a small engine running at the highest efficiency rpm can save much weight of oxidizer and its expensive reusable rocket engines would have more time to cool down. -- Toytoy (talk) 09:11, 21 February 2017 (UTC)


 * Falcon 9 First Stage Reusability Graphic.jpg I imagine SpaceX decided there were plenty of challenges in developing a re-usable space vehicle with rocket engine technology alone, and that adding a second engine type would have been too ambitious. Also, you have to use the rocket engine to de-orbit and for the entry burn that slows the vehicle's descent in the upper atmosphere. So the fuel savings in using an air-breathing engine just for the final landing may not be significant. Hybrid jet/rocket engines are being researched - see SABRE (rocket engine) for one example - but they seem to be several years away from a working prototype. Gandalf61 (talk) 10:09, 21 February 2017 (UTC)


 * The air-breathing engine would be dead weight on the way up. Not only do you have to carry that dead weight up, you also have to carry the weight of the fuel to carry the weight of the air-breathing engine. This means your dry mass of the stage went up, which means your payload went down. See rocket equation. In Falcon 9, you use the same dry mass for launching and landing, saving significant amounts of weight. Other considerations are that the air-breathing engines would need intake and exhaust ports, which would cause drag, again lowering payload. They would also have a significantly lower thrust than the rocket engines (a three engine landing burn from a GTO launch pulls >10g), leading to higher gravity losses, requiring more fuel and, again, lowering payload. Fgf10 (talk) 11:02, 21 February 2017 (UTC)
 * Also, to add, the rocket engines don't need to cool down. In fact, they are fired on the way down in a re-entry burn to prevent the rocket from overheating due to the aerodynamic stresses. Fgf10 (talk) 11:04, 21 February 2017 (UTC)
 * Of course, it wouldn't have to be dead-weight on the way up. You could theoretically redesign the whole craft so that an air-breathing engine used both ways. But then you're talking about moving drastically away from a traditional, proven rocket design into some sort of much more complicated space plane design. ApLundell (talk) 18:07, 21 February 2017 (UTC)
 * The only part they could really use an air-breathing engine for is the vertical landing - they can't use it for the two earlier burns because they are high in the atmosphere. Gliding down to the landing site can be done by a flying brick a.k.a. Space Shuttle.  But they want to VTOL onto a barge in the ocean rather than being dependent on a runway, so they need something at the very end.  I suppose it could be a special VTOL jet engine used just for landing a rocket, but ... it's a rocket, with a built in VTOL engine already. Wnt (talk) 20:45, 21 February 2017 (UTC)


 * "Wouldn't it be more efficient..." is a really complicated question in space flight... I, and many others, are still not convinced that it is efficient to recover the rocket by any method! Wikipedia has an article on the economics of SpaceX rocket re-use.
 * A lot of the "efficiency" questions boil down to the relative macro-economic costs of fuel, costs of metal (and similar raw material commodities), and costs of non-recurring engineering. I am of the mindset that commodities are cheaper (in dollars) than enormous-quantities-of-skilled-labor; and that enormous commodity quantities of rocket metals are cheaper (in dollars) than enormous commodity quantities of rocket-fuels, ... and if I am correct, the reusable launch system is not an efficient use of resources (or dollars).
 * For those who want to take an independent stab at the problem: jet fuel falls near $1 per pound, and aluminum falls near $1 per pound; so if you spend a few extra pounds of fuel but you recover a few extra pounds of aluminum, and you also have to hire an extra engineering team at a cost of around $1 per total pound of recovered rocket... well, it's not quite that simple, because you have to carry extra fuel to carry the extra fuel, and you have to carry extra aluminum to hold the extra fuel and build the landing gear ... before long, you're in a quagmire of spherical cow estimates of marginal cost, and there's actually a lot of room for convincing arguments on all sides! Regrettably, the case for SpaceX is made with a lot sloppier math than I have used: Elon Musk actually estimated, in a very public and grandiose speech in October 2016, that SpaceX would soon launch something like 10,000 spherical-cow rockets to Mars.  ...For the purposes of cost reduction.
 * Here's a good presentation: NASA's Cost-Benefit Analysis Used In Support Of The Space Shuttle Program (1972) explaining the benefits of a reusable launch system; and a different opinion published in 2011.
 * Efficiency can be measured in lots of ways: dollars-per-launch, dollars-per-mass-of-payload; fuel-per-launch, fuel-per-mass-of-payload, ...; man-hours-per-launch, man-hours-per-mass-of-payload...
 * Until you define your metric, it's not practical to evaluate the efficiency of SpaceX's intent to land its first-stage rocket booster (and presumably, to re-use that rocket booster).
 * Nimur (talk) 21:17, 21 February 2017 (UTC)
 * It's more than just "a few pounds of aluminum (sic)" that are recovered. There's the small matter of returning 9 very complex rocket engines too.... You are of course right it's not yet known whether reuse will be economical. What we do know is that the first returned core has already been through 8 test firings, and that the SES10 mission will be flown on a reused core. So it's a bit more than some tankange. Your example of the space shuttle is misleading, as the external tank was expended, the SRBs were rebuilt segment by segment for each flight (including shipping from Florida to Utah and back), and the shuttle was pretty much ripped apart and reassembled for each flight. Hardly 'reusable'. Fgf10 (talk) 23:03, 21 February 2017 (UTC)
 * "The only part they could really use an air-breathing engine for is the vertical landing" isn't a limitation, if they use something like LACE or SABRE, concepts which have been around since the '50s. Those are dual-mode airbreathing jet or stored oxidiser rocket engines, usually powering a high-speed horizontal flight spaceplane.
 * However such engines are still a long way from the sort of development stage that SpaceX's more conventional rocket engines were already at. Andy Dingley (talk) 22:54, 21 February 2017 (UTC)
 * It comes down to the engineering adage "Keep it simple, stupid!" (that's not directed at OP or anyone else personally). Space X is in the business of putting stuff into space and doing so less expensively than their competitors.  By reusing their  first stages, they've got a chance to beat Arianespace on price despite the disadvantage of not being able to launch from French Guyana and save on propellant. More importantly, they're already beating the United Launch Alliance on price - to the point they can afford to share costs with NASA on the scheduled Mars shot with their Dragon lander - partly because they can look forward to economies by reusing their first stages.
 * Doing all this is hard enough without introducing a new air-breathing engine, getting it to work (many trial runs, and a few not-profitable test launches), certifying it to work (so that customers will risk their ghastly expensive satellite payloads, ISS supplies, orbital/microgravity experiments, whatever on it), annnnnd... getting it as cheap or cheaper than Arianespace, or the Russian or Chinese equivalents. I'm sure Elon Musk took a hard look at the Skylon/SABRE system and said "no thanks".
 * Reaction Engines Limited has shown the way to do air-breathing SSTOs... you have to build the spacecraft around the air-breathing system entirely, and make it do double-duty as rocket thrusters. In doing so, it looks like they've also put themselves in a niche of only making a profit if they have customers needing 11 tonnes of goods placed in profitable orbit per flight, or a major government paying for something else that requires Skylon's secret sauce.
 * Space X, however, has no such limitations. They can - now - do Falcon 9 launches, make money, or strap them together into Falcon Heavy boosters,     and make even more money - and get 22 tonnes to GTO (to Skylon's 11 tonnes), and credibly estimate getting over 13 tonnes to Mars.  And all of that without air-breathing engines, which don't seem ready for prime-time space travel yet. loupgarous (talk) 21:24, 22 February 2017 (UTC)

If you donate a kidney to someone and then somehow manage to acquire your kidney back and to have it be implanted back inside of your body, would your kidney work and function just fine afterwards?
If you donate a kidney to someone and then somehow manage to acquire your kidney back and to have it be implanted back inside of your body, would your kidney work and function just fine afterwards? Futurist110 (talk) 22:43, 21 February 2017 (UTC)
 * That would depend on how well it has been looked after in the meantime.   D b f i r s   22:50, 21 February 2017 (UTC)


 * this paper seems to indicate that "reuse" of donated organs is possible, and has been done a small number of times. I doubt it's ever been tried to give it back to the original donor, though. There usually wouldn't be a need. The original donor would either be dead, or have another perfectly healthy kidney. ApLundell (talk) 23:23, 21 February 2017 (UTC)


 * Kidney_transplantation outlines problems that can occur. Theoretically, since its your own tissue, tissue rejection might be less likely. But there are plenty of other potential issues. ←Baseball Bugs What's up, Doc? carrots→ 23:34, 21 February 2017 (UTC)


 * Much would depend on whether, after the first transplant, significant amounts of the first recipient's own tissue had grown in and around the donated kidney. That could cause histocompatibility problems if the kidney were removed (one suspects after the recipient's death) and replaced into the donor - who would then also be a recipient of the first recipient's tissue.  The donor might even need to take immunosuppressants because of little bits of the first recipient growing either inside the vasculature of the kidney or on its exterior. And as others have indicated, the need usually isn't there unless the donor's remaining kidney begins to fail.  But the likelihood of that happening when the donor's other kidney is available to be retransplanted is very small.  There's the chance, as well of the first recipient having acquired a kidney infection, or damaging the donated kidney through the disease process which did both of his original kidneys in, by poor lifestyle choices, or not taking his immunosuppressants (as many as 50% of transplant patients don't, at some point after their transplants), after which the cellular immune system takes over and starts rejecting the kidney.  That kidney has a sadly high chance of going back to the donor as damaged goods. loupgarous (talk) 21:46, 22 February 2017 (UTC)