Wikipedia:Reference desk/Archives/Science/2011 July 3

Science desk
< July 2	<< Jun | July | Aug >>	July 4 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

July 3

Planets

Looking at this picture (second one) made me wonder, on the paths of orbit, why the planets do not - given the very rare chance it would happen - crash into one another. Obviously planets in our solar system travel at different speeds and therefore have different solar years but is it not possible at least once that one planet would crash into another? 86.150.155.115 (talk) 00:19, 3 July 2011 (UTC)

That's gotta sting.

A few possible answers that come to mind:

1. Most of the universe is pretty empty as far as we know. The planets are really far apart from each other even in our solar system. Across galaxies, even more so. So the odds of straight-up collision are not as high as you probably imagine from those types of images alone, which make everything look pretty dense and occupied.

2. Planets have collided in the past, they might do so in the future. It is believed that Earth's moon was created by just such a collision between the Earth and a Mars-sized planet.

3. Entire galaxies do collide, but because they are rather diffuse with matter (see point #1), it isn't necessarily all big explosions.

4. The universe as we experience it (locally) is pretty old. So most of the big colliding that has happened has already occurred. In other words, if the orbits of Mars and the Earth were collision prone, they would have probably collided already sometime in the past billion years. That is, we can be reasonably confident that the orbits in our solar system are stable because if they weren't stable, there's been plenty of time already for them to crash into each other. (This is a variation of the anthropic principle.)

5. Lastly, it's entirely possible that very large bodies could collide with the Earth. There are a lot of astronomers who think an asteroid (not planet) collision is probably in the next few tens or hundreds of thousands of years, which is really not that long on a geologic or universe scale, even if it seems like a long time to humans. --Mr.98 (talk) 00:30, 3 July 2011 (UTC)

As stated in #4 above, the period of heavy bombardment is in the past. The current planets are what they are today because their heavy mass largely cleared their orbit of excessive debris. Large asteroid impacts are inevitable but with far less frequency than in the past. See Near-Earth object. --George100 (talk) 02:08, 3 July 2011 (UTC)

See also Worlds in Collision. -- Jack of Oz ^{[your turn]} 00:39, 3 July 2011 (UTC)

Which is, incidentally, pseudoscientific nonsense... --Mr.98 (talk) 01:20, 3 July 2011 (UTC)

Now for some specific comments on our own solar system:

1) Most of the planets' (all, now that we exclude Pluto) orbits are close to circular, so they don't overlap.

2) Pluto has a more elliptical orbit, which does bring it inside the orbit of Neptune periodically. Pluto's orbit is also tilted relative to Neptune's orbit, so, even if it was at the same distance and angle of revolution about the Sun, it might pass well above or below Neptune.

3) There's also the concept of orbital resonance, where two nearby orbits adjust until they find stable orbits that don't conflict. As noted above, by now they would either have had to find a stable orbital configuration, or they would have already collided.

4) Two planets sharing the same orbit would revolve about the Sun at approximately the same speed, so wouldn't strike each other soon, unless going in opposite directions, in which case they would collide on the first orbit. Note that the only stable positions are if they were in each other's L4, or L5 Lagrange points (and possibly a halo orbit or Lissajous orbit about L3). Any other location would cause them to move into different orbits or collide. StuRat (talk) 02:39, 3 July 2011 (UTC)

It's pretty obvious that collisions have happened in the past on a grand scale in our solar system. Evidence such as Barringer Crater on our Earth, or the craters on the Moon, are a little close to home for us. Uranus and its moon Miranda show evidence of a massive impact having taken place a long time ago (millions if not billions of years). As has been noted above, planets seem to have found stable orbits now, so an impact on that scale is improbable. However, planetesimals could still cause havoc: if the comet Shoemaker-Levy 9 had hit Earth instead of Jupiter, we probably wouldn't be here to discuss it. There is a theory that says that impacts such as that are less common in our part of the Solar System purely because of the attractional effect of Jupiter and the other gas giants. --TammyMoet (talk) 08:06, 3 July 2011 (UTC)

And also Earth is a much smaller target with a smaller gravitational field, so is far less likely to pull in any passing comets. StuRat (talk) 08:17, 3 July 2011 (UTC)

See here:

A long-term numerical integration of the classical Newtonian approximation to the planetary orbital motions of the full Solar System (sun + 8 planets), spanning 20 Gyr, was performed. The results showed no severe instability arising over this time interval. Subsequently, utilizing a bifurcation method described by Jacques Laskar, two numerical experiments were performed with the goal of determining dynamically allowed evolutions for the Solar System in which the planetary orbits become unstable. The experiments yielded one evolution in which Mercury falls onto the Sun at ~1.261Gyr from now, and another in which Mercury and Venus collide in ~862Myr. In the latter solution, as a result of Mercury's unstable behavior, Mars was ejected from the Solar System at ~822Myr. We have performed a number of numerical tests that confirm these results, and indicate that they are not numerical artifacts. Using synthetic secular perturbation theory, we find that Mercury is destabilized via an entrance into a linear secular resonance with Jupiter in which their corresponding eigenfrequencies experience extended periods of commensurability. The effects of general relativity on the dynamical stability are discussed. An application of the bifurcation method to the outer Solar System (Jupiter, Saturn, Uranus, and Neptune) showed no sign of instability during the course of 24Gyr of integrations, in keeping with an expected Uranian dynamical lifetime of 10^(18) years.

Count Iblis (talk) 23:45, 3 July 2011 (UTC)

---

Excuse me I never believe giant impact and late heavy bombardment AND HAVE GOOD REASON FOR THEM ,I WILL SAY IF YOU WANT.A. mohammadzade Iran --78.38.28.3 (talk) 08:27, 5 July 2011 (UTC)

chemistry-its uses

what is the use of chemistry to the human beings. — Preceding unsigned comment added by Rajib pulu (talk • contribs) 04:37, 3 July 2011 (UTC)

Chemistry made the plastic that made the keyboard that you typed this question on. Looie496 (talk) 05:55, 3 July 2011 (UTC)

Chemistry brought most of us out of the stone age and into the bronze age. Plasmic Physics (talk) 10:17, 3 July 2011 (UTC)

Long before that, we discovered how to preserve foods with salt. Plasmic Physics (talk) 10:21, 3 July 2011 (UTC)

It creates McDonalds shakes. HiLo48 (talk) 10:24, 3 July 2011 (UTC)

I don't quite think that is a major contribution to humanity. Plasmic Physics (talk) 10:36, 3 July 2011 (UTC)

Actually I thought the key ingredient in the MacDonald's shake was simply adding clay to make it "thicker", not chemicals... Wnt (talk) 02:53, 4 July 2011 (UTC)

Technically any combining of substances, including those in a McDonald's shake, would necessarily involve an understanding of chemistry. --George100 (talk) 18:45, 4 July 2011 (UTC)

Chemistry is useless. We should all be living in caves and clubbing wildebeasts for food. --Jayron32 13:31, 3 July 2011 (UTC)

'cept we wouldn't be able to digest them without chemistry. DMacks (talk) 13:33, 3 July 2011 (UTC)

Chemistry is about creating chemicals. Those could be new materials for construction, new medicines, new cosmetics, new cleaning products, new fuels, etc., etc., etc.. --Tango (talk) 14:07, 3 July 2011 (UTC)

Or, of course, creating more of previously invented chemicals. I was thinking of research in chemistry, but work using previous research is still chemistry. --Tango (talk) 14:09, 3 July 2011 (UTC)

Chemistry creates drugs for medical and other purposes. HiLo48 (talk) 20:51, 3 July 2011 (UTC)

The old saying goes that biology is really just applied chemistry. Chemical reactions are fundamental to all life itself. In addition, every single object in your house that is not the product of a plant or an animal was made using some form of chemical process. This goes from your glass windows to your TV screen to the preservatives in your food to all medications. HominidMachinae (talk) 05:25, 4 July 2011 (UTC)

It's all just applied math. Mitch Ames (talk) 13:23, 4 July 2011 (UTC)

This year is the International Year of Chemistry. -- Jack of Oz ^{[your turn]} 12:43, 4 July 2011 (UTC)

The formation of cell membranes and DNA is really biochemistry, so without chemistry we wouldn't even exist. - 194.60.106.17 (talk) 13:56, 4 July 2011 (UTC)

When a man and a woman have good chemistry, they sometimes date, mate, reproduce, and help to perpetuate the human species. —SeekingAnswers (reply) 07:42, 5 July 2011 (UTC)

One could ask, what isn't affected by it? Chemistry and physics pretty much cover the physical world. Astronomers use spectroscopy to determine the chemical composition of light from distant stars or planetary surfaces. Metallurgists need to test and understand the chemical properties of metals. Biochemists study the chemicals in muscles, such as the ones that moved your fingers when you typed the message. Neurochemists study the chemical reactions that send signals from your brain to those muscles, and back from the sense organs, and provide an understanding of the neurons in your brain that allow you to think things in the first place.

Bottom line: every physical substance can be studied through chemistry, and all physical phenomena are associated with chemistry in one way or another. --George100 (talk) 09:17, 5 July 2011 (UTC)

"... all physical phenomena ..." - really ? Does chemistry determine the period of a pendulum, the orbits of the planets, or the formation of a black hole ? Are beta decay and neutrino oscillations chemical reactions ? Gandalf61 (talk) 10:38, 5 July 2011 (UTC)

That's why I said "chemistry and physics" at the beginning of the statement. --George100 (talk) 13:55, 5 July 2011 (UTC)

Peeing on the lemon tree

In The World's Fastest Indian you will see reference to the main character peeing on his lemon tree every morning. That's in New Zealand. Here in Australia it's also a custom of some people, believed to be good for the tree.

Firstly, is it really good for the tree?

Secondly, is this custom practised elsewhere? HiLo48 (talk) 04:48, 3 July 2011 (UTC)

Well, the nutrients in urine could be helpful, if the soil happens to be deficient in those nutrients. With smaller plants, there's a danger that the high acidity of urine could kill them, but by the time the urine makes it down to the tree roots, it would be diluted enough to not be a problem. Then there's also the small amount of water in the urine, which could help during a drought. There might also be an effect that all the neighborhood dogs and cats smell the urine and feel the need to add their own. So, it might help, a bit, I suppose. StuRat (talk) 06:23, 3 July 2011 (UTC)

See Urine#Agriculture. Urine contains a significant amount of nitrogen, mostly as urea, as well as some phosphates and potassium, the three macro-nutrients typically in fertilizer. Buddy431 (talk) 15:56, 3 July 2011 (UTC)

Would probably be better to put the urine on the compost heap and then put the compost round the tree as a mulch. Itsmejudith (talk) 17:14, 3 July 2011 (UTC)

Just so the providers of lemon juice aren't using some yellow acidic liquid not derived from the lemon to stretch the output a bit. Edison (talk) 02:34, 4 July 2011 (UTC)

So that's why the fruit of the poor lemon is impossible to eat. Mitch Ames (talk) 12:07, 4 July 2011 (UTC)

Nitrogen oxide, Carbon dioxide, Sulfur Oxide, Hydrocarbon

I would like to know how these following gasses (In the form of gass) would react with water (In the liquid form):

♠ Hydrocarbon ♠ Sulfur oxide ♠ Carbon dioxide ♠ Nitrogen oxide

Please give me a simple explanation and a chemical equation with each gass. Thank you very much. — Preceding unsigned comment added by 41.18.64.179 (talk) 12:04, 3 July 2011 (UTC)

Hydrocarbons do not appreciably react with water without some tricks, assuming you mean alkanes. Alkenes and Alkynes will hydrolyze in the presence of acid or base, producing alcohols. See Hydration reaction. Sulfur oxides and nitrogen oxides will react to form the corresponding oxyacids, you'd need to know what the oxidation state of the sulfur or nitrogen atom was to predict the exact oxyacid formed. Carbon dioxide forms carbonic acid with water, and if you read that page, it will give you a reaction; you can use that same reaction as a model for the nitrogen oxides and sulfur oxides reaction as well. --Jayron32 13:29, 3 July 2011 (UTC)

Diet soda and type 2 diabetes

I just took an online news quiz. One of the questions was about a disease linked to diet soda (type 2 diabetes was the answer). Search the web turned up news about the connection from 2009. Correlation was established but a casual link was not. Do scientists now know how consumption of diet soda is linked to type 2 diabetes? — Preceding unsigned comment added by 98.114.98.18 (talk) 15:32, 3 July 2011 (UTC)

According to a recent large scale study, PMID 21430119: "Sugar-sweetened beverage consumption is associated with a significantly elevated risk of type 2 diabetes, whereas the association between artificially sweetened beverages and type 2 diabetes was largely explained by health status, pre-enrollment weight change, dieting, and body mass index.". However this question does not seem to be definitively settled. Looie496 (talk) 16:52, 3 July 2011 (UTC)

In other words, if you drink diet soda, you're likely to do so because you're fat, and to get diabetes for the same reason. StuRat (talk) 18:21, 3 July 2011 (UTC)

I would paraphrase Looie496's quote as "Drinking sugary beverages is associated with type 2 diabetes. However the similar relationship found between drinking artificially sweetened beverages and diabetes was due to its consumers being overweight." Which implies that artificially sweetened drinks are still less harmful than sugary drinks. 92.28.244.187 (talk) 16:30, 4 July 2011 (UTC)

I doubt it's been proven that you can become diabetic just from being "fat", as you put it, Stu. -- Jack of Oz ^{[your turn]} 12:40, 4 July 2011 (UTC)

I believe it definately has been proven, although I cannot quote references off the top of my head. Lack of excercise is another risk factor. 92.28.244.187 (talk) 16:32, 4 July 2011 (UTC)

Obesity is a major risk factor for developing diabetes. StuRat (talk) 14:51, 4 July 2011 (UTC)

Another theory might be that tasting something sweet makes your body anticipate it, and, when no sugar appears, this drives it to crave the real thing, causing you to eat something containing real sugar, or perhaps some other form of calories. StuRat (talk) 18:21, 3 July 2011 (UTC)

In other words, all of the above conclusions, as well as any others, represent a fallacy known as post hoc ergo propter hoc. --Jayron32 18:43, 3 July 2011 (UTC)

Not necessarily. The theory I suggested would make artificial sweeteners an actual cause, albeit an indirect one. There could also be a direct cause we haven't found yet, such as if those artificial sweeteners damage the pancreas. StuRat (talk) 20:28, 3 July 2011 (UTC)

A priori that would strike me as fairly implausible, but actually I think I remember reading about a study where rats given artificial sweetener had physiological responses similar to those from sugar. I think it might have been an insulin spike, which would be directly on point. Does anyone remember this study, and can give better details? --Trovatore (talk) 21:21, 3 July 2011 (UTC)

There's a cottage industry of aspartame conspiracists. Perhaps the quiz author was taken in by it or something similar. Imagine Reason (talk) 21:17, 3 July 2011 (UTC)

No, StuRat's points are good. 1) correlation isn't causation but may indicate possible causation - the direction of causation needs to be established, and it may well go "obese leads to consumption of soda" rather (or as well as) "consumption of soda leads to obese". 2) I recently read news of a study that says that consumption of something very sweet leads to you experiencing other foods as less sweet, and that makes sense too, i.e. you drink lots of artificially sweet sodas and then you get a taste for sweet foods, and experience foods that are only a little sweet as bland. These are potential mechanisms. Research is ongoing. Itsmejudith (talk) 22:06, 3 July 2011 (UTC)

This is just a thought, and to my knowledge I don't have type II diabetes (I've tested within the normal range while fasting, though I suspect overeating brings my sugar to unreasonably high levels now and then). Still, it's my impression, being a rather heavy person, that ordinary sugar soda is not satisfying in allaying thirst. It just has too high an osmolality or something. Diet soda is more appealing because it is "wetter" to the tongue and thus more satisfying for thirst. Wnt (talk) 23:40, 3 July 2011 (UTC)

There were recent news stories[1], [2] reporting a scientific finding that drinking diet soda raised blood sugar and caused weight gain. This no doubt gladdened the hearts of the makers of cane sugar and corn sweetener. The results were presented at an American Diabetes Association meeting, but have not been published or peer reviewed. The study did not compare caloric intake. The conclusion by the researchers that drinking diet soda make one fat or raises blood sugar seems a monumental case of assuming that correlation proves causation, while ignoring potential confounds. Someone with "a sweet tooth" might chose diet soda over regular soda because he has a history of gaining and losing weight, or because he has a family history of diabetes. Those same confounds could ultimately result in weight gain. Had the person drunk the same amount of sugar pop, he would likely have gained even more weight or experienced a greater elevation of blood sugar over time. Had he been constrained from drinking diet pop, and been given only water to drink, but had the same diet otherwise, he might well have had the same effects as drinking diet pop, or he might have gained weight faster due to eating more sweets, to satisfy the "sweet tooth." A rebuttal by an obesity specialist said that "extra-calorie food consumed along with diet soda is the main culprit." She did advise people to limit consumption to 2 diet sodas a day. Edison (talk) 02:14, 4 July 2011 (UTC)

It's rather obvious that if they aren't getting the calories from the diet soda and are still gaining weight, then they must be getting the excess calories elsewhere. The question, though, is whether the diet soda causes them to crave additional high calorie foods. In any case, the marketing of diet soda, which claims that it helps you lose weight, doesn't appear to be correct. StuRat (talk) 06:50, 4 July 2011 (UTC)

If someone drinks 2 sugar sodas a day and wishes to lose weight, switching to 2 diet sodas a day while keeping other activity and food intake the same would help with weight loss since the 2 diet sodas might contribute 8 calories or so, while the 2 sugar colas would contribute about 194 calories. The daily caloric difference of 186 or so food calories would lead to a weight loss of a pound every 19 days, or about 19 pounds per year, since a caloric deficit totalling 3500 calories produces a one pound weight loss. To urge a fat person or a diabetic to switch from diet cola to sugar cola is to do him a horrible disservice, and it all sounds like propaganda from the corn sweetener industry. Edison (talk) 22:33, 4 July 2011 (UTC)

Yes, but the assumption that you can switch to diet soda "while keeping other activity and food intake the same" is where the flaw lies. You're body will still expect the same number of calories, and if it doesn't get them from the diet soda, it will make you crave other sweets. Also, less sugar will mean less energy for those other activities. If switching to diet soda really worked as advertised, then the weight loss would be obvious by now, since millions of people have been drinking them for decades. Nobody is suggesting regular soda as an alternative, but perhaps real fruit juice is better (since there you at least get nutrients), or something else calorie-free, like water or unsweetened tea. I drink unsweetened herbal tea (really tisane), myself. StuRat (talk) 02:00, 6 July 2011 (UTC)

The study mentioned by Looie496 is of 40,389 men, and shows that just changing to diet drinks doesn't reduce the chance of diabetes unless other other risk factors are changed as a consequence (i.e lower weight). Where changing to diet drinks (or even coffee) was associated with weight loss, the risk of diabetes was reduced. There is not just correlation between obesity and type II diabetes but a causal connection. The increased chance of diabetes with extra weight becomes even higher over time, so ultimatly the muscular and athletic obese people eventually have a much higher chance of diabetes than their lazy thin friends.

I wish the scientific articles were available without having to log in to databases, then I could just provide a link to some of the many papers to make it easy to see for yourselves. E.g Sugar-sweetened beverages and risk of obesity and type 2 diabetes: Epidemiologic evidence, Physiology & Behavior, Volume 100, Issue 1, 26 April 2010. Polypipe Wrangler (talk) 01:51, 6 July 2011 (UTC)

Agreed that they should remove the impediments to accessing scientific articles. Science should be about free, easy access to data, not erecting obstacles to the flow of information. StuRat (talk) 02:05, 6 July 2011 (UTC)

Wrong speed indicator

Why do aircrafts use probes (which could malfunction, for example, due to ice) to indicate speed? Wouldn't it be a much more reliable approach to use the GPS navigation system? Wikiweek (talk) 16:49, 3 July 2011 (UTC)

I'm not sure how reliable GPS systems are, but in any case airspeed is often more important that ground speed, and up in the zone of the jet stream the two values can easily differ by more than a hundred miles an hour. Looie496 (talk) 16:57, 3 July 2011 (UTC)

Pitot-static systems , usually in the forms of "tubes" are generally used because they report the airspeed, as well as other parameters. They are generally considered very reliable. However, yes, there is the concern of a block, usually by ice. For this reason, each cockpit has a "pitot heat" button, which de-ices the tubes. Lynch7 17:18, 3 July 2011 (UTC)

I suspect this is a reference to Air France Flight 447. Without wishing to pre-judge the formal report, the cause of the accident seems to be not so much the blocked pitot tube, but the pilot chasing the needle into the sea. From the article, "Multiple sensors provide the pitch (attitude) information and there was no indication any of them were malfunctioning." Anything safety-critical in a commercial airliner will (or should) have backup systems, so the failure of one system shouldn't compromise the flight - _if_ the pilot recognizes the failure and responds appropriately. See Kegworth air disaster for a similar case of pilot error. Tevildo (talk) 17:19, 3 July 2011 (UTC)

To clarify the other answers: GPS would tell you your speed relative to the ground (ground speed), which is useful for working out how long it is going to take to get to your destination, but that's about it. What pilots really need to know is their speed relative to the air around them (air speed). This is important because it is what determines whether the plane is going to stay in the air or not (see stall speed). --Tango (talk) 20:00, 3 July 2011 (UTC)

Also, the air speed should indicate the average rate of fuel use. (The instantaneous rate of fuel use can, of course, be measured directly, provided the proper gauges and measurement devices are in place.) Air speed may be important in many other ways, too, like determining the optimal position for control surfaces and the turn radius. StuRat (talk) 22:32, 3 July 2011 (UTC)

The article says there was a "NAV ADR DISAGREE" warning early on - I would assume, but don't know, that this implies some use of GPS data to check the speed. Wnt (talk) 23:32, 3 July 2011 (UTC)

I agree that GPS shows ground speed whereas the pilot is interested in true airspeed for purposes of navigation and fuel management. But there is another consideration too, and one that is probably more important. An airspeed indicator does not show true airspeed - it shows indicated airspeed. At altitudes significantly above sea level, indicated airspeed is significantly slower than true airspeed, due to the decreasing air density with increasing altitude. The pilot is actually more interested in indicated airspeed than true airspeed because:

• the airspeed limitations on the aircraft are expressed in indicated airspeed (they cannot be readily expressed in true airspeed)
• the stalling speed in straight, level flight can be expressed in indicated airspeed, and the pilot needs to avoid flying at such a slow speed that the aircraft is at risk of stalling.
• the high speed at which buffeting and other undesirable flight characteristics commence are expressed in indicated airspeed (or indicated Mach number), and the pilot needs to avoid flying at such a high speed that the aircraft is at risk of these undesirable characteristics.

I specially agree with Lynch7 that pitot tubes are automatically heated to avoid contamination by ice. World-wide, pitot tubes have proven to be highly reliable. Dolphin (t) 00:26, 4 July 2011 (UTC)

Perhaps not as highly reliable as they should be. See also Birgenair Flight 301, Eastern Air Lines Flight 375, Austral Líneas Aéreas Flight 2553, Northwest Airlines Flight 6231, Florida Commuter Airlines crash...--Shantavira|^{feed me} 07:56, 4 July 2011 (UTC)

Of those five, only one (Austral 2553) was solely due to an ASI failure. Eastern 375 was a birdstrike and shouldn't really be on the list at all, the pitot blockage on the Florida Commuter aircraft was repaired before takeoff and the crash was probably due to bad weather, and the other two went in with (at least) one functioning ASI that the pilots ignored in preference to the failed unit. No mechanical system can be 100% reliable, but humans are a great deal less reliable than any mechanical system... Tevildo (talk) 21:26, 4 July 2011 (UTC)

Also, and this might be a minor point, but GPS systems rely on a satellite signal. At some latitudes or at high speeds relative to the signal source (which is made more dramatic by closer proximity to orbit) it could be hard to get sufficient signal resolution to determine airspeed with enough accuracy. I'm sure they know where they are good enough to know where they are, but may not be enough for inertial navigation. HominidMachinae (talk) 07:29, 4 July 2011 (UTC)

GPS systems also rely on (relatively) complicated electronics and computing systems - I would say that this would probably make them less reliable that a (heated) pitot tube as they are many more potential points of failure. This is added to the other good points about airspeed vs. speed over ground. Equisetum (talk | email | contributions) 10:41, 4 July 2011 (UTC)

One should be cautious not to equate internal complexity with a lack of ruggedness. Single-chip solid-state integrated electronics will nearly always be much more tolerant of vibration, changes in pressure, temperature swings, impact, and other forms of abuse than any electromechanical system. A GPS receiver also doesn't require long runs of airtight plumbing or multiple penetrations of the fuselage, and it doesn't leave tubes and ports exposed on the exterior of the plane where they can be damaged or plugged by ice, bird strikes, nesting insects, and passing baggage trucks. GPS receivers can be placed pretty much anywhere within the aircraft, and their light weight and minimal electrical demand means than multiple redundant receivers can be used.

That said, it would be inappropriate (and impossible) to rely exclusively on a GPS-based system for the reasons noted above. (HominidMachinae's comment about altitude is a red herring, however—commercial airliners cruise at altitudes of around 10 km; GPS satellites orbit at around 20,000 km.) Natural and artificial radio interference (from transmitters, space weather, or thunderstorms) can degrade and distort GPS signals, as can Selective Availability. There's also no guarantee that GPS will even remain available for the life of a modern aircraft; there are a lot of jets built 40 years ago still flying today. TenOfAllTrades(talk) 14:46, 4 July 2011 (UTC)

There have been incidents such as Aeroperú Flight 603 in which maintenance persons put masking tape or duct tape over the "static ports" or pitot tubes while painting or cleaning the plane and negligently failed to remove it, resulting in inaccurate air speed indication. If an airliner had inoperative pitot tubes, and functioning GPS or radar coverage, couldn't ground control approximate the air speed at their altitude and location and combined with their GPS speed or radar speed tell them their "indicated air speed," to a close enough approximation for safe flight? Satellite weather views should provide some information of local windspeed even in midocean. Various types of clouds characteristically appear at typical altitudes, and in a crisis, it should be possible to provide some airspeed adjustment to the GPS information based on such meterological information, as well as on reports of windspeed from other planes in the region. If the jetstream is a high altitude unknown, perhaps a plane could descend to a lower altitude not subject to the jetstream. Once they got near an airport, air traffic control should be able to provide the pilot with adequate "indicated air speed" for the approach and landing, especially if another plane could shadow the one which had bad instrumentation, once it got into a high traffic density area. Edison (talk) 22:49, 4 July 2011 (UTC)

Aviation history is rich with accounts of flight crews resorting to their ingenuity to get themselves and their aircraft out of potentially fatal predicaments, so I'm sure that this sort of strategy, using GPS or INS, could be made to work in a life-or-death situation. (Have a look at the Windsor incident.) Air traffic controllers are very remote from the scene but suitable information could come from another aircraft in the vicinity - if the crew of another aircraft compared their GPS/INS ground speed and their true airspeed they could quickly determine a wind speed and direction and advise the crew of the stricken aircraft who could then check the accuracy of their own airspeed indicating system. It just might be sufficient to keep the stricken aircraft safely airborne long enough to diagnose the problem. Dolphin (t) 06:15, 5 July 2011 (UTC)