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Cold Sun?


Tobbzzzz

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There's practically nothing around you to be heated up, since you are in space.If i recall correctly, all that heat is coming from the light alone that falls directly on the thermometer.

edit: oh, wait a second.Meters?

Edited by SoldierHair
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There's practically nothing around you to be heated up, since you are in space.If i recall correctly, all that heat is coming from the light alone that falls directly on the thermometer.

edit: oh, wait a second.Meters?

Actually, the vaccum AROUND our own sun is a few million degrees in temperature. Scientists aren't exactly sure why

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Vacuum itself doesn't have a temperature, but being in a direct sunlight would obviously heat things up quite a bit in real life. On the other hand, in shadow the temperature would slowly radiate away. Maybe this'll get implemented in the game at some point.

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Vacuum itself doesn't have a temperature, but being in a direct sunlight would obviously heat things up quite a bit in real life. On the other hand, in shadow the temperature would slowly radiate away. Maybe this'll get implemented in the game at some point.

The temperatures are just placeholders for now. Thermometers in shadow and sunlight has the same values.

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Actually, the vaccum AROUND our own sun is a few million degrees in temperature. Scientists aren't exactly sure why

It's pretty simple, and yes, we're sure why. "Temperature" is defined by how much thermal energy each molecule has, as evidenced by its random motion. The area just around the sun is MOSTLY vacuum, but there are a few hydrogen molecules floating there, and those few molecules are getting bombarded with a tremendous amount of energy coming off the "surface" of the star. This gives them a lot of thermal motion, so we say they have temperatures of several million degrees. But you could fly a spaceship through that region without any appreciable amount of heating, because there's just so little there to heat you up.

The visible surface of the sun (using the word "surface" very loosely) is emitting at ~6000 degrees, so that's what you'd have to worry about more if you got close; the energy density from radiation from the surface is many orders of magnitude higher than the little convection you'd get from the molecules you're running into. But even there, an object above the surface wouldn't record a temperature of 6000, because up to half would be in shadow, and more would be only heated obliquely, to say nothing of how much heat the interior can absorb.

But yeah, 900 degrees is unrealistically low. Of course, Kerbin's Sun (and "Kerbol" is a fan-made name, not official in any way) is impossibly small, due to the 10:1 scaling issue; its mass is 1/100th of our own Sun's, and you need a mass of 0.08 solar masses just to get hydrogen to fuse, although you can do deuterium fusion at about 0.06. So I wouldn't worry about it too much.

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It's pretty simple, and yes, we're sure why. "Temperature" is defined by how much thermal energy each molecule has, as evidenced by its random motion. The area just around the sun is MOSTLY vacuum, but there are a few hydrogen molecules floating there, and those few molecules are getting bombarded with a tremendous amount of energy coming off the "surface" of the star. This gives them a lot of thermal motion, so we say they have temperatures of several million degrees. But you could fly a spaceship through that region without any appreciable amount of heating, because there's just so little there to heat you up.

The visible surface of the sun (using the word "surface" very loosely) is emitting at ~6000 degrees, so that's what you'd have to worry about more if you got close; the energy density from radiation from the surface is many orders of magnitude higher than the little convection you'd get from the molecules you're running into. But even there, an object above the surface wouldn't record a temperature of 6000, because up to half would be in shadow, and more would be only heated obliquely, to say nothing of how much heat the interior can absorb.

But yeah, 900 degrees is unrealistically low. Of course, Kerbin's Sun (and "Kerbol" is a fan-made name, not official in any way) is impossibly small, due to the 10:1 scaling issue; its mass is 1/100th of our own Sun's, and you need a mass of 0.08 solar masses just to get hydrogen to fuse, although you can do deuterium fusion at about 0.06. So I wouldn't worry about it too much.

I wouldn't call 10^10 particles per cm^3, "a few hydrogen molecules" or vacuum =) . Sirrobert is correct in his statement, as the solar corona can reach temperatures as high as 10MK(10 million Kelvin) and as of now, two theories are leading as to trying to explain where the corona gets its massive amount of energy from within the Sun.

With that said, you're correct in the definition of temperature (motion of molecules/atoms), but I would worry a lot more about radiation, CMEs (Coronal Mass Ejections) and gravitational tidal forces than temperatures, if I were to go close to our Sun :P

Making a spacecraft survive the temperatures of the surface of the Sun wouldn't be a big as a task as to create one that would survive the gravitational forces. Kerbals have it backwards though =)

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I wouldn't call 10^10 particles per cm^3, "a few hydrogen molecules" or vacuum =)

I would. 10^10 is a TINY density. Earth's atmosphere reaches nine or ten orders of magnitude denser than that in terms of number density (and the fact that our atmosphere is nitrogen/oxygen instead of hydrogen/helium adds another order of magnitude to the mass density ratio). Once you start working in astrophysics, you have to stop looking at numbers like 10^10 as being automatically big. Pretty much every number in astrophysics is huge, so you have to look at them all in context. I mean, it's 6.02*10^23 atoms of hydrogen per gram, so 10^10 per cm^3 means one gram of hydrogen per ~6*10^13 cm^3. That's a cube 400 meters on a side, containing a single gram of gas on average. In astronomical terms, yes, that's pretty close to vacuum; sure, it's still a lot thicker than interstellar space, but that's not saying a whole lot.

So now, look at the effect on a rocket flying through that layer of gas, which was the original point of the thread. A plane flying through the air on Earth worries about frictional heating or compression, NOT heat transfer from the molecules it passes through, so the temperature of the gas is basically irrelevant to the discussion. Now extrapolate that to a gas that's less than a BILLIONTH of that density; it doesn't matter if the gas molecules are moving with a temperature of a million degrees, it just won't have any real impact on the temperature of the rocket. Hence, to the original topic, the million-degree layer won't result in an inflated reading on an onboard thermometer.

Making a spacecraft survive the temperatures of the surface of the Sun wouldn't be a big as a task as to create one that would survive the gravitational forces. Kerbals have it backwards though =)

The bigger challenge would be making a craft whose CREW can survive the near-solar environment, given the radiation involved. Just because our star produces most of its energy in the visible spectrum doesn't mean it isn't cranking out plenty of ultraviolet, X-ray, and gamma-ray photons, to say nothing of the high-energy particles and such. This'd also have a serious impact on any onboard electronics, so you can't get around this just by going unmanned.

(Also, I'd wonder how much radiation pressure would have pushed the rocket on the way in. Probably wasted a lot of fuel just getting that close.)

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Well, I don't think the Sun puts out gamma rays, nor do I think radiation pressure is going to have a significant effect on your spaceship.

The Sun does produce gamma rays. Anything that has a temperature above absolute zero produces a full array of "black body" photons, with the distribution of photon energies depending on the temperature. (The plot of photons vs. wavelength looks a lot like the Rock of Gibraltar, with the peak's frequency directly proportional to the surface temperature, a steep dropoff on one side, and a shallow dropoff on the other.) The million-degree inner layers of the Sun would produce quite a large number of gamma photons; the catch is that nearly all of those photons interact with the opaque outer layers of the Sun before reaching the surface, so the number that escape from the surface is extremely small. The thing about radiative astrophysics is that there are very few absolutes in these sorts of interactions; it's more like a half-life relationship. A photon might have an X% chance to interact with matter every Y kilometers it travels, and if enough of those scale lengths are inside the star it's very unlikely a given photon can escape without interacting along the way. But there's no way, in a finite distance, to raise the odds to 100%, so some WILL escape over time.

Alternately, the cooler 6000-degree surface layer would produce a much smaller number of gamma rays, but they'd have less distance to travel before escaping so their odds are a bit better. I think the few that escape from our own Sun are from the first group, ones made deep inside the star that managed to make it all the way out, but Kerbin's Sun might be different. (More on that below.)

So in a Sun-sized star the total gamma emission is admittedly very small. But if you're getting within a couple thousand METERS of the surface (like in the original post), even that small number might still be enough to kill you. For big stars, the number goes way up; the main difference between big stars and little stars is the number of high-energy photons that were produced in the first place. The larger stars produce far more total photons (energy output scaling as the FOURTH power of mass), AND their black-body curves are shifted to where the high-energy photons make up a larger fraction of the initial output. So, even if those stars also require the photons to travel a larger distance before reaching the surface, they'll still output much more ionizing radiation. (The largest stars are about 100 times the mass of our own Sun, so if 100 million times more photons are being created per second, and a much higher fraction of those are gamma/X-ray/far-UV, the fact that they have to travel ~5 times as far to reach the surface is not going to be significant.)

And that's where Kerbin's Sun comes in; due to the 10:1 scaling that star is physically impossible. It's 10 times denser than our own Sun, while maintaining the same surface temperature, but with a mass far too low to undergo fusion at all. A star like that just can't exist in the real world, and therefore we can't definitively state how it'd behave. With that density, it might act more like a large star than a small one, or it might produce almost no gamma rays at all due to the low mass.

Regardless, the X-rays and hard UV photons DO escape pretty easily, and those'd fry you as well. So we don't really NEED to include gamma rays in our discussion; I just didn't want to leave them out entirely. Those other wavelengths would also do a number on inorganic parts; a UV photon with wavelength below 912 Angstroms will ionize atomic hydrogen, for instance, which would probably do bad things to a hydrogen-oxygen fuel system...

As for radiation pressure, it'd depend on exactly how you planned to get close to the surface. Real-world ion drives, for instance, would be so low in acceleration that these effects might actually make a difference even out at planetary distances. But again, we're talking about objects coming right up to the visible surface... light pressures that are negligible at 1 AU might be significant at those distances. I'd have to run the numbers, but (again) Kerbin's sun is simply impossible, so a lot of the physics can't be derived by using real-world relationships.

Guys, if I recall correctly, Spatzimaus is a real-life space scientist.

True, but my recent work is all in far-infrared observations, mainly of large nearby galaxies (mostly quasars). My thesis did involve measuring ultraviolet photons from really big stars in nearby galaxies, but it does mean that most of this particular discussion is coming from memory. For instance, I'd forgotten how few gamma rays do escape from the surfaces of stars the size of our Sun, since I rarely worried about anything more energetic than a far-UV photon or any stars smaller than about 20 solar masses.

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Guest Brody_Peffley
Actually, the vaccum AROUND our own sun is a few million degrees in temperature. Scientists aren't exactly sure why

Yes we are, Its because of the magnetic field and plasma touching together at really high speeds witch provide friction which that friction gets turned into heat.

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Reading that was quite interesting, thankyou sir! (+rep)

So, basically... the thermometer isn't broken, it's just that there's no enough stuff there to heat the rocket up appreciably. Hmm. It certainly is contrary to most people's expectations, though. :D

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Hmm. It certainly is contrary to most people's expectations, though. :D

There's a lot of that in physics, especially if you get into quantum mechanics. Most people try to visualize complex concepts by finding analogies within the framework of what they experience in their daily lives, but what happens when the thing you're discussing is so fundamentally different from the "normal" world that no good analogies exist? Quantum is based around inherently alien concepts (unless you're a philosopher, then you'll feel right at home), while most of astrophysics deals with things on scales too large (or small) to comprehend. I can tell you that there are 7*10^22 stars in the universe, or that the mass of the Sun is 2*10^33 grams, or that it's about 150 billion meters from the Sun to Earth (plus or minus a couple billion depending on the season), but those numbers are just too big to visualize. So we use units like AUs, light-years, solar masses, and so on, but even so the scales are still mind-boggling once you try to figure out things like the distances to the nearest galaxies.

Bottom line, to study this stuff you basically have to suppress your expectations, and fight against that instinct to put everything in terms of what you're familiar with in the mundane world. The environments in space are so exotic that you just have to deal with them purely on a mathematical basis, even if the conclusions make no sense at first glance.

(That being said, 900 degrees IS on the low side. I'd expect an equilibrium temperature of more like 4000 degrees, given how radiation energy density scales as T^4 and there's on average a factor of 4 difference between cross-sectional area and surface area.)

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The Sun does produce gamma rays. Anything that has a temperature above absolute zero produces a full array of "black body" photons, with the distribution of photon energies depending on the temperature. (The plot of photons vs. wavelength looks a lot like the Rock of Gibraltar, with the peak's frequency directly proportional to the surface temperature, a steep dropoff on one side, and a shallow dropoff on the other.) The million-degree inner layers of the Sun would produce quite a large number of gamma photons; the catch is that nearly all of those photons interact with the opaque outer layers of the Sun before reaching the surface, so the number that escape from the surface is extremely small. The thing about radiative astrophysics is that there are very few absolutes in these sorts of interactions; it's more like a half-life relationship. A photon might have an X% chance to interact with matter every Y kilometers it travels, and if enough of those scale lengths are inside the star it's very unlikely a given photon can escape without interacting along the way. But there's no way, in a finite distance, to raise the odds to 100%, so some WILL escape over time…

There are indeed vast numbers of gamma ray photons produced by the nuclear reactions in the Sun's core. But the mean free path of those photons is way below 0.1 cm. The mean free path of a diffusing photon is the inverse of the product of the opacity and density in the Sun…and it stays below 0.1 cm within the inner 50% of the Sun's radius (see Mitalas, R. & Sills, K. R., Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 401, no. 2, p. 759, 760.). These gamma ray photons have to work their way out from the Sun's interior by random walk to the base of the convection layer (at about 70% of the Sun's radius, from which point the convective transfer is comparatively quick). A photon from the core would need to take about 10^25 steps during its random walk out, and the photon's energy is down-scattered to keep the photon's energy in line with the thermal equilibrium temperature of the plasma inside the Sun at the current position of the photon. So it typically takes the energy from a gamma ray photon formed in the core 1.7 x 10^5 years to reach the base of the convection zone (yes…over one hundred thousand years). And although you could calculate a number for the likelihood that a gamma ray photon could make it from the center of the Sun to its surface in one hop through that huge optical depth, the result would be so tiny as to be completely insignificant for any practical discussion. Just because something is POSSIBLE does not mean it is anywhere near likely enough to worry about. It's POSSIBLE that all the atoms in my body will suddenly end up moving upwards and smash me through the roof of my house, but that's also so amazingly insignificant of a probability that there's no point in worrying about it. The X-ray flux from the Sun's corona would be worth worrying about -- the possibility of gamma rays getting out from the center is not.

As for radiation pressure, it'd depend on exactly how you planned to get close to the surface. Real-world ion drives, for instance, would be so low in acceleration that these effects might actually make a difference even out at planetary distances. But again, we're talking about objects coming right up to the visible surface... light pressures that are negligible at 1 AU might be significant at those distances. I'd have to run the numbers, but (again) Kerbin's sun is simply impossible, so a lot of the physics can't be derived by using real-world relationships.

The best way to get close to the surface of the Sun is to boost far out in the solar system, then kill your speed and drop in. Even better to get a gravity assist from Jupiter to slow you down. The radiation pressure from the Sun at one AU is about 9.08 microNewtons per square meter (just using the values given on the Wikipedia page about radiation pressure). This simply scales inversely as the square of the distance, so it's 227 microNewtons per square meter at 0.2 AU. We don't need to worry about the physics of how Kerbol produces its energy to use that same information for the Kerbol system -- if we assume that the devs want Kerbin to have Earth-like illumination, it would have a similar photon flux and radiation pressure at its orbit, and the radiation pressure at other distances can be found from the inverse square law using the distances in terms of the "Kerbin AU." So you needn't waste a lot of fuel even if you wanted to crash your probe into Kerbol.

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So, basically... the thermometer isn't broken, it's just that there's no enough stuff there to heat the rocket up appreciably. Hmm. It certainly is contrary to most people's expectations, though. :D

That's pretty much what i said in the first reply.

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I'm not sure we're all talking about the same thing. A thermometer on a ship near the sun should be freaking HOT!!!, no matter what the local gas density is like and even if the instrument is in the shade, because heat would be imparted to it by radiation and conduction through the vessel itself as well as from the convecting gas. (Am I using those terms correctly, Spatzimaus?) The planned new space telescope needs to have multiple levels of shade/protection for the IR sensors to work even though it is not going to be anywhere near the sun.

PV0BKPK.jpg

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no matter what the local gas density is like and even if the instrument is in the shade, because heat would be imparted to it by radiation and conduction through the vessel itself as well as from the convecting gas.

Not quite. Convection is a non-issue; that's a function of the gas density in contact with the vessel itself. Radiation from the surface could only get a nearby insulated object to about 6000 degrees, the same temperature as the surface of the Sun; at that point the object's thermal black-body emission would exactly balance the incoming energy. Thermal conduction within the object would DECREASE that; the more parts of the vessel that aren't getting heated directly, the cooler the average would be. (For a perfect sphere, the equilibrium would be about 4000 degrees.)

The part that's in shadow is really important, because heat is transferring to that area from the parts directly lit by the Sun, but the entire surface (shadowed and lit parts alike) are emitting energy to stay in equilibrium. Take Earth as an example; the night side of Earth isn't much colder than the day side (if the day temperature averages 300K, the night might only be 290K), because energy is transferred to it to keep it from freezing overnight. Some energy is stored by the rocks and water, some is carried by atmosphere movements. The net result is that a lit, nonconductive object at Earth distance would have a temperature close to the boiling point of water, but by transferring energy to the "cold" side, the "hot" side of an object ends up quite a bit cooler. This assumes the object isn't reflective, of course; if it is, then it'd be even cooler still.

The planned new space telescope needs to have multiple levels of shade/protection for the IR sensors to work even though it is not going to be anywhere near the sun.

The reason for the shielding has nothing to do with keeping the vessel as a whole from overheating, that's solely to keep the CCD (charged-couple device, basically the same mechanism as a digital camera) from warming up to even room temperature. And that has nothing to do with whether it's an IR telescope, UV telescope, etc; the detector is the same for all, only the optics and filters really differ. (One of Hubble's instruments has dozens of filters ranging from far-UV to mid-IR, using the same detector for all.)

You see, in electronics you get what we call "thermal noise", where the random thermal motion of the atoms in the wires and such create interference. The colder the wires, the lower the noise (scaling as the square root of the temperature in Kelvin). For the hypersensitive detectors used by modern telescopes (ground-based or space-based), the electronics are usually chilled with liquid nitrogen or liquid helium (usually cheap nitrogen for ground-based ones and more expensive helium for space) to much, much colder temperatures than they'd reach otherwise. When you're trying to detect very faint galaxies and such, that lower noise is an absolute necessity. For something like a manned spaceship, it's not important.

Ever since the Spitzer telescope it's become standard practice to put a "sun shield" on one side of a space telescope, to minimize how much coolant you have to expend in normal operation; in the case of Spitzer, this change extended its original designed 3-year mission out to about 6.5 before it ran out (early 2003 to mid-2009). Basically, those shields keep the temperature well below the "typical" temperature for that distance from the Sun, without having to expend a constant amount of coolant. Spitzer continued to function for a couple years after that (what was called the "warm mission"), but the much higher thermal noise meant that only two out of the seven wavelength bands were useable at all during that period, and only for purposes that didn't need as high of precision.

In the case of the Webb telescope, those extensive shields are going to be the primary way to keep cold, with little or no coolant needed at all. That's a necessity since the Webb won't be in Earth orbit like the Hubble was; a good thing, too, since a Hubble-type orbit can't really use a shield like that in the first place. Basically, it's designed to operate at about 53 K (-220 C), whereas without a shield it'd be at more like 350 K. The JWST is just the first design to go that far with the shielding.

Edited by Spatzimaus
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Yeah, I shouldn't have thrown around terms without a clear idea of their meaning in this context, but the point I was trying to get at, in my clumsy way, was that the thermometer on such a ship would read "freaking HOT!!!," and I think

the equilibrium would be about 4000 degrees

qualifies as freaking hot. :)

The reason for the shielding has nothing to do with keeping the vessel as a whole from overheating
That's also the point I was trying to make, and apparently failing. If the Webb needs that parasol to keep its instruments at a working temp way out where it's going to be orbiting, a ship near the sun would need something even more elaborate to keep a thermometer from reading a cool temp, as some of the posts in the thread seemed to suggest it might be doing.

Really, I had a point. It was here a minute ago. I should probably just cut my losses now. :)

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If the Webb needs that parasol to keep its instruments at a working temp way out where it's going to be orbiting, a ship near the sun would need something even more elaborate to keep a thermometer from reading a cool temp, as some of the posts in the thread seemed to suggest it might be doing.

If the Webb's shielding is reflecting enough energy to lower the "natural" temperature of 300K (the temperature of a conductive, nonreflective object at that distance from the Sun) down to a working temperature of 53K, then that's actually MORE of a decrease than you'd need to lower an object near the Sun (where the natural temperature would be 4000 degrees) down to 900 Celsius (~1200 K). It's a ratio thing; to go from 300 down to 53 you have to reflect about 99.9% of the incoming photons, since energy density goes as T^4, whereas it's more like 99% to go from 4000 to 1200. So you'd need less shielding than the Webb has, although not by much, especially if you add some onboard cryogen to help keep things cold.

(Side note: Webb is basically going to be at the same distance from the Sun as Earth is. It'll be at the L2 Lagrange point, which is slightly further out, but the difference is small. So the same basic math applies to any Earth-orbiting satellites as well.)

As to whether 4000 degrees qualifies as "freaking hot", that's a different discussion. I'd generally agree that it's ridiculously hot, and I definitely wouldn't want to be in a manned vessel in that area, but I'm sure it's possible to make an unmanned probe that could survive it for brief periods. Besides, if you want to land on the Sun without burning up, just wait until night.

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