Comet Lovejoy Plunges Into the Sun and Survives

boldie writes with a link to NASA's account of comet Lovejoy's close encounter with the sun. Excerpting: "This morning, an armada of spacecraft witnessed something that many experts thought impossible. Comet Lovejoy flew through the hot atmosphere of the sun and emerged intact. ... The comet's close encounter was recorded by at least five spacecraft: NASA's Solar Dynamics Observatory and twin STEREO probes, Europe's Proba2 microsatellite, and the ESA/NASA Solar and Heliospheric Observatory. The most dramatic footage so far comes from SDO, which saw the comet go in (movie) and then come back out again (movie)." Here are larger QuickTime versions of the comet's entrance (22MB) and exit (26MB).

Comments

Re:we are all doomed!

[jginspace]

It's going to come back and smash into the earth in 2012.

Quite possible as, quoting TFA:

"There is still a possibility that Comet Lovejoy will start to fragment,"

No telling which directions those pieces might fly off.

You can all thank Dr. Reyga...

[Anonymous Coward]

for a successful demonstration of Metaphasic Shields.

Re:You can all thank Dr. Reyga...

[CrackedButter]

Actually, that comet was Gomtuu trying to kill itself again.

Misleading title

[aneroid]

Sounds a lot more sensational when you compare the title's "comet plunges into sun and survives" event vs the actual "comet flew through hot atmosphere of the sun".

/. worthy event nevertheless.

Re:Misleading title

[MaskedSlacker]

You realize that the sun doesn't actually have a surface, right? It's increasingly dense atmosphere all the way down.

Re:Misleading title

[tqk]

Sounds a lot more sensational when you compare the title's "comet plunges into sun and survives" event vs the actual "comet flew through hot atmosphere of the sun".

Isn't the Sun's atmosphere supposed to be holy freakin' hell hotter than the Sun itself? Me, I'll just say "Way to go, Lovejoy!" (as in "Hunt for Red October").

Cool stuff.

Re:Misleading title

[jd]

Iron takes more energy to fuse than it releases. Any star with more than a trivial amount of iron tends to go kablooey, for that reason. Mainstream stars like the sun don't do that because they just don't have iron cores.

Re:Misleading title

[Anonymous Coward]

http://www.thesurfaceofthesun.com/
Maybe that depends on your definition of a surface?

Re:Misleading title

[Anonymous Coward]

I was under the impression that stars, any star really, is continually increasing its stock of heavier elements. From Helium to Iron, and that these elements settle in the core in a layered fashion ordered by their atomic weights.
Then as it goes through its life cycles the star is progressively consuming heavier and heavier elements until there's little more than Iron left in the core and only then does it go kablooey.

I am not a phycisist, but i guess i don't see how you can avoid a build up of heavier (than hydrogen) elements when the product of fusion is heavier elements?

Re:Misleading title

[symbolset]

Cool stuff.

Or maybe, not.

Re:Misleading title

[snowgirl]

Sounds a lot more sensational when you compare the title's "comet plunges into sun and survives" event vs the actual "comet flew through hot atmosphere of the sun".

Isn't the Sun's atmosphere supposed to be holy freakin' hell hotter than the Sun itself? Me, I'll just say "Way to go, Lovejoy!" (as in "Hunt for Red October").

Cool stuff.

I think it might be something like the Leidenfrost effect. The sun's atmosphere vaporizes comet, and these vaporized comet parts shield the rest of the core from vaporizing. Only, this would have to work with the vapor blocking the radiation heat rather than the convection/conductive heat that the typical Leidenfrost uses. a.k.a. a sort of über-Leidenfrost effect.

Re:Misleading title

[reverseengineer]

The sun's corona is intensely hot- about 1 million kelvin, much hotter than the photosphere beneath, but the plasma is very diffuse. The photosphere, the layer that appears to us to generate the opaque disk of the sun (and is the closest thing it has to a surface) is a mere 6000K, but it's 10^12 times denser than the corona. In turn, the photosphere is about one ten-thousandth of the density of Earth's atmosphere at sea level. This really skews notions of "temperature" when we talk about a star. On Earth, we're used to objects placed in a medium fairly rapidly equilibrating to the temperature of that medium. We realize that some substances conduct faster or slower than others, but overall putting something in a hot environment makes it hot.

For all but the most finicky of physics experiments, if we had pressure conditions of the density of the sun's corona, it would be "high vacuum." Very little conduction of heat from the plasma to a comet is going to take place. The bombardment by solar photons and the gigantic magnetic and gravitational fields of the sun play a greater role here than the actual material of the sun, and thus NASA can be pleasantly surprised by Comet Lovejoy's survival of its close encounter. But it's the wrong idea to picture this comet plunging into some sort of molten inferno. Of course, the sun's core is another story. 15 times denser than lead and 16 million kelvin. I'll like to see the comet that survives that.

Re:Misleading title

[schroedingers_hat]

This is true to a small extent, but there's a feedback loop where the star gets bigger and cooler if the fusion rate increases. This results in (predominantly) only one element being fused at a time, so you have Hydrogen->Helium (with some slightly heavier elements, C,N, and O can be involved) until the star is almost out of hydrogen, then Helium->Things between LIthium and Oxygen, and after that the star goes boom pretty quickly (or lacks the mass to go any further).
By comparison the amount of fusion of elements heavier than the one it is burning at the time is extremely small.
The majority of all stars (such as our own) are burning Hydrogen.
The majority of all heavy elements come from supernovae (the stars that our sun was made of before it became a star again).
That being said, there is probably a large chunk of iron and other heavy elements (from past supernovae) in the middle of our sun, but we generally ignore it because it is such a small percentage.

Re:Misleading title

[schroedingers_hat]

note to self, read preview when it comes up

Re:Misleading title

[Zarhan]

I was under the impression that stars, any star really, is continually increasing its stock of heavier elements. From Helium to Iron, and that these elements settle in the core in a layered fashion ordered by their atomic weights.
Then as it goes through its life cycles the star is progressively consuming heavier and heavier elements until there's little more than Iron left in the core and only then does it go kablooey.

"The onion of elements" happens at the end of star's life (or exiting of main sequence), but until then, there's plenty of hydrogen in core. Once star goes red giant, it has a helium core fusing to carbon (and hydrogen still fusing to helium in the mantle). If there's enough mass, the carbon core can start fusing neon, and so on all the way to iron. However, the full range with all the layers only happens in the most massive stars that finally explode as supernovas. And the main sequence is all about fusing hydrogen.

Re:Misleading title

[Anonymous Coward]

BWAH-hahahaha! Oh my, that's hilarious, the things people will believe.

Re:Misleading title

[thegarbz]

I was under the impression that stars, any star really, is continually increasing its stock of heavier elements. From Helium to Iron, and that these elements settle in the core in a layered fashion ordered by their atomic weights.

This happens once during the life of certain stars. Typically supergiants, and typically a second or two before a massive solarsystem devastating explosion.

The entire mess of nuclear reactions in a star make the core heavier and heavier UNTIL it fuses to iron. At that point there's no where left it go, it collapses, and violently ejects most of the outer layers. From what I've read over the billion year life of a star this all holds together for no more than a few seconds.

Re:Misleading title

[Anonymous Coward]

Sounds a lot more sensational when you compare the title's "comet plunges into sun and survives" event vs the actual "comet flew through hot atmosphere of the sun".

My first take on the 'headline' was: I just passed my finger through a candle flame and it survived.

Wooo-peee-dooo

Re:Misleading title

[PyroMosh]

The effect we're discussing is easily observable to anyone who's reasonably familiar with a kitchen.

Ever fry french fries in oil? This is typically what? 350F?

Baking a pizza will typically be around 450F.

Yet it's easy to reach into a 450 degree oven and remove the pizza. As long as you use a towel or a tool, your hand can be in the same environment that just cooked the pizza for a relatively long time..

But any fool knows that reaching into the oil with your bare hand *at all* will burn your skin in less than a second. Even though the oil is 100 degrees cooler than the oven.

It's just a dramatic, every-day example of the difference in heat transfer between mediums (in this case, oil vs air).

Re:Misleading title

[turing_m]

And here I thought it was turtles.

Re:Misleading title

[Anonymous Coward]

Is there any observable evidence to support the theory that the core is so dense and hot?

Neutrino measurements and extensive helioseismology.

Re:Misleading title

[Anonymous Coward]

That's quite a bold statement!

Re:Misleading title

[MaskedSlacker]

That was my first thought after I posted the comment too. I wish I could remember where I first heard that story. I want to say it was during a tour at the Palomar Mountain Observatory, but I'm probably mixing memories. It's only in the last five or six years that I've realized how many people know it, and how common a comment it is.

Re:Misleading title

[roman_mir]

Of course, the sun's core is another story. 15 times denser than lead and 16 million kelvin. I'll like to see the comet that survives that.

- you probably wouldn't like to see the comet that does that with our Sun and survives, because for a comet to do that, it would be a neutron star or a black hole and not a comet and it's not clear that our Sun wouldn't then have an explosion within it, that would cause massive problems for our little rock here at the very least with the enormous amount of radiation, even in gamma spectrum and a possible orbit change causing some real problems later on.

Re:Misleading title

[MaskedSlacker]

No you couldn't.

At a certain point you'd reach neutral bouyancy and stop sinking (ignoring the part about you vaporizing from the temperature). This is still not the same as there being a surface (and in point of fact is COMPLETELY unrelated to what is usually referred to by the term "surface of the sun"). There is no meaningful line of demarcation between "below the surface" and "above the surface," and the term surface NECESSARILY implies a demarcation. The use of the term "surface" in the case of stars (and gas giants) is purely terrestrial metaphor, and it's fine as far as that goes--but only that far. Take the metaphor further (as my parent did in taking it literally), and you wind up reaching physically absurd conclusions (as you also have).

As for tomatoes, all I care about is that they're fucking delicious.

You'd be better off trying to claim that black holes have a surface. They don't, but at least they do have a clear demarcation between above and below their "surface." Stars do not.

FWIW, real astrophysicists define the surface of the sun as the radius which equals an optical depth of 2/3. This is a useful demarcation (though not a terribly clear one, since the optical depth is wavelength dependent) for observational purposes, but not for kinematic ones (such as when talking about comets flying into, and out of, the sun).

Re:Misleading title

[jrumney]

I thought the GPs post was more about deep fried ice cream than putting hands in the oven.

Re:Misleading title

[jrumney]

Then again, I might be looking at the wrong GP, a post about the Leidenfrost effect appeared above the GP when I was reading this.

Re:Misleading title

[Surt]

That iron core is still a gas. We're just not used to iron being a gas because we live at such cold temperatures.

Re:Misleading title

[JTW]

A thought experiment worthy.

If you took a ball of Iron the size of say planet Earth, and it were to plunge into the heart of the Sun. What would be the result.. a Nova, SuperNova.. fizzle.

And what if it were slightly off target and merely circled the center for a while.. would it retain its shape or spinout into a smeared ball of plasma.. undoing the star?
Something somewhat like this probably already happened.. the Lithium content of our star for example.. guess it just wasn't significant emough.

Re:Misleading title

[berbo]

You realize that the sun doesn't actually have a surface, right? It's increasingly dense atmosphere all the way down.

an increasingly dense atmosphere of turtles, to be precise.

Re:Misleading title

[corbettw]

Quick question to someone who seems like he might know: is the current stock of hydrogen in the universe all that will ever exist, or does the stock get replenished (so to speak) during a supernova as heavier elements get split into lighter ones? Just wondering if there could ever be a point in the lifetime of the universe when light elements like hydrogen and helium simply don't exist any more.

Re:Misleading title

[ClickOnThis]

Sounds a lot more sensational when you compare the title's "comet plunges into sun and survives" event vs the actual "comet flew through hot atmosphere of the sun".

Isn't the Sun's atmosphere supposed to be holy freakin' hell hotter than the Sun itself? Me, I'll just say "Way to go, Lovejoy!" (as in "Hunt for Red October").

Cool stuff.

It is true that the Sun's corona is extremely hot, but it also has an extremely low density. Together that means that the corona may not impart much of an energy flux to the surface of the comet. Without knowing the numbers, my guess would be that radiation from the sun is a more important contributor to melting of the comet.

Consider another situation: the temperature of the plasma in the earth's magnetosphere can be thousands of degrees, and yet spacecraft don't melt in it, for the same reason.

Re:Misleading title

[dwye]

> It's increasingly dense atmosphere all the way down.

Actually, at the "surface" it is an increasingly less hard vacuum. As I recall, the glowing surface is a vacuum, harder than we could achieve before 1960 or so, with the transparency or lack thereof of a London Pea-souper fog.

As long as the comet has a low enough albedo, I can see no problem with it diving below the chromosphere surface, given that it is a surface in the mathematical sense more than anything else, with very little heat conductance due to its low (effectively zero) pressure.

Re:Misleading title

[Tablizer]

Any star with more than a trivial amount of iron tends to go kablooey

I'd like to see that tested on Mythbusters, especially when they jack the iron way up for an encore.

Or, did you mean Hollywood stars?
 

Re:Misleading title

[michelcolman]

As far as I know, supernova only fuse elements, they don't do fission. The only way to get more hydrogen in a natural way is probably waiting for elements to decay by themselves. Maybe if the supernova makes heavy, unstable elements, those will fall apart and give you some hydrogen back. But apart from that, if I am not mistaken, supernovas only make heavier elements.

Re:Misleading title

[CubicleView]

Wild guessing, but if there was any significantly increased out-gassing it might have further reduced the heat conductance.

Re:Misleading title

[Waccoon]

Or perhaps something like this?

Re:Misleading title

[Ginger Unicorn]

There are a whole host of unexplained phenomena related to the sun's activities that still baffle gas model theorists to this day because they fail to recognize the existence of an iron alloy transitional layer that rests beneath the visible photosphere.

Crank alert.

Re:Misleading title

[justthinkit]

Quoting wiki: "In larger stars, fusion continues until the iron core has grown so large (more than 1.4 solar masses) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay, or electron capture. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova."

So don't expect much to happen unless you have 1.4 solar masses worth of iron to toss into the sun. Expect a fizzle.

Composition?

[martas]

What the hell is that thing made of? Article doesn't seem to say, and I'm sure nobody is 100% certain, but any guesses as to its composition based on its orbit? Also what would the temperature of such an object likely be?

Re:Composition?

[galaad2]

Orbit? that thing no longer has a stable orbit... at least for a while it won't have, until it stabilizes.

have you seen those two movies? its exit is like an out-of-control fire hose with afterburners.

Re:Composition?

[Eternauta3k]

Stabilize? Once it's out of the corona it should have a normal orbit.

Re:Composition?

[riverat1]

Just because the tail of Lovejoy was bouncing around in the solar wind doesn't mean the comet's orbit is unstable. Perhaps the orbit was changed a bit by it's close encounter with the Sun but that doesn't mean it isn't stable..

Klingon Bird-of-Prey

[JO_DIE_THE_STAR_F***]

That wasn't a comet it was Kirk and company in a Klingon Bird-of-Prey trying to get back to the 23rd century.

Re:Klingon Bird-of-Prey

[The Grim Reefer]

That wasn't a comet it was Kirk and company in a Klingon Bird-of-Prey trying to get back to the 23rd century.

Those thieving bastards just took off with a pair of my whales too. -Ismael

Re:Klingon Bird-of-Prey

[SteveFoerster]

People who did a little too much LDS in the '60s.

Re:Klingon Bird-of-Prey

[jbengt]

Who let greenpeace write a Star Trek movie plot?

People who did a little too much LDS in the '60s.

What did the Mormons have to do with Start Trek?

Re:Klingon Bird-of-Prey

[SteveFoerster]

Nothing -- that was Battlestar Galactica.

Re:Klingon Bird-of-Prey

[Tablizer]

Did Kirk ever visit planet Kolob?

Better title:

[circletimessquare]

"The Sun somehow survives close call with badass comet Lovejoy. Meekly vows to be more respectful next time."

Re:Better title:

[lister king of smeg]

hmm, lovejoy is that a new name for chuck noris?

Re:Better title:

[Eunuchswear]

Pah, Chuck Norris is a wimp.

http://www.wnd.com/?pageId=101063

Afraid of Obama!

Re:Better title:

[Surt]

Show a little respect. Reverend lovejoy is a badass preacher who can take on a tribe of baboons bare handed. Being gay wouldn't be compatible with his christian values, and you can tell he's not gay because he has kids with his beard, I mean, his wife.

Re:Better title:

[HyperVerbal]

lol

Energy Depleted

[Anonymous Coward]

The comet's fuel reserves were low; flew into a star to recharge.

Re:Energy Depleted

[lister king of smeg]

nice Stargate Universe reference

Re:Energy Depleted

[Anonymous Coward]

No, it is an Elite reference.

Re:Energy Depleted

[PlasmaEye]

It could also be a reference to Arthur C. Clarke's Rendezvous with Rama.

Re:Energy Depleted

[Sponge Bath]

I think Ren and Stimpy did something similar in the Space Madness episode.

Re:Energy Depleted

[barry99705]

That was a pretty cool game for it's time.

Of course it is possible ....

[Anonymous Coward]

Just as you can plunge your hand in a dewar of liquid nitrogen and not have your hand immediately frozen, a comet will survive for the same reason. With your hand, the liquid nitrogen boils from the heat of your hand creating an insulating layer of air between your hand and the liquid nitrogen. With the comet, the comet evaporates creating an insulating layer of gasses that protect the entire from immediately evaporating.

I've kept my fist in liquid nitrogen for a total of 38 seconds. (Not the smartest thing I've done.) I had a touch of frost bite on the pads of my fingers where liquid nitrogen seeped into my fist and the gasses escape properly and couldn't insulate as needed. The rest of my hand was just fine and I could have probably left it in there longer had I chose with little ill effects -- other than on the pads of my fingers.

Re:Of course it is possible ....

[Anonymous Coward]

Indeed, that's a direct correlation to the hour the comet spent in the corona.

Re:Of course it is possible ....

[Anonymous Coward]

I've kept my fist in liquid nitrogen for a total of 38 seconds. (Not the smartest thing I've done.)

May not be the smartest thing you have done. But I suppose it is the coolest thing you have done.

Re:Of course it is possible ....

[Overzeetop]

Though, to be fair, the comet had slightly more mass than his balled fist.

Re:Of course it is possible ....

[circletimessquare]

you forgot the "don't try this at home, kids"

no, seriously

Re:Of course it is possible ....

[c6gunner]

you forgot the "don't try this at home, kids"

Especially since his story is almost certainly bullshit. I've never worked with liquid nitrogen, but I do work with liquid oxygen. To freeze an apple to the point where it will shatter when thrown takes about 45 seconds to a minute. I've seen guys get the equivalent of second-degree burns just from getting splashed - they couldn't have been in contact with the liquid for more than 5 seconds.

His explanation for WHY he didn't get injured would have made sense if he claimed to have done it for a split-second ... but 35 seconds? Bullshit.

So I'll do it for him: DO NOT TRY THIS AT HOME. People on the internet lie, all the time. Don't be a gullible idiot.

I'm surprised they're surprised.

[jd]

Basic chemistry tells us that heat transfer isn't instantaneous, that solid objects remain at melting point until fully melted, and that heat != temperature. It's why you can walk over hot coals without burning yourself. The composition of the comet would be easy to determine, since absorption spectrometry will tell you what the tail is made of. We also know, from the Giotto probe, that comets don't evaporate from the outside. That was one of the biggest blunders in the mission. Never, ever make assumptions in science because it WILL bite you. Facts are the only acceptable currency.

Re:I'm surprised they're surprised.

[Anonymous Coward]

That's the weirdest combination of terrible and excellent science education ever. Congrats.

Re:I'm surprised they're surprised.

[Anonymous Coward]

It's why you can walk over hot coals without burning yourself.

A perfect analogy for a comet that spent approximately one hour immersed in temperatures of several million degrees.

Re:I'm surprised they're surprised.

[Dunbal]

Especially since often the "hot coals" turn out to be christmas lights.

Re:I'm surprised they're surprised.

[CODiNE]

It's why you can walk over hot coals without burning yourself.

Actually it's the Leidenfrost effect. Try that with dry feet and let me know how long it is before you can walk again.

Re:I'm surprised they're surprised.

[CODiNE]

Yes and ice cubes are even easier to walk over. Perhaps wearing shoes can prevent getting paint on ones feet, thereby eliminating the Leidenfrost effect entirely.

Re:I'm surprised they're surprised.

[pjt33]

Wet skin conducts heat better than dry skin, so it's often recommended to firewalk with dry feet rather than wet feet.

Re:I'm surprised they're surprised.

[chocapix]

And the best is to have cold feet.

Re:I'm surprised they're surprised.

[jd]

See CODiNE's response for why humans do indeed ablate material.

Re:we are all doomed!

[Colonel Korn]

It's going to come back and smash into the earth in 2012.

Quite possible as, quoting TFA:

"There is still a possibility that Comet Lovejoy will start to fragment,"

No telling which directions those pieces might fly off.

Absolutely. When a single object slowly fragments due to thermal gradients, it ignores conservation of momentum and sometimes even conservation of mass. It's possible this ~100-500 m radius comet will launch a 50000 m chunk at us with a velocity of over half the speed of light!

No Audio

[Cylix]

I'm pretty sure it was shouting, "Hot hot hot hot!"

I'm fairly certain comet love joy won't be taking on any more dares for a while.

Re:No Audio

[Krishnoid]

I'm pretty sure it was shouting, "Hot hot hot hot!"

Shortly after which it reported, 'I got a little cooked but I'm ok'.

Re:No Audio

[michelcolman]

And before that, it was saying "What's this thing coming towards me very fast? So big and fat and round, I think I'll call it... Sun. That's a good name - Sun! I wonder if it will be friends with me"

Re:No Audio

[MLease]

Oh, no, not again.

Velocity of Comet

[dispersionrelation]

I went ahead and calculated the velocity of the comet at its Perihelion (closest distance to the sun) to be or 618km/s which is the same as 383 mi/s which is the same as 0.2% the speed of light, very fast!

Re:Velocity of Comet

[MichaelSmith]

Sound like the ideal place to start your interstellar ramjet engine.

Re:Velocity of Comet

[Dunbal]

I think that's slightly above the solar escape velocity, so we can kiss this one goodbye. Don't worry boys, he won't be coming back.

Re:Velocity of Comet

[michelcolman]

Which explains why it didn't evaporate. Time contraction!

Re:Velocity of Comet

[michelcolman]

But wasn't it in some sort of orbit around the sun before? Where did it pick up the extra energy to get away from the sun, then? If anything, I would have expected its orbit to shrink.

Re:Velocity of Comet

[f()rK()_Bomb]

The gases boiling off the comet effectively give it a rocket engine. One of the proposed methods to deflect a comet on a collision with earth is to shine sun on it with giant mirrors.

Re:Velocity of Comet

[f()rK()_Bomb]

The time dilation factor is 1.0000021136121362, so no.

Re:Velocity of Comet

[michelcolman]

But that "rocket engine" would work in opposite ways on arrival and departure, either slowing it down or speeding it up (not sure) when it's coming in and doing the exact opposite when it's emerging again. So that would not give it a net extra energy. Or am I missing something?

Re:Velocity of Comet

[michelcolman]

I wasn't entirely serious, but thanks for checking ;-)

Re:Velocity of Comet

[f()rK()_Bomb]

I didn't think you were really, but I just wanted to show how tiny the time dilation effect is, its barely noticeable until very high speeds. 86% light speed gets you only double the time dilation.

Re:Velocity of Comet

[pjt33]

I haven't worked through this in any detail, but it seems plausible that there could be a net push away from the Sun on the basis that the incoming comet is larger and hence has a lower surface area to mass ratio.

Re:Velocity of Comet

[Dunbal]

From a near miss. The sun's gravity accelerates it as it falls close in to the sun - gravity is dependent on the inverse of the distance squared. So as it gets very close the gravitational pull grows along square function. By not actually hitting the sun, it manages to keep some of that energy (and the sun loses the same amount in angular momentum - but for the sun it's a negligible amount of spin so you won't notice). Because its velocity is far, far greater than the (I believe it's 33km/s) escape velocity - it will slow down a little on its way out, but it's never, ever coming back. Not only that, but inverse of distance squared. It's not going to slow down much more due to gravity - most of the slowing down was done within a few solar radii.

This technique of hitching a ride on another celestial body's gravity well has been used for space probes, and you can read a dramatization of "aerobraking" in Arthur C Clarke's 2011 novel.

Re:Velocity of Comet

[Solandri]

The comet shrinks as it gets sublimated away by the heat. This leads to both a decrease in mass and surface area. Modeling it as a sphere:

m ~ V (mass is proportional to volume)
V = 4/3 pi * r^3
so r ~ m^(1/3)
surface area = 4 pi * r^2
so SA ~ m^(2/3)

Assuming the velocity of a vaporization jet is constant (constant sublimation temperature means gas molecules have constant kinetic energy), the force is proportional to the amount of escaping gases. The amount of escaping gases is proportional to surface area, so:

F ~ SA = m^(2/3)
a = F/m
a ~ m^(2/3) / m
a ~ m^(-1/3)

The smaller the mass of the comet, the greater the acceleration.

When the comet is approaching the sun, m is large, so vapor jets contribute acceleration away from the sun (slows the comet down).
When the comet is leaving the sun, m is small, so vapor jets contribute greater acceleration away from the sun (speeds the comet up more than it was slowed down before).

Re:Velocity of Comet

[DadLeopard]

Forget the comets "jet"! For a simple explanation of the effect on the comet, see The Rollings Stones by Heinlein! To gain velocity dive toward a planet or sun and reduce the mass of your craft on the way in, via whatever means, then you have a net gain in velocity on the way out!

Re:Velocity of Comet

[michelcolman]

No, that's definitely not how it works. For a small object passing close to a massive object, if no other forces like friction or electric forces exist, and disregarging relativistic effects, the exit velocity will be exactly equal to the approach velocity. The sum of potential and kinetic energy remains the same: as it loses potential energy in the gravitational field by coming closer, it will speed up and it will lose this speed again when going back up in the gravitational field. The decelleration while going away is exactly the same as the accelleration on the way in. Any extra speed would have to be caused by some sort of magnetic effects or solar wind. The most likely reason seems to be one that someone else has pointed out elsewhere in this thread: the comet gets smaller, so the ratio between surface area and mass gets larger (mass being proportional to radius^3 and surface area only to radius^2), so the push from the solar wind will be stronger on the way out. Gravitational accelleration will be the same though, no matter what the mass is.

Also, I don't think the spin of the sun would have any effect apart from a very, very tiny relativistic frame dragging effect. So that's not it either, I would think.

Slingshot manoeuvers work quite differently: they work by approaching a planet from a different direction and leaving it in the same direction as the planet is moving in. The exit velocity relative to the planet is the same as the approach velocity, but since you are leaving in the same direction the planet is moving in, you are basically adding its speed to your own. The ideal example would be approaching a planet in the opposite direction of its trajectory and turning around it 180 degrees to leave in the same direction it's going in. This would add twice the planet's speed to your own. A less ideal but more realistic example is approaching perpendicularly and leaving in the direction of the orbit, adding only once the planet's speed to your own. But in the case of the sun, which we would consider pretty much stationary in our solar system, you wouldn't gain anything.

Re:Velocity of Comet

[michelcolman]

But surely accelleration in a gravitational field does not depend on your mass? The force of gravity is proportional to your mass, so the accelleration is the same! Any object with any mass will, given the same starting position and velocity, leave the planet or sun with exactly the same speed. Shedding mass will only work if you shed it in the direction opposite to the direction you want to accellerate in. Simply "losing weight" does nothing to change your orbit.

Re:Velocity of Comet

[michelcolman]

That's a very logical explanation. And maybe the solar wind could have something to do with it too, because that would also be proportional to surface area.

Re:Velocity of Comet

[sFurbo]

Rotation can change that. Depending on the direction, rotation will either move the hottest (most active part of "rocket engine") to the front, slowing down the comet, or to the back, speeding it up. I have no idea whether that is the explanation here, though, or, indeed, whether we are in need of an explanation, or in need of checking calculations.

Maybe it's made of a great material

[Squiddie]

It would be cool if that were the case, then we would just have to make some ships or probes from that. Indestructible space-craft. Might be nice.

Re:That's not a comet.

[Inigo Montoya]

...maybe it was the Destiny.

Whaaat?

[rev0lt]

Is this a Simpson's prank?

Re:we are all doomed!

[jhoegl]

It may end the earth as we know it!...

and I feel fine.

Re:That's not a comet.

[Anonymous Coward]

Uh, the tail WAS blowing away from the sun. Take a look at the coronograph footage for a view that isn't wildly foreshortened:

Oh my

[lxs]

"What," said Trillian in a small quiet voice, "does sundive mean?"

"It means," said Marvin, "that the ship is going to dive into the sun. Sun... Dive. It's very simple to understand. What do you expect if you steal Hotblack Desiato's stunt ship?"

"How do you know..." said Zaphod in a voice that would make a Vegan snow lizard feel chilly, "that this is Hotblack Desiato's stuntship?"

"Simple," said Marvin, "I parked it for him."

"The why... didn't... you... tell us!"

"You said you wanted excitement and adventure and really wild things."

If DNA was still alive he'd have to do a lot of rewriting.

Re:Oh my

[MichaelSmith]

Fortunately any occupants of that comet have long since used the partly working teleport to get the hell out of our solar system.

Re:Coolness factor re: comet passes "through the S

[Runaway1956]

I keyed on that "armada" myself. Had to read TFS, so see where this "armada" came from. Unfortunately, the word has no bearing on the story - it was just thrown in there, much as the word "decimate" is oftentimes improperly used to generate attention.

One would expect an "armada" to, at the least, come under one common authority, and to share a common mission.

Re:That's not a comet.

[Runaway1956]

TFA has some speculation regarding that phenomena. Solar wind, magnetics, gravity, who knows at this point? Are you suggesting that it was really a rocket?

Misleading subject

[harris s newman]

Into Near If the comet had truly plunged into the Sun, it would not be in orbit around it. It's trajectory never was into anything.

Re:Awesome

[durrr]

It's actually an aerobraking alien spacebattleship. We're so fucked.

Re:Awesome

[Jeremiah Cornelius]

Hey that line about "comet Lovejoy plunging into the sun.."

I just used it on some bird in the pub, and it worked!

Re:we are all doomed!

[Anonymous Coward]

It may end the earth as we know it!

Man, everything ends the earth as we know it.

I could go out there and shit in the bushes and BAM, the earth as you knew it where that bush was shit free? GONE.

btw, don't go out to your bushes for another few minutes. Bring toilet paper.

Re:That's not a comet.

[michelcolman]

That's weird, in the other video of the comet emerging from the sun, it seemed to be dragging its tail behind it. I was wondering exactly the same thing as catmistake.

Re:we are all doomed!

[bryan1945]

Great, you just gave SyFy it's next disaster movie. And yet I'm still a sucker for them, heh.

Re:we are all doomed!

[impaledsunset]

Meter is a unit of length, not mass, so you're obviously wrong! I'm already living in my bomb shelter

Re:time travel

[maxwell demon]

No, it didn't go at Warp speed.

ZOMG!!1

[tusam]

Youtube PLS!!1

Re:ZOMG!!1

[tusam]

Might have guessed, all of them seem to feature ridiculous music, unnecessary commentary and/or run of the mill conspiracy theories/end of the world prophecies.
Would've hoped http://www.youtube.com/user/nasatelevision had something but so far the most informative I came across was http://www.youtube.com/watch?v=9DE6sPnm4Pk (at least you can mute it)

Re:Awesome

[Anonymous Coward]

Still playing Mass Effect, I see.

Re:That's not a comet.

[Whiteox]

Uh, the tail WAS blowing away from the sun. Take a look at the coronograph footage [nasa.gov] for a view that isn't wildly foreshortened:

Exactly. Therefore the matter in the vids was the comet losing mass.

Re:we are all doomed!

[sempir]

If I had points to use, that would have got you a few more than one! Hit my funny bone...BAM.... I'm still laughing every now and then...Missus thinks I'm nuts.

SIGI!!

[b_dover]

I love Seeing It Go In and come back out again.

Re:we are all doomed!

[Arancaytar]

Stop hassling Hollywood with your "physics"! :P

Wow!

[Ferretman]

What an awesome solar system we've got!

Reminiscence of a Jello Biafra-for-mayor slogan

[retiarius]

When the Dead Kennedy's Jello-of-"California Uber Alles"-and-"Holiday in Cambodia"-fame
(amongst other faves) was running for mayor of San Francisco,
one of his heartfelt pleas was that he'd be the first politician to spearhead the idea of
"landing a man on the sun".

Re:we are all doomed!

[Ihmhi]

Oh great. A big ball of ice passes through fire, survives, catches on fire, and now it's coming for us. Ice on fire. How do we even stop that?

Snowball in a Hot Place

[Anonymous Coward]

Snowball's chance to survive in Hell was just experimentally verified. Snowball survived, myth busted!

I don't want to know

[p51d007]

Wouldn't it be better, since there isn't a d*mn thing we can do to stop it, to NOT KNOW if some stupid asteroid is going to wipe us out like the dinosaurs? We'd just run around like a bunch of idiots doing the rape, pillage and plunder, and for what? Nothing. How about NOT telling us anything, and we'd just wonder why the sky is getting so dark, then BOOM! We'll all toast. ;)

Re:I don't want to know

[repapetilto]

No.

And now we know...

[Chelloveck]

So, a snowball really does have a chance in hell.

Something is wrong

[Corson]

There is something wrong with the exit movie. Normally, the comet tail runs aways from the Sun but in this video it appers to run in the opposite direction.

Re:we are all doomed!

[ae1294]

Comet Lovejoy flew through the hot atmosphere of the sun and emerged intact. ..

Should it have not went back in time? I seem to remember that being a rule...

Someday I hope we can follow

[wisebabo]

Like in Arthur C. Clarke's story "Sunrise" (I think), a spaceship gets very close to the sun by remaining in the shadow of a sun grazing asteroid.

Probably wouldn't be a good idea to use a comet because of all the outgassing (in addition to being dangerous and literally blowing you away, it would mess up the measurements). Also, a quickly rotating asteroid wouldn't be good as the surface would be re-radiating the heat directly below you. If we could find a sufficiently large asteroid (for it's heat capacity) that wasn't rotating at all (could we stop one?) we could bury the ship inside and get really really close to the sun, much closer than even the upcoming Solar Probe to be launched in 2018 (I think).

If no such asteroid exists that's in the proper orbit, I guess we could move one. Then again, with that kind of technology we could probably just turn on our metaphasic shields and go.

Re:Awesome

[flappinbooger]

It's actually an aerobraking alien spacebattleship. We're so fucked.

Or it's refueling....

Re:we are all doomed!

[dwye]

Comet Lovejoy flew through the hot atmosphere of the sun and emerged intact. ..

Should it have not went back in time? I seem to remember that being a rule...

Only if it was using Star Fleet technologies. Apparently, there is some interference between the warp or impulse engines and the solar core that causes it - perhaps there is a quantum black hole in the center or something, or else everyone would have done it at their own star and the Klingons, Romulans, or Borg would have conquered the galaxy by thousands or million of years before their founding.

Lovejoy and Tinker...

[Maxmin]

...must be laughing right now over another narrow escape.

I'd give it a new name ...

[Skapare]

... Comet Annealed.

Re:we are all doomed!

[Mateorabi]

Uh, ST-IV they used a captured Bird of Prey, not a Star Fleet vessel. So at least the Klingons could do it...if their scientists were thoughtful enough.

Warp factor 9

[madhi19]

Next stop San Fran to save the whale!

Re:icce melts

[arkane1234]

Everyone knows they're made of turtles.
Turtles, all the way down.

Re:That's not a comet.

[riverat1]

The exit video I saw was very close to the Sun. If the comet's moving moving faster than the solar wind can push the tail particles the tail will stay behind. At some point the comet slows enough for the tail to rotate to away from the Sun. That would be interesting to see.

Re:Awesome

[RockDoctor]

I just used it on some bird in the pub, and it worked!

That was the stuffed grouse in a box with the bottle of whisky.

How did you know it worked? Apart from waking up this morning with a smile on your lips and feathers round your mouth?

Re:we are all doomed!

[Coren22]

The problem as they put it in the movie was the calculations needed to make the time travel work. Spock had to do it in his head as the computer couldn't do it.

lovejoy!

[joleonard1]

My new favorite comet

Nasa.gov website unavailable in Russia

[pipy]

Strangely enough, I'm unable to reach neither http://nasa.gov/ nor http://science.nasa.gov/. Other web sites and services are working perfectly well. Anyone experiencing the same issue? I'm using Corbina/Beeline, one of the largest internet providers in Moscow...

Re:Nasa.gov website unavailable in Russia

[pipy]

Switching to Google Public DNS fixed the issue.

Strangely enough, I'm unable to reach neither http://nasa.gov/ nor http://science.nasa.gov/. Other web sites and services are working perfectly well. Anyone experiencing the same issue? I'm using Corbina/Beeline, one of the largest internet providers in Moscow...