Bodies hitting Sol

[Kitsune]

I recieved some material from a person who wants me to post his materials.
In the project, he has an asteroid the size of a moon hitting the local star and causing a disaster in a storyline. As far as I understand, an asteroid the size of Luna would do absolutely nothing to the sun and would simply be absorbed. Am I correct and how large an object could the sun be hit by without causing disruption in the solar system? Could it be hit by Jupiter, for example?

[White Haven]

...and here I was picturing mass executions in some bizarre, massively-wasteful process. My limited knowledge of astrophysics says that the effects would be minimal at best, but I'd suggest hitting some more expert parties up before you take my word on that.

[Kuroneko]

Perhaps so, perhaps not; your question is not specific enough. An object as massive as Luna falling into Sol from rest at infinity would have as much energy as the Sun puts out in fourteen months. It is easy to see that such an event would be utterly catastrophic. On the other hand, a more placid (and plausible) trajectory might, at the most, make Sol a little brighter, in that breaking the cooler outer layers would give the hotter interior some exposure. As for Jupiter, it is massive enough to guarantee a major disturbance no matter what the specifics of the trajectory are, but theoretically it could still be gentle enough not to be devastating. The short answer to your question is 'it depends on the trajectory'.

[Guy N. Cognito]

Perhaps so, perhaps not; your question is not specific enough. An object as massive as Luna falling into Sol from rest at infinity would have as much energy as the Sun puts out in fourteen months. It is easy to see that such an event would be utterly catastrophic. On the other hand, a more placid (and plausible) trajectory might, at the most, make Sol a little brighter, in that breaking the cooler outer layers would give the hotter interior some exposure. As for Jupiter, it is massive enough to guarantee a major disturbance no matter what the specifics of the trajectory are, but theoretically it could still be gentle enough not to be devastating. The short answer to your question is 'it depends on the trajectory'.

Are you talking about momentum or light and heat? They are different. If a moon sized object hit the sun, then it would become part of the sun and that is something I'd have to look at regarding the effect of the cool body hitting the sun. As for momentum, that is strictly mass and velocity vs mass and velocity. And seeing as the sun is 'soft' and freaking huge in comparison I don't think there would be any real shift in the sun's orbit. We would just have a moon sized body enter the sun and get heated up.

[GrandMasterTerwynn]

I recieved some material from a person who wants me to post his materials.
In the project, he has an asteroid the size of a moon hitting the local star and causing a disaster in a storyline. As far as I understand, an asteroid the size of Luna would do absolutely nothing to the sun and would simply be absorbed. Am I correct and how large an object could the sun be hit by without causing disruption in the solar system? Could it be hit by Jupiter, for example?

If it fell in on a natural trajectory, then the damage it would do would be entirely minimal. Sure you might get a bright spot on the surface of the star, and the star's heavy metal abundance would increase by a small fraction, but there won't be enough to cause a disaster.

If the asteroid fell into the star at 90% the speed of light, then it might have a considerable effect, given that (taking a naive approach) you'd be hitting the star with some tens of Death Star blasts all at once. But this effect will likely be the triggering of a giant flare at the point the asteroid went in. Of course, we'll ignore how one gets a Moon-sized asteroid up to 90% the speed of light.

If a Jupiter-sized body fell into the star, then it's passage through the inner system alone would cause a major catastrophe. Depending on where the inner planets were with respect to this Jupiter-sized body, their orbits will either be changed into more elliptical orbits, or they'll be sent into the star, or ejected from the system outright.

[Duckie]

On the note of catastrophic sun events and the Death Star, didn't somebody say what would happen if you fired the Death Star Superlaser into the Sol-like sun of an Earth-like planet? I forget exactly what happens and can't think of a keyword to search the archives for. Does anyone remember?

As has been said by everyone else before, on the subject of Luna hitting Sol, it all depends on velocity. Someone once said "At .99c, it doesn't matter whether a spitball or a mile-wide rock hits the planet." [<-- Misquoted, but I got the idea.]

With Jupiter, it's a no win since fast it screws the Sun over and slow it flings planets around while it stays close to their orbits.

[GrandMasterTerwynn]

With Jupiter, it's a no win since fast it screws the Sun over and slow it flings planets around while it stays close to their orbits.

No, not quite. Jupiter spiraling into the Sun would make it about a tenth of a percent more massive, causing the planets to orbit slightly closer, and its overall luminosity to increase by a very, very small fraction. The destructive effects would come from having a Jupiter-sized mass (and the accompanying gravitational field) careening through the inner system on its way to its doom.

[Crossroads Inc.]

Yea no joke, We all have to remember that Big as Jupiter is, it's 96% GAS. So, Imagine something it's size that was SOLID metal and rock.

[Kitsune]

This is the storyline:

Then a great tragedy befell them; one of their long-range telescopes on the outermost planet in their system had detected a huge chunk of cosmic debris heading towards their system. At first it was no cause for alarm, merely a scientific curiosity, but as more accurate information was developed, a chilling discovery was made. On its current course, this huge asteroid, easily the size of a small moon would enter their system in only 20 years! Scientists argued the effect this would have on their system, but perhaps the most telling blow came when after 2 years of constant monitoring, and refining the course of the asteroid, it was shown that no matter what else it did, life in their solar system would be destroyed. For in the end it came down to the simple fact that it would impact with their sun in approximately 18 years. The impact that such a huge asteroid would have on the sun would be devastating, and while there was some argument over the exact effect, no one could doubt that it spelled the end of the Moraii as a race. At first this news was viewed with disbelief by the population, then wide spread panic. But eventually, the Moraii came to view this as the penultimate challenge, and threw all their efforts into meeting it.

I don't think from the description that it is travelling at a significant fraction of light.

[Zero]

Yes, if they have 18 years, it's likely traveling much slower. That doesn't make any sense. But if he still wants it to, he can have it be a neutron star or something. That'd fuck things up... a LOT... actually, I aint sure on this, but wouldn't that actually create a supernova?

[GrandMasterTerwynn]

Yes, if they have 18 years, it's likely traveling much slower. That doesn't make any sense. But if he still wants it to, he can have it be a neutron star or something. That'd fuck things up... a LOT... actually, I aint sure on this, but wouldn't that actually create a supernova?

If you had a neutron star pass through your system, then it would be game-over for every other body in the system, as it would be like a solar sized mass passing through. Every planet would be ejected. If the neutron star collided with a Sunlike star, then the result would be a plain nova, not a supernova, and more likely than not, several novas, since the neutron star would pass unimpeded through the Sun and enter into a close orbit, siphoning hydrogen off the Sun into an accretion disc . . . until enough hydrogen had been siphoned off to create a powerful nuclear explosion as the hydrogen on the surface of the neutron star gets sufficiently hot and dense to undergo fusion.

[GrandMasterTerwynn]

This is the storyline:

Then a great tragedy befell them; one of their long-range telescopes on the outermost planet in their system had detected a huge chunk of cosmic debris heading towards their system. At first it was no cause for alarm, merely a scientific curiosity, but as more accurate information was developed, a chilling discovery was made. On its current course, this huge asteroid, easily the size of a small moon would enter their system in only 20 years! Scientists argued the effect this would have on their system, but perhaps the most telling blow came when after 2 years of constant monitoring, and refining the course of the asteroid, it was shown that no matter what else it did, life in their solar system would be destroyed. For in the end it came down to the simple fact that it would impact with their sun in approximately 18 years. The impact that such a huge asteroid would have on the sun would be devastating, and while there was some argument over the exact effect, no one could doubt that it spelled the end of the Moraii as a race. At first this news was viewed with disbelief by the population, then wide spread panic. But eventually, the Moraii came to view this as the penultimate challenge, and threw all their efforts into meeting it.

I don't think from the description that it is travelling at a significant fraction of light.

If the "small moon" impacted the sun, then, again, the results would be minimal, at best. Unless the Moraii sun happened to be a red dwarf (as a red dwarf need only be somewhere between a tenth of Sol's mass to a third of Sol's mass. The result being that a habitable planet would orbit well inside the orbit of Mercury . . . close enough to tide-lock, in fact,) and the interloper impacted in such a way as to generate a superflare that was aimed right at the planet. (Since most red dwarves are flare stars which can produce immense flares which boost the star's overall brightness by up to an order of magnitude for a short duration.) The problem with this theory is that any habitable planet of a red dwarf would have to have a thick atmosphere which would likely serve to attenuate the effects of such superflares.

[Kuroneko]

Are you talking about momentum or light and heat? They are different. ... As for momentum, that is strictly mass and velocity vs mass and velocity.

Yes, I'm aware of what momentum is. That has nothing to do with my statements, which were regarding kinetic energy only.

If a moon sized object hit the sun, then it would become part of the sun and that is something I'd have to look at regarding the effect of the cool body hitting the sun. And seeing as the sun is 'soft' and freaking huge in comparison I don't think there would be any real shift in the sun's orbit. We would just have a moon sized body enter the sun and get heated up.

Of course the Sun's orbit won't be affected in any non-negligble manner (about a hundrendth of a percent of its orbital velocity about the galactic center). That's not the point. For a Moon-sized object falling from rest from infinity, the collision velocity would be about 6.176e5m/s (given the ~1/r drop of gravitational potential, even 1AU initial distance would be fairly close to this, however). The kinetic energy of such an object would 1.402e34J, surpassing the Sun's yearly output. Now, it is hard to say how much of that would be released near the Sun's surface (photosphere), but with an energy of roughly a billion large solar flares, a fraction as small as 0.01% would be very signficant indeed. I would expect more than that, really--compression of the plasma would be immense when encountering hypersonic object of such a size, leading to comparable increase in absolute temperature. Stefan and Boltzmann will take care of the rest.

Given an unfortunate trajectory, a Moon-sized body appears to have the potential to broil an aspiring civilization.

[Guy N. Cognito]

Of course the Sun's orbit won't be affected in any non-negligble manner (about a hundrendth of a percent of its orbital velocity about the galactic center). That's not the point. For a Moon-sized object falling from rest from infinity, the collision velocity would be about 6.176e5m/s (given the ~1/r drop of gravitational potential, even 1AU initial distance would be fairly close to this, however). The kinetic energy of such an object would 1.402e34J, surpassing the Sun's yearly output. Now, it is hard to say how much of that would be released near the Sun's surface (photosphere), but with an energy of roughly a billion large solar flares, a fraction as small as 0.01% would be very signficant indeed. I would expect more than that, really--compression of the plasma would be immense when encountering hypersonic object of such a size, leading to comparable increase in absolute temperature. Stefan and Boltzmann will take care of the rest.

Given an unfortunate trajectory, a Moon-sized body appears to have the potential to broil an aspiring civilization.

Yes, but realistically, how would a moon sized object get accelerated to nearly the speed of light? And as his synopsis points out, it is very doubful that the moon sized object would have any real effect on a Sol sized object. When given a question I always factor out the near impossible until I get better details. And if any one says the Death Star was a small moon sized object and travelled fast, quiet!. So as GrandMasterTerwynn pointed out, it would have to be a realitivelt smlall object. And as an after thought, how much more would a moon sized object mass travelling at near the speed of light? Damn, where are my physics books?

[Kuroneko]

Yes, but realistically, how would a moon sized object get accelerated to nearly the speed of light?

What speed of light? The speeds mentioned in my posts are less than 0.21% lightspeed, achieving which is rather easy for a radially infalling object. In fact, since this is the escape velocity for the Sun, any object initially at rest 'far away' will achieve this speed due to the Sun's gravitational field alone.

And as [GrandMasterTerwynn's?] synopsis points out, it is very doubful that the moon sized object would have any real effect on a Sol sized object.

On the long term, that's completely correct--Sol's thermal energy is on the order of 1e40-1e42J, so it is not likely to care at all. However, that does not mean there won't be an extreme superflare at the moment of impact. Assuming the object has mass m = 7.3477e22kg and Sol M = 1.9891e30kg, with collision velocity v = 6.1754m/s and kinetic energy k = 1.4010e34J. The situation described is a perfectly inelastic collision. Momentum should be conserved, so in Sol's rest frame at the time of impact, Sol obtains velocity mv/M = 2.2812e-2m/s, corresponding to a kinetic energy of K = 1.0351e27J, meaning that virtually all of the impactor's kinetic energy would be transferred into light and heat.

Again, it is not clear what fraction of this will be released in the initial 'superflare' and how much of it will go deeper into the Sun (which won't do all that much, being many orders of magnitude away from Sol's total thermal energy), but even 0.01% of this energy would be analogous to around a hundred thousand very large solar flares occuring simultaneously (and over a shorter timeframe), the radiation from which, both electromagnetic and particle, would be very problematic for the biosphere of a nearby planet caught in an unfortunate position (due to the impact energies involed, particle radiation is much more likely to be highly relativistic than that caused by normal flares, and thus would have a correspondingly much greater penetration into the magnetosphere). Since the object is so large and hypersonic, this is likely to be a severe underestimate--the compression of the gas around the impactor will be extreme, leading to tremendous heating of the surrounding matter, not to mention the impactor is not likely to stay in one piece throughout this ordeal, which would lead to greater energy released near the surface.

Note: 'Very large' events are taken to be on the order of 1e25J, which I hope is fairly accurate.

And as his synopsis points out, it is very doubful that the moon sized object would have any real effect on a Sol sized object. When given a question I always factor out the near impossible until I get better details.

This is not 'impossible'. Unlikely, yes--I've acknowledged that this scenario is implausible from the beginning--but not impossible. It violates no laws of physics that spring to mind, and involves a perfectly ordinary star and a perfectly ordinary Moon-sized object undergoing gravitational acceleration.

And as an after thought, how much more would a moon sized object mass travelling at near the speed of light?

How much more what?

[Guy N. Cognito]

Sorry I was having a brain fart day yesterday. I really should have read the numbers closer. And I was trying to say how much more would a moon sized object mass. Agreed the superflare would be the biggest issue. Unfortunately, I don't have much in the way of knowledge when it comes to the properties of the sun and solar flares. I guess the resulting issue comes down to, where on the sun relative to the planet in question is the moon sized object hitting? And how much radiation would be generated from a super flare would effect the planet. In other words, is it's magnetic field strong enough to deflect the majority of the radiation. Im not sure how strong our field is in relation to other planets that sustain life, but I imagine it wouldn't stand up to the blast too well. But seriously, how bad would it be? Worst case scenario, half of the planet gets a really bad day. That wouldn't destroy all life in the system, just cause some issues.

[GrandMasterTerwynn]

Sorry I was having a brain fart day yesterday. I really should have read the numbers closer. And I was trying to say how much more would a moon sized object mass. Agreed the superflare would be the biggest issue. Unfortunately, I don't have much in the way of knowledge when it comes to the properties of the sun and solar flares. I guess the resulting issue comes down to, where on the sun relative to the planet in question is the moon sized object hitting? And how much radiation would be generated from a super flare would effect the planet. In other words, is it's magnetic field strong enough to deflect the majority of the radiation. Im not sure how strong our field is in relation to other planets that sustain life, but I imagine it wouldn't stand up to the blast too well. But seriously, how bad would it be? Worst case scenario, half of the planet gets a really bad day. That wouldn't destroy all life in the system, just cause some issues.

Yes, the absolute worst-case scenario would be one where an object the size of Earth's Moon (which is, as far as moons go, freakishly large. It is the fifth largest moon in the entire solar system, and the second-largest in terms of size relative to its parent planet. A 'small moon' will probably be much less than a tenth of our Moon's diameter.) fell into the star and impacted in such a way where the resulting flare was aimed straight at the planet. But even then, the flare would expand rapidly, and the planet would only absorb a relatively small fraction of the total energy release. Though the energy absorbed by the planet in the form of particle radiation is still going to be extremely high. At the very least, our imaginary civilization can kiss its entire space infrastructure goodbye, as well as its power grids (this is just based on what ordinary large-scale solar flares can do to the planet.) Such a tremendous blast will likely erode away the planet's ozone layer, resulting in a lot of long-term trouble for anything living on the surface of the planet.

These effects only get worse if the star is smaller and less luminous than our Sun, since a planet would have to orbit much closer, in order to be habitable, and will likely have a slower rotational period, due to tidal forces.

[Kuroneko]

And I was trying to say how much more would a moon sized object mass.

Oh, you're talking about relativistic mass dilation. The effect is wholly negligible; the velocity of the impactor may be have a Mach number is the high-80s, assuming typical photosphere speed of sound of about 7km/s [1], but compared to the speed of light, the relativistic mass dilation is still about two parts per trillion--far below the precision that the masses of astronomical objects can be measured.

Yes, the absolute worst-case scenario would be one where an object the size of Earth's Moon (which is, as far as moons go, freakishly large. It is the fifth largest moon in the entire solar system, and the second-largest in terms of size relative to its parent planet. A 'small moon' will probably be much less than a tenth of our Moon's diameter.)

True, but the OP question is on the effect of an "asteroid the size of Luna," i.e., that of the Earth's moon, and the effect in such a case is not really minimal. However, since Ms. Kitsune is unsure of what the impact paramaters, let's try obtain a lower bound by neglecting gravity after impact, and assume that the impactor is spherical and very hypersonic. With a mass m, radius r, and initial velocity v₀, the drag force is F = -kv², and since F = mdv/dt, it follows that -dv/v² = [k/m]dt; hence 1/v = 1/v₀ + kt/m; hence v = [v₀ m]/[m + ktv₀]. The amount of energy released into the environement is the change in kinetic energy, E = K₀[1-m²/(m+ktv₀)²], where K₀ = mv₀²/2. Unfortunately, it is very if ktv₀ is relatively small, so let's expand as a Maclaurin series in δ = ktv₀: E = K₀[2(δ/m) - 3(δ/m)² + 4(δ/m)³ - ... ], so that E = δv₀² for δ<<m.

Under hypersonic speeds, the drag is relatively well-behaved, with the base drag of the object negligible and the forebody drag dominating. For the sphere under hypersonic conditions, there is almost no variation over Reynolds numbers (fortunate!) with drag coefficient C = 0.92 [2]. Thus, k = ρAC/2 = Cπr²ρ/2 and δ = (Cπr²ρv₀)t/2. Assuming that the the impactor has the same parameters as Luna with radial infall, m = 7.3476e22kg, r = 1.7381m, v₀ = 6.1754e5m/s, it takes about t = 11s to completely submerge the impactor and t = 11s for the impactor's edge to reach 1.0% of Sol's radius. Further assuming that the average density in Sol's outer convective layer (only around 1% depth) is ρ as betwen 1.0e-2kg/m³ or 1.0e-1kg/m³ [4], the energy released becomes 1.1e29J < E < 1.1e30J, depending on which ρ is chosen.

Putting gravity back in does not affect the outcome much at all, despite the change in gravitational potential being several orders higher than this. The total force that the object experiences, assuming that the distance x from the stellar center is high enough to have the vast majority of the stellar mass to be within it, is F = μm/x² - kv². Since we are dealing of depths on the order of 1% of the stellar radius, that condition is true to far beyond measurement error (stellar mass is concentrated in the core, with roughly exponential dropoff in density beyond it). For Sol, μ = 1.3271e20m³/s², while the above treatment places k = 4.4e11kg/m, and so the first term dominates by a factor of about 120, and we more or less ignore it (it is just barely within our significant digits anyway). The gravitational potential at stellar radius x is then simply μm/x, making the squared velocity for a radially infalling object from rest at infinity v² = 2μ/x. To find how much energy is dissipated due to drag, we integrate the drag force over the distance: E = 2kμ Int{Rf,R}[ dx/x ] = -[2kμ]log(d), where d is the final depth as a fraction of stellar radius. For 1.0% depth, d = 0.99; hence 1.2e29 < E < 1.2e30J, depending on the ρ.

However, the situation is more complicated. While significant fusion is unlikely, it is trivial to verify that the gravitational binding energy of a Luna-sized impactor is about the same as the low-end figure above. Far from all of it will go into separating the impactor; nevertheless it would be incredible for it to stay in one piece. Thus, the entire picture of the Sol-Luna collision as the star smoothly absorbing the impactor is fatally flawed, and it is much more likely that much of the impactor's 1.4e34J kinetic energy will be delivered near the surface of the star.

For an unfortunately placed planet at 1AU distance assuming a typical Earthlike bond albedo of ~0.3, the average incident radiation would be up to 3.1cal/cm² for the 1.2e29J blast, which is about midway between the energy required to cause first and second degree burns for thermal radiation [5], while the 1.2e30J blast would completely a hemisphere in a gigantic conflagration. Having an exceptionally clear day would make this much worse, and the above discussion makes it likely that the collision would be on the order of 1e34J--at 2e5cal/cm², that would simply sterilize the land.

The above also completely ignores particle radiation, which would be very dangerous in its own right. I have not performed any calculutations regarding this, but I expect the magnetic field to be ineffective in stopping the sheer number and high energies of the particles involved. In any case, I consider that issue moot--the EM radiation alone would be apocalyptic.

[1] As I said previously, I have no idea of the speed of sound distribution in the Sun, a websearch revealed the following paper fairly quickly: Berger, Löfdahl, et. al., "Measurements of Solar Magnetic Element Motion from High-Resolution Filtergrams", The Astrophysical Journal, 495:973-983 (available on the web). It makes this claim.
[2] Hoerner, Sighard F., Fluid-Dynamic Drag, 1965ed: Ch. 16 Fig. 20 (p.16.16)
[3] Hoerner 18.20.
[4] If there is an astrophysist reading this, I'd appreciate this could be confirmed, corrected, or made more precise.
[5] U.S. Atomic Defence Agency, The Effects of Nuclear Weapons, 1962, 11.62 (p.571)

[The Dark]

In theory, it could cause problems if it somehow was generating a magnetic field and interfered with the field of the Moraii's star. That could cause all sorts of increases in radiation streaming from the star, since solar flares occur when magnetic lines snap naturally. Snapping large numbers of them simultaneously might well cause an increase in radiation sufficient to cause sterilization of the system.

Hey, it's really far-fetched, but I think it'd work.

[Kuroneko]

In theory, it could cause problems if it somehow was generating a magnetic field and interfered with the field of the Moraii's star. That could cause all sorts of increases in radiation streaming from the star, since solar flares occur when magnetic lines snap naturally. Snapping large numbers of them simultaneously might well cause an increase in radiation sufficient to cause sterilization of the system.

No. For astronomical bodies, magnetic fields are generated through geological activity, so any body with a large magnetic field would be itself large enough as an impactor. Furthermore, none of them can compete with the magnetic field (3-4 orders of magnitude stronger than Earth's) over an even larger surface areas of the star that are present for the typical sunposts that cause stellar flares. Above all, conservation of energy must hold: how is the object to generate internally 1e24+ joules of energy? The energy excess must come from somewhere, and it won't be given by the star unless it is already about to undergo such a process anyway (fusion does not occur anywhere near the surface).

[Kuroneko]

For an unfortunately placed planet at 1AU distance assuming a typical Earthlike bond albedo of ~0.3, the average incident radiation would be up to 3.1cal/cm² for the 1.2e29J blast, ... .

Ah, the perils of typing and calculating concurrently. The above intensity figures are too low by a factor of 2.3 (7.1cal/cm²), due to an incredibly inane mistake.