Comment author: turchin 05 October 2013 06:01:35PM 0 points [-]

I think that the comet would desintegrate quicker than with speed of sound, and more like with the speed of incoming gases that is 600 km/sec, so it would be destroed in less than 1 second. Comet is not asteroid - it is very fragile and it will start to desinegrate even before the impact bacuse of gravitational forces - it will be inside Roshe limit of Sun and tidal forses will start to elongate it.

Comment author: Adam_Crowl 06 October 2013 02:16:05AM 0 points [-]

The mass of whatever the impactor is has to be brought to a halt and for a 100 km chunk massing as much as you've computed, that's not happening in an instant in the photosphere of the Sun. Computing exactly how long is a non trivial task and at that speed is more like the injection of a supersonic fluid through a much lower density medium since the internal strength, heat capacity and even the ionization energy of the object is trivial against the kinetic energy it is dissipating.

Comment author: turchin 05 September 2013 07:46:59PM 0 points [-]

It is important in my idea that it is a comet - that is large chunk of not connected ice, not a hard rock. As we know from examples of Tunguska event 1908 and Chelabinsk event this year, such bodieas tend to desintegrate on high altitude in large explosion because they quickly fall apart. Chelabinsk flash video. https://www.youtube.com/watch?v=OPSzpnHHwos

Estmation of the energy is based on the speed of imact, that is 600 000 meters per second (second cosmic speed on the Sun surface), mass of the object - that is 10++18 kg (based on water density and size of a cube with 100 km rib) and formula for kinetic energy, that gives us enegry of impact 3.6x10xx29 J.

Energy output the Sun is 10x26 J per second. So, total energy of impact would be 3600 times more than Sun's output. Not all energy will go in radiation so 1000 times seems to be good estimate.

In fact I started from the question «What is the size of the body, which could cause harm to the Earth if it fall on Sun?" And find that 1 km will not be even visable, but 100 km is dangerous.

Now I need estimation of the frequency of such impacts.

Comment author: Adam_Crowl 05 October 2013 01:16:08PM 1 point [-]

First correction, the Sun's luminosity is ~3.827E+26 W, so the falling ice-cube would have a kinetic energy of ~1,000 seconds of solar output. However an object falling into the Sun - such as a 100 km ice-cube - would only release its kinetic energy in such a burst if it was brought to a sudden halt. At 600 km/s the object is a solid surface moving through a fairly diffuse gas - the outer layers of the Sun are thin, hot plasma. A good estimate of the braking effect would be Newtonian Flat Plate drag - i.e. the stuff of the Sun immediately in front of the object is ramming into it, causing drag, with essentially no flow around the object. At 600 km/s the dynamic pressure on the front face is 360 GPa times the plasma density - enough to decelerate the ice-cube (with 100,000 tonnes per square metre areal density) at 3600 m/s^2 if the plasma density was just ~1 kg/m^3. Of course the object won't decelerate until the gas drag is greater than the Sun's gravity - equivalent to ~28 GPa pressure on the front face, which is achieved about 4,000 km below the Photosphere. The dynamic pressures would obliterate the mass eventually, but sound in ice only travels at ~3 km/s, so the main mass should travel for about ~30 seconds before starting to fragment as pressure waves travel from the front to the back of the 100 km block. But if it ablates, then it may travel some distance after that. Seems likely to be a rather protracted process, which might produce a flash, but the only hazard to Earth would be if it was in the line of sight of the impact, I suspect.

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