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A question for you physics experts on the oort cloud and comets


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A friend of mine wants to know the answer to two questions regarding a book he is wants to write..

1. How long would a small rock say 2m in diameter in the Oort cloud take too come in through the solar system and reach the orbit of the earth. Assume it gets a shove that starts it out at walking pace and then it gradually speeds up under gravity.

2. Would it be possible for the blast wave from a relatively near supernova to be powerful enough to provide the above shove to such a rock in the Oort cloud...

I told him there will be someone on SGL who can answer the questions so can you? :cool:

Yours hopefully

Mark

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1) Must depend on the direction of the initial shove, if directly towards the sun then "quick", if at a tangent then somewhat slower. The first could be approximated to by simple newtonian mechanics.

2) Whats close? 10 lightyears, 100 lightyears both close in astronomic terms, which sun, again different sizes. Another assumption is that the supernova explosion is spherical, which looking at hthe variouis planetary nebula is that common.

If your friend is writing this book shouldn't they be doing the research and getting the answers?

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The light from a supernova would reach us a long long long time before any blast wave would, look at the crab nebula it was the supernova of 1066 when Harold got it in the eye.

A more plausible scenario is that a very long period comet returns after a vacation in the outer reaches and smashes into the Oort cloud or asteroid belt dislodging rocks in all directions, may be I should write my own book. :evil:

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a supernova can't cause a blast wave to reach us as a blast wave is an airborne pressure wave and the interstellar space is too hard a vacuum

To support pressure waves.

It can if course cause radiation pressure effects and within its immediate environment can cause some physical direct pressure effects.

Regards

Rob

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a supernova can't cause a blast wave to reach us as a blast wave is an airborne pressure wave and the interstellar space is too hard a vacuum

To support pressure waves.

A supernova can cause a blast wave, it typically emits a vast amount of material into the interstellar medium, causing heating and an expanding shock ring. A star of perhaps 12 solar masses, may end up as a neutron star of 1 solar mass, so 11 solar masses of gas and dust will be blown out into a shock wave. So it makes its own media to carry the shock wave. It does also heat up any interstellar gas it meets on the way too. Hence images like the crab nebula.

250px-Crab_Nebula.jpg

Of course it takes a while for it to reach other star systems.

It can if course cause radiation pressure effects and within its immediate environment can cause some physical direct pressure effects.

This is true too - a vast amount of energy given off as radiation. Its hypothesised that a lot of radiation is emitted as neutrinos, which even though they hardly interact there are so many of them that they are partly responsible for lifting the explosive shell of material. Nothing about a supernova is small :)

I haven't read it, but have heard good things about http://www.amazon.co...d/dp/0670019976 which might be a useful book for your friend.

The other thing that is thought to dislodge comets are close (relatively speaking!) encounters with other stars. The Oort cloud is a long way out, so a little tug from a passing star is enough to start things tumbling in sometimes.

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My point was that what people know as a blast wave from say conventional high explosives is completely different to the effects seen around supernovae at distances beyond a couple of light years.

As I said there is no medium to carry the supersonic congressional wave only the expanding shell of gas itself. The shell can cause direct pressure effects but these are not blast waves but material flow.

Radiation pressure is what does most of the heating, acceleration and compression effects beyond the immediate environment - teachers just use the phrase blast wave to encourage interest.

Regards

Rob

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Thanks for all the responses. I think you are saying that if a star exploded say <100> light years away the shell and radiation pressure could knock a load of 2m rocks out of the Oort cloud and they would shower on to the earth after approximately <1000/100/10> years.

But is it possible to do the maths to see if it would be about 1000/100/10 years for the objects to reach the earths orbit? If it got a shove directly towards earth at say 2mph?

Mark

P.S Yes my friend who is writing the book was doing his research by asking me to ask around.!

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