Physics/Astronomy Question

[N Levers]

I pose two questions for budding Physicists

Please note in the following questions I am not asking for the probability of a comet hitting the earth.

1. Given an asteroid/comet that is on a collision course with the Earth what is the probability of it hitting the moon and either being destroyed by the moon or deflected away?

2. Given the above scenario what is the probability of the asteroid/comet causing the moon to leave its orbit and start on a collision course towards the Earth?

 

[DaveNewcastle]

I think we'd need a little data, such as mass, velocity, size and vectors of the movement of the 3 bodies, before any probabilities could be calculated.

Both probabilities are likely to be extremely low values. In fact I'd guess that the conditions necessary for either outcome to be true would be hard to determine.

 

[Eagle]

If the comet were to hit the Moon you wouldn't really describe it as "on a collision course with Earth". The path can be determined with an accuracy of tens of kilometres. It's astronomically unlikely that the Moon would be exactly between the comet and the Earth (or even that the Moon's orbit would).

No comet could cause the Moon to stop orbiting the Earth. You'd need something the size of a small planet to do that.

 

[strange6]

I pose two questions for budding Physicists

Please note in the following questions I am not asking for the probability of a comet hitting the earth.

1. Given an asteroid/comet that is on a collision course with the Earth what is the probability of it hitting the moon and either being destroyed by the moon or deflected away?

2. Given the above scenario what is the probability of the asteroid/comet causing the moon to leave its orbit and start on a collision course towards the Earth?

1: Very, very small. If it hit the moon, it would not hit the earth.

2: You would need an asteroid of at least 2/3 the size of the moon to make it leave it's orbit by a fraction. And to make it happen, it would need to be travelling faster than its terminal velocity, which is impossible knowing our present day understanding of Newtonian mechanics


Don't worry!

 

[tony_mac]

Here is my very rough calculations, which may be complete nonsense!

Assume a sphere of radius 380,000 km (moon's average distance from earth) - any object reaching earth must pass into this sphere, which has a surface area of 1.814582384*10^12 km^2.
Consider the moon as a circle on the surface of this sphere, with a radius of 1,735km - hence a cross-section of 9.456892758*10^6 km^2.

So, a probability of a random point (when it crosses the sphere) hitting that cross section of about 1 in 190,000.

(Ignoring very many things like size of comet, velocity, gravity and the planar tendency of solar-system etc. etc.)

would need to be travelling faster than its terminal velocity

I don't understand how this would be relevant; is this a meaning of terminal velocity that I don't know? (http://en.wikipedia.org/wiki/Terminal_velocity)

 

[strange6]

Here is my very rough calculations, which may be complete nonsense!

Assume a sphere of radius 380,000 km (moon's average distance from earth) - any object reaching earth must pass into this sphere, which has a surface area of 1.814582384*10^12 km^2.
Consider the moon as a circle on the surface of this sphere, with a radius of 1,735km - hence a cross-section of 9.456892758*10^6 km^2.

So, a probability of a random point (when it crosses the sphere) hitting that cross section of about 1 in 190,000.

(Ignoring very many things like size of comet, velocity, gravity and the planar tendency of solar-system etc. etc.)


I don't understand how this would be relevant; is this a meaning of terminal velocity that I don't know? (http://en.wikipedia.org/wiki/Terminal_velocity)

Sorry, I meant at the speed of light. Just come back from the pub

 

[Xenophon PCDGS]

I think we'd need a little data, such as mass, velocity, size and vectors of the movement of the 3 bodies, before any probabilities could be calculated.

This point that you have made is similar a point raised on the "spoof" thread, Delay Attribution Board, recently posted on the NR General Discussion Forum, which referred to the mass, forces and velocity of a peacock :roll:

 

[bb21]

So, a probability of a random point (when it crosses the sphere) hitting that cross section of about 1 in 190,000.

10^-5 is actually a quite high probability.

 

[ralphchadkirk]

Given that the probability of being stuck by lighting is 1 in 600,000 I find it hard to believe that the probability of a comet hitting the moon is more than double that of lightening.


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- Sent from my iPhone 4 using Tapatalk

 

[Greenback]

Given that the probability of being stuck by lighting is 1 in 600,000 I find it hard to believe that the probability of a comet hitting the moon is more than double that of lightening.


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- Sent from my iPhone 4 using Tapatalk

So do I. The moon and the earth are both in 3D space, which must complicate the calculations!

 

[bb21]

You know what they say - Garbage In, Garbage Out.

Not criticising tony_mac's method. Just pointing out that we have absolutely no meaningful information for this calculation, so any work done on top is totally and utterly meaningless.

 

[Greenback]

I think tony_mac has done a good job of trying to make mathematical sense out of a very open question.

I think very few of us would disagree that it is impossible to provide any sort of answer given that there so many variables involved.

 

[jrhilton]

I pose two questions for budding Physicists

Please note in the following questions I am not asking for the probability of a comet hitting the earth.

1. Given an asteroid/comet that is on a collision course with the Earth what is the probability of it hitting the moon and either being destroyed by the moon or deflected away?

You would need to take into account all the paths it could take that do or don't cross the moon’s orbit and then factor in time for when the moon may be there (if it even is).

Just for hitting the moon you have something crazy like:

Probability of asteroid crossing the path of the moon's orbit for each impact point on earth (which is a function of the asteroid size and asteroid speed and the orbit/path it has and the size and orbit of the moon and time of year) X chance the moon is actually at the cross over point (which is a function of the asteroid's speed, astroid orbit path and moon's size, orbit and time of year) = answer.

Bonus marks would have to be awarded to candidates taking in account the moon’s slight outward velocity casing its orbit to slow and position to change.

There are probably countless other factors too! Direction and size are the two big unknowns here and would probably be the biggest influencing factors. The problem is for every point on Earth there are at any one point in time a huge number of paths the asteroid could take to avoid the moon depending on its size.

For a 1km wide one with all the variables it must be billions to one.