We were lucky today

[mooncliff]

A 1/4 mile asteroid passed fairly near Earth today. It was not detected until just three days ago.
An asteroid would not have to actually be on a collision course to cause big trouble. The target is actually much larger: gravitational capture would send it into a chaotic orbit that would eventually impact.
There are thousands of these objects.

http://www.universetoday.com/95815/b...earth-june-14/

[c1ue]

Re: We were lucky today

The object is so bright, the Slooh Observatory will attempt to have a live webcast showing the object sneaking past Earth at about 5.3 million km (3.35 million) miles away, or about 14 times the distance between Earth and the Moon.

3.35 million miles away? Doesn't seem so close to me.

And at under 500 meters - gravitational capture is hardly an issue. The moon is 1737 kilometers in radius, or slightly under 3,500 times larger in diameter and more than 12 million times heavier - even assuming identical specific weights.

[mooncliff]

Re: We were lucky today

No, there was no threat this time, but now it's orbit has been changed. There are many of them. Many more than were expected, and in spite of the automated tracking, it was detected only 3 days in advance, which would mean that in a similar situation in which we actually were hit, we would be unable to do anything about it, not even evacuate.

If hale bopp had been going a meter per second slower, it would have hit the north pacific, and we would not be having this discussion.

If the Tunguska impact or had arrived eight hours later, it would have exploded over europe, not over a sparsely populated part of Siberia.

Comets and asteroids are often gravitationally captured and eventually wind up impacting. Moons of mars, comet shoemaker levy. Comet fragments from shoemaker levy, when they hit Jupiter, produced explosions bigger than earth.

[lektrode]

Re: We were lucky today

We were lucky today...
A 1/4 mile asteroid passed fairly near Earth today. It was not detected until just three days ago.

and heres a thurs(ty) nite toast to being(gittin) lucky!

methinks we'll soon regret shutting down the last shuttle (and turning it into a floating museum)

[mooncliff]

Re: We were lucky today

One promising scheme is an armadas of small laser satellites that would shoot many small pulses, vaporizing the surface and producing small amounts if thrust.

But three days would give us no time.

This is going to happen again eventually. It has been a century since Tunguska, and something like that seems to have happened every couple of hundred years.

A solar flare like the carrington event would do much more damage and kill many more people because we would lose electricity for months or years.

Damn low frequency high impact events!

[c1ue]

Re: We were lucky today

No, there was no threat this time, but now it's orbit has been changed. There are many of them. Many more than were expected, and in spite of the automated tracking, it was detected only 3 days in advance, which would mean that in a similar situation in which we actually were hit, we would be unable to do anything about it, not even evacuate.

Perhaps the reason it wasn't detected was because it is so far away.

If hale bopp had been going a meter per second slower, it would have hit the north pacific, and we would not be having this discussion.

Hypotheticals are all fine and good, but I'm confused how you can assert a 1 m/s speed would have made any difference when the actual speed of the comet is not even known - only estimated, and changes continuously as it got closer then further away from the sun. You're also assuming that all changes would have been in favor of said comet getting closer to the Earth when in reality most random changes would be away from the earth.

If the Tunguska impact or had arrived eight hours later, it would have exploded over europe, not over a sparsely populated part of Siberia.

Actually you're assuming a lot more than that. You don't know (because no one does) what the actual angle of the Tunguska meteor was, nor its size, mass, etc etc.

Comets and asteroids are often gravitationally captured and eventually wind up impacting. Moons of mars, comet shoemaker levy. Comet fragments from shoemaker levy, when they hit Jupiter, produced explosions bigger than earth.

Jupiter is more than 300 times the mass of earth, with 10 times the radius. This means it has a hugely greater volume in which Jupiter's gravity sweeps - and more importantly this volume moves around the solar system and clears out a significant amount of moving objects that might affect the Earth. Effectively between Jupiter and the Sun, any object with any significant pass through time in the ecliptic plane of the solar system is likely to have long since been swept up and destroyed.

Of course most of the solar system's volume is not along the ecliptic, but the Earth's orbit is in turn a very, very tiny portion of the ecliptic. It would be like hitting the bullseye of a bullseye of a bullseye - were the Earth's orbit to be expressed as a function of the overall ecliptic in the form of concentric rings.

Pluto is 5.9 billion km from the sun, Jupiter is 778 million km, Mars is 228 million km, Venus is 58 million km and the earth is 150 million km.

If we take the entire area between Mars and Venus, it is only 8% of the area inside Jupiter, and 0.14% of the area inside Pluto.

As for the moons of Mars, well, Mars is the closest planet besides Jupiter, and the asteroid belt lies mostly between the two. It isn't surprising that the breakup of the actual 5th planet resulted in some objects being captured by the nearest planets.

Lastly a look at the relative sizes and placements of the solar system:



You can see that the huge relative sizes of the Sun and Jupiter, over millions of years, blocks off very large portions of possible collision trajectories.

Add that to the Earth's atmosphere - and the wonder is that we see any events at all in recorded human history.

I've noted elsewhere that the estimated chance of a dinosaur killer hitting earth is 0.0001% or 1 in 100,000. If anything, we'd be lucky to be hit!