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Old February 20 2013, 03:32 PM   #87
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Re: Russia reports amazing meteorite strike

Gary7 wrote: View Post
I can appreciate NASA's ability to create precise estimates on the distances and paths of celestial objects, but I'm mystified as to how they could possibly determine mass. Wouldn't they need to be able to penetrate the object with some kind of sensor to determine density and composition, and in multiple places throughout the object (because density and composition vary from spot to spot)? Not having a precise measurement of mass will skew calculations of gravitational effects, which ultimately affects the trajectory.
Generally it'd work the other way around -- if you observe how two objects interact gravitationally, how they affect each other's motion, you can calculate both their masses using simple Newtonian physics. And there are other ways. Taking a spectroscopic reading of an object can tell you something about its composition, and you can compare it to other objects that are better understood, all of which can help you estimate density.

In this case, we're talking about an object in a fairly circular orbit between Earth and Venus. The circularity tells us it isn't a recent capture that came in from further out; it's been in the inner system for a long time, long enough for its orbit to circularize. And that means it can't be an icy body, since any volatiles would've vaporized long ago. So it must be rocky or metallic. Spectral readings show it's a subclass of S-type, silicate, asteroid. We know the density of silicate rock, because, hey, it's what most of the Earth's crust is made of. And we've studied plenty of other similar asteroids.

And like I said, it's not like we're not going to keep watching the damn thing. If our current estimates of its mass are wrong, then further observations of its trajectory will diverge from our estimates and we will correct our estimates!

jmc247 wrote: View Post
You missed my point, one of these smaller ones that fly under the radar could strike a bigger one that we think will miss us and change its trajectory slightly to a trajectory that would not be conducive to the survival of the human race.
Maybe once every few million years, sure. You're talking about an immensely low-probability event. Even with all the debris out there, space is still huge and empty. That's why Earth gets hit by large objects so rarely even though there are so many of them out there.

Not to mention that you're talking about a hell of a sweet spot there -- hit by something large enough to change its course significantly within our lifetimes or that of our civilization, yet small enough not to shatter it? That makes it even more vanishingly improbable. Your odds of being struck by lightning are considerably higher. Hell, your odds of dying in a traffic accident are immensely higher.

That is why developing new tech to track these little guys along with better computers to look at the course of all the space rocks in our solar system matters in the coming decades.
Exactly. We've already found this one, and we're not going to stop tracking it. And the events of this past week are already goading politicians to pay more attention to the need for better spacewatch systems, so hopefully they'll finally get more funding.

Given we don't have anti matter weapons yet or anything that can explode in the 200 gigaton range its a bit speculative as to if the pieces left would be small enough to wipe out life on Earth...
Nonsense. The whole reason the laws of physics exist is to let us extrapolate the outcomes of events that have not been directly observed. We know the conversion rate of matter to energy -- it's E=mc^2. We've synthesized and observed small quantities of antimatter for decades and we can extrapolate upward. We can calculate how much energy it would take to vaporize an object of a given composition. We can calculate how much energy would be imparted to the Earth by an impactor of a given mass and velocity. All of this is quite easily done with a little physics. That's what it's for!

Of course, there is a margin for uncertainty, a range of error, so there's always the possibility that an object wouldn't be completely vaporized -- which is why the far saner and more practical approach is to deflect rather than destroy. And that's why we should put our effort into detection first and foremost, to ensure we have plenty of advance warning, rather than taking our asteroid-defense philosophy from Michael Bay.

Even if it doesn't work all that well I would rather let the atmosphere work its thing on tens of thousands of pieces of rock instead of dealing with just one half mile wide rock.
That's because you're not considering the physics of it. Remember that huge, 500-kiloton explosion over Chelyabinsk five days ago? That wasn't the meteoroid that exploded -- it was the air in front of it. As the impactor broke up in the atmosphere and was decelerated, it transferred its immense kinetic energy to the atmosphere in front of it in the form of immense heat, resulting in a fireball nearly 30 times more powerful than the Hiroshima bomb and a shock wave that caused serious damage to the city below and injured over a thousand people. Now imagine that happening tens of thousands of times all over the Earth within the span of a few minutes or hours. All that heat getting dumped into the atmosphere all at once, the equivalent of thousands of nuclear bombs detonating in the air all over the world.

Mass and energy can't be destroyed, only redistributed. So if the mass of that asteroid hits the Earth, it doesn't matter much whether it does so in one single clump or in thousands of separate pieces. You're still imparting the same amount of total kinetic energy to the Earth. Spreading it out over space and time might change the way it does damage, but it wouldn't have that much effect on the total amount of damage. It would still be a planetary catastrophe either way.
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