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View Full Version : How close can something get to the Earth and not impact?



utenzil
2010-Oct-28, 02:00 PM
We see a lot of asteroids whizzing by lately and so I wonder how close something can get without hitting the Earth, so that if it gets any closer it is pretty much a sure thing that it will be at the very least an atmospheric impactor.

I recall that anything less than about 60 feet across will tend to explode in the atmosphere, and anything bigger than that has a better chance of at least pieces hitting the surface.

To narrow the question a little, it seems likely that trajectory and orbit paths have something to do with this, making imminent impacts more likely at different distances depending on the path the object it taking, but it also seems like there would be a distance, despite trajectory, that will make an impact a foregone conclusion.

Also, when something enters the atmosphere, is that always the end of it, could it ever "bounce out and away" once it entered, and/or could it "spiral in", going around a few times before it got close enough to enter the atmosphere and ultimately hit the surface?

Hornblower
2010-Oct-28, 03:03 PM
We see a lot of asteroids whizzing by lately and so I wonder how close something can get without hitting the Earth, so that if it gets any closer it is pretty much a sure thing that it will be at the very least an atmospheric impactor.

I recall that anything less than about 60 feet across will tend to explode in the atmosphere, and anything bigger than that has a better chance of at least pieces hitting the surface.

To narrow the question a little, it seems likely that trajectory and orbit paths have something to do with this, making imminent impacts more likely at different distances depending on the path the object it taking, but it also seems like there would be a distance, despite trajectory, that will make an impact a foregone conclusion.

Also, when something enters the atmosphere, is that always the end of it, could it ever "bounce out and away" once it entered, and/or could it "spiral in", going around a few times before it got close enough to enter the atmosphere and ultimately hit the surface?

If I am not mistaken, the grazing fireball over Wyoming and Montana in the summer of 1972 came with 40 miles of the ground before skimming back out of the atmosphere and out into space. I have no means of estimating how much lower a really large one could go without breaking up and causing some sort of ground impact.

If the body makes a grazing entry at just the right height, it could be slowed enough to go back out in an eccentric elliptical orbit, and then lose some more velocity at the next perigee. In such a case its orbit would decay quickly. The tendency would be to circularize and then spiral in. The exact height of the subsequent perigee passes would be critical. Perturbations by the Sun and Moon could raise or lower those passes depending on the orientation of the orbit.

samkent
2010-Oct-28, 03:13 PM
I would expect it has to do with . . .
Speed
Size
Composition

In theory a very VERY large iron ball (moon) could pass within feet if it were going fast enough. But there is bound to be damage to us.

slang
2010-Oct-28, 03:37 PM
In theory a very VERY large iron ball (moon) could pass within feet if it were going fast enough. But there is bound to be damage to us.

M-O-O-N! That spells DUCK!

RAF_Blackace
2010-Oct-28, 07:42 PM
If the speed at ejection was high enough the object would never return to the Earth. In fact most atmosphere grazing objects do not return, only a very few that have the right amount of speed bled away by the atmosphere and end up in orbit are doomed to come back and burn away or crash. These are few and far between.

chornedsnorkack
2010-Oct-28, 08:09 PM
If I am not mistaken, the grazing fireball over Wyoming and Montana in the summer of 1972 came with 40 miles of the ground before skimming back out of the atmosphere and out into space. I have no means of estimating how much lower a really large one could go without breaking up and causing some sort of ground impact.

If the body makes a grazing entry at just the right height, it could be slowed enough to go back out in an eccentric elliptical orbit, and then lose some more velocity at the next perigee. In such a case its orbit would decay quickly. The tendency would be to circularize and then spiral in. The exact height of the subsequent perigee passes would be critical. Perturbations by the Sun and Moon could raise or lower those passes depending on the orientation of the orbit.

The 1972 bolide approached at about 15 km/s and lost about 875 m/s, so it left at only 3 km/s above escape speed. How much lower could it have flown to be captured by Earth?

korjik
2010-Oct-28, 08:29 PM
The 1972 bolide approached at about 15 km/s and lost about 875 m/s, so it left at only 3 km/s above escape speed. How much lower could it have flown to be captured by Earth?

If the object grazed so that it was travelling slower than escape velocity, it would have a apogee still within the atmosphere. That would bleed off energy every orbit until the object hit the Earth.

Assuming it dosent break up first

chornedsnorkack
2010-Oct-28, 09:17 PM
If the object grazed so that it was travelling slower than escape velocity, it would have a apogee still within the atmosphere. That would bleed off energy every orbit until the object hit the Earth.


No. A body with speed between first and second cosmic velocity has perigee in atmosphere, but not apogee.

Hornblower
2010-Oct-28, 10:34 PM
The 1972 bolide approached at about 15 km/s and lost about 875 m/s, so it left at only 3 km/s above escape speed. How much lower could it have flown to be captured by Earth?

I would guess just a few miles. The atmospheric density increases about tenfold for each ten miles of descent.

korjik
2010-Oct-29, 06:10 AM
No. A body with speed between first and second cosmic velocity has perigee in atmosphere, but not apogee.

nuts. I always get those backwards :(

astromark
2010-Oct-29, 06:43 AM
Answering the question with a yes... and then adding that it is calculable.

Back there 'Samkent' said. Speed, Size, Composition.

Neutrino's do it all the time and they avoid the collision aspect by being sub atomic and just keep right on going.....

With enough velocity a atmosphere entering object can bounce away.

This is a mathmatics question. Given the right angle and composition and a balance of mass and density. Yes.

utenzil
2010-Oct-29, 06:58 PM
Thanks for all the input, this is very interesting. So speed has a lot to do with it, and atmospheric drag.

The reason I'm wondering is that it seems like a lot of the asteroids that have been discovered lately are a) mostly on the smaller side and b) only discovered a few days before they are at their very closest. That is sensible of course because it is most likely you'll see something small is when it is closer.

But if something can get as close as 40 miles and still skip out... that is 0.0001674642149905592048799072248249 of a lunar distance... then the speed is a big factor. The angle seems less a factor, because if the speed is slow enough, it will spiral in even at a shallow angle once it experiences some atmospheric drag.

Is it right to say, then, that out of objects that get close, larger "lumbering" asteroids are the most dangerous? Can those be detected by the same methods used to catch the smaller, faster ones?

astromark
2010-Oct-30, 01:02 AM
Yes... and of course the larger are the most likely to be detected. Simply because of the size. Reflected light and so on...

fcunnane
2010-Oct-31, 03:29 PM
I think you are looking for the Roche limit (http://en.wikipedia.org/wiki/Roche_limit)???

Any seconds out there???

The Roche limit (pronounced /ˈroʊʃ/), sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction.[1] Inside the Roche limit, orbiting material will tend to disperse and form rings, while outside the limit, material will tend to coalesce. The term is named after Édouard Roche, the French astronomer who first calculated this theoretical limit in 1848.

Edit: Maybe not exactly, but this is a prerequisite for impact I imagine...

Hornblower
2010-Nov-01, 11:35 AM
Thanks for all the input, this is very interesting. So speed has a lot to do with it, and atmospheric drag.

The reason I'm wondering is that it seems like a lot of the asteroids that have been discovered lately are a) mostly on the smaller side and b) only discovered a few days before they are at their very closest. That is sensible of course because it is most likely you'll see something small is when it is closer.

But if something can get as close as 40 miles and still skip out... that is 0.0001674642149905592048799072248249 of a lunar distance... then the speed is a big factor. The angle seems less a factor, because if the speed is slow enough, it will spiral in even at a shallow angle once it experiences some atmospheric drag.

Is it right to say, then, that out of objects that get close, larger "lumbering" asteroids are the most dangerous? Can those be detected by the same methods used to catch the smaller, faster ones?

I would not bother trying to estimate the hazard of various combinations of size and velocity for a hypothetical borderline grazing encounter. Such an encounter is far less probable than a direct hit for objects coming in on random trajectories.