APOD and General Astronomy Discussion Forum
https://asterisk.apod.com/
No.BDanielMayfield wrote:On the final incursion, yes, but if a body’s orbit (unless it’s non-elliptical) passes within 1 Earth – Moon distance once isn’t it bound to do so repetedly?Chris Peterson wrote:Only the tiniest fraction of all the craters on the Moon were created by impactors that would have hit the Earth otherwise.
While two orbital planes do intersect in a line, the way I see it, the orbits themselves don't have to intersect at two points if the orbital angles deviate significantly.Chris Peterson wrote:Spif wrote:Again, "vertical separation" is meaningless. Every body is orbiting on its own plane, with the Sun at the center, and crosses Earth's orbit perfectly in two spots.This 2D map is dramatic. But when you consider vertical separation of those orbits, the threat ought to be less than the 2D diagram implies.
Another way to imaging two orbits not interesecting, EVER, is this... take two hula hoops. Squish one so it is eccentric. The circle lies flat (Earth orbit). The eccentric one then inclines (say 30 degrees for illustration). Insert the eccentric one into the circle (clinical geometry please, no jokesChris Peterson wrote: Again, "vertical separation" is meaningless. Every body is orbiting on its own plane, with the Sun at the center, and crosses Earth's orbit perfectly in two spots.
Edit: that's still not quite clear, I think. Not every pair of inclined eccentric orbits have to intersect at two points (or any points). I'll try to think of a better way to visualize the geometry. In any case, all of these asteroids have orbital paths which intersect Earth's.
) so that they share the same orbital focus. The eccentric orbit can share the same focus as the circular one without the two ever intersecting... This is true even if you drift the semimajor axis around over time. The two orbits never intersect simply because one is eccentric (skinny) and has an angle of inclination that ensures vertical separation out of the plane of the circularized orbit.In case 2, some people are assuming that when an asteroid approaches the moon, the odds are 50-50 that the moon will either provoke a collision with the Earth or provoke a miss. What I'm saying is that the odds are way WAY in favor of provoking a miss in this case. Given a random distribution of incident angle and radius of closest approach, the factor that really matters in the dynamic is that the Earth that is behind the moon is a very tiny target to hit compared to all the other space that the asteroid can fly through harmlessly.Chris Peterson wrote:The problem is with your case number 2. That describes almost all asteroids, but contains a small subpopulation... the ones that actually are deflected into the Earth. The small percentage of misses that get redirected to hits probably balances out the small number of hits that redirected to misses.Spif wrote:Right, assume pure randomness... So given that, any interaction with the moon will almost always cause a deflection along an angle that misses the Earth. Because on the dartboard, the Earth is a tiny bullseye compared to the vast area that constitutes the miss zone.
So there are two possible cases:
1) asteroid is going to strike the Earth -- encounters with the moon will tend to act to deflect the asteroid away from the Earth.
2) asteroid is not going to strike the Earth -- encounters with the moon will almost always result in a deflection that misses the Earth.
So my thinking is we are better off having a moon than not.
The Moon provides a small degree of direct shielding simply because it blocks a fraction of the sky. But in terms of gravitational deflection, it's pretty much ineffective as a shield.
As I noted, not all overlapping orbits intersect.Spif wrote:While two orbital planes do intersect in a line, the way I see it, the orbits themselves don't have to intersect at two points if the orbital angles deviate significantly.
Because intersecting orbits can have any difference of inclination.Even if that were the case, I just don't see how it is true that "inclination is meaningless" in terms of considering the risk of collision.
Every asteroid on this display has an orbit that intersects Earth's. That's what defines them as "potentially hazardous". Not every asteroid shown has a low inclination- many PHAs are 20 or 30 degrees (or more) off of Earth's orbital plane.Spif wrote:So if the asteroids in the 2D map are simply a random selection of orbits that "cross" Earth's orbit in a top-down view, odds are that only a fraction of those are an actual threat to the Earth because they happen to share an inclination that is very co-incident with Earth's orbit. The odds of such close coincidence seem fairly small to me.
Alright, I agree with that.Chris Peterson wrote:Because intersecting orbits can have any difference of inclination.Even if that were the case, I just don't see how it is true that "inclination is meaningless" in terms of considering the risk of collision.
Then there is the Tunguska event. Assuming it was asteroid related, it would be devastating if it happened again to a populated area. While that happened in 1908, a long time from a human stand point, in astronomical time frames, that is a blink of an eye.Boomer12k wrote:Pretty Spirograph...
Russia had that little brush with one awhile back, and it did not even hit, and still caused damage and harm....I don't think I want to be where one hits, especially if it is bigger....
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Yeah, we better start taking this threat seriously. Like, budget serious.tkc wrote: Then there is the Tunguska event. Assuming it was asteroid related, it would be devastating if it happened again to a populated area. While that happened in 1908, a long time from a human stand point, in astronomical time frames, that is a blink of an eye.
It is being taken pretty seriously for something that actually has rather low risks associated with it. We are pretty rapidly identifying all the hazardous asteroids. At this point, we're getting close to having all of them bigger than city busters. Not nice if one of those hits, but the global consequences to humans are small. And in a few decades at most, we'll have all of the ones that could produce any damage at all. We already know that there is no immediate major threat from any asteroids.Spif wrote:Yeah, we better start taking this threat seriously. Like, budget serious.
No, any secondary effects have been extremely small. Even the largest lunar impacts would have had negligible effect on its orbit or rotation.Adolfo Domínguez wrote:My question: Astronomers know if these asteroids striking on the Monn changed its orbit or affected the system Earth/Moon?
Ed Lu from B612 Foundation asserts that they believe there are as many as half a million undiscovered asteroids that are Tunguska class. Of course, he is trying to raise funding.Chris Peterson wrote:It is being taken pretty seriously for something that actually has rather low risks associated with it. We are pretty rapidly identifying all the hazardous asteroids. At this point, we're getting close to having all of them bigger than city busters. Not nice if one of those hits, but the global consequences to humans are small. And in a few decades at most, we'll have all of the ones that could produce any damage at all. We already know that there is no immediate major threat from any asteroids.Spif wrote:Yeah, we better start taking this threat seriously. Like, budget serious.
I guess this is where "budget serious" really comes in to play. We need to develop some new capability. We'd have to spot these things while they're still in the outer solar system; And have spacecraft pre-positioned out there prepared to ride in to the inner solar system with them... Gravitational tractors perhaps?Chris Peterson wrote: Although rarer, the bigger threat is from a comet. These can come in on a highly eccentric orbit, and from the daytime sky. If so, they are essentially undetectable from the Earth, and even if detected, would require technology well beyond what we currently have to do anything about. The strategy with such objects is to look for them as far in advance as possible, which requires using two or more fairly large IR space telescopes, in different orbits around the Sun. But there's still the unsolved problem of deflection.
Adolfo,Adolfo Domínguez wrote:All of us know Earth has been hit by many "potentially hazardous asteroids". As far as I know, Rusia has had "very close encounters" at least two times: one at the very beginning of XX century and the last one Russians had this year and caused serious damages in buildings and personal injuries to thousand of people. You people of US have a big crater near Winslow, Arizona. So we are really at risk. Many of us watched how the Moon was shaped out by these asteroids in an astounding video released by Apod several months ago. My question: Astronomers know if these asteroids striking on the Monn changed its orbit or affected the system Earth/Moon?
Maybe, maybe not. The point is, events of that class occur about once every thousand years, and even so, don't represent any global risk- their effect is quite local. Objects don't start producing major damage until they exceed 100m or so in diameter- now we're talking about events that are statistically separated by thousands of years, and a size in the range we will probably have detected in a few decades.Spif wrote:Ed Lu from B612 Foundation asserts that they believe there are as many as half a million undiscovered asteroids that are Tunguska class. Of course, he is trying to raise funding.
Long period comets might be detected a few years away. That's very little time to do anything about it.I guess this is where "budget serious" really comes in to play. We need to develop some new capability. We'd have to spot these things while they're still in the outer solar system; And have spacecraft pre-positioned out there prepared to ride in to the inner solar system with them... Gravitational tractors perhaps?Chris Peterson wrote: Although rarer, the bigger threat is from a comet. These can come in on a highly eccentric orbit, and from the daytime sky. If so, they are essentially undetectable from the Earth, and even if detected, would require technology well beyond what we currently have to do anything about. The strategy with such objects is to look for them as far in advance as possible, which requires using two or more fairly large IR space telescopes, in different orbits around the Sun. But there's still the unsolved problem of deflection.
If a long period comet makes a straight shot from out there right into the Earth, I dunno how plausible it would be for us to deflect one. Perhaps the gravitational tractor idea would only work with asteroids over several years of orbits?
Anyone know what the in-fall time of a comet is from, say, Neptune? A year maybe?
In the long term, if we want to be sure, perhaps we need to map the entire Kuiper belt and even the Oort Cloud. I guess long period comets can theoretically come from a light year out?
Here is a brief database of NEO's. Selecting a designation will bring up an orbital diagram that represents 4D space with the time component.Spif wrote:Yeah, I'm wondering about that 3rd dimension... Do all of these Earth-crossing asteroids have inclinations that are dead-on in Earth's plane?ta152h0 wrote:we found the enemy and they are everywhere ( in 2D anyways )
It seems to me that it would be easy for an Earth-crossing asteroid to have an inclination that deviates out of Earth's plane by several Earth diameters in altitude (at Earth's orbit). It seems to me that it ought to be rare for an asteroid to be exactly in Earth's plane, even if it does happen to cross our orbit right where we are.
Secondly, I wonder how much of a vertical separation would be required to avoid striking the Earth? As it falls into Earth's gravity well, an asteroid would be pulled down (or up) out of it's orbital plane toward the Earth. So I think such asteroids would need more than 1 Earth radius of vertical separation... perhaps a fair bit more to avoid striking the Earth.
-s
Perhaps you didn’t understand the point I was making Chris. Are you suggesting that a sun orbiting object that passes one lunar distance from Earth will never threaten Earth again? If so, why bother tracking objects to -Chris Peterson wrote:No.BDanielMayfield wrote:On the final incursion, yes, but if a body’s orbit (unless it’s non-elliptical) passes within 1 Earth – Moon distance once isn’t it bound to do so repetedly?Chris Peterson wrote:Only the tiniest fraction of all the craters on the Moon were created by impactors that would have hit the Earth otherwise.
APOD Robot wrote:... find and catalog those asteroids that may one day affect life on Earth. Pictured above are the orbits of the over 1,000 known Potentially Hazardous Asteroids (PHAs). These documented tumbling boulders of rock and ice are over 140 meters across and will pass within 7.5 million kilometers of Earth -- about 20 times the distance to the Moon.
Yes...but only at infrequent intervals of N asteroid orbits that approximately equal M Earth years (±1 day).BDanielMayfield wrote:On the final incursion, yes, but if a body’s orbit (unless it’s non-elliptical)Chris Peterson wrote:
Only the tiniest fraction of all the craters on the Moon were created by impactors that would have hit the Earth otherwise.
passes within 1 Earth – Moon distance once isn’t it bound to do so repeatedly?
Ever again is a long time. But an object that passes so close to Earth will have its orbit significantly perturbed. After one such pass, it might not even be in an Earth-crossing orbit anymore. And if it is, it might be hundreds or thousands of years before it comes close again, even if its orbital period is short. And the orbits of PHAs tend to be quite unstable, given their frequent interactions with inner system planets. We can't reliably predict future collisions with any known PHA because of that orbital instability, just probabilities.BDanielMayfield wrote:Perhaps you didn’t understand the point I was making Chris. Are you suggesting that a sun orbiting object that passes one lunar distance from Earth will never threaten Earth again?
