APOD: Orbits of Potentially Hazardous... (2013 Aug 12)

[Chris Peterson]

Only the tiniest fraction of all the craters on the Moon were created by impactors that would have hit the Earth otherwise.

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?

No.

[rstevenson]

Anyone trying to visualize PHA orbits 3-dimensionally might want to have a look at this page, and this one. The first is an "overhead" map of the inner Solar System showing known asteroids. The second is a closer view, this time with vertical lines illustrating where the "local" (as of last January) asteroids happen to be, relative to the plane of the ecliptic.

Rob

[Spif]

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.

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.

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.

Try this... form two rings with your index finger and thumb in each hand. Then intersect the two rings with your hands twisted at different angles... like the magician's intersecting rings trick. Do your fingers have to occupy the same space at the same time? No. Likewise, what I am imagining is that two orbits of different inclination (and twist) can interlink like links in a chain, sharing the same orbital focus, without themselves connecting with each other.

Unless the semimajor axis of the orbits themselves rotate over time so that over billions of years the trajectories of the orbits do sometimes cross one another?

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.

-s

[JohnD]

Thank you Chris and excuse my ignorance. The brightest (=biggest?) observed lunar impact was only last March, when a 40 ton object impacted in the Mare Imbrium. NASA has a monitoring programme that has seen over 300 smaller impacts since 2005.

See: http://www.youtube.com/watch?v=BxewkRwY_uY

John

[Spif]

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.

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 jokes ) 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.

From the top down, they appear to be "intersecting" orbits. But when you take the 3rd dimension into account, it is clear that the two bodies do not threaten each other unless they are perturbed by a third body sometime in the future.

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.

-s

[Spif]

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 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.

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.

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.

I imagine that you're right in that the moon as an impact shield probably dominates in it's protective affect over the moon as a deflector.

I'm not sure about how deflections would affect the future odds of collision... Perhaps inclinations induced by such a deflection, even if tiny, would dramatically reduce the future odds of a collision?

And, of course, the odds of an asteroid suffering a significant interaction with the moon in the first place are quite tiny.

But the issue at hand was... given an interaction with the moon, what are the likely outcomes?

I believe the moon is definitely a guardian, though a weak one overall.

-s

[Chris Peterson]

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.

As I noted, not all overlapping orbits intersect.

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.

Because intersecting orbits can have any difference of inclination.

[Chris Peterson]

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.

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]

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.

Because intersecting orbits can have any difference of inclination.

Alright, I agree with that.

Given two orbits that intersect, the angle of inclination has no impact on whether or not the orbits intersect.

-s

[Dustin M.]

speaking of slingshots and if the moon could protect us or be of greater danger I would suppose it could be of both depending on the trajectory. If the rock flies behind the moons direction then a gravitational slingshot would enhance the rocks speed. If the rock flies in front of the moons direction then the moon would slow it down a bit. The moon could also deflect towards or away from the earth depending on the trajectory but I am guessing its relative size would statistically minimize its chance of deflection due to a smaller gravity field.

I think we should look into predicting asteroid paths and steerable lasers that provide a curved and very tight beam over long distances to deflect future collisions unless I am mistaken about the laser that they are using. http://physicsworld.com/cws/article/new ... nd-corners This way we could immediately, as fast a light travels, start affecting the trajectory.

[tkc]

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....

:---[===] *

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.

[Spif]

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.

Yeah, we better start taking this threat seriously. Like, budget serious.

Neil DeGrasse Tyson has a funny way to put this... The dinosaurs didn't have a space program and so in some way they can be forgiven for not saving themselves. But if millions of years from now the aliens discover that we got wiped out by a rock WHILE we had a space program, we'd be the laughing stock of the galaxy.... Like Lemmings or something.

We'd be the butt-end of all the alien jokes that involved fools that got themselves killed.

Silly Humans.

-s

[Chris Peterson]

Yeah, we better start taking this threat seriously. Like, budget serious.

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.

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 Domínguez]

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?

[Chris Peterson]

My question: Astronomers know if these asteroids striking on the Monn changed its orbit or affected the system Earth/Moon?

No, any secondary effects have been extremely small. Even the largest lunar impacts would have had negligible effect on its orbit or rotation.

[geckzilla]

The PHA list is annoyingly formatted using spaces rather than an HTML table and there seems to be no spreadsheet version available. http://www.minorplanetcenter.net/iau/li ... erous.html

[Spif]

Yeah, we better start taking this threat seriously. Like, budget serious.

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.

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.

I suppose if things run smoothly for them on the time-table they dream of, that might qualify as "finding everything within a couple decades".

They seem to be on track for significant private funding, but in part I think it's shameful that it takes a private effort to address the problem when governments could achieve the necessary funding in short order if only they had the rational wits and willpower to solve real problems instead of manufacturing their own problems that get in the way of progress (partisan bickering).

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.

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?

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?

-s

[JohnD]

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?

Adolfo,
Don't panic!

The effect of the Barringer crater impact was probably only local, 2-300mkms around, and no global effect at all.
See: http://www.lpi.usra.edu/science/kring/e ... tpage.html
The land area of either Russia or the comboned USA with Canada are the same, but two events over Russia in living memory just isn't significant.
And the event that shaped the Moon we see today was the Late Great Bombardment, 4 Billion years ago in a planet-forming era of the Solar System that just doesn't apply now.

It is sensible and prudent to investigate this possibility, but the probability that there is a big impactor on its way in the forseeable future is very low.

John

[Chris Peterson]

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.

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.

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.

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?

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?

Long period comets might be detected a few years away. That's very little time to do anything about it.

But all the technology you suggest may simply not be worth the expense right now, given the extremely low risk involved. This kind of risk analysis is tricky... in absolute terms, the chance of a huge impact is incredibly tiny. But the damage if one occurs is huge (potentially destroying the human race). That makes for some peculiar analysis.

[BMAONE23]

we found the enemy and they are everywhere ( in 2D anyways )

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?

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

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.
http://neo.jpl.nasa.gov/neo/close.html
The smapp body database has numerous orbital diagrams in 4D space
http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2013 LM31;orb=1
Just enter the designation number or name in the "Search" field at the top
ahere is a larger list from the NEO program
http://neo.jpl.nasa.gov/risk/
If the object is numerically designated like 1999 RQ36, enter in the search field of link 2 above 1999RQ36 (no spaces)
If it is Named then you can enter the name like "apophis" in the search field then click on "show orbit diagram" to see the orbit in 4d space

[retrogalax]

There are very few sungrazer into them.

[BDanielMayfield]

Only the tiniest fraction of all the craters on the Moon were created by impactors that would have hit the Earth otherwise.

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?

No.

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 -

... 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.

[neufer]

Only the tiniest fraction of all the craters on the Moon were created by impactors that would have hit the Earth otherwise.

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 repeatedly?

Yes...but only at infrequent intervals of N asteroid orbits that approximately equal M Earth years (±1 day).

Any interaction of the asteroid with another body
(or radiation effects for smaller asteroids) will eventually break this pattern.

[Chris Peterson]

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?

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]

Thanks Art and Chris. Those explanations make sense.