Starship Asterisk*

Orbits of Potentially Hazardous Asteroids

Explanation: Are asteroids dangerous? Some are, but the likelihood of a dangerous asteroid striking the Earth during any given year is low. Because some past mass extinction events have been linked to asteroid impacts, however, humanity has made it a priority 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. Although none of them will strike the Earth in the next 100 years -- not all PHAs have been discovered, and past 100 years, many orbits become hard to predict. Were an asteroid of this size to impact the Earth, it could raise dangerous tsunamis, for example. Of course rocks and ice bits of much smaller size strike the Earth every day, usually pose no danger, and sometimes creating memorable fireball and meteor displays.

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GADZOOKS!! We're surrounded.

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

Any thought of plotting this in 3D with the third dimension being time? Would be way more informative!

I've searched for but never found anything discussing what may happen if a PHA strikes the Moon. Depending on size, speed, angle, and location it seems like any effects could range from visually spectacular to deadly.

Extending the question from the Moon, where I too know of no observed significant impact in human memory, and excepting Jupiter and Shoemaker, have we seen impacts on other planets?
Would they be visible on Venus with it's intense cloud cover?
I read of a new crater on Mars, but it was less than 100 meters across.

John

ta152h0 wrote: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

ta152h0 wrote:
we found the enemy and they are everywhere ( in 2D anyways )

• They are our PHOs.

This doesn’t have to be seen as bad news, IMO. First, it is a very good thing that these objects are being discovered, their orbits are being determined and that they are being studied. Such efforts should allow the threatening nature of NEOs to be mitigated, should any be found to be on earth intersecting paths.

Secondly, since these are relics from our solar system’s formation, their being nearby should advance studies into how solar systems like ours were formed. This is very important now that planetary systems are thought to be so common. It’s easy to see how planetesimals grow larger once they achieve enough mass to attract their neighbors, but how do grains of dust coalesce into planetesimals? The asteroid belt should give us the answers, but we don’t have to go that far to get samples.

Last but not least, these objects could be very valuable both for the materials they contain and for their location in space. Precious metals and rare earth elements that are becoming increasingly hard to come by on Earth will be found in these objects, for it was from the asteroid belt that the earth’s crust received many of these elements in the first place. But even common elements and compounds like AL, FE and H2O will be valuable in space for future construction, since they will not need to be hauled up from Earth.

So, learn to look on the bright side of things. (Cue the Jamaican music man, “Don’t worry, be happy!”) When the solar system hands you lemons, make asteroidade.

dkp wrote:I've searched for but never found anything discussing what may happen if a PHA strikes the Moon. Depending on size, speed, angle, and location it seems like any effects could range from visually spectacular to deadly.

One day we'll get to see a high res video of a large strike on the moon. That would be an awesome sight... you know, like the ones that leave a central mountain in the middle of the crater?

Anyway, it seems to me that, statistically, over many encounters, the moon would have more of a protective effect than a risk effect on us.

In other words, if an asteroid is going to hit the Earth but it encounters the moon first, the moon will almost always act toward causing a miss (depending on the strength of the interaction). But it would be exceedingly rare that a random interaction with the moon would cause a deflection into the Earth (directly or on a subsequent orbital cycle).

Unless the asteroid is truly massive, it seems to me that ejecta from an impact on the moon would all be small fragments, only a fraction of which would actually strike the Earth. And most of those would be blunted/absorbed by the atmosphere.

-s

Spif wrote:

dkp wrote:I've searched for but never found anything discussing what may happen if a PHA strikes the Moon. Depending on size, speed, angle, and location it seems like any effects could range from visually spectacular to deadly.

Unless the asteroid is truly massive, it seems to me that ejecta from an impact on the moon would all be small fragments, only a fraction of which would actually strike the Earth. And most of those would be blunted/absorbed by the atmosphere.

Just thinking about this some more... Stuff would probably be raining down on us for years afterwards. Maybe a few buildings would be struck and perhaps some satellites would be taken out.

-s

Spif wrote:
Anyway, it seems to me that, statistically, over many encounters, the moon would have more of a protective effect than a risk effect on us.

In other words, if an asteroid is going to hit the Earth but it encounters the moon first, the moon will almost always act toward causing a miss (depending on the strength of the interaction). But it would be exceedingly rare that a random interaction with the moon would cause a deflection into the Earth (directly or on a subsequent orbital cycle).

I can't see why the effect of the moon would be anything other than random...neither helping nor hurting terrestrial impacts.

(Jupiter OTOH is large enough that it can fling asteroids & comets off into interstellar space never to return.)

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.

-s

The last time I saw a diagram like that, I was playing with gear wheels and pens... that's one awfully big Spirograph! http://en.wikipedia.org/wiki/Spirograph

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?

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.

You seem to be visualizing the geometry incorrectly.

Earth-crossing asteroids can occur at any inclination, although most are at fairly small inclinations- not surprising given their origin in the asteroid belt (which lies on the ecliptic). These asteroids have perihelions that lie inside Earth's orbital radius, and aphelions that are outside it. That means their orbital paths and Earth's orbital path intersect. They have to. So the only thing that determines whether or not they hit us is timing. If both bodies are at one of the points of intersection at the same time... bang.

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.

As an asteroid gets close to Earth, its orbit is perturbed by Earth's gravity. That can either result in deviating a perfect hit into a miss, or a miss into a hit. Deciding which it will be requires numerical integration of the orbit, and depends on details of the orbital parameters. There is no rule (like x Earth radiuses) that can be generalized.

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....

:---[===] *

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 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.

JohnD wrote:Extending the question from the Moon, where I too know of no observed significant impact in human memory, and excepting Jupiter and Shoemaker, have we seen impacts on other planets?

Hundreds of small impacts have been observed on the Moon, at least three on Jupiter, and I think that's all.

Would they be visible on Venus with it's intense cloud cover?

A meteor would be detectable. Whatever it might do to the surface if it reached that far, probably not.

dkp wrote:I've searched for but never found anything discussing what may happen if a PHA strikes the Moon. Depending on size, speed, angle, and location it seems like any effects could range from visually spectacular to deadly.

For the most part, it isn't a concern. The Moon's gravity well is deep enough that most material would fall back. It would require a very large body (a few kilometers or more) to create a shower of large enough debris to result in significant meteoritic hazards on Earth.

Spif wrote:So my thinking is we are better off having a moon than not.

Absolutely Spif, but not just as an asteroid catcher/deflector. The Moon’s diameter is 27% of Earth’s, but it’s mass is only 1.23% of Earth’s. If something is headed toward the Earth/Moon system (which is 1.23% more likely due to the additional pull from the Moon) the odds would be more than 80 to 1 that if something was going to be hit, it would be the Earth taking the blow. Still, every crater on the moon is a case of something that could have hit the Earth but didn’t because the Moon was there.

The main reasons that the Moon’s presence is of vital importance for life on Earth is that it stabilizes the Earth’s axial tilt and that it generates tides in our oceans and seas.

Plus it’s a romantic nightlight. Would any of us be here without it? I doubt it.

Clicking on today’s APOD brings up the larger view, showing that some of these Earth crossers go even a bit farther out that Jupiter’s orbit, while some pass inside that of Mercury. So Mercury, Venus, Mars, our Moon, and Jupiter are each going to be taking out a piece of this action. We better go out and get ‘em while they last.

BDanielMayfield wrote:Absolutely Spif, but not just as an asteroid catcher/deflector. The Moon’s diameter is 27% of Earth’s, but it’s mass is only 1.23% of Earth’s. If something is headed toward the Earth/Moon system (which is 1.23% more likely due to the additional pull from the Moon) the odds would be more than 80 to 1 that if something was going to be hit, it would be the Earth taking the blow. Still, every crater on the moon is a case of something that could have hit the Earth but didn’t because the Moon was there.

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

What we can't easily see, because of the way Earth remodels its surface, are the impacts on our own planet that only occurred because the Moon focused something that would otherwise have missed.

Chris Peterson wrote:

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?

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.

Earth-crossing asteroids can occur at any inclination, although most are at fairly small inclinations- not surprising given their origin in the asteroid belt (which lies on the ecliptic). These asteroids have perihelions that lie inside Earth's orbital radius, and aphelions that are outside it. That means their orbital paths and Earth's orbital path intersect. They have to. So the only thing that determines whether or not they hit us is timing. If both bodies are at one of the points of intersection at the same time... bang.

Well, with basic trigonometry, if I'm doing this right, at 1 AU, an altitude of 1 Earth radius translates to an inclination of 0.12 degrees.

I have that Earth's orbit has an inclination of 1.57 degrees.

So by my reading of the geometry, in order to be a significant threat, an Earth-crossing asteroid must have an inclination close to the very tiny range between 1.45 degrees and 1.69 degrees. (That's one Earth radius ... perhaps a significant threat would be an altitude of several Earth radii).

It seems a stretch to me to assume that all of these asteroids fall so close to Earth's orbital plane.

I don't have much actual data, and would love to see some, but the Wikipedia page on the asteroid belt suggests that "most" asteroids have inclinations of less than 4 degrees. I assume by "most" they mean more than 50% of the population (1 sigma perhaps?). But that's still a huge distribution around the exact plane of Earth's orbit (1.57 degrees).

Also, it seems to me that eccentric asteroids that cross planetary orbits are perhaps eccentric because they've interacted with something in the past. Or, anyway, such asteroids would likely have had some interactions during their billions of years of life? I imagine then that any given random encounter with another body is likely to impart some inclination to an asteroid's orbit. If you randomly incline an asteroid's orbit, the odds of shifting it right into Earth's plane seem very small to me.

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.

(Unless all those asteroids in the chart have been selected precisely BECAUSE they lie in Earth's plane?)

-s

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.

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?

Spif wrote:Well, with basic trigonometry, if I'm doing this right, at 1 AU, an altitude of 1 Earth radius translates to an inclination of 0.12 degrees.

I have that Earth's orbit has an inclination of 1.57 degrees.

So by my reading of the geometry, in order to be a significant threat, an Earth-crossing asteroid must have an inclination close to the very tiny range between 1.45 degrees and 1.69 degrees. (That's one Earth radius ... perhaps a significant threat would be an altitude of several Earth radii).

It seems a stretch to me to assume that all of these asteroids fall so close to Earth's orbital plane.

You're still getting it wrong. None of the asteroids lie in Earth's orbital plane.

The intersection of two planes (Earth's orbital plane and the asteroid's orbital plane) define a line. The intersection of that line with the two orbits define a pair of points where the orbits intersect. That's where you get a collision of both bodies are near that point at the same time.

Also, it seems to me that eccentric asteroids that cross planetary orbits are perhaps eccentric because they've interacted with something in the past. Or, anyway, such asteroids would likely have had some interactions during their billions of years of life? I imagine then that any given random encounter with another body is likely to impart some inclination to an asteroid's orbit. If you randomly incline an asteroid's orbit, the odds of shifting it right into Earth's plane seem very small to me.

All asteroids have had their orbits altered by interacting with planets- mainly Jupiter. The very structure of the asteroid belt is defined by orbital resonances with the gas giants. Perturbations of bodies on the ecliptic, by bodies on the ecliptic, tend to change the eccentricity and semi-major axis of the orbit, but leave the inclination largely unchanged. That's why most Earth-crossing asteroids still have low inclinations. A high inclination either implies a direct interaction with a planet (a slingshot) or an origin outside the asteroid belt, such as a comet.

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.

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.