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

[geckzilla]

I finally managed to take that list of PHAs and convert it to xml format which I opened in Universe Sandbox and created a stereograph.



(Milky Way background is not oriented correctly to the solar system. It's just there for depth.)

[Beyond]

It looks nice, but for some reason, it reminds me of the spaghetti monster.

[alter-ego]

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.

There is no guarantee that an object will (ever) threaten Earth again if it passes within one lunar distance. There are mathematics and models that do define small regions of space near Earth that will impart a near-term (relatively) orbital resonance condition, and leading to another near pass or a possible impact. They are different regions, and they a very small compared to the volume of space bounded by a sphere containing the lunar orbit. The recent scare about a future Apophis impact with Earth led to further modeling and understanding of these Gravitational Keyholes.

It turns out the conditions for predictable, close PHA re-encounters require very precise parameter sets. Exacerbated by intrinsic orbital instability (as pointed out), a simplified, broad-brush conclusion for a future re-encounter really does not exist.

[neufer]

I finally managed to take that list of PHAs and convert it to xml format which I opened in Universe Sandbox and created a stereograph.

Even if the dinosaurs did have a space program the larger carnosaurs of the Cretaceous Period didn't have very good binocular vision so in some way they, at least, can be forgiven for not saving themselves.

[DavidLeodis]

The chart brought up through the "not all PHAs have been discovered" link is very interesting.

The mesh nature in the APOD reminded me of a Star Trek episode where the Enterprise was being increasingly caught in a grid. Kirk thankfully managed to find a way to get free.

[Spif]

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?

Ok, when I got a moment last night I did some back of the envelope calculations on the feasibility of using the "gravitational tractor" idea on a comet.

NOT.

To get a deflection of 1 Earth diameter after one year, I got a required force of about 4MN (mega newtons) and if you use gravity to apply that force, the "spacecraft" would have to be something like 36GT (giga tons). Ridiculous.

I used Halley's comet as a model ... average 11km radius, 2.2E14 kg mass. Just WAY too heavy to kick around using gravity alone.

Ok, so two fallback plans I can think of...

1) NUKES! ... yeah, everyone says you just make a million smaller problems when you try this. But wait a sec... If you're a year out from impact, perhaps dispersion of the comet via nukage would be a reasonable approach. If things go well, there would still be some small debirs passing close to Earth but perhaps the brunt of the mass could be dispersed beyond a few Earth radii after one year?

Would nukes have enough energy to disperse something this massive? Not entirely sure... I'll try ballparking it by calculating total gravitational potential energy of Halley's comet.

2) Other plan... Lasso and long term thruster. Depends on the rotation, oblateness, and structural integrity of the comet. Some objects are sort of loose gravel piles. Perhaps pitons and lassos can be sunk into a solid ice ball. Perhaps a giant net could be used. Tricky. But, if you can secure the thing on a tether, then you solve the problem of requiring a billion ton spacecraft for gravity effect and instead just have to worry about long term thrust.

Ok, 4MN is a LOT of thrust over a 1 year period. But perhaps you could continually refuel the thrusters by sending out tankers every few days. Very expensive. Requires impressive space based industrial capacity. But if Earth were at stake, economic inconvenience gives way to necessity, assuming the capacity is there in the first place. Perhaps it is plausible.

Can anyone chime in on how plausible 4MN would be with near future advanced propulsion technology? I know that ion drives are super low thrust (negative exponents) but have very high efficiency. To get MN's of output you'd probably need something most like a plasma fire hose that spits out very high propellant volume. Fission rockets might have the thrust but not the staying power. Fusion?

-s

[Spif]

I finally managed to take that list of PHAs and convert it to xml format which I opened in Universe Sandbox and created a stereograph.



(Milky Way background is not oriented correctly to the solar system. It's just there for depth.)

Slick looking visuals.

What's the distinction between the two sets? -- depth perception?

-s

[geckzilla]

Slick looking visuals.

What's the distinction between the two sets? -- depth perception?

-s

Universe Sandbox did most of the work.

The two images are intended to be viewed cross-eyed. You combine the two images like this and it creates a 3d image. It can be a bit challenging to do if you've never done it before.

[Beyond]

Boy! What a goofball am i I didn't even think of doing that, till you mentioned it, geckzilla. It works good With this particular picture, for me anyway, it also gives the impression that the two side images are also 3-D... until i try to focus on them. Then the whole thing goes down the crapper. So it's only the third image in the middle where you can actually see the 3-D effect directly.

[BMAONE23]

The chart brought up through the "not all PHAs have been discovered" link is very interesting.

The mesh nature in the APOD reminded me of a Star Trek episode where the Enterprise was being increasingly caught in a grid. Kirk thankfully managed to find a way to get free.

I think that one was "The Tholian Web"

[BDanielMayfield]

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.

There is no guarantee that an object will (ever) threaten Earth again if it passes within one lunar distance. There are mathematics and models that do define small regions of space near Earth that will impart a near-term (relatively) orbital resonance condition, and leading to another near pass or a possible impact. They are different regions, and they a very small compared to the volume of space bounded by a sphere containing the lunar orbit. The recent scare about a future Apophis impact with Earth led to further modeling and understanding of these Gravitational Keyholes.

It turns out the conditions for predictable, close PHA re-encounters require very precise parameter sets. Exacerbated by intrinsic orbital instability (as pointed out), a simplified, broad-brush conclusion for a future re-encounter really does not exist.

Thanks alter-ego, that was very informative too.

In one of my favorite Star Trek Voyager episodes the ship (which is trying to fly back from the far side of the galaxy) is being spied on by a scientist who actually IS a duck-billed dinosaur descendent. He is working on a theory that his race didn’t originate on the planet they inhabit, which is a heretical view to the dinosaurs in charge. He proves that his race and the newly discovered humans share DNA, indicating that both races did ordinate back on Earth. This view is so unsettling that many of the dinos in charge want to destroy the evidence, (the starship Voyager and its crew) which they could easily do since their technology is tens of millions (thanks geckzilla) of years ahead of these primitive primates from Earth. They finally consent to release Voyager after being informed about how dinosaurs are viewed as magnificent creatures back on Earth and how happy people will be to learn that one of their species had developed space flight prior to the CT extinction event.

…

But seriously, (some may feel that I should never be taken seriously, I fear) having NEOs is not all bad. Some serious dough is being invested in plans to actually mine asteroids:

http://www.skyandtelescope.com/communit ... 22815.html

http://www.skyandtelescope.com/communit ... 38361.html

[neufer]

Ok, when I got a moment last night I did some back of the envelope calculations on the feasibility of using the "gravitational tractor" idea on a comet.

NOT.

To get a deflection of 1 Earth diameter after one year, I got a required force of about 4MN (mega newtons) and if you use gravity to apply that force, the "spacecraft" would have to be something like 36GT (giga tons). Ridiculous.

I used Halley's comet as a model ... average 11km radius, 2.2E14 kg mass. Just WAY too heavy to kick around using gravity alone.

Halley's comet has an average diameter of 11km (or about the size of the the impacting bolide that wiped out the dinosaurs.

I doubt that gravitational tractor would be attempted on:

• 1) any active comet or on
  2) any asteroid larger than 1km in diameter
  3) or with a lead time of less than a decade.

A gravitational tractor works most efficiently (i.e., maximal {F ⋅ v} power) when the asteroid's velocity is, itself, maximal (e.g., at perihelion and/or during a planetary encounter).

• Note that a change in the asteroid's (sun) radial velocity by 1 cm/s will permanently change the orbit by only ~50km.
  
  However, a change in the asteroid's orbital velocity by 1 cm/s will not only permanently change the orbit, itself, by ~100km but (more importantly) it will permanently modify its sidereal period by ~30 seconds. This permanent ~30 second change in sidereal period will cause the asteroid to drift ~900km per year in its orbital location and thus prevent any predicted future collision with Earth more than a decade or so out.

[geckzilla]

In one of my favorite Star Trek Voyager episodes the ship (which is trying to fly back from the far side of the galaxy) is being spied on by a scientist who actually IS a duck-billed dinosaur descendent.

I thought that was one of the silliest episodes, Bruce. I mean, it was a good one, but still, so silly! Especially since humans and many other species originating from various planets are able to reproduce with and evidently share their DNA with all sorts of species in the Star Trek universe. It annoyed me that DNA was used as a plot device for that particular episode but seemingly ignored for other instances. BTW, they are a few million years ahead, not billions. That's still a long time, though.

[Chris Peterson]

Note that a change in the asteroid's (sun) radial velocity by 1 cm/s will permanently change the orbit by only ~50km.

Could you elaborate? I don't know what this means. Are you referring to some particular orbital element like the semi-major axis?

However, a change in the asteroid's orbital velocity by 1 cm/s will not only permanently change the orbit, itself, by ~100km ...

Same question.

[ta152h0]

I hope the martians have a space program for it appears they might get a high velocity visitor who is planning on staying next year

[BMAONE23]

I hope the martians have a space program for it appears they might get a high velocity visitor who is planning on staying next year

I would like to see that particular visitor make a landing near enough to Opportunity or Curosity or as Hirise pases over for the imaging opportunity of a life time and a possibility of checking out a really fresh crater

[DavidLeodis]

The chart brought up through the "not all PHAs have been discovered" link is very interesting.

The mesh nature in the APOD reminded me of a Star Trek episode where the Enterprise was being increasingly caught in a grid. Kirk thankfully managed to find a way to get free.

I think that one was "The Tholian Web"

Cheers for that BMAONE23. I seem to recall it was quite a good episode, but then weren't all!

[neufer]

Note that a change in the asteroid's (sun) radial velocity by 1 cm/s will permanently change the orbit by only ~50km.

Could you elaborate? I don't know what this means. Are you referring to some particular orbital element like the semi-major axis?

A circular orbit becomes an elliptical orbit whose aphelion is ~50km greater and whose perihelion is ~50km less.

However, the orbital period remains the same.

However, a change in the asteroid's orbital velocity by 1 cm/s will not only permanently change the orbit, itself, by ~100km ...

Same question.

A circular orbit becomes an elliptical orbit whose aphelion is ~100km greater or whose perihelion is ~100km less.

And , the orbital period increases (or decreases) accordingly,
thereby, allowing for the asteroid to miss the Earth after a decade or so.

[Chris Peterson]

A circular orbit becomes an elliptical orbit whose aphelion is ~50km greater and whose perihelion is ~50km less.

A circular orbit becomes an elliptical orbit whose aphelion is ~100km greater or whose perihelion is ~100km less.

Thanks. That makes sense. I'll note, however, that your comments were to somebody addressing my comments about the real danger coming from a long period comet. He was considering approaches for deflecting one of those. Such a comet would be in a highly eccentric (near hyperbolic) orbit, so there's no good way I can think of to generalize the effects of small gravitational nudges; it depends where and in what direction they are applied, and even very small velocity changes could profoundly affect the position as the comet reached the inner system.

[BDanielMayfield]

In one of my favorite Star Trek Voyager episodes the ship (which is trying to fly back from the far side of the galaxy) is being spied on by a scientist who actually IS a duck-billed dinosaur descendent.

I thought that was one of the silliest episodes, Bruce. I mean, it was a good one, but still, so silly! Especially since humans and many other species originating from various planets are able to reproduce with and evidently share their DNA with all sorts of species in the Star Trek universe. It annoyed me that DNA was used as a plot device for that particular episode but seemingly ignored for other instances. BTW, they are a few million years ahead, not billions. That's still a long time, though.

Yes geckzilla. I meant to write millions, but must have had billions on the brain. “Tens of billions” is non-sensible as the whole universe is less that 14 BY old. The CT demise of dinosaurs was 65 MILLION years ago. Thanks for the correction, which has now been applied.

I also found all the alien interbreeding in the Star Trek universe a stretch too, but they had started it right from the start with Mr. Spock being a Vulcan/Human cross, so it was a built in plot tool. Anyway you need to be able to suspend disbelief for a little while with any science fiction in order to enjoy it, IMO.

As for the aforementioned Voyager episode, I couldn’t help but love it, as dinosaurs and therefore paleontology was the first science that I fell in love with as a small boy. (This may have been my mom’s fault, because as a toddler I never had a teddy bear; I had a stuffed brontosaurid.) The only gripe I had with the episode was that they portrayed the super-advanced dinosaurs as being cold blooded. I’m a big fan of Robert T. Bakker’s book The Dinosaur Heresies which made the case back in 1986 that dinosaurs were closer to warm blooded birds than to cold blooded reptiles. So 66 plus million year old trans-warp inventing dinosaurs are still cold blooded??? No way.

So, what does this have to do with this PHA discussion? Well like I’ve been trying to point out, this story is not all doom and gloom. Asteroids and their cometary kin can be beneficial too. Would we be here without the CT impact event? No. But we do need to learn how to pluck ‘em, dodge ‘em, duck ‘em, shake ‘em and shuck ‘em if we don’t want to go the way of the dinos.

[Beyond]

But we do need to learn how to pluck 'em, duck 'em, shake 'em and shuck 'em if we don't want to go the way of the dinos.

Sounds like you're making a cometary/astroidal salad.

[Dustin M.]

Ok, when I got a moment last night I did some back of the envelope calculations on the feasibility of using the "gravitational tractor" idea on a comet.

NOT.

To get a deflection of 1 Earth diameter after one year, I got a required force of about 4MN (mega newtons) and if you use gravity to apply that force, the "spacecraft" would have to be something like 36GT (giga tons). Ridiculous.

I used Halley's comet as a model ... average 11km radius, 2.2E14 kg mass. Just WAY too heavy to kick around using gravity alone.

Ok, 4MN is a LOT of thrust over a 1 year period. But perhaps you could continually refuel the thrusters by sending out tankers every few days. Very expensive. Requires impressive space based industrial capacity. But if Earth were at stake, economic inconvenience gives way to necessity, assuming the capacity is there in the first place. Perhaps it is plausible.

Can anyone chime in on how plausible 4MN would be with near future advanced propulsion technology? I know that ion drives are super low thrust (negative exponents) but have very high efficiency. To get MN's of output you'd probably need something most like a plasma fire hose that spits out very high propellant volume. Fission rockets might have the thrust but not the staying power. Fusion?

-s

I did some figuring with the laser idea. The energy over one year to move Halley's Comet (I assume you mean earth radius 6371000.0 m) is said to be 4 MN or 4000000 N and E=F*dx so 4E6 * 6.371E6 = 2.5484E13 Joules over the course of one year. 1J/s=1W so a 1E6 or 1MW laser over the course of one year should pump out what is needed. That or use multiple lasers at the same time of lesser power. And this is for moving Haley's Comet. Were not factoring in reflected light which provides twice the force and the evaporation of liquid from the surface of the comet.

[JohnD]

I hope the martians have a space program for it appears they might get a high velocity visitor who is planning on staying next year

I presume you mean this:
http://science.nasa.gov/science-news/sc ... marscomet/

At 1-3kms in diameter, that's between 7^11 and 7^12 litres of water, or less depending on how much rock is in the comet.
How much of that water will stay on Mars and what changes might that drop effect?


John

[neufer]

I did some figuring with the laser idea. The energy over one year to move Halley's Comet (I assume you mean earth radius 6371000.0 m) is said to be 4 MN or 4000000 N and E=F*dx so 4E6 * 6.371E6 = 2.5484E13 Joules over the course of one year. 1J/s=1W so a 1E6 or 1MW laser over the course of one year should pump out what is needed. That or use multiple lasers at the same time of lesser power. And this is for moving Haley's Comet. Were not factoring in reflected light which provides twice the force and the evaporation of liquid from the surface of the comet.

A 1MW laser has a kickback force of:

10⁶newton*meters/sec divided by
the speed of light: 3x10⁸meters/sec

or just 3.33 mN.

A solar powered photon drive spaceship may be thrifty of fuel but it is extremely feeble
since almost all of the energy, itself, goes into the fast photons rather than the slow spaceship.

The energy loss of photons reflected off of a comet is simply the redshift Doppler loss of the photons.

(The energy loss of photons absorbed by a comet producing a propulsive geyser jet is a different matter.)
------------------------------------------------------------------------------------
Note: NASA's 2.3 kW NSTAR ion thruster for the Deep Space 1 spacecraft has a thrust of 92 mN.
Modern ion rockets would require ~ 17 kilotons of fuel to effectively move Halley's Comet.

[BMAONE23]

So if you landed a heat source on the comet and produced a long term geyser, the force of the geyser would also change the orbit like a rocket engine