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

[Chris Peterson]

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

Of course, but the force would be extremely weak. Probably not an effective way to divert a long period comet. Indeed, all of the weak reaction methods are mainly suitable for deflecting bodies that are multiple orbits away from collision.

[neufer]

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

Of course, but the force would be extremely weak. Probably not an effective way to divert a long period comet. Indeed, all of the weak reaction methods are mainly suitable for deflecting bodies that are multiple orbits away from collision.

A nuclear heat source on the comet (e.g., an out of control fission reactor) might be as reasonable a means as any of diverting a comet. Of course, one would have to make sure that the comet remains more or less intact and that the geyser always points in more or less the same direction (e.g., by positioning it near the north or south pole of the comet).

[Chris Peterson]

A nuclear heat source on the comet (e.g., an out of control fission reactor) might be as reasonable a means as any of diverting a comet. Of course, one would have to make sure that the comet remains more or less intact and that the geyser always points in more or less the same direction (e.g., by positioning it near the north or south pole of the comet).

Doesn't sound very reasonable to me at all. The output of a geyser provides weak propulsion at best... not what you want for a body just a few months away from impact! If you have the capability of getting a nuclear heat source to the comet in the first place, it makes more sense to incorporate it into some sort of rocket and simply use material on the comet as reaction mass. I'd guess that would buy you several orders of magnitude greater thrust, as well as more control over it.

Any comet might well have more than one axis of rotation (i.e. tumbling) meaning there would be no poles where you could place a thruster, which further complicates things.

[neufer]

Doesn't sound very reasonable to me at all. The output of a geyser provides weak propulsion at best... not what you want for a body just a few months away from impact!

Well...if you restrict the situation to "just a few months away from impact"
all that can be done is hit it with a buckshot of thermonuclear weapons and hope for the best.

[geckzilla]

Must not post video of Armageddon end scene...

[Chris Peterson]

Well...if you restrict the situation to "just a few months away from impact"
all that can be done is hit it with a buckshot of thermonuclear weapons and hope for the best.

The restriction is reasonable for a long period comet. These aren't typically detected until about a year before perihelion. Even allowing for a bit longer, there's still the response time to get something to it that could alter its path. Even a few months might be optimistic.

I agree that throwing nuclear bombs at it probably represents the best (if not only) strategy in this scenario for the foreseeable future.

[Dustin M.]

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.

Not sure what I was thinking. There is no way a 1MW laser could put out 4MN of force. Thanks for correcting me.

[ta152h0]

would that not be a bummer if Murphy steps in and the beast , instead of barely missing the Earth naturally, would be directed at hitting the Earth, due to these efforts ? the law of unintended consequences.

[neufer]

Click to play embedded YouTube video.

would that not be a bummer if Murphy steps in and the beast , instead of barely missing the Earth naturally, would be directed at hitting the Earth, due to these efforts ? the law of unintended consequences.

[Chris Peterson]

would that not be a bummer if Murphy steps in and the beast , instead of barely missing the Earth naturally, would be directed at hitting the Earth, due to these efforts ? the law of unintended consequences.

There is a good argument to be made against developing technology that could deflect asteroids and comets, based on the assumption that the risk to humanity is greater from its weaponized use than the risk from a large asteroid. It's kind of like the way that you don't give a little kid a gun to protect himself from bad people. The gun is more dangerous than the criminals.

[neufer]

There is a good argument to be made against developing technology that could deflect asteroids and comets, based on the assumption that the risk to humanity is greater from its weaponized use than the risk from a large asteroid. It's kind of like the way that you don't give a little kid a gun to protect himself from bad people. The gun is more dangerous than the criminals.

It's a little late to be concerned about that: http://www.relativityonline.com/from-th ... ittle-kim/

[Galaxian]

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.

"Moonfall", Jack McDevitt. Highly recommended, though there is a slight iffiness with the description of lunar comestibles near the start.

[Galaxian]

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

Rather like the orbits of the two planets Neptune and Pluto? Pluto[1] is usually way farther out from Sol than is Neptune but for a tiny fraction of its orbit, 20 years out of about 250, the order of distances is reversed. Pluto gets *closer* to Sol than Neptune would be. That last happened from 1979 to 1999and will next happen around 2227 to 2247 -ish. However, Pluto's orbit is *very* tipped relative to the other planets' and when it "crosses Neptune's orbit", when it is as close to Sol as Neptune gets, it is a long way "below" Neptune (or "above"?), many millions of miles away.
PHA's and Earth often have somewhat the same relationship, only the angles and distances are smaller. Still, as should be obvious from the fact that they are still around and aren't crashing into us, when a PHA "crosses Earth's orbit" it's often many millions of miles away.
Of course, as we famously found out in 2013, twice, that is not always true.
Sometimes they do come close. Sometimes they come so close it counts as an intersection.
Still, it only ever seems to happen in Russia so the rest of us can feel safe and happy. http://asterisk.apod.com/posting.php?mo ... 9&p=205223#

[1] Pluto was a planet when I was younger, so, to me, it always will be a planet no matter what the IAU says. Technically, using the "cleared its orbit" rule, none of Earth, Luna, Neptune or Jupiter would be IAU planets. Earth certainly hasn't cleared its orbit of that rather large lump, Luna, Luna hasn't cleared her orbit of Earth [which may be a good thing] and both Neptune and Jupiter have Trojans. "Planet" is a sentimental term, not a scientific one and I'm rather sentimental about poor wee Pluto. [2]

[2] Tongue firmly in cheek. http://asterisk.apod.com/posting.php?mo ... 9&p=205223#

[Galaxian]

Anyone know what the in-fall time of a comet is from, say, Neptune? A year maybe?

I suspect it's one over root two of the orbital period, assuming it starts from Neptune's orbit and zero velocity. That seems like a very long time for a comet to fall from such a tiny distance so the initial velocity which it retained from its fall from the cometary zones must cut it down a lot.

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

LPC's (more accurately, non-repeating comets) can come from M31. Truly, they can though I would expect that to be a rare event. From M31 this galaxy is a small target and Earth is so small a target that the entire lifespan of the universe is probably not long enough for such a collision to be probable. More sanely, comets can be dragged by Sol from the cometary zones surrounding passing stars, so they can, theoretically, come from any direction and fall through the System in any hyperbolic trajectory at any inclination. Slightly more likely than that, comets falling from Sol's cometary zone can come from a good couple of light years, maybe more if their previous orbit was perturbed by one of those handy passing stars into something highly elliptical.
To be very, very safe, Earth would need to map everything moving in the entire galaxy. To be very safe long-term it would need a map of everything moving within a few diameters of the Local Cluster. Just in case something like the Magellanic Clouds perturbed the trajectory of something small and distant. Which, of course, they may have done several hundred million years ago - just in time for it to zoom through the Solar System next month.
Did I say "next"?
There is no such thing as "long term safety". Indeed, in the long term Earth will get plastered by something large enough to pasteurise the planet. It is inevitable and probably unavoidable with any technology short of divinity. Long term, this is a lethal place to live. The only way to avoid getting smeared by an infalling snowball the size of Texas is to not be here when it happens.
Of course, you as an individual could get vaporised no matter how many worlds we seed with the Children of Man, but if we make these human galaxies the cultures could survive anything.
Long term, safety is getting off this rock.

[Chris Peterson]

Long term, safety is getting off this rock.

That's probably not going to happen.

I'm for extinction. Every organism, every species has its day. Most species that have ever existed on Earth are long gone, and I have little doubt that the same is true elsewhere in the Universe.

Humans will die out, or will evolve into something else. That's the natural course of things, and I can't see any problem with it. Even if we were to somehow spread off the planet, humans will still be gone in a few million years.

[BMAONE23]

Click to play embedded YouTube video.

[Beyond]

I wonder if that's in English somewhere. Now I'll have to go prowling around the strange land of youtube, where I'll probably get so sidetracked that I'll forget what i was looking for.

[Beyond]

IN ENGLISH.

Click to play embedded YouTube video.

I think i liked it better when it wasn't in English.

[neufer]

Anyone know what the in-fall time of a comet is from, say, Neptune? A year maybe?

I suspect it's one over root two of the orbital period, assuming it starts from Neptune's orbit and zero velocity.

Halley's Comet aphelion lies half way between Neptune & Pluto and it only takes 37.5 years to come in.

Kepler states that a full period goes as R^(3/2)
and the in-fall time is only half that so: (1/2) x (1/2)^(3/2) = (1/2)^(5/2):

• Period/[sqrt(32)] ~ 29 years.

A comet passing by Neptune from the distant Oort cloud would be considerably quicker:

• Period/[π*sqrt(18)] ~ 12.4 years.

[alter-ego]

Kepler states that a full period goes as R^(3/2)
and the in-fall time is only half that so: (1/2) x (1/2)^(3/2) = (1/2)^(5/2):

• Period/[sqrt(32)] ~ 29 years.

A comet passing by Neptune from the distant Oort cloud would be considerably quicker:

• Period/[π*sqrt(18)] ~ 12.4 years.

Yup. I agree with your derivations. Here, Kepler's law is applied to a degenerate ellipse with eccentricity = 1 (not a parabola in this case). The semi-major axis = R/2 where R = orbital radius of Neptune. The calculations describe times for free fall. First from starting from Neptune, and second starting from the Oort Cloud. It's quite the travels though from the Oort Cloud: It takes 10,269,623 years to fall to Neptune, then only 12.4 years to get to the sun.

[alter-ego]

...
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.

Yes, the accumulative NEO & Earth position changes over time is the basis for a successful collision avoidance, at least in a controlled way (no nukes). However, I was surprised by how low the calculated asteroid orbital ΔV (change in velocity) is needed if the following conditions are met: 1. A threatening asteroid has made previous close pass, and 2. A collision is identified within 10 years of impact.

Even for sizeable asteroids (Apophis @ 320m, and 2004VD17 @ 580m) an integrated ΔV < 10 µm/sec will substantially divert them from collision with Earth. For example, for a modeled 2036 collision with Apophis (NOT going to happen!), a 53mN, 20-day deflection engagement (using a 1000kg tractor) is required to achieve a miss. This tractor engagement would theoretically occur in 2027. (Note for 2004D17, I believe many decades are needed) 
 

Threat Mitigation: The Gravity Tractor

The above paper, and the more technical NEAR-EARTH OBJECT (NEO) ANALYSIS OF TRANSPONDER TRACKING AND GRAVITY TRACTOR PERFORMANCE (JPL Final Report, 2008) detail scenarios, keyholes, tractor control efficiencies, stabilities and more. One common caveat is avoiding the already-stated threat of being blind-sided by large comet. We don't have a controlled response capability for that scenario - barring a last-ditch, hope-for-the-best deflection/fragmentation approach using nuclear explosives.