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Posted by: nprev Dec 26 2006, 10:17 PM

Here's a Christmas present for us all...a 0.7 km NEO may make a 4200 km altitude Earth flyby on Halloween, 2041: http://neo.jpl.nasa.gov/risk/2006xg1.html

Posted by: volcanopele Dec 26 2006, 11:33 PM

hmm, I tried simulating this in Celestia, but even with orbital elements from Horizons at the time of the encounter, this asteroid appears to be about 1.56 au from Earth.

Posted by: dilo Dec 26 2006, 11:58 PM

http://newton.dm.unipi.it/cgi-bin/neodys/neoibo?quicksearch:0;main gives only a close approach for another date (2060) at 0.155AU - but with great uncertain...
I think we have to wait for the next few weeks and, we hope, refined measurements will declass this object to a zero Torino scale as usual

Posted by: nprev Dec 27 2006, 01:54 AM

Yeah, I'm sure that will be the case. I noticed that the NASA NEO site already had 76 observations logged between 11-22 Dec; it must be very favorably placed, so we should see it drop off the radar screen after the holidays.

Good thing, too. 1900 megatons of impact energy could be a bit unpleasant. With that in mind, does anybody think that we should get serious about devising a way to deflect these things, if it's ever needed? I like the idea of "docking" a DS1-style propulsion system to a threatening rock & steering it clear...or, ideally, steering it slowly into an exploitable orbit for future mining!

And on a somewhat related topic, is there anything at all we could do about an imminent long-period comet impact? I can't think of a single countermeasure. (Okay, I confess...I rented the classic When Worlds Collide two days ago, and it's been bugging me ever since... )

Posted by: Bob Shaw Dec 27 2006, 08:59 AM

The most attractive proposal I've seen is for a 'gravitational tractor' where a vehicle with an ion-engine is held aloft 'above' the target rock/dustpile/snowdrift and gradually applies a small force (equivalent to it's local weight, so merely ounces in the case of a test vehicle) for years at a time. In the case of a more urgent removal mission, you could send more spacecraft rather than being stuck with a Saturn V/Icarus scenario. By remaining perched above the sweet spot you could have the best possible trajectory adjustment, constant sunlight, and the ability to move loose structures.

Bob Shaw

Posted by: tty Dec 27 2006, 06:23 PM

I agree that the "gravitational tug" concept is the most promising when there is plenty of time to deflect an object. In the cometary case where only months may be available an Orion-style deflection using nuclear charges is probably the only remotely feasible method.

tty

Posted by: RJG Dec 27 2006, 06:50 PM

Can someone explain to me why it is preferable to hover over an object exerting a tiny force rather than simply coming into contact and giving it a good old fashioned shove? I understand the arguments against shattering the object into many potentially dangerous objects but even the most loosely held together bunch of rocks should withstand pounds /kilos rather than ounces /grams of force -particularly if spread over a large area by, say, a net arrangement? As Bob point out, we may not have the option of time to play with very small forces.

Rob

Posted by: tuvas Dec 27 2006, 06:57 PM

It's simple. If you use a gravitational pull method, you don't have to do several very complex things, namely:

1. Land on the astroid, in a very precise location.
2. Fire a rocket in the correct orientation.
3. Worry about shaking off debris, it is quite possible to knock of large rocks which could be potentially hazerdous.

But the main thing to keep in mind is that asteroids rotate, and quite often do so erratically. So the bottom line is, you might not be able to fire in the correct direction to shove it in the nessicary way to move it.

Posted by: RJG Dec 27 2006, 07:09 PM

Thanks Tuvas -sounds like lots of good reasons. Though if it is irregularly shaped and rotating you'd have to stand off some distance -or do some pretty fancy station keeping to avoid collision. And you'd want to be as close as possible or the inverse square law seriously diminishes the effect.

Not a bad idea to get some practice in before we need to do it for real!
Rob

Posted by: nprev Dec 27 2006, 07:22 PM

...and even then we'd probably have to try to severely fracture the nucleus al a Lucifer's Hammer in order to disperse the terminal impact energy. However, the Stardust results are kind of scary in this respect: at least some comets may be pretty mechanically solid instead of just big snowballs as we've assumed for many years...

On the lighter side, per the NASA NEO site 2006XG1 now has 99 observations under its belt, miss distance is now 0.72 Earth radii, the Palermo rating has decreased (good) as well as the estimated impact energy, and the sigma LOV now indicates much less certainty re the close-encounter trajectory: http://neo.jpl.nasa.gov/risk/2006xg1.html

Posted by: JRehling Dec 27 2006, 08:18 PM

I wonder if a good long-term defense would be to try to give the Earth some Phobos-class satellites diverted into elliptical orbits from the asteroid belt. Then stop an incoming impactor by moving the blocker into the way. The advantage is that you wouldn't have to launch any "solutions" out into solar orbit all of a sudden as the threat arose. Disadvantages abound, of course, including debris from the pre-collision and the threat of just plain missing.

The "push" strategies for dealing with a short-period threat, of course, leave the menace there to eventually swing back towards Earth after decades or centuries. A strategy for dealing with those would be to push them into the Moon.

Posted by: nprev Dec 27 2006, 08:31 PM

Interesting idea, JR, though I think trying for lunar collisions might be a bit risky; the Moon's path from a heliocentric viewpoint is rather complex, and fine-tuning the trajectories might prove problematic (especially if we ever have lunar colonies! )

I still like the thought of ultimately placing them into orbits that can be accessed from Earth using minimal delta-V for future mining applications.

Posted by: tuvas Dec 27 2006, 09:10 PM

There is a very good reason why both ideas (Crashing it into the moon and putting it into earth orbit) would be very difficult. While it is relatively easy to move a rock away from the earth (Think of how easy it is to make a dart miss a bullseye, and multiply it by 10,000 or more), it's much harder to get it into a precise lunar trajectory, and even harder still to get it into a perfect keyhole to orbit the earth. Most of the time one must depend on rockets to slow down the approach near the closest point it will be, and with an asteroid, to put it into a useful orbit would require way too much rocket fuel, not to mention other stuff like being riskier for the earth and timing and all of that fun stuff.

Posted by: ugordan Dec 27 2006, 09:22 PM

Apart from the nasty possibility of missing you mentioned, the biggest problem I see with this scenario is how a Phobos-style moon would absorb the hundreds or thousands of megatons of impact energy. It seems such an energy density is way too much for a small body of an order of 20 km in diameter to structurally hold it together. Whether the moon be a rubble pile or a solid rock I'd expect it to simply blow up. Spectacularly. Then you get many smaller projectiles (say several meters diameter) possibly spraying back on Earth. That's still at least kilotons of energy per fragment, not something you want to be at the receiving end of.

Posted by: Bob Shaw Dec 27 2006, 09:32 PM

Yeah; it wouldn't work - it's the technique developed at the Wily E. Coyote Memorial Physics Dept, ACME University!

You want to move these things *well* away from here, not least because the further away, and sooner, you move it, the more chances you have.

Posted by: tty Dec 28 2006, 12:37 AM

You definitely do NOT want to use any violent technique that might break an impactor up into small fragments. Remember that according to recent research the main killing mechanism of the Chicxulub impact was the vast cloud of dust-sized secondaries that deposited enough energy in the stratosphere to cause wordwide fires and fry unprotected organisms.

That, paradoxically, is one reason I think using nuclear charges would be safer than a series of "deep impacts". The shock wave from a directed-energy nuclear weapon set off some distance from an impactor would act more or less in parallell and equally on all parts of it and therefore run less risk of breaking up an "orbiting rubble pile".

Safest of all (but very slow) is of course "gravity tugs", which apply an equal force on each atom of an impactor.

tty

Posted by: ugordan Dec 28 2006, 12:57 AM

Except that would be a very cost ineffective way to divert an object. Probably wouldn't work, either. Some 80% percent of the energy released by a nuclear weapon is soft x-rays, the rest being mostly kinetic energy of the fragments. Were you to arrange detonation some distance away from the object, the surface area of the expanding debris that did useful work would diminish rapidly. In essence you'd waste a large fraction of the bomb's yield on nothing. It's arguable that the fragments would push upon the object with any significance when they hit. Let's say you set the bomb off so 10% of the spherical debris cloud impacted the object - a fairly close burst. Given a warhead in the (conservative) range of 1 ton, that's 100 kg of material pushing on a kilometer or so sized object. Even given the high nucleus speeds (something like thousands of km/s) that wouldn't do much. The fragments would still deliver a shock to the object, the front being parallel wouldn't help much. As for the bulk of the energy released - the x-rays, I imagine they'd be rapidly absorbed by the first couple of millimeters of surface regolith which would explosively flash into steam, generating some impulse thrust and producing a shock wave on its own in the object, in advance of the much slower fragment debris cloud that is yet to arrive. The x-ray generated shock wave shape wouldn't be dependant on whether the bomb went off near or far -- what angle the x-rays were absorbed wouldn't matter as the material would blow-off vertically to the surface at that point. There's really no easy way to convert the sudden release of many megatons into a gentle push.
A nuclear charge would basically only be good at destroying an object and we don't want that. Moving the detonation point away from the object will rapidly diminish your return, if any.

BTW, what's a directed-energy nuclear weapon?

Posted by: nprev Dec 28 2006, 04:14 AM

Actually, I meant putting threatening bodies in accessible heliocentric orbits that would require little more than cislunar delta-V to reach for future use. I agree that putting the things in orbit around the Earth would be WAY risky given the current state of the art & our limited knowledge about these bodies.

Posted by: dilo Dec 28 2006, 06:26 AM

Agree with you, nprev. Also considering that, in order to put such a body in Earth orbit, you need an high delta-V impressed in a relatively small time frame with further technological issues and asteroid-integrity risk.
Unless you want to use airbraking in Earth atmosphere!

Posted by: nprev Dec 28 2006, 09:35 AM

Thanks, Dilo. I think it's prudent to save these objects when feasible rather than destroy them. Our civilization will shortly have an exponential appetite for resources...better to keep any such handy, no matter how remote the application!

EDIT: BTW, I personally witnessed the 10 Aug 1972 'aerobraking' of an NEO over western Montana...gotta say that it definitely looked both risky & nerve-racking, can't imagine initiating such an event voluntarily... This pic isn't mine on the link (I was only 9 yrs old & had no camera), but it passed right over my local zenith & produced two impressive sonic booms...brr.

http://gep.alien.de/ifo/images/ifo_meteor01.jpg

Posted by: tty Dec 28 2006, 02:02 PM

It is not well known, but it is possible to build a fission weapon so that nearly all the energy is concentrated into two relatively narrow cones (conversely it should also be possible to concentrate it into a "sheet", though I don't know if this has ever been verified experimentally). Of course this would considerably increase the efficiency of the energy transfer to an impactor.

tty

Posted by: nprev Dec 28 2006, 11:05 PM

Personally, I think it'll be a long, long time (if ever) until the 1967 UN Treaty on Outer Space is amended to allow thermonuclear devices in space even for peaceful purposes, so I don't see this as a feasible alternative.

Given global proliferation of both nuclear technology & launch capability, that's probably a genie best left in the bottle anyhow. We sure don't need a new trigger for major political crises to cloud space development. However, I'm not sure if manufacturing steering bombs from, say, indigenous lunar resources is expressly prohibited by the Treaty; if I recall correctly, most of the relevant language is focused on launching weapons into Earth orbit.

Posted by: tuvas Dec 28 2006, 11:12 PM

It hasn't been completely tested, but I can tell you that most nuclear weapons (At least to my knowledge) were shaped to exert more force towards the central parts, to give more force along the ground and not so much up and down. I do know it's possible if you have something the shape of a tuna can or even more exagerated to essentially release the force in a forward and backward direction, which would increase the efficiency considerably.

As to allowing the peaceful use of nuclear weapons in space, I'm sure something could be ammended if it's outside of the moon's orbit, as I recall, no law is valid beyond that point (I could be wrong...)

Posted by: nprev Dec 28 2006, 11:28 PM

Nope...there are a number of provisions in the UN agreements that apparently extend into infinity. For example, no nation can make a territorial claim on the Moon "or any heavenly body" greater than (I think) a 50 km radius around a permanent base. A lot of it was based on the treaties that cover Antarctica.

I don't know if asteroid mining has ever been directly addressed in the UN, but I'm sure that it'll be a real dogfight someday if & when any real profit potential becomes evident.

Posted by: ugordan Dec 29 2006, 11:53 AM

Where did you hear this? By up and down you mean ground or?

Posted by: tuvas Dec 29 2006, 02:00 PM

Basically the explosion of a typical nuke expands outwards. I did a research study on Nuclear Pulse Propulsion some time ago, it was in one of the sources I found, basically the explosion for a typical nuke looks like a "pancake" for lack of a better word. It's more eliptical, but there is more explosion going to the ground level then up and down. Just a thought, that's all.

Posted by: ugordan Dec 29 2006, 02:59 PM

If you're referring to the Mach stem where the shock wave reflected off the ground produces a "skirt" when combined with the direct wave, enhancing the ground damage, then yes. As far as directed energy weapons go, they probably do exist for special purposes, but once that energy (x-rays and fragments) exits the bomb casing and hits the atmosphere it loses this directional component. From there on you have a classic spherical fireball which quickly evens out an irregular energy distribution. Apart from gamma rays and neutrons that is, which interact poorly with the air and can largerly travel freely.

Posted by: Nyx Jan 1 2007, 11:59 PM

This is my first post, so welcome everybody!! And happy new year!

May we say that 2006XG1 is more dangerous than Apophis?, or what is the level of danger we should expect from this asteroid as compared to other know NEAs... As a physicist, I can answer that, comparing fly-by distances or Torino scale, 2006XG1 is more dangerous, but Apophis' orbit seems to be more Earth-close that the other one, because of the apogee distance. What do you think about?

Posted by: nprev Jan 2 2007, 12:57 AM

I think the jury's still out, but there's no significant risk; it'll probably come in as less risky than Apophis, if for no other reason than there's only one potential encounter. The latest update as of today has increased the probable miss distance to 0.72 Earth radii and the sigma LOV is decreasing, so the asteroid's orbit is becoming better known.

EDIT: just to cap off this thread, it's now down to a Torino 0 with a miss distance of 0.93 Earth radii & a 1 in 15 million chance of impact...no surprise!