[X] My Assistant

[nprev]

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

[volcanopele]

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.

[dilo]

NeoDys 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

[nprev]

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... )

[Bob Shaw]

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... )

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

[tty]

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... )

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

[RJG]

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.

Bob Shaw

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

[tuvas]

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

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.

[RJG]

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

[nprev]

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

...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

[JRehling]

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.

[nprev]

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.

[tuvas]

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.

[ugordan]

Then stop an incoming impactor by moving the blocker into the way.

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.

[Bob Shaw]

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.

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.