http://www.jiba-jaba.com/Asteroid-On-A-Collision-Course-With-Earth.aspx

It makes me think about do I really do enough in my life to actually make a difference…

Agreed. I never stated “no nukes” ever, just as a last resort.

And, also, we have several thousands nuclear warheads but no working nuclear electric rocket yet.

According to this document, the Sun could be a source of unlimitled power for a Gravity Tractor:

A solar sail gravity tractor (SSGT) spacecraft is proposed as a viable option for deflecting a certain class of near-Earth asteroids (NEAs) such as highly porous rubble piles rather than solid monolithic bodies. Solar sails are large, lightweight reflectors in space that are pushed by sunlight. The SSGT spacecraft concept is based on the so-called Gravity Tractor (GT) for towing asteroids by using gravity as a towline, which was proposed by Lu and Love in the 10 November 2005 issue of Nature. It exploits the “propellantless” nature of solar sails; consequently, its probable advantage over a GT spacecraft propelled by ion engines is its longer mission life times (> 10 years) with a larger “propellantless” ΔV capability. Furthermore, it has no concern of rocket plume impingement on the asteroid surface. This paper demonstrates the practical hovering control feasibility of an SSGT spacecraft for towing NEAs. For example, a 5-year towing of asteroid Apophis using a 2500-kg SSGT spacecraft (equipped with a modest 90-m, 50-kg solar sail of a 0.03-N solar thrust with a 35-deg sun angle) and an additional 3-year coasting time will result in an orbital deflection of 30 km in 2029. A 30-km deflection is more than sufficient to move Apophis out of its 600-m keyhole.

“And, also, we have several thousands nuclear warheads but no working nuclear electric rocket yet.”

How many of those warheads are attached to rockets or probes that could travel 150 million kilometers or more?

Actually, that works out at 35,850.86 kilotons;

comparison. 475 kiloton warhead weights only 270 kg

proposed gravitational tractor may be — such as a Nuclear Electric Rocket. Even a conventional rocket powered gravitational tractor could be refuelled by robot ‘gas stations’.

comparison ISP :
VASIMR 36 000
nuclear pulse propulsion > 100 000

And, also, we have several thousands nuclear warheads but no working nuclear electric rocket yet.

Phil, when the bloody hell are we gonna get a preview/edit facility here?!

Writing in Nature^*, physicists Edward T. Lu and Stanley G. Love of NASA’s Johnson Space Center show that a gravitational tractor is a reasonable proposition: it is robust to the asteroid’s structure and rotation rate, and a technologically feasible plan that could impart a delta-v of a few millimeters per second to an asteroid with a diameter of 200 metres over the course of several years, sufficient to prevent an Earth impact.

My response to your java applet post is awaiting moderation.

A : No, but you can download demo java applets doing N-body simulation of trajectories http://www.cs.cmu.edu/~scandal/alg/nbody.html
It is really no big deal, any such mission woud start at LEO so all you would need is position and velocity of the of the asteroid, orbital parameters of the probe, and work out trajectory of the warhead by numerical simulation The gravity of any other body than earth, sun, moon, and the asteroid would be negligible so you could do them really fast.

Greg already addressed this, but I have some thoughts as well.

So, we need to start with X probes in orbit, all armed with nuclear warheads. Great, so where are these probes? Who is going to build them? Who is going to launch them? What will happen if something goes wrong with the launch? Who will control them once they are in orbit? And, more pertinent to an impending collision, just how long does it take to figure out the exact location of each of these probes for purposes of calculating the trajectory of the warheads? As I understand it, getting the precise location of a satellite is a mite on the fuzzy side.

My other questions still stand, which Greg reiterated. Are we using 1 100-megaton nuke? 10 10-megaton nukes? 100 1-megaton nukes? If there is more than one, then I assume that they will all hit at the exact same time to avoid situations like Greg describes, in which subsequent missiles miss or do only negligible damage, at best, and at worst send debris hurtling more directly toward Earth when it may otherwise have been on course to miss the planet. So, how long does it take to calculate the timing required to hit the asteroid at the exact same moment? Certainly more than milliseconds, and I’d wager it’s a somewhat non-simple bit of math.

So, really, your “milliseconds” claim was pulled out of thin air and has nothing to back it up. Nor does your claim that it is simple to determine.

Perhaps I’m being a bit unfair. The calculation itself may take milliseconds on a properly robust computer, but the initial legwork of determining position of everything involved, all object speeds and directions, etc., followed by inputting everything into the calculating module, would take a wee bit more than a few milliseconds.

And from your earlier post:

No one would really bother to estimate how long would 100 of them take.

Actually, I think they probably would, as it would affect the timing of every part of the process. You need to have some idea of how long it would take to figure out something requiring that level of precision so that the overall timing doesn’t get screwed.

Some follow-up questions: what happens when the orbital probes collide with something else? What happens if someone that controls one of those probes gets a bit twitchy and decides to use them for a different purpose? What happens when the timing is off by a few seconds? Will the asteroid still pose a danger to Earth?

“A : No, but you can download demo java applets doing N-body simulation of trajectories (cs.cmu.edu/~scandal/alg/nbody.html)
It is really no big deal, any such mission woud start at LEO so all you would need is position and velocity of the of the asteroid, orbital parameters of the probe, and work out trajectory of the warhead by numerical simulation The gravity of any other body than earth, sun, moon, and the asteroid would be negligible so you could do them really fast.”

Which of those java applets did you use to determine the trajectories of 100 rockets would only take a few milliseconds to calculate?

Wait, so we would need 100 nuclear warheads in low-earth-orbit waiting to be sent to intercept an asteroid? How many times have we sent a probe to intercept an asteroid or comet? Were those missions simplistic, like you said? And if we were going to hit one asteroid with 100 nukes, would we plan to hit the asteroid with all 100 at once, 10 at a time, 1 at a time? What happens when one nuke hits, fractures the target in two, and the next 10 fly right on thru the gap? Is calculating those course corrections 1,000 times a second trivial as well?

This sounds way more complicated than you claim.

The same problem is with a kinetic impactor. 30 ton used against a comet speeding at 100 km/s will produce 50 kiloton explosion which is 20 times less. Against much slower asteroids it would be maybe just 5 kiloton. Which is really pathetic.

Actually, that works out at 35,850.86 kilotons; however, the point of the kinetic impactor is not to pulverize the asteroid/comet, but to ‘nudge’ it out of the way — the feasibility of which is what ESA’s Don Quijote mission aims to determine.

So, do you dispose of your car and buy a new one every time it runs out of gas?

my car i neither nuclear powered, neither most of its mass is fuel, neither the fuel is of comparable price to the car, and also, I don’t usually travel outside of earth gravitational well.

Todd : my link is stuck in the moderation you will have to wait

If the asteroid is small, and collision decades ahead, surely gravitational tractor is a possibility (no, it can not be used again and again, because it expends fuel, and expends more of it per 1m/s deltaV than nuke).

So, do you dispose of your car and buy a new one every time it runs out of gas?

So you have no citation for your claim? Upon what do you base your conclusion that the trajectory computations are simplistic? Do you have experience calculating such trajectories, taking into consideration the Earth’s rotation, position of the moon, positions of satellites and other orbiting objects, atmospheric conditions, etc.?

A : several miliseconds.

Can you provide a citation for that figure? Thank you.

The same problem is with a kinetic impactor. 30 ton used against a comet speeding at 100 km/s will produce 50 kiloton explosion which is 20 times less. Against much slower asteroids it would be maybe just 5 kiloton. Which is really pathetic.

Even if we reduce it to ten 10 megaton warheads, the logistics are still pretty nasty.

And if we go with one 100 megaton warhead, what kind of delivery vehicle is necessary to not only get the warhead away from Earth, but keep it heading in the right direction? Does such a delivery vehicle exist already? If not, how long would it take to design, engineer and build?

Not at all. I didn’t invent it. Its cool!