OT: Space Station Fun

Los Alamos and LLNL are packed with guys who could work this out on their nuke-simulating supercomputers. Let them figure out how to save the world instead of destroying it. They could collaborate with our former enemies, the Russians and the Chinese and the French.

In fact, there are directional nukes. But if you have years to spare (from a really good telescope system) the energy transfer from a megaton nuke to a 100m radius rock is, if I did the math right, in the ballpark of 10 PPM. That could be done with some gentle ablation. If you're short on time, get closer.

Why cleave off a chunk? Vaporize a meter off the surface at high velocity. Nukes have radical physical effects, and in a vacuum none of the good things, like neutrons and gammas, are attenuated. Surface temperatures on the target will be impressive.

They're really just staing the obvious:

A NASA analysis of deflection alternatives, conducted in 2007, stated:[39] Nuclear standoff explosions are assessed to be 10-100 times more effective than the non-nuclear alternatives analyzed in this study. Other techniques involving the surface or subsurface use of nuclear explosives may be more efficient, but they run an increased risk of fracturing the target NEO. They also carry higher development and operations risks.

In 2011, Bong Wie, director of the Asteroid Deflection Research Center at Iowa State University, studied strategies to respond to a threatening asteroid on short notice of a year or so, and determined that to provide the required energy, a nuclear explosion is likely the only thing that would work against a very large asteroid in this short time frame. Other systems designed to divert an asteroid such as tugboats, gravity tractors, solar sails and mass drivers require 10 or 20 years of advance notice.

Our record intercepting comets is surprisingly good so far although it requires complex slingshot trajectories and a few years transit time to reach it and match orbits. I guess for a hard intercept you can skip the last step you just need speed and a high precision proximity fuse.

Unlikely to be an easy option in the foreseeable future.

I expect nuking a comet will make it a lot brighter for a while.

I doubt if we will ever test this option by nuking a comet so it will either be used in anger as a last resort or not at all.

One thing the "nuke it" fiends don't seem to realise is that the top surface ablates in the flash to form an optically dense plasma and that does a very good job of protecting the rest from the remaining incoming.

The main detonation fireball has to touch the surface if you are to impart serious momentum into the target. Though buried shallow would be much better in the bunker buster style.

You seem happy enough when it goes to NIF

I wasn't talking so much about the technical difficulty as the political. G iven the greenies' constant outcry against people who are trying to keep th em from freezing in the dark, launching H-bombs into orbit on a hypothetica l would make for an interesting fight.

Cheers

Phil Hobbs

surface ablates in the flash to form an optically dense plasma and that does a very good job of protecting the rest from the remaining incoming.

The neutron scattering cross section of most materials at 14 MeV is a few b arns. A barn is 10**-24 cm**2, and there are about 10**22 nuclei per cm**2, so the mean penetration depth is a foot or so. The absorption cross sectio n is much smaller, so those neutrons will be rattling around in there for a while.

Thus the notion of ejecting a metre or so of asteroid seems pretty feasible if the dose is high enough.

Cheers

Phil Hobbs

Given the greenies' constant outcry against people who are trying to keep them from freezing in the dark, launching H-bombs into orbit on a hypotheti cal would make for an interesting fight.

You must have been reading the Murdoch media with your critical faculties t urned off. The environmental movement isn't against generating power, it's just against generating power in ways that foul up the environment. There a re plenty of only slightly more expensive ways of generating electrical pow er that are perfectly fine with environmentalists

The fossil fuel extraction lobby tries to generalise the objection to pollu ting power generators into an objection to all power generation, and the mo re gullible readers of the lower end of the popular press fall for this fro m time to time - John Larkin has been known to recycle that particular piec e of idiocy, but I had thought that you were smarter than that.

The gammas and x-rays will penetrate the rock in microseconds and be absorbed faster than material can move. If there is an optically dense plasma that somehow manages to absorb some of the incoming, it adds to the momentum transfer, or at least half of it does.

In a vacuum, most of the energy from an h-bomb is in radiation. The mass ejected from the bomb is tiny compared to the mass that could be ablated from the target. The fireball from an atmospheric blast is mostly gammas being absorbed by the air.

Yes, even saving the Earth has momumental political challenges.

I'd think that the gammas would arrive a bit before the neutrons, but both would dump most of their energy into a thinnish layer of rock or ice. The ejecta would be impressive.

By the time any atoms arrive, the "blast", it's all committed.

I'd prefer that we didn't need NIF, namely that we had banned all nuclear weapons at the end of WWII.

On a sunny day (Wed, 04 Mar 2015 08:34:04 -0800) it happened John Larkin wrote in :

If the Chinese, who did come up with gunpowder, had banned it, I think the Europeans would not have giving a * about the ban. You want to stop physics?

Agreed but it isn't very efficient compared to cleaving a chunk off.

You are still stuck with the problem that most of the incident energy goes into latent heat of melting and vapourising rock and ice.

Not for a groundburst or an embedded one it isn't.

I agree. It might well be safer, though, from the point of view of keeping the rest of the asteroid together.

If we had a year's time between the nudge and the projected impact, the nudge would have to be on the order of 40000 km / 3e7 sec ~ 1 m/s, which would need something like

For a 10 MT bomb (4e16 J), that's about 10 ppm of the yield. That sounds small, but to get the same momentum by ejecting 0.1% of the mass, the mass has to be going at 1 km/s as it leaves, which costs 10**6 times more energy, i.e. 5E17 J, which is about 120 megatons, and that's the amount actually delivered to the ejecta, not counting what gets lost vapourizing and ionizing it. Big number.

For a 100 m class object, it would be a lot easier--only about 100 kT would have to be delivered as kinetic energy to the ejecta.

A lot of the ejecta would still be very hot, and the body of the asteroid wouldn't contain the fireball for long, so to leading order I think the effect would be similar to putting the bomb between two asteroids of half the size, touching. At no time would the pressure be anywhere near hydrostatic, I don't think, so it's sort of difficult to calculate.

Cheers

Phil Hobbs

Problem? That's the entire point: stuff ablated off the surface, at insane velocities, produces impulse to move the bulk of the rock in the opposite direction.

The bomb itself contributes trivial amounts of momentum. The rock itself donates most of the reaction mass.

There isn't much "blast" when a nuke is fired in vacuum. Most blast is from the absorption of radiation by the atmosphere.

The optimum distance can be calculated by nuclear experts with supercomputers; Goodness knows we have lots of them on the payroll. But a nuke gives about 6 orders of magnitude more energy than a similar mass chemical device.

I was just rereading this, and find that I made a silly mistake--ejecting 0.1% of the mass 1000 times faster takes 10**3 times more energy, not 10**6 times. So with a year's leeway, you'd only need 120 kT delivered as kinetic energy to the ejecta.

Cheers

Phil Hobbs

I estimated (tm) 10 PPM energy transfer efficiency is needed to move a