What happens to asteroids when heated into a plasma?

Question

I'm looking to build a scientifically plausible mining laser for my science fiction setting. The first approach that came to mind was a high-power laser that heats the surface asteroid until it becomes a plasma then pulls in the ionized material in with an electromagnetic tractor beam.

I'm already imagining the energy demands of such a system being astronomical. Though this is in a world where the energy demands for FTL travel have been reached so maybe compared to that this energy demand is nothing on that scale.

What I would like to know is what would happen to the asteroid itself. Could this ionized asteroid material be more easily compressed before it cools for improved storage density?

Would the ionized/gaseous states make it easier to separate the valuable materials such as metal from rock and any impurities? For that matter, would the system be easy to turn into an all-in-one extractor/refinery system?

And, lastly, what temperatures and conditions would I be looking at for turning asteroids into a plasma in the first place?

Answer 1

Using a laser may seem cool, but any mining company is going to look at cost effectiveness, and give your laser idea a pass.

While heating materials to plasma temperatures is a pretty important step to running a "Santa Claus Machine", the energy needed should come from something cheap and simple to operate: a solar mirror. Since you are in free space, a mirror of any arbitrary size can be made and extended out of simple metal foil, and focused on a point on the asteroid to heat, vapourize and ionize the material to create the plasma stream.

Since the composition of the asteroid is likely to be a mixture of materials, and the ejection of a stream of gas or plasma is going to cause the asteroid to move, a few more steps need to be taken.

Firstly, the asteroid should be encased in a foil "pouch" to capture any out streaming gasses, water vapour etc. which are also valuable resources. The heating of the asteroid is likely to cause materials to escape through fissures in uncontrolled ways that you as a miner want to avoid. The solar energy can be sent into the asteroid through a transparent port in the pouch.

Secondly, the stream of gasses should be collected into a chamber to be heated to plasma temperatures by a secondary mirror. This allows for finer control of the process than erecting a continent sized mirror and doing it in one step.

Finally, the plasma can be streamed into the Santa Claus Machine to be separated through a mass spectrometer via magnetic fields, and the separate ions of purified materials (separated by atomic weight) collected in cold traps. If the company is simply interested in mining, the process ends there with purified materials being bundled together for shipping to industrial centres, otherwise the materials are used directly via 3D printers or other assembly devices to make products for use or for sale.

There will still be station keeping issues as jets of gas are collected from the asteroid, and manipulating the mirrors won't be "easy", but the essential simplicity of the scheme compared to the mass and complexity of a laser (and all its associated hardware, power generation, heat rejection apparatus etc.) will make it compelling to any profit minded company.

Answer 2

Asteroid Laser Ablation is a related technique, where a set of lasers heats up part of an asteroid, applying a torque that can change its orbit and (hopefully) send it moving away from Earth. Looking at that might be helpful. Here are some statistics:

• Temperatures needed: 3000 K, at the most.¹
• Power source for lasers: solar power, possible reaching 50% efficiency¹, or nuclear power^[2].
• Power of lasers: 30 kW - 75 kW or more, depending on technological development.^{1, [2]}
• Asteroid sublimation temperature: 1700 K.^[2]

This is only to deflect the asteroid, though - not destroy it. We can expand on some of the results, though, using the same models. The second group, Gibbings et al., used an expression for the change in mass of the asteroid, $\dot{m}$: $$\dot{m}=\frac{1}{E}(P-Q_{rad}-Q_{cond})$$ where $E$ is enthalpy, $P$ is the power input, and $Q_{rad}$ and $Q_{cond}$ are the heat losses due to radiation and conduction.

Some rough numbers:

• $E=1.97\times10^7\text{ J kg}^{-1}$
• $P=75000\text{ J s}^{-1}$
• $Q_{rad}=3.79\times10^7\text{ J s}^{-1}$, if the laser bank is focused on 100 square meters of the asteroid at a time.
• $Q_{cond}=2.07\times10^{19}\text{ J s}^{-1/2}t^{-1/2}$, where I've used an asteroid density of $\rho=2000\text{ kg m}^{-3}$ and $t$ is the time during which the laser is applied, in seconds.

We see then that $$\dot{m}=\frac{1}{1.97\times10^7}(75000-3.79\times10^7-2.07\times10^{19}\times t^{-1/2})\quad\text{ kg s}^{-1}$$ which is a simple differential equation. If the initial mass of the asteroid is $\sim10^{19}\text{ kg}$, then the solution is, approximately, $$m(t)=10^{19}-1.92005t-2.10152\times10^{12}\sqrt{t}$$

This actually would take quite a while to destroy the whole asteroid; setting $m(t)=0$ suggests it would take $\sim10^{13}\text{ seconds}$ - millennia! Plots from one article about a system support result, give or take:

The point is, you're not going to be destroying asteroids with lasers any time soon. You can only chip away at them, bit by bit.