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NASA proposes new propulsion to cut Mars voyage to 3 days

wired.co.uk

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Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#61

Post by stcredzero »

rubidium wrote: post #9

back-of-envelope: conservation of momentum: 2 (num photons) h/lambda = (100 kg)*(c/3), so 5.9×10^36 photons needed to propel a 100 kg object to 1/3 speed of light. 1.5×10^18 joules of energy, which is 11% of the energy output of the US in 2001 (according to wolfram alpha). That's a 5.8 Tera Watt laser running for 3 days straight. I haven't even gotten into dispersion of a laser beam. The ludicrousness of this proposa…

11% of the energy output of the US...I don't doubt the theoretical possibility, I doubt the economic and experimental reality.

If we're going to be doing space travel on time scales where decades don't come into the picture, we're going to need devices with this kind of energy. Access to that kind of energy isn't ridiculous if your infrastructure is already in space, so it's only ridiculous in our current context.

The notion of creating a web of railway networks across an entire country was completely impractical before the Bessemer steel process.

I haven't even gotten into dispersion of a laser beam.

Also, diffraction lenses of ridiculous size become practical in micro-gravity so diffraction isn't an issue either, so long as your infrastructure is already in space. This is precisely how Planetary Resources will eventually make their giga-fortunes; by pioneering entirely new forms of infrastructure for an entirely new context of civilization. (This new context may not have much to do with earthly civilization, or even with humans.)

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#62

Post by stcredzero »

ldpg wrote: post #10

I think the "Mars in 3" days comment is just for scale, not a serious proposal. This is more about getting probes to whiz past other solar systems. As obviously, you can't stop the craft.

you can't stop the craft.

Zubrin and someone devised a way to use magsails to decelerate starships without using onboard fuel. That's magic, as you're not subject to the rocket equation. (Really, it was a failed attempt to develop a Bussard ramjet, but they figured out you'd never overcome the "friction" with the interstellar medium. So they were like, let's go with the friction!)

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#63

Post by kemiller »

rubidium wrote: post #9

back-of-envelope: conservation of momentum: 2 (num photons) h/lambda = (100 kg)*(c/3), so 5.9×10^36 photons needed to propel a 100 kg object to 1/3 speed of light. 1.5×10^18 joules of energy, which is 11% of the energy output of the US in 2001 (according to wolfram alpha). That's a 5.8 Tera Watt laser running for 3 days straight. I haven't even gotten into dispersion of a laser beam. The ludicrousness of this proposa…

How would it slow down at the other end?

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#64

Post by mcguire »

vkou wrote: post #54
TickleSteve wrote: post #33

Shooting a 100kg mass off at 30%C wouldn't be taken lightly by whoever it hit! Planet killer?

Not quite. Considering an inelastic collision, the impact would dissipate roughly mv^2/2 worth of energy, so... 1/2 * 100kg * (10^8 m/s)^2 = 5 * 10^17 Joules of kinetic energy. In contrast, the amount of solar energy that impacts the Earth each second is 10^17 Joules. For a more relevant comparison, the Tsar Bomba (50 megatons) released ~2*10^17 Joules. So, worst-case, the probe would cause the equivalent of a ~100 m…

But still pretty impressive.

I say we start flinging these things at all the extra-solar planets we've discovered. Think of it as grafitti or an art project. Leaving our mark on the universe.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#65

Post by dr_zoidberg »

nkrisc wrote: post #57

So please help me out here. The article suggests a craft could be accelerated to .3c; surely this would not be for a Mars intercept!? This is just an example of the potential, right? Otherwise slowing down a craft at relativistic speeds for Mars capture sounds... difficult to say the least. EDIT: OK, after thinking it through further at .3c the trip would only take about 50 minutes, roughly. So clearly that's not wha…

Sounds like Kerbal Engineering at its best... :P

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#66

Post by [deleted] »

vilhelm_s wrote: post #44
rubidium wrote: post #9

back-of-envelope: conservation of momentum: 2 (num photons) h/lambda = (100 kg)*(c/3), so 5.9×10^36 photons needed to propel a 100 kg object to 1/3 speed of light. 1.5×10^18 joules of energy, which is 11% of the energy output of the US in 2001 (according to wolfram alpha). That's a 5.8 Tera Watt laser running for 3 days straight. I haven't even gotten into dispersion of a laser beam. The ludicrousness of this proposa…

Here is the relevant passage in the paper, for the convenience of other readers: > The photon driver is a laser phased array which eliminates the need to develop one extremely large laser and replaces it with a large number of modest (kW class) laser amplifiers that are inherently phase locked as they are fed by a common seed laser. This approach also eliminates the conventional optics and replaces it with a phased a…

[deleted]

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#67

Post by CHsurfer »

kemiller wrote: post #63
rubidium wrote: post #9

back-of-envelope: conservation of momentum: 2 (num photons) h/lambda = (100 kg)*(c/3), so 5.9×10^36 photons needed to propel a 100 kg object to 1/3 speed of light. 1.5×10^18 joules of energy, which is 11% of the energy output of the US in 2001 (according to wolfram alpha). That's a 5.8 Tera Watt laser running for 3 days straight. I haven't even gotten into dispersion of a laser beam. The ludicrousness of this proposa…

How would it slow down at the other end?

Crash into the planet.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#68

Post by alejohausner »

falcolas wrote: post #52
leecarraher wrote: post #2

Jane, Stop this crazy thing! or in other words, how do you decelerate the spacecraft once you near Mars orbit. You'd need either a ton of conventional propulsion fuel , or a mars based laser there too. Worse is that you have a limited energy sources and no one to mine them. Solar might help, but it'd take a helluva alot of planning to install them via some automated system.

In the story "Flight of the Dragonfly", which used the same propulsion system, they simply detached a portion of the sail, and used it to reflect the beam back at the spacecraft to decelerate it.

This doesn't make sense to me. The detached portion would be subject to radiation pressure from the ground-based propulsion lasers, and would quickly be pushed away from the spacecraft it was meant to act on.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#69

Post by vilhelm_s »

kemiller wrote: post #63
rubidium wrote: post #9

back-of-envelope: conservation of momentum: 2 (num photons) h/lambda = (100 kg)*(c/3), so 5.9×10^36 photons needed to propel a 100 kg object to 1/3 speed of light. 1.5×10^18 joules of energy, which is 11% of the energy output of the US in 2001 (according to wolfram alpha). That's a 5.8 Tera Watt laser running for 3 days straight. I haven't even gotten into dispersion of a laser beam. The ludicrousness of this proposa…

How would it slow down at the other end?

Further Research Is Needed™. The paper says:

> A very difficult challenge is to slow the spacecraft to typical planetary orbital speeds to enable orbital capture once arriving. This task is extremely difficult as the initial entry speeds are so high (~ c) and the orbital speeds are so low (~ 1e-4 c). Dissipating this much energy is challenging. We have considered using the stars photon pressure, the stellar wind (assuming it is like our own solar system), using the magnetic coupling to the exo solar system plasma. None of these techniques appears to be obviously able to accomplish this task and much more work and simulation is needed. A simple fly-by mission is clearly the first type of mission to explore in any case to assess the environment in a given system to design (if possible) an optimized braking strategy.

Re: NASA proposes new propulsion to cut Mars voyage to 3 days

#70

Post by dr_zoidberg »

mcguire wrote: post #64
vkou wrote: post #54

Earlier quoted context omitted.

Not quite. Considering an inelastic collision, the impact would dissipate roughly mv^2/2 worth of energy, so... 1/2 * 100kg * (10^8 m/s)^2 = 5 * 10^17 Joules of kinetic energy. In contrast, the amount of solar energy that impacts the Earth each second is 10^17 Joules. For a more relevant comparison, the Tsar Bomba (50 megatons) released ~2*10^17 Joules. So, worst-case, the probe would cause the equivalent of a ~100 m…

But still pretty impressive. I say we start flinging these things at all the extra-solar planets we've discovered. Think of it as grafitti or an art project. Leaving our mark on the universe.

That sounds like the kind of art that Black Hat Guy from XKCD would aprove of.
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