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Comment by rubidium | Hacker News Reader

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 proposal is left as an exercise to the reader.

Edit: here's their proposal with physics. Just to be clear, I don't doubt the theoretical possibility, I doubt the economic and experimental reality. http://www.deepspace.ucsb.edu/wp-content/uploads/2015/04/A-R...

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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 array of small optics that are thin film optical elements. Both of these are a follow on DARPA programs and hence there is enormous leverage in this system. The laser array has been described in a series of papers we have published and is called DE-STAR (Directed Energy System for Targeting of Asteroids and ExploRation). Powered by the solar PV array the same size as the 2D modular array of modest and currently existing kilowatt class Yb fiber-fed lasers and phased-array optics it would be capable of delivering sufficient power to propel a small scale probe combined with a modest (meter class) laser sail to reach speeds that are relativistic. [...] As an example, on the eventual upper end, a full scale DE-STAR 4 (50-70 GW) will propel a wafer scale spacecraft with a 1 m laser sail to about 26% the speed of light in about 10 minutes.

(I like the idea of DARPA putting together a 70GW orbital laser array — "no no, this is purely for scientific purposes, trust us!")

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.)

I like exercise ;-)

If we wanted a self-assembling orbital solar array to provide that much power, it would need to be almost half the surface area of the moon.

http://www.wolframalpha.com/input/?i=1.5%C3%9710^18+J++%2F+8...

we could use a suitable size black hole to create lensing effect - that way we could capture sizable chunk of the sun's energy

probably making a reasonably large lens would be somewhat easier than creating and controlling a black hole

The .3c estimate was a best theoretical case, not proposed for Earth to Mars voyage. Note 3 days to Mars is ~50M miles/day ~2M miles / hour ~500 miles/sec or .3%c - 1/100 of his best case estimate.

Good point. Cuts the acceleration/deceleration down to about 1 G, which would actually be useful.

Exactly how do the deceleration work... ? Hoping someone put a really big haystack on mars?

I assume they can change the angle so the thrust vector is negative. It depends on the geometry between Earth, Mars, and the spacecraft. Or maybe they accelerate to just match the orbital velocity of Mars as they arrive.

Another thing to consider: mirrors are never perfect. 100% of the light is not reflected.

If the spaceship uses high quality laser mirror with 0.00001% absorption rate, absorption from 5.8 terawatts is still 5.8 megawatts of power to get rid of in space.

Nit: 0.00001% of 5.8 TW is 580 kW. (I assume you just misplaced a decimal point.)

    5,800,000,000,000 W × 0.00001 =

           58,000,000 W  = 58 MW

I misplaced decimal point, but it's for different direction.

0.00001% != 0.00001

> 100% of the light is not reflected.

That isn't a mirror, it's a black hole!

How would it slow down at the other end?

Crash into the planet.

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.

Wonder if you could use a one shot variant of Orion to slow down?

Fling a device ahead of the craft, detonate it and catch the energy on the back of the sail.

Aerobreaking could net you some gain. Obviously, this requires a more engineeered vehicle, including possibly an ablative shield.

Even at that, however, you'll be skimming very close to Mars' surface and incurring very high temperatures.

Perhaps (and this is a big perhaps) we could engineer lifting body surfaces to increase the time-in-atmosphere and dissipate heat better.

This is why I love playing Kerbal Space Program.

Aerobraking from .3c would require materials many orders of magnitude more heat-resistant than known to humanity.

Yes. Stopping from 90000km/s in 100ms (more or less earth radius) might prove to be a bit hard :-)

We just build a second megalaser on mars. Super easy.

I think it is good that people are thinking very far out of the box for this. Reason? What we think we can do now isn’t enough, so if we want to go to Mars, some ridiculous idea must eventually turn out to be practical.

For that reason, I would not worry too much about that 11%. _If_ we go into space, we will have to build enormous solar arrays in space, and they could provide order of magnitude more power than we produce now.

I would not aim for Mars, though. Instead, send it out to overtake Voyager. To do that, you do not need to decelerate, and you do not need to goto 0.3c, either.

(Actually, some billionaire should put a huge price on the first one to overtake Voyager and send a video of it back to earth. That could become a fun race, with later, faster, spaceships overtaking early entries.)

Not to mention the ~350 G's of acceleration/deceleration you'd experience over those three days.

Sounds like fusion and better lasers need to be viable before this is viable.

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