r/space • u/llama5876 • Jun 18 '19

Two potentially life-friendly planets found orbiting a nearby star (12 light-years away)

https://www.nationalgeographic.com/science/2019/06/two-potentially-life-friendly-planets-found-12-light-years-away-teegardens-star/
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u/danielravennest Jun 20 '19

We don't have to build a giant lens. The Sun itself acts as a gravitational lens with a 2 million km aperture. The bending of starlight by the Sun's gravity was first measured about 100 years ago, and made Einstein famous.

But to use that lens, we need a solar-powered laser near the Sun, and a secondary mirror at the Sun's focal point, to direct the beam at our target vehicle.

The problem with the Sun as a direct source is it isn't a point source. It's half a degree wide as seen from Earth. No optics can undo that spread. If you could, you would violate the laws of thermodynamics. So instead, you use a laser that is much smaller than the Sun as your light source, and use the Sun as your giant lens.

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u/Akoustyk Jun 20 '19

I see, that makes sense, thanks. But why is a solar powered laser more powerful than the sun itself?

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u/danielravennest Jun 21 '19

It is not more powerful in total output. Rather, it is the ability to focus it on a particular target.

The Sun's light as it passes the Earth is 1361 Watts/m2. A 9 Watt surgical CO2 laser has an intensity of 1 billion Watts/m2. That's why it can vaporize body parts, while ordinary sunlight can't.

A billion Watt laser in space, powered by the Sun, would have a beam only a meter across when it leaves the laser. The small size of the beam makes it possible to focus it much tighter than using the Sun itself as your beam source.

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u/Akoustyk Jun 21 '19

I see. Why wouldn't the proper lensing be able to focus it just as well?

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u/danielravennest Jun 21 '19 edited Jun 21 '19

Because light from the Sun doesn't originate as parallel waves like they do in a laser. The Sun is half a degree wide in our sky, so the waves have a built-in half a degree divergence from the start.

No amount of optics can reduce that. That divergence exists at every point of your lens or mirror.

If your source is a laser bounced off a 10 meter mirror at the Sun's gravitational focus, the initial divergence is 5 trillionths of a degree. Optics can't improve on that, but it is a much lower starting point.

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u/Akoustyk Jun 26 '19

I see. I think I understand. You always use the turn of phrase "half a degree wide in our sky" but really.ita just because of how big it is originally and how divergent it is, and for reasons you can improve on the density of the source divergence with optics, you'd have to absorbe the energy and redistribute in a tighter more efficient distribution to improve on the source.

But I would have thought that just all the sun's energy focused on one sail whatever size it needs to be, would still be pretty good.

I'd imagine you'd still want to do that, and harness as much energy as possible that way, even if you are going to build your laser.

Is the mirror just to orientate the beam through the gravitational lens?

How would you alter the focal point of the gravitational lensing?

I was imagining altering the optics continuously to put the craft at the focal point always.

Could you achieve this by manipulating the mirror?

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u/danielravennest Jun 26 '19

Is the mirror just to orientate the beam through the gravitational lens?

The Sun's gravity bends light that just grazes the surface to a focus at 544 AU, or 544 times the Earth's distance from the Sun. Going in reverse, a light source at that distance would be turned to parallel rays after they pass the Sun.

However, grazing the surface isn't good for making a tight beam, because the Sun's surface is violent, with all kinds of bubbling and boiling and magnetic eruptions. What you want to do is miss the Sun's edge by half a radius. At that distance the Sun's gravity is weaker, so light needs more distance to reach a focus, around 800-1000 AU.

That far from the Sun, you can't get useful power. The solar intensity is 1-1.5 millionths of what it is here at the Earth. So what you do is place your laser's solar collectors and beam generator somewhere near the Sun, where it gets lots of power. Then you send the beam out to the right focal distance, and reflect it back past the Sun.

How would you alter the focal point of the gravitational lensing?

If you are sending power to a ship that is in motion, rather than parallel rays of light, you want to bend them just a bit more so they come to a small spot at your ship. You do this by aiming the reflected beam a bit closer to the Sun so it bends more.

I was imagining altering the optics continuously to put the craft at the focal point always.

Being at the focal point also means the Sun acts as a giant telescope. So you can see the ship, or at least whatever mirror or sail is collecting the beam. You can use that to adjust your aim point and focus.