# Laser-boosted Solar Sails to 61 Virginis?

I've just read this article and watched the accompanying video. Heady stuff, for sure. I'm particularly intrigued by the concept of a "solar gravitational lens telescope" somewhere out near the heliopause. I'm surprised this isn't bigger news.

I don't know how to effectively read the charts, nor interpret what they're telling me even if I did. Conceptually I grasp it, though; at least I think I do.

So, let's say we got really serious about sending a manned mission to a nearby star. In this scenario, we've determined without a doubt that there is an advanced civilization living on a planet orbiting 61 Virginis, which is (per Wikipedia) 27.9 light years away.

Many questions come to mind, and I need to break this idea up into several "questions", but here they are a few that are related:

1. Given sufficient funding to build and deploy up to three space-based lasers in the solar system, where should they be "placed" (orbited) to best accelerate a ship?

2. Given the above placements, how large are the launch windows to use the lasers?

3. How often do the launch windows come around?

• Do solar systems have launch windows? I know there are for planets because you want to go when they are going to be close together. I'm not sure you need to worry about that for something that is 27.9 light years away, since you're going to aim for where that star is going to be in 1000 years. Mar 30 '16 at 20:09
• No, I meant that the varying orbits of the laser arrays would necessitate timing your launches so the spaceship would pass the second two arrays when they were in position to provide boost. My sense is that you'd plot a curve between here and the destination, and the alignment would be somewhat natural, but I'm often wrong about these things. Mar 30 '16 at 20:26
• Ah. I gotcha now. Mar 30 '16 at 20:42
• Have you given any thought as to how the ships will slow down when they reach their destination? Lasers don't suck, you know. Mar 31 '16 at 2:19
• Once again, I misread the tittle. "Laser-Boobed Solar Snails to 61 Virgins". I tried to imagine the cover. Mar 31 '16 at 20:16

Using light to launch an interstellar probe has a number of things going for it, but it's not all peaches and cream.

Most importantly. it's wildly inefficient in the short term. For a light beam bouncing off a mirror, the thrust applied to the mirror amounts to 150 MW of optical power per newton of force. Note that a newton isn't much. For the video's proposed 70 GW array, that amounts to about 467 N, or about 100 pounds of force. Of course, the advantage is that the thrust just doesn't stop. The linked video talked about sending a package to Mars in 8 hours, and this is correct. What you may not have realized is that the package under discussion is a CubeSat, and these things have a maximum mass of 1.33 kg. For such a light object, the acceleration is about 35 g's so yes, it really does get moving. Also note that nothing is said about how to slow it down once it reaches target. This is not, in principle, difficult: you just have another 70 GW array orbiting Mars which applies deceleration. It is unfair to ask how the target array got there in the first place.

So, there are (at least) 2 things to ask about an interstellar launcher. How big is the laser array, and how big is the probe? Let's say, just as a starting point, that the probe has a 1 $km^2$ light sail, and weighs 1000 kg. This is clearly not a manned probe, and the technology is beyond what we can do (if nothing else, we can't guarantee reliable operation for a century or more, and that assumes average velocities of about 0.3 c - more on that later). Let's say that the laser arrays are the video's 70 GW. As the video points out, solar power makes the most sense, especially for long-duration power production. Conceptually, each array consists of a 10 km x 10 km solar cell array which will orbit oriented to point directly at the sum. The back side of the platform is a phased-array of laser emitters producing a total of 70 GW, with a beam steering capability of +/- 60 degrees. This limits the illumination time for an object to about 1/3 of the array's orbit. Fortunately, you specified that 3 launchers will be built, so if the 3 arrays are in solar orbit at 120 degrees spacing, one will always be available for use. An obvious requirement in this case is that the array orbit must be inside earth's orbit, since the arrays can only fire outward from the sun.

With a probe mass of 1000 kg, acceleration will be nominally about 0.467 m/$sec^2$.

How long will the array be able to supply power? Assuming a 1 um laser wavelength, the diffraction angle for the beam is the Rayleigh criterion $$\theta = 2.44\frac{\lambda}{D} = \frac{2.44 \times 10^{-6}}{10^4} = 2.44\times10^{-10}\text{ radians}$$ and this will produce a spot size of 1 km at $$R = \frac{d}{2 \theta} = \frac{1000}{2\times 2.44\times 10^{-10}}= 2\times10^9\text{ meters}$$ or about 7 light-seconds. After this range, the thrust will drop off as the square of the range, since the beam will get larger and large and the mirror will intercept a progressively smaller portion of.

The high-boost phase will take$$t = \sqrt{\frac{2s}{a}}= \sqrt{\frac{4\times10^9}{.467}}=857,000\text{ sec}$$, or about 10 days and velocity at that point will be $$v = at = .467\times 8.57\times10^5 = 4\times10^5\text{ m/sec}$$ I am, frankly, too lazy to do the math for the post-peak acceleration, but let's round up the final velocity to about $10^6$ m/sec. Note that this is only about 0.3% of c, and time to 61 Viginus is about 8600 years.

It's clear that we need bigger guns.

Now, as promised, the question of how to slow down at journey's end. It's very, very clear from the previous that there is no way affect the final trajectory with the specified array. It simply will not produce an appreciable power density over 30 light years. But let's say that we could, somehow, do this. Does that help? The answer is yes. During the voyage the probe turns around and ejects a second, much larger mirror which precedes the probe. The braking beam impinges mostly on the secondary mirror, accelerating it, but the reflected beam hits the probe mirror and provides a braking force. This is not exactly a friendly move towards the target system, since it produces an expended secondary mirror which whips through the target system at (for a secondary mirror equal in mass to the payload probe) about twice the transit velocity. Admittedly, the braking mirror is presumably some extremely lightweight material, but still...

Assuming a launch acceleration adequate to produce low-relativistic velocities, launch window is fairly forgiving, about 4 months/year. The immediate issue is to eliminate the cross-target velocity of the probe. Doing this immediately will, of course, result in a small radial velocity for any probe launched with a velocity near Earth's escape velocity, since the orbital velocity of the earth is about 30 m/sec. The ideal launch point occurs when the sun/earth vector is about 45 degrees to the target vector. Then the cross-target velocity is relatively small, and angling the mirror to eliminate this will also produce decent down-range acceleration. The exact optimum and window will depend on the thrust available and the launch velocity of the probe from earth. In principle, there is nothing to prevent using the laser beam to provide all of the thrust and the probe assembled in low earth orbit, but the numbers need to be worked out.

EDIT - Oh yes, and about the gravitational lensing thing. You can probably forget it. I haven't been able to get at the underlying calculations, but it seems pretty clear that the author simply doesn't know what he's talking about. A discussion of this is beyond the scope of this question, but I'm fairly certain that it won't work. His claims and explanations are to some degree self-contradictory, and he seems to have overlooked a few very important issues. I could be wrong (as history has shown) but I'm fairly sure I'm not in this case.

This sort of system was used by the book Rocheworld, a.k.a The Flight of the Dragonfly.

https://en.wikipedia.org/wiki/Rocheworld

The author had a very well thought out system where collector stations orbiting Mercury (and crewed by people working from a sunhook in Mercury's shadow gathered up the power and sent it out to a lens further out in the solar system. That lens then focused the light and sent it on to the lightsail.

With modern laser technology the lens stage may not even be needed, so we could just send the power directly from Mercury. This would be quite a dangerous thing to have and control though, be careful where you aim it!

In that example the ship got up to 0.2c, which is similar to that in your article which mentions 0.26c. Even at those speeds though it would still take 107 years. This is not fast enough for relativity to really kick in either, elapsed time for people on board would still be over 100 years, especially once you factor in acceleration and deceleration times.

For "launch windows" you need Mercury to be on the right side of the sun as the sail and the target, and stay in it for a while, which means that traveling to a target in line with the plane of the solar system you would be able to launch for say 20 days out of each 88 day Mercurian year and have enough acceleration time before you lost the laser. Multiple sails could be launched to different destinations though and the lasers would switch between them in turn as the planet orbited so you could keep continuous uptime accelerating something.

If you were launching up or down then that would not be a problem and you could launch at any time.

• I was thinking along the order of one laser in Earth orbit, one maybe at the asteroid belt, and another around Neptune. I'm sure there's some sort of long math equation that figures out what proper placement would be like, given reasonable effective distances (the laser is going to spread; eventually you'll only be capturing a small fraction of its power), so putting another laser out there somewhere would give you a kick. But the further out you put it, the less often your launch window comes around. However it lasts longer. At least I think so. Mar 30 '16 at 22:30

The logical place for the lasers is near the Sun, so they can absorb the maximum amount of solar energy. Many proposals suggest they orbit Mercury so the gravity of the planet keeps them aligned, and also has the advantage of providing raw materials for building the project in the first place.

Since the amount of energy needed increases exponentially as the ship gets farther away, the construction site will be busy building and launching lasers around Mercury, starting with a 43,000TW array and finishing with a monster 73,000TW array. Authorities on any inhabited planet will be watching this very closely, since you are building a real life Death Star that could fry any planet crossing the beam.....

The lasers will need energy. A lot of energy. Possibly several terawatts of energy each. So you'll need to put them somewhere they can get energy. So put them in orbit around one of the gas giants, probably Saturn because of how hostile Jupiter's radiation is, and use the gas giant as fuel to power the laser.

The lasers will be in orbit, which means that you'll have between 1 and 2 with line of sight to the space ship at all times. So as they get line of site they will aim their laser at where the ship will be when the light gets that far.

Maintenance can happen while the laser is behind the planet. The people working on the lasers can live on Titan, which actually has a dense atmosphere, though it's very cold.

The ship will be traveling toward where Virginis will be a thousand years, give or take, but the lasers will only really be pushing them for the first hundred or two, before they get to far away for the light to do much good.

• What makes you say a thousand years or so? Mar 30 '16 at 21:06
• The article specifically referred to a 70 GW laser array about 10 km on a side. Mar 30 '16 at 23:17
• @J.D.Ray he must have an average speed of 3% light speed. Mar 31 '16 at 13:23