Launch | Keeping beam pointed on meter-scale lightsail

There are a number of effects that make this task difficult. These include beam instabilities, laser mode issues, differential forces on the sail, differential heating of the sail, and instabilities in the atmosphere induced by the energy of the beam.

The above challenges can be mitigated by spinning the sail, and by shaping both the sail and the beam. Feedback from the sail to the array helps, but the short time of flight requires a self-stabilizing system. The firing time of the beamer is on the order of 10 minutes. During this time the Earth rotates, requiring some beam agility on the order of 2 degrees. Coarse sub-element pointing and fine pointing via phase corrections at the array system can provide beam agility on the order of 30 degrees in order to accommodate various targets.

One promising approach is to shape the sail so that its position on the beam is stable— i.e. the spinning sail itself experiences torques and forces that restore its position and orientation as low-frequency pointing errors move the beam away from the sail’s centroid. High-frequency jitter degrades the overall power imparted to the sail, but the sail’s dynamics limit its susceptibility to such disturbances above a certain bandwidth.

Since a phased array would be used to form the spot, the beam profile could be shaped to maximize the sail’s ability to maintain its own position on the beam without active feedback control.

Apr 22, 2017 17:54 Nathan Bemis Posted on: Breakthrough Initiatives

RE: Apr 04, 2017 22:42 Breakthrough Initiatives

Thanks again for being responsive to not only myself but all of us who post ideas here.

The dimple idea comes from not only through fluid but through the air, Mythbusters tested the idea on a vehicle (video provided in main post) and it was successful. I'm not one to know the results of how it would work in a space environment. Which is why I pose the question here. I also try to come up with a solution to minimize the drag a sphere design would have from it's trailing side, perhaps a way to have the drag space occupied or some solution for it to negate drag. The ideas could be far off, and I'm far from knowing the science. Just posing some ideas from an average interested persons perspective.
Personally I think a sphere design is worth the sacrifice in weight and time. I question how easy it can be to send off a flat target in comparison to a sphere. With a sphere, it doesn't matter how it moves/rotates it should stay on track and easily controlled. Plus then you can use the Coanda Effect with the beam onto the target/s. I know there are pros and cons to it. That goes with everything.

Jul 15, 2017 03:27 Breakthrough Initiatives Posted on: Breakthrough Initiatives

Apr 22, 2017 17:54Nathan Bemis Posted on: Breakthrough Initiatives

While there are certainly some analogies that can be made between fluid mechanics and the optics of the beam-sail system, at the end of the day they are quite different. The Coanda effect is fundamentally fluid-mechanical and doesn’t show up in our case.

- Zac Manchester, Breakthrough Starshot

Sep 26, 2017 22:36 Nathan Bemis Posted on: Breakthrough Initiatives

With a sphere (ish) design, what if you can manipulate the beam with a (*lens type effect) to wrap around the craft (maybe trapping reflective film or shell around the craft?); to Also Pull the craft on the opposite side like a vacuum. I believe this can provide two main appreciative results, 1. Relieving much of the G-Force stress for structural integrity. 2. Provide more use of the beam during launch, ideally resulting with a noticeable improvement of the crafts cruising speed.
A possible bonus effect of this idea is that the immediate path may be cleared of debris by the laser streaming ahead.

The idea of a sphere design is for the launch phase stability and control challenge, not necessarily for the whole trip. Once the launch is complete it could be flattened or folded into a narrow shape and used as barrier from dust impacts for the craft, Then possibly redeployed to assist with stopping and/or making an eventual 180* U-turn to return when the project comes to developing that.

Nov 01, 2017 12:48 Nathan Bemis Posted on: Breakthrough Initiatives

There is a concern of too much spin of the craft (at least with a sphere design). There could be many ways to approach this, by the surface layer microscopically, by larger changes with dimples, or otherwise.

Microscopic surface modification example: A Diffractive Meta-Sailcraft -

Larger scale modification Example:

You can also have the chip located away from the center to create a small addition of weighted drag to favor a side to face the beam. The further away from the center, the greater the effect becomes, I imagine.

Related Article of interest:
"Dr Yoshihiki Arita, Dr Michael Mazilu and Professor Kishan Dholakia of the School of Physics and Astronomy at the University of St Andrews were able to levitate and spin a microscopic sphere, purely using laser light in a vacuum, briefly up to 600 million RPM before it broke apart." - "The rotation rate is so fast that the angular acceleration at the sphere surface is 1 billion times that of gravity on the Earth surface– it's amazing that the centrifugal forces do not cause the sphere to disintegrate!"

Encouraging preliminary results from the TVIW talks by Giancarlo Genta, Professor, Department of Mechanical and Aerospace Engineering, Polytechnic University of Turin, who spoke on the Breakthrough Starshot craft stability.
His testing was basically to test stability, and was not including all factors expected. With positive results so far, he plans to continue testing and find the limits of what the beamed craft can do. on page 7 the graph looks like the beam testing has 4 main areas of contact. (Apologies if I am misunderstanding the graphic)
I hope testing is also performed for an evenly distributed array (form fitting) like a cone, where it gradually gets slightly weaker toward the center.
In this example, the "beam" ends after contact on one side, but I hope there are ways to manipulate the beam to wrap around and include a pulling effect.
I realize if this is possible, the research would be incredibly hard to analyse with the additional equations in this scenario. I feel it's an idea worth exploring that may have positive benefits.

Feb 26, 2018 21:29 Breakthrough Initiatives Posted on: Breakthrough Initiatives

Thank you for your contribution and the links.

Sep 13, 2018 16:26 Breakthrough Initiatives Posted on: Breakthrough Initiatives


The stability of the sail in all aspects - both its ability to remain on the beam, and its resilience to perturbations introduced by the beam as it is accelerated, are, as you say, of great concern to the success of the project. The sort of modeling discussed by Professor Genta in the video you shared will be a critical component of understanding the nonlinear response of the sail. We anticipate that this will be a long process of iteration, subject to constraints and technological developments in the design of the material itself, the laser system, and the shape and structure of the sail. The spherical sail concept as proposed by Manchester & Loeb at is indeed a promising direction, and various versions of the methods you suggest to mitigate concerns about its spin are under consideration. Ultimately the microstructure of the sail may even be manipulated such that the spherical sail itself is not necessary for stability, but just one of many plausible designs.
Just a note about the shape of the beam itself - Manchester & Loeb used the sum of four Gaussian peaks to construct their cone-like beam. This choice may seem somewhat arbitrary, but in fact a Gaussian beam is the natural shape produced by most plausible designs for the laser array - it's the best way to concentrate laser power in a small area given an array of some size. The more Gaussian beams that are added together to get a closer approximation of a true conical potential, the more beam power is wasted on the outer part of the cone, and the more complicated the laser array must become. Tradeoffs like this between the expense of the laser array and sail stability may be unavoidable, and the final design of the mission will have to make choices depending on cost and technological constraints at the time.

- John Forbes and Avi Loeb (Harvard), Breakthrough Starshot

Jun 03, 2020 12:56 Nathan Bemis Posted on: Breakthrough Initiatives

*Basic concept here*, design and other details to be explored. A gradually speeding up over time approach for the craft.

Can an onboard laser setup be able to push the craft (itself included) in space in any way possible?? Perhaps this way the laser can be active for far longer than a blast from the array. If it can be figured out how it can be self sustaining, it may provide a way to have it to return back.

I understand there are quick write off questions like "too heavy" and such. This is merely an idea worth exploring.

Having the laser beam recycled and reflected multiple times over during an extended period of time. Like a trapped beam inside reflecting mirrors. Instead of the beam hitting the face of the target just one time. It'll capture, trap, and reflect it many times over. Again it's just an idea and perhaps there are creative answers to problems that people see in this idea. Thanks for your time.

A few positive things for this idea:
1. Control and corrections more manageable
2. G-Forces concern not an issue
3. Heat will be much more manageable compared to the array.
4. Possible? Way to redirect the craft and have it return back.
5. Even if this idea is flawed, a good one might be created from it.

v Thank you for your reply v :)

Jun 25, 2020 15:49 Breakthrough Initiatives Posted on: Breakthrough Initiatives

Dear Nathan,

Thank you for your insight and question. This is a very good question with many parts. It sure would be handy if we could bring along a power source to operate a laser. This would allow us to stop at the target star and maybe even return to earth. We have been unable to find a power source that is light enough and provides enough power to propel the craft.
Placing a laser onboard the Starchip is a great idea, however, we ran into a few problems when trying to realize this design. You must carry your fuel with you in this approach. This quickly becomes a problem. In interstellar propulsion design we call this the tyranny of Tsiolkovsky or the tyranny of the rocket equation. In this scenario you box yourself into a corner. You need to bring a very large nuclear or fusion machine with you in order to produce the energy to use the laser. Our analysis suggest you need a spacecraft of 100’s of tons to make this work. The top velocity you can achieve is on the order of 0.1% C. Which is much lower than our desired 20%C.
We even considered using an antimatter annihilation drive to obtain the required energy. But this was also too heavy. The current solution we are working is to leave the fuel on the ground and take no fuel with you. This way we can lower the mass of the craft to 1g and achieve 20% the speed of light. For a more complete discussion of Nuclear rockets please see

Pete Klupar
Director of Engineer, Breakthrough Initiatives

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