Challenges to the Breakthrough Starshot Dream

I hate to be an old skeptic about revolutionary technological ideas such as the Breakthrough Starshot laser driven sails to send exploration computer chips to Alpha Centauri. Especially since groups of brilliant scientists are already at work solving these problems, and I have only read about it tonight. But it is OK to mention the many challenges that they are going to have to face and solve. My source is just four pages of a Special Edition of How It Works about Mars from HP.COM/GO/MARS.

Whereas I had heard of giant and slow solar sails driven by the solar wind slowly accelerating around the solar system, the Starshot is just the opposite. Here, a set of lasers of 100 GigaWatts is going to accelerate a computer and instrumented chip attached to a one meter squared sail, for just two minutes to reach 20% of the speed of light. Each object will be accelerated at 60,000 g’s, or feel an equivalent weight of 60,000 times its weight at the surface of the Earth. I’m sure the reader is already also saying Whaaat?  The Wikipedia article says 4m X 4m sails, and up to 10 minutes acceleration.

Let’s start with the power source. Each nuclear reactor in the US has about 1.1 GigaWatts of power. There are still about 100 nuclear reactors in the US, which make up about 20% of US electrical power. How they are going to concentrate all of this current flow in one spot is already hard to imagine. In a figure, they show a 9 X 20 laser array, or 180 lasers. Are they planning a half a GigaWatt laser? Sending all of that energy as a single coherent beam is good for an equally distributed push on the sail. Sending uncoordinated lasers or just microwave light could create disruptive forces on the sail.  The maximum power ever generated across all of California was 50 GigaWatts.

The sail is shown perfectly reflecting all the light hitting it. Putting 20% of US power on a square meter would burn up anything in a flash, so it must be perfectly totally reflecting, another challenge. Even a tiny spot not reflecting would absorb so much energy to totally overheat the rest of a metallic conducting reflector. The coherent laser beams add their in phase electric fields together in phase, making it so much easier to eject electrons from the sail and chip.

How are they going to attach a chip to a sail in such a way that the sail can undergo 60,000 g’s acceleration and transmit it to the chip?

How do you create miniature instruments and electronics to withstand 60,000 g’s? That is roughly the force of a hammer strike, and it will be continuously pressed, for two minutes. The only thing I can easily think of that can withstand that is a hammer head. On top of that, the sail is only a few molecules thick (why not).

The laser beams have to go through the turbulent atmosphere, even from a high altitude location. The same atmospheric movements that make stars twinkle and have to be corrected by adaptive optics in telescopes, will also have to be corrected for in the lasers to keep the beam coherent and uniform. Also, there will be heightened disturbance in the atmosphere when part of 100 GigaWatts is absorbed into heating the atmosphere.

At 20% of the speed of light, to reach the Alpha Centauri system at 4 light years away, will take 20 years. The sail is really not needed then, but could be reshaped to an antenna for sending signals back to the Earth.

With long missions, there is always the possibility of the space traveler being overtaken by new technologies. Say the first Starshot was only moving at 10% the speed of light, with the mission taking 40 years. During the next 20 years we will convert to much better quantum computers. (Let’s ignore the fact that the quantum nature requires very low temperatures.). Say the lighter capsule could then be launched at 20% the speed of light. That would arrive 20 years later at the same time as the initial 40 year flight.

Miniature and Micro satellites would be great for exploring the solar system, but you still need large antennas and plentiful power to send signals back to Earth. Although, the small explorers can first send to a local large antenna.

Forgetting a relativistic acceleration formula, we just use a direct non-relativistic one to estimate the final velocity.

V = a t = 60,000 X 10 m/sec^2 X 120 sec = 7200 X 10^4 m/sec = 72,000 km/sec. The speed of light is 300,000 km/sec, so this give 24% of the speed of light.

The distance traveled while accelerating is:

s = 1/2 a t^2 = 0.5 X 60,000 X 10 m/sec^2 X 120^2 sec^2 = 4.3 X 10^9 m = 4.3 million km. The moon is 400,000 km away, the acceleration works until the Starshot is 10 time the distance to the moon. That requires amazing aiming to a meter square sail.

Lots of challenges. Maybe that’s why we haven’t seen any space alien chips flying by at a fraction of the speed of light.

While there may be a plethora of planetary and life varieties in the galaxy, the real jewel would be an advanced civilization, which could easily communicate with us at the speed of light with giant sending and receiving antennae.

 

About Dennis SILVERMAN

I am a retired Professor of Physics and Astronomy at U C Irvine. For a decade I have been active in learning about energy and the environment, and in lecturing and attending classes at the Osher Lifelong Learning Institute (OLLI) at UC Irvine.
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