#### Re: Calling Isaac Newton...

There is a huge problem, just the article doesn’t explain it well.

The sails don’t slow down, but the acceleration drops off quickly, which imposes basic limits that aren’t immediately obvious.

If you start off with a certain acceleration from the Earth (distance from the Sun 1AU), once you get to 1.4 AU from the Sun the thrust and acceleration halves (inverse square law). It halves again @2 AU, plus you spend much less time there so there’s less velocity change. Etc, etc. Almost all the velocity the velocity the sail will ever gain, is achieved within the first 0.4 AU. Light sailing is marketed as constant thrust, allowing ultra-low acceleration to get you somewhere useful, but it really doesn’t live up to that.

Some numbers: to escape the solar system, you need delta-v 12km/s from near-Earth-but-not-gravitationally-bound, on a 60 million km effective runway. This requires 0.0012 m/s2 acceleration, which for a light sail is a high hurdle to jump, but achievable. To get to the nearest star within a century needs delta-v 12,000 km/s. To achieve that on only a 60 million km runway, you need 1200 m/s2. Acceleration 120g is just impossible for solar sailing.

That’s why people want to use lasers, to increase the incident power per square meter, and get the acceleration done on a reasonable runway. These guys are achieving 0.1g acceleration, which is an amazing achievement. The problem is, it still doesn’t scale well. Remember the runway length of 0.4AU? Now you have to be able to focus your laser onto the spacecraft sail at 60 million km distance. Focusing is diffraction-limited, the tighter beam you want, the bigger the mirror. However you scale things, there’s problems.

The Breakthrough Starshot wants to make a sail of 14m2, weighing 4 grams, to reach 15% of speed of light with 8.5 GW lasers. That gives a very respectable 7000m/s2 acceleration. It also requires electronics that can survive 700g acceleration (feasible), and given that you are focusing a *8 GW laser* on it sufficient to vaporise most things, either: 99% reflective sail (feasible) with payload that can survive 2000Centigrade (not feasible), or 99.99% reflective sail (not feasible) with payload surviving 380C (marginal, payload mass <<4g, limited thermal insulation)

But the main problem is that they need to focus the lasers onto a spot-size of 4meter diameter from 1AU distance. That’s the same optics problem as a telescope resolving 4m at a 1AU distance. Atmospheric twinkle distortion prevents you doing that, so it has to be a *space* laser. And fundamental optical diffraction limit requires you to make an 18 kilometer diameter mirror to focus the laser. The largest mirror ever made is a 10 meter mirror, on the ground, even with adaptive optics, and James Webb space will be 6.5m.

An alternative mission concept has the spacecraft sail 4km diameter to have an 18 meter launch mirror. Then, the spacecraft mass x million, and even on the Starshot hyper-aggressive mass assumptions needs an 8 petawatt space laser firing for two hours continuous, which isn’t going to happen. In other words, people are tending to focus (pun intended) on the spacecraft side of laser sailing, which isn’t really the problem.