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https://phys.org/news/2024-02-laser-driven-lightsails-stable.html
It's a long way to the nearest star, which means conventional rockets won't get us there. The fuel requirements would make our ship prohibitively heavy. So an alternative is to travel light. Literally. Rather than carrying your fuel with you, simply attach your tiny starship to a large reflective sail, and shine a powerful laser at it.
The impulse of photons would push the starship to a fraction of light speed. Riding a beam of light, a lightsail mission could reach Proxima Centauri in a couple of decades. But while the idea is simple, the engineering challenges are significant, because, across decades and light-years, even the smallest problem can be difficult to solve.
One example of this can be seen in a recent arXiv preprint paper. It looks at the problem of how to balance a lightsail on a laser beam. Although the laser could be aimed directly toward a star, or where it will be in a couple of decades, the lightsail would only follow the beam if it is perfectly balanced.
If a sail is slightly tilted relative to the beam, the reflected laser light would give the lightsail a slight transverse push. No matter how small this deviation is, it would grow over time, causing its path to drift ever away from its target. We will never align a lightsail perfectly, so we need some way to correct small deviations.
For traditional rockets, this can be done with internal gyroscopes to stabilize the rocket, and engines that can dynamically adjust their thrust to restore balance. But a gyro system would be too heavy for an interstellar lightsail, and adjustments of the beam would take months or years to reach the lightsail, making quick changes impossible. So the authors suggest using a radiative trick known as the Poynting–Robertson effect.
The effect was first studied in the early 1900s and is caused by the relative motion between an object and a light source. For example, a dust grain orbiting the sun sees light coming at a slight forward angle due to its motion through sunlight. That little forward component of light can slow down the asteroid ever so slightly. This effect causes dust to drift toward the inner solar system over time.
In this paper, the authors consider a two-dimensional model to see how the Poynting–Robertson effect might be used to keep our lightsail probe on course. To keep things simple, they assumed the light beam to be a simple monochromatic plane wave. Real lasers are more complex, but the assumption is reasonable for a proof of concept. They then showed how a simple two-sail system can use the effects of relative motion to keep the craft in balance. As the sails tilt off course slightly, a restorative force from the beam counters it. Thus proving the concept could work.
More information: Rhys Mackintosh et al, Poynting-Robertson damping of laser beam driven lightsails, arXiv (2024). DOI: 10.48550/arxiv.2401.16924
(Score: 1) by Chrontius on Monday February 12 2024, @02:04AM (1 child)
You’re sort of recapitulating the Starwisp concept. Each ship would mass only a kilogram, between the massive sail and an instrument package the size of a Coke can. They would be light enough and cheap enough that you could launch entire swarms of the fuckers, which will be nice to have when inevitable attrition causes the failure of a portion of your swarm. Because a single speck of dust won’t kill the entire mission, you can use considerably less shielding per probe which makes them, again, lighter faster (both to make, and their cruising speed) and cheaper. This gets you into factory-style mass production, which again cuts down the cost of your space probe program.
Let’s conservatively say that a Starship can launch 100 metric tons to a parking orbit. That’s 100,000 Starwisps! For the price of one bigass interstellar probe, you could blanket every system within ~20 light years with precursor probes. Plus, because they’re so small and can accelerate so fast, you can use them as improvised relativistic missiles in case you need to shoot down the alien mothership or something! ;)
(Score: 2) by VLM on Monday February 12 2024, @12:32PM
Also you could vary the mass very slightly resulting in slightly different acceleration and, eventually, arrival times, so as to snap multiple pics of possible planets, etc.