(ORDO NEWS) — For centuries, people have dreamed of being carried by light winds at great speed across the oceans of space.As bizarre as the idea sounds, slowly pushing reflective sails up to the speed of light with just photons could be the only realistic chance of reaching another star within a single human lifetime.
It’s also much easier said than done. Particles of light can be fast, but they don’t push very hard. If you make a sail light enough to feel the inertia of the radiation, then a constant barrage of photons can inadvertently damage its material.
Creating a sail that can withstand the dangers that threaten spacecraft on such a long journey will require smart decisions; perhaps such as are proposed in two studies published recently in the journal Nano Letters.
Developed by engineers at the University of Pennsylvania and UCLA in the US as part of the Breakthrough Starshot initiative, the innovations seek to find ways to achieve the balance of strength and mass needed for an interstellar spacecraft.
“The idea of a light sail has been around for a long time, but we’re only now figuring out how to make these designs survive the voyage,” says Igor Bargatin, a mechanical engineer at the University of Pennsylvania.Like air particles hitting a cloth sail, radiation waves exchange momentum with whatever objects they hit. Unlike air molecules, photons, or light molecules, have no rest mass, so any force they create will be small.
For example, the light reflected off your body when you sunbathe is about one thousandth of a gram in strength.
There are several ways to increase this pressure to make the object move. One of them is to make a bigger sail that can catch more light. Another way is to make the light hitting the object more intense, for example by shooting multiple lasers at it.
But there are some problems here. Big sails mean big mass. Reducing the mass will allow it to be pushed more easily, but the potential cost of doing so will be a reduction in strength, putting the sail at risk of breaking.
More weight also creates some problems. For example, as the sail accelerates, the wavelength of the radiation hitting it will slowly shift towards the red part of the rainbow, which imposes restrictions on the types of materials that will not absorb too much infrared radiation and overheat.
Finding the right material to make sails strong, light and able to withstand the heat generated by gigawatts of laser radiation has been the subject of previous research. But none of them really focused on the trade-off between low absorption and high momentum at a certain distance needed to accelerate a ship.
In the latest proposal, the engineers propose to make a sail of two layers, consisting of compounds of molybdenum disulphide and silicon nitride, which can be made in the form of sheets and have optical properties that balance the minimum absorption and emission of light when it is stretched.
In the second work, the problem is not considered in terms of materials, but in terms of a structure designed to withstand the load from the increased photon pressure that a laser array will exert.Curving the sail will improve stability, as happens in parachutes, but little research has been done on how much stress light pressure would place on such a structure, the study authors note.
By modeling a square meter round, spherically curved sail – one that could tow a payload weighing several grams – the team demonstrated that sufficient curvature is definitely a necessary condition.
As in the other study, the scientists also varied the acceleration time to find the optimal balance of mechanical and thermal loads and travel time.Ideally, the Breakthrough Starshot aims to make the ship light enough that it can reach about 20% of the speed of light; this is enough to cover the distance of 4.2 light years from Proxima Centauri in just a couple of decades.
It is important to note that this technology may never be able to carry passengers. This will remain the fruit of science fiction for some time to come.But perhaps already during our lifetime we will be able to get to know the alien planetary system closely.
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