Spacecraft could fly to distant stars using sails with surfaces similar to those of CDs and DVDs to help them stay centered on laser beams, a new study finds.
Conventional rockets driven by chemical reactions are currently the dominant form of space propulsion. However, they are nowhere near efficient enough to reach another star within a human lifetime. For example, although Alpha Centauri is the nearest star system to Earth, it still lies about 4.37 light-years away, equal to more than 25.6 trillion miles (41.2 trillion kilometers), or more than 276,000 times the distance from Earth to the sun. It would take NASA's Voyager 1 spacecraft, which launched in 1977 and reached interstellar space in 2012, about 75,000 years to reach Alpha Centauri if the probe were headed in the right direction (which it's not).
The problem with all thrusters that current spacecraft use for propulsion is that the propellant they carry with them has mass. Long trips require a lot of propellant, which makes spacecraft heavy, which, in turn, requires more propellant, making them heavier, and so on. That problem gets exponentially worse the bigger a spacecraft gets.
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Previous research has suggested that "light sailing" might be one of the only technically feasible ways to get a probe to another star within a human lifetime. Although light does not exert much pressure, scientists have long suggested that what little it does apply could have a major effect. Indeed, numerous experiments have shown that "solar sails" can rely on sunlight for propulsion, given a large enough mirror and a lightweight-enough spacecraft.
The $100 million Breakthrough Starshot initiative, which was announced in 2016, plans to launch swarms of microchip-size spacecraft to Alpha Centauri, each of them sporting extraordinarily thin, incredibly reflective sails propelled by the most powerful lasers ever built. The plan has them flying at up to 20% the speed of light, reaching Alpha Centauri in about 20 years.
One concern with using laser sails is that if they drift out of alignment with the propelling laser beams — which will be based here on Earth, at least initially, in Breakthrough Starshot's plan — they may veer wildly off course from their targets. Now scientists have designed and tested a new sail that could in principle automatically keep itself centered on a laser beam for the required few minutes, allowing a spacecraft to stay on course for interplanetary or even interstellar journeys.
The new sail relies on structures known as diffraction gratings, the most familiar versions of which are seen in CDs and DVDs. A diffraction grating is a surface covered with a series of regularly spaced microscopic ridges or slits that can scatter or diffract light, making different wavelengths or colors of light travel in different directions.
A recording on a CD or DVD is encoded in the form of microscopic pits of different lengths that are placed in rows of the same width and equal distances, and laser beams can scan these disks to read their data. These rows form a diffraction grating on the mirror surfaces of CDs and DVDs that can split white light into the many colors that make it up, resulting in the rainbow patterns that one can see on these disks.
"If you've ever examined the beautiful play of light from a compact disk, you will have seen the effects of diffraction," study senior author Grover Swartzlander, an optical physicist at the Rochester Institute of Technology in New York, told Space.com.
The researchers built a sail consisting of two diffractive gratings placed side by side. Each grating was made of aligned liquid crystals that were contained in a plastic sheet. Similar liquid crystals are often used in the electronic displays of video screens and digital watches.
Previous light sail designs act like mirrors that reflect beams of light back at their sources. In the new design, the liquid crystals in each diffraction grating deflect the light rays at an angle, generating forces that send the sail both backward and sideways.
The grating on the left side of the new sail deflects light to the right of the laser beam, whereas the grating on the right side deflects light to the left. If the sail drifts so the laser beam fall on either side of the sail, that pushes the sail back into position with the light falling on the center of the sail.
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In tests of their experimental sail, the scientists had to detect the microscopic forces the sail generated in response to a laser while distinguishing those forces from disturbances such as building vibrations or air currents.
"We were frustrated to find that our measurements were not reliable if the floor sagged from the weight of a small person," Swartzlander said. "Eventually, we found adequate locations and methods of avoiding disturbances."
The researchers successfully detected the sail generating re-centering forces that pushed it back into alignment with a laser beam.
"It was very satisfying to find that the experimental results agreed with our theoretical predictions," Swartzlander said. "This agreement suggests that we can confidently design more complex diffractive structures for light sails driven by either sunlight or a laser beam."
The researchers are now experimenting with sails capable of centering themselves if they drift in any direction, not just left or right. "Interestingly, these may have optical properties very similar to the diffractive nature of compact disks," Swartzlander said.
In the future, the researchers suggested, their sails could be tested on the International Space Station or on a small satellite around Earth. They detailed their findings online Dec. 13 in the journal Physical Review Letters.
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