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科学家开发出新型激光帆 偏离激光束时可自动纠正方向 目前，由化学反应驱动的传统火箭仍然是太空推进方法的主流形式。然而，它们的效率还远不足以使航天器在人类有生之年到达另一颗恒星。比方说，虽然半人马座阿尔法星是离地球最近的恒星系统，但它距离地球仍然有4.37光年，这相当于41.2万亿多公里，是地日距离的27.6万多倍。这相当于什么概念呢?美国国家航空航天局(NASA)的旅行者1号(Voyager 1)探测器于1977年发射，2012年才抵达星际空间，如果探测器的方向正确的话(更何况现在并不正确)，它需要大约7.5万年的时间才能抵达半人马座阿尔法星。  目前宇宙飞船使用的所有推进器都有一个共同的问题，那就是它们携带的推进剂都有质量。长途太空旅行需要大量的推进剂，使得航天器变得很重，而这反过来又需要更多的推进剂，使它们变得更重。宇宙飞船越大，这个问题就越严重。 此前的研究表明，“光帆”技术可能是其中一种可以在人类有生之年将探测器送到另一颗恒星的技术可行方法。虽然光不会产生太大的推力，但科学家们一直认为，光产生的一点点推力可能也会产生重大影响。事实上，大量的实验也表明，只要有足够大的镜子和足够轻的飞船，“太阳帆”就可以依靠阳光来推进。 耗资1亿美元的“突破摄星计划”(Breakthrough Starshot)曾经于2016年宣布，计划向半人马座阿尔法星(Alpha Centauri)发射大量微芯片大小的宇宙飞船，每艘飞船都配有非常薄、反射性极强的帆，而且这些飞船将会由有史以来最强大的激光推进。按照该计划，这些小飞船将会以20%光速的速度飞行，大约在20年之后就可以到达半人马座阿尔法星。 但使用激光帆有一个问题，如果激光帆偏离了推进激光束的方向，它们就可能会严重偏离目标(在“突破摄星”计划中，至少在最初阶段，激光束将会以地球为基地)。而现在，科学家们已经设计并测试了一种新的帆，这种帆在原理上可以自动地在所需的几分钟内保持在激光束的中心，使航天器能够在星际旅行甚至是恒星际旅行中保持航向。 这种新型的帆依赖于一种被称为衍射光栅的结构，这种结构最常见的地方就是在CD和DVD上。衍射光栅是一种覆盖一系列规则细微凸起或狭缝的表面，它们可以散射或衍射光，使不同波长或颜色的光向不同的方向传播。 在CD或DVD上，信息以不同长度的细微凹坑作为编码形式，这些凹坑排列在相同宽度和相同距离的行中，然后激光束就可以扫描这些磁盘来读取数据。这些行在CD和DVD的镜面上形成一个衍射光栅，可以将白光分解成许多颜色，形成人们在这些光盘上看到的彩虹图案。 这项研究的资深作者、纽约罗切斯特理工学院的光学物理学家Grover Swartzlander：“如果你曾经研究过光盘上美丽的光，你就会看到衍射效应。” 研究人员制造了一个由两个衍射光栅并排放置的帆。每个光栅均由排列整齐的液晶构成，这些液晶包含在一张塑料薄膜中，类似的液晶常用于电子显示屏和电子表。 以前的光帆设计就像镜子一样会将光束反射回其光源。但在新的设计中，每个衍射光栅中的液晶会使光线以一定的角度偏转，产生的推力会使帆向后方和侧方运动。 新帆左侧的光栅会将光束偏转到激光束的右侧，而右侧的光栅会将光束偏转到左侧。因此，如果帆的位置移动了的话，激光束落在帆的两边之后就会把帆推回原来的位置，使激光重新落在帆的中心。 在实验激光帆的测试中，科学家们必须检测出帆在激光作用下产生的微小作用力，同时将这些力与诸如建筑振动或气流扰动区分开来。 Swartzlander说：“我们沮丧地发现，在某些情况下我们的测量结果会变得不可靠，但最终，我们找到了足够的位置和避免干扰的方法。” 研究人员成功地探测到帆产生了令其重新回到激光中心的推力，将它推回到与激光束对齐的位置。 Swartzlander说：“实验结果与我们的理论预测一致，这非常令人兴奋，这表明我们可以自信地设计出由阳光或激光束驱动的更复杂的衍射结构。” 目前，研究人员正在试验一种新的帆，无论这种帆向任何方向漂移，它都可以自动居中，而不仅仅只限制于向左和向右。Swartzlander说：“有趣的是，它们可能具有与密集磁盘衍射特性非常相似的光学特性，”。 研究人员建议，未来他们的帆可以在国际空间站或地球周围的小卫星上进行测试。12月13日，他们在《物理评论快报》(Physical Review Letters)在线版上详细介绍了他们的发现。（来源：前瞻网） 分享到：
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最新评论
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dushunli 2020-01-14 00:41新型激光帆！
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mang2004 2020-01-14 03:59
Fly by light. A light sail using diffractive gratings, as depicted in this artist’s conception, generates sideways forces that could keep it aligned with a laser beam or allow it to be steered towards a target.
Figure caption
To snap close-up photos of planets outside our Solar System (exoplanets), some researchers are proposing the use of a fleet of “light sails” propelled by powerful Earth-bound lasers. A new experiment has demonstrated a possible sail design that uses diffraction gratings, rather than the reflective mirrors that have been used in previous sail designs. The gratings deflect an incoming laser beam at an angle, creating a sideways force that keeps the sail aligned with the beam’s center. Further testing is needed, but the team developing the sail is hopeful that their “beam-rider” technology could guide probes to faraway stars or to closer targets in our own Solar System.
Chemically powered rockets continue to be the dominant form of space travel. However, venturing out of the Solar System will require more energy than chemical fuel propellants can supply. “Rockets have done just as much as they can do,” says Grover Swartzlander from Rochester Institute of Technology in New York. Light sails are one alternative, with the earliest proposals dating from the 19th century. The simplest idea is to deploy a large mirror that reflects sunlight. With such a sail, the force providing propulsion is small compared with that of conventional rockets, but the available energy is limitless.
Both the Japanese space agency and NASA have flown light sail missions, and the privately funded Planetary Society is currently operating a light sail in orbit around Earth. “The technology is advancing,” Swartzlander says. In 2016, the Breakthrough Starshot project, which was founded by the late physicist Stephen Hawking and billionaires Yuri Milner and Mark Zuckerberg, announced a plan to send 1000 light sails carrying postage-stamp-sized cameras to Proxima b, the closest known exoplanet. To reach their target in 20 years, the sails would be accelerated to 20% of the speed of light by an array of lasers on Earth supplying 100 gigawatts of total power.
Laser-based propulsion can achieve much greater acceleration than solar-based propulsion, but one concern is that the sail can drift out of alignment with the laser beam. Swartzlander’s team has come up with a way to maintain alignment using diffraction gratings. Rather than reflect the laser light, their sail transmits the light at an angle. The deflected light exerts both a forward force to propel the sail and a sideways force to keep it aligned.
To demonstrate this alignment capability, the team built a centimeter-wide sail consisting of two gratings side-by-side. Each grating was made up of nematic liquid crystals contained in a plastic sheet and arranged in a periodic pattern. The pattern on the left side deflected light to the right, whereas the right-side pattern deflected light to the left. The researchers placed the sail in a sensitive force-measuring setup and aimed a laser beam at the center of the sail. Any left- or right-shift of the sail resulted in a nanonewton-scale re-centering force that moved the sail back into alignment with the beam.
Swartzlander says that liquid-crystal gratings are simpler and more cost-effective than other sail-centering solutions, such as nanofabricated metamaterials that would be prohibitively expensive to produce at the large scale needed for a light sail. Using liquid crystals also has the advantage that the grating pattern can be changed with an electrical signal, so that with future designs, one could potentially steer a light sail, just as a pilot steers an airplane. This control could work even when the light source is sunlight rather than a laser beam. Swartzlander imagines using a diffractive light sail on a mission to the Sun or an asteroid. However, he is proposing that the technology first be tested on the International Space Station or on a small satellite in Earth orbit.
Keeping the sail aligned is “a critical step” toward making laser-driven sails practical, says Les Johnson from NASA Marshall Space Flight Center in Alabama. He is leading a NASA-sponsored feasibility study of Swartzlander’s light sail project, but he was not involved in this specific experiment. Johnson says that there are still many questions, such as whether the technology can work in more than one direction (up-down in addition to left-right, for example) and whether these materials can survive high laser intensities. Space scientist Bruce Betts from the Planetary Society agrees that this technology is still in its infancy. “Diffractive sails are not the next steps in space for the young field of space sail propulsion,” he says. “But if proven out and demonstrated [in space], diffractive sails may someday represent a revolutionary leap forward in sail propulsion.”
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zx1986925 2020-01-14 07:35新型激光帆！
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从业者007 2020-01-14 07:58宇宙飞船可以利用一种激光帆飞向遥远的恒星
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songshaoman 2020-01-14 08:13有点意思
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daite1978 2020-01-14 08:29新型激光帆！
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thorn12345 2020-01-14 08:40偏离激光束时可自动纠
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mmttxiaoxiao 2020-01-14 08:44期待，星空航行
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leihen 2020-01-14 08:48科学家开发出新型激光帆 偏离激光束时可自动纠正方向
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james951 2020-01-14 08:55新型激光帆！