In the example of solar sails, perfecting the technology would allow spacecraft to travel through our solar system using very little fuel.

NASA has been making strides with solar sail technology. Using the NanoSail-D mission, NASA continues to gather valuable data on how well solar sails perform in space. The Planetary Society will also be testing solar sail technology with their LightSail-1 project sometime next year.

How will NASA (and others) test solar sail technology, and develop it into a common, reliable technology?

The second of three recently announced technology demonstrations, The Solar Sail Demonstration, will test the deployment of a solar sail in space along with testing attitude control. The solar sail will also execute a navigation sequence with mission-capable accuracy.

In order to make science fiction into reality, NASA engineers are testing solar sails that could one day provide the propulsion for deep space missions. Spacecraft using solar sails would travel in our solar system in a similar manner to a sailboat through water, except spacecraft using solar sails would rely on sunlight instead of wind. A spacecraft propelled by a solar sail would use the sail to capture photons emitted from the Sun. Over time, the buildup of the solar photons provides enough thrust for a small spacecraft to travel in space.

NASA’s solar sail demonstration mission will deploy and operate a sail area 7 times larger than ever flown in space. The technology used in the demonstration will be applicable to many future space missions, including use in space weather warning systems to provide timely and accurate warnings of solar flare activity. The solar sail demonstration is a collaborative effort between The National Oceanic and Atmospheric Administration (NOAA), NASA and contractor L’Garde Inc.

NASA lists several capabilities solar sails have to offer, such as:

Orbital Debris: Orbital debris can be captured and removed from orbit over a period of years using the small solar-sail thrust.

De-orbit of spent satellites: Solar sails can be integrated into satellite payloads so that the satellite can be de-orbited at the end of its mission.

Station keeping: Using the low propellantless thrust of a solar sail to provide station keeping for unstable in-space locations.

Deep space propulsion: Payloads free of the Earth’s pull can be continuously and efficiently accelerated to the other planets, or out of the solar system, such as proposed in Project Encounter.

As an example, the GeoStorm project considers locating solar storm warning satellites at pseudo Lagrange points three times further from the Earth by using the solar sail to cancel some solar gravitational pull, thus increasing warning time from ~15 minutes to ~45 minutes.

Providing a satellite with a persistent view of northern or southern latitudes, i.e., a “pole-sitter” project. This allows the observational advantages of today’s geosynchronous satellites for orbits with view angles of the northern and southern high-latitudes.

A solar sail system, measuring 66 feet on each side was tested in 2005 in the world's largest vacuum chamber. Image Credit: NASA

 

The Solar Sail demonstration mission will deploy and operate a sail area 7 times larger than ever flown in space with potential applicability to a wide range of future space missions, including use in an advanced space weather warning system to provide more timely and accurate notice of solar flare activity. The National Oceanic and Atmospheric Administration (NOAA) is collaborating with NASA and L'Garde Inc. on the solar sail demonstration. Solar sails offer many potential game-changing mission capabilities including:

• Orbital Debris: Orbital debris can be captured and removed from orbit over a period of years using the small solar-sail thrust.
• De-orbit of spent satellites: Solar sails can be integrated into satellite payloads so that the satellite can be de-orbited at the end of its mission.
• Station keeping: Using the low propellantless thrust of a solar sail to provide station keeping for unstable in-space locations.
  • As an example, the GeoStorm project considers locating solar storm warning satellites at pseudo Lagrange points three times further from the Earth by using the solar sail to cancel some solar gravitational pull, thus increasing warning time from ~15 minutes to ~45 minutes.
  • Providing a satellite with a persistent view of northern or southern latitudes, i.e., a “pole-sitter” project. This allows the observational advantages of today’s geosynchronous satellites for orbits with view angles of the northern and southern high-latitudes.
• Deep space propulsion: Payloads free of the Earth’s pull can be continuously and efficiently accelerated to the other planets, or out of the solar system, such as proposed in Project Encounter.

The Solar Sail demonstration will:

• Demonstrate the deployment of a 38m x 38m solar sail in space (quadrupling the area of the largest sail deployed and tested on the ground of 20m x 20m by L’Garde at NASA’s Plumbrook facility in Ohio).
• Demonstrate attitude control plus passive stability and trim using beam-tip vanes.
• Execute a navigation sequence with mission-capable accuracy.

太阳帆简介：

What is a Solar Sail?

A solar sail is a spacecraft with a large, lightweight mirror attached to it that moves by being pushed by light reflecting off of the mirror instead of rockets. Look here for pictures of possible solar sails. The light to push a sail can come from the sun or large lasers we could build. Satellites in orbit around the Earth can survive for many years without any maintenance while using only a little bit of rocket propellant to hold their positions. Solar sails can be made to survive in space for many years as well. But, because solar sails use sunlight that never runs out like rocket propellant, during those years the sail can move around as much as you want it to, such as from Earth to Mars and back, possibly several times if the sail remains in good condition. A similarly equipped rocket would either be ridiculously huge because it has to carry the fuel for each trip, or would need to be refueled regularly.

How Does Light Push a Solar Sail?

Electromagnetism

Before anyone ever took a beam of light and measured how much it could push, there were predictions that light could exert a very gentle push on objects it hits. James Clerk Maxwell developed the laws describing electromagnetism and concluded that light is an electromagnetic wave. Maxwell predicted that when light hits an object and is absorbed or reflected, the light wave pushes on the electrons in the surface of the object, which in turn push on the rest of the object. If the light is reflected, the object gets pushed twice as hard, just as if you would be pushed twice as hard by a rubber ball hitting you as a ball of clay. In 1901-1903, the Americans Nichols and Hull and Russian Lebedev were able to measure light pressure as predicted by Maxwell. Find a physics text on electromagnetism, like Physics, by Halliday, Resnick, and Krane, to see how the force is derived from Maxwell's equations.

Einstein

When Einstein developed his theories of relativity, and gave us the equation E=mc², it allowed us to calculate light pressure a lot easier. E=mc² compares energy, which can be easily measured in light, to mass and movement, which can easily be used to find forces.

• E is an amount of energy
• m is an amount of mass
• c is the speed of light, about 300 million meters per second

If you fiddle around with E=mc², you find that the force light exerts is the power of the sunlight divided by the speed of light. Like I stated above, you get twice as much force from an object that reflects all the light as you do from an object that absorbs all the light. In order to get this simple formula, force equals power divided by speed of light, the steps taken by Maxwell and others had to be taken first.

Very, Very, Gentle Force

Sunlight exerts a very gentle force. The power of sunlight in space at Earth's distance from the sun is between 1.3-1.4 kilowatts per square meter. When you divide 1.4 kilowatts by the speed of light, about 300 million meters per second, the result is very small. A square mirror 1 kilometer on a side would only feel about 9 Newtons or 2 pounds of force. Fortunately, space is very empty and clean compared to Earth, so there is plenty of room for a 1 kilometer wide sail to maneuver, and there is no noticeable friction to interfere with your 9 Newtons of thrust. A sailboat on Earth wouldn't be going anywhere with that little force because of drag from the water and air. Some rockets can push millions of times harder, but the sail keeps pulling so long as light shines on it. Months or years after the rocket runs out of fuel, the sail is still pulling.

Why Don't Solar Sails Use the Solar Wind?

Many people assume that because here on Earth they feel wind but not sunlight, that solar sails must be pushed by the solar wind. However, there is a very big difference between space and Earth. Earth is wrapped in a thick layer of gas that is felt as wind whenever it moves. In space, there is no air to move around and cause strong winds like we feel on Earth. The solar wind is an extremely tenuous flow of particles ejected by the sun which exerts very little force on anything it hits. The reason people worry about the solar wind is because many of the particles have an electric charge that can hurt people and electronics, or can push a magnetic sail.

NASA：太阳帆将测试飞行 或成未来太空动力

http://scitech.people.com.cn/GB/15942459.html

    说到太阳帆，你的第一反应是不是和反物质发动机或者离子发动机一样，感觉就像是未来科幻世界中的技术？不过现在正有越来越多的科幻事物正逐渐成为现实。就拿太阳帆来说，一旦这项技术成熟，我们将只需很少的能量就可以让飞船进行行星际的旅行。

　　在这一全新的领域，美国宇航局正全力开拓。借助之前的NanoSail-D项目，美国宇航局持续收集有关太阳帆在太空中表现状况的相关数据。而就在明年，美国的一个民间机构——行星学会也将对他们的太阳帆技术——LightSail-1项目进行测试飞行验证。那么美国宇航局和其它机构将如何发展太阳帆技术并最终使这一技术成为普及的实用技术呢？

　　在美国宇航局即将进行的测试飞行中，有一个技术科目是验证太阳帆在太空中的展开与部署能力，并测试其姿态操控性能。同时太阳帆飞行器还将模拟未来实际任务情形与地面进行导航信号控制调试。

　　为了尽快将太阳帆这一最早出现在科幻作品中的新生事物变成实用的技术，美国宇航局正加紧进行测试，希望有朝一日它将驱动人类未来的飞船。使用太阳帆驱动的飞船就像在大海中航行的帆船，只不过后者的动力借助的是风，而太阳帆借助的是阳光。飞船将张开一张大大的网，用以收集从太阳辐射的光子，随着时间的推移，积聚的光子将产生足够推动飞船前进的动力。

　　此次美国宇航局将进行测试验证的太阳帆展开面积将是之前任何一个太阳帆的7倍以上。在此次测试中掌握的各种技术将对未来的该项技术发展起到关键作用，包括使用空间天气预警系统来提供及时和精确可靠的太阳耀斑爆发信息。这一实验验证计划是由美国宇航局和美国国家海洋和大气管理局(NOAA)和系统承包商加德公司(L’Garde Inc)共同进行的。

　　雄心壮志

　　美国宇航局还专门为这一计划开列了一些其设想中的太阳帆卫星必须具备的条件，包括：

　　轨道碎片清理：可以在数年的运行周期内帮助捕获并清理轨道碎片

　　退役卫星清理：将太阳帆整合进卫星设计中，这样当卫星抵达设计寿命之后便可以打开太阳帆，实现卫星减速并最终坠入大气层销毁

　　轨道自持力：能够使用携带的有限推进剂保证太阳帆卫星能在太空中保持设定的轨道

　　深空探测项目：一旦脱离地球引力场，太阳帆飞船将能够在阳光驱动下持续加速，一直飞抵太阳系的边缘甚至飞出太阳系。

　　除此之外，太阳帆还有其它重要的实际用途，比如在一个名为“地磁暴”的项目中就有科学家提出将太阳风暴预警探测卫星发射到比拉格朗日点远3倍的位置上，使其更加接近太阳，这样一来人类就能比现在提前大约15~45分钟对危险的太阳爆发发出警告。

　　目前这一阶段我们只能将卫星部署在拉格朗日点上，因为这是地球和太阳之间的一个引力平衡点，而假如为了提升预警能力而将卫星进一步放置到更加接近太阳的距离上，其轨道位置就将无法稳定。但是如果有了太阳帆技术，我们就可以借助打开的太阳帆获得持续的动力，从而不断抵消太阳的“拉力”并实现预警卫星在这一位置上的稳定部署。

　　或者，还可以设想在未来借助太阳帆技术扩展目前对“地球同步卫星”的概念。目前的技术只能将卫星发射并部署在距离地面赤道上空约3.6万公里的轨道上实现与地球的同步运行，一般会被用作通讯卫星使用。但是一旦有了太阳帆技术，我们便可以不再受到纬度的限制，而是可以在南半球或北半球上空部署同样的“同步卫星”，这样一来，其应用价值将无法估量。

 

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