# The History Channels idea for traveling at half the speed c.

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#### venator_3000

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Hello & welcome to SDC,<br /><br />I'm afraid I don't have a TV so I missed the show on the Discovery Channel. It sounded interesting, though.<br /><br />Your question about "drag" is quite relevant to solar sailing. As you may have noted above I crudely attempted to explain the function of a solar sail with basic algebra. If you assume a beam of sunlight full of photons has an area of 1 square meter that beam will have a power level of around 1400 watts in that one square meter. This is what falls on the Earth so if we start in LEO that is what will power our solarsail. These photons fly at the speed of light, say, 3 x 10^8 meters per sec or 300 meters in one micro-second. If you divide that energy by the speed of light you can determine the momentum of the beam which will be lost when the beam strikes and is then reflected by the sail. Assuming the sail is very reflective the value works out to several micro-newtons. <br /><br />Micro-newtons doesn't seem like much but it does add up for a very large sail. But before we lose ourselves in sails that stretch for kilometers, let's stay with that initial one square meter. Pretend we have a solar sail made of some material that is one square meter and assume it weighs one gram. So the beam whose momentum we have calculated falls directly upon it. If the momentum gained by the photon striking the surface of the sail were 10 micronewtons, or one dyne, then an acceleration of 1 cm/sec^2 could be derived. <br /><br />Again very small! And in reality roughly 1-tenth of 1 percent of the Earth's surface gravity. But remember our solarsailors are in space above the earth. 1 cm/sec^2 is almost 2 times greater than the Sun's gravity at the Earth's orbit. <br /><br />Someone has defined a metric for solar sails that relates the ratio of the sail's ability to accelerate to solar gravity at a given orbit. Remember, the Sun's intensity of radiation (how many photons in a given space) and its force of gravity both obey the inverse-square <div class="Discussion_UserSignature"> </div>

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