Missions for Near Term Solar Photon Sails

Because of the inverse-square nature of solar radiant flux, the solar-photon sail will accelerate fastest in the inner solar system. intuition might suggest that an Earth-launched sail could never closely approach the Sun because the solar flux is directed radially outward from the Sun. But in this case, intuition is wrong for the following reason.

Everything in the solar system is in some sort of orbit and therefore, thanks to Mr Newton, we know that they will continue to move with the same orbital energy unless acted upon by some outside force. A sailcraft leaving the Earth would still be in orbit around the Sun. It would not "fall" toward the Sun unless some force acts upon it to change its velocity. A sail could be used to reflect sunlight in such a way as to slow the spacecraft in its orbit, thus causing it to spiral in toward the Sun. Once the new, presumably desired orbital distance from the Sun has been achieved, the sail could be reoriented to produce thrust in the proper direction in order to allow the craft to remain in this new orbit or further propel it to another location.

As presented in Figure 13.4, the sail can spiral either closer to the Sun or farther out in the solar system, depending upon its tilt angle (also called the aspect angle).

Perhaps the premier scientific mission for the solar-photon sail will be station-keeping for solar observatories. For many years, solar-storm early warning satellites, such as NASA's Advanced Composition Explorer

(ACE), have been permanently stationed 1.5 million kilometers closer to the Sun than the Earth's orbit. The main function of these craft is to warn of massive solar flares directed toward Earth that have the potential of increasing orbital radiation levels and disrupting intercontinental communication. The lifetime of solar observatories is limited by the requirement to carry maneuvering fuel. The solar-photon sail could be used for all post-Earth-escape maneuvers, reducing mass and cost and increasing operational lifetime.

If you construct a three-dimensional drawing in the manner of Figure 13.4, you'll readily see that the solar-photon sail can also direct a component of reflected solar flux perpendicular to the ecliptic, the plane of the Earth's solar orbit. Because of this capability, solar sails are also capable of propelling extra-ecliptic missions such as solar-polar observatories and comet rendezvous probes.

FIGURE 13.4 Two solar sails (A and B) orbiting the Sun. Because solar radiation pressure on sail A produces a thrust component tangent to and in the same direction as sail A's orbit, sail A spirals outward. Sail B will spiral inward toward the Sun because the solar radiation thrust component opposes the orbital direction.

direction of sail solar orbit Sun

FIGURE 13.4 Two solar sails (A and B) orbiting the Sun. Because solar radiation pressure on sail A produces a thrust component tangent to and in the same direction as sail A's orbit, sail A spirals outward. Sail B will spiral inward toward the Sun because the solar radiation thrust component opposes the orbital direction.

direction of sail solar orbit Sun

Solar-photon sails in a number of configurations also have advantages for designers of communication, navigation, and Earth-observation satellites. It is possible to design a sail capable of maintaining a more-or-less constant high-latitude position analogous to an equatorial geosynchronous satellite, but at a greater distance from the Earth.

Most sail designers expect that atmospheric drag might limit sail operations to orbital heights greater than 1,000 kilometers. But it is not impossible that parabolic-rigged sails could utilize Earth-reflection to unfurl at lower altitudes.

Atmospheric drag on sails might actually be a good thing. Analysis indicates that certain sail configurations could withstand accelerations greater than one Earth gravity. Sails could therefore serve as aerocapture devices.

But for the present, sails, like ion drives, will be low acceleration devices. Months or years must be devoted to the propulsive phase of any scientific mission propelled by the solar-photon sail.

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