Patent that introduced the concept of the Statite

Statite: Spacecraft that Utilizes Sight Pressure and method of use, Patent US5183225, 1989 by Robert Forward

An artificial space object capable of operating usefully adjacent to, but not in orbit about, a celestial body such as the Earth, comprising: payload means for providing useful services from a position in space adjacent to the Earth, light pressure propulsion means for intercepting light pressure and directing the resulting force to oppose the gravitational force between the Earth and the space object; and attachment means for attaching the propulsion system to the payload, whereby the force generated by the propulsion system may be transmitted to payload. The invention is designated a “Statite”, i.e. a useful space payload maintained by light pressure in a position adjacent to the surface of a celestial body, but not in orbit around it. The propulsion system may be a solar sail or it may be a solar photon thruster. The useful payload may be the space segment of a communications, broadcasting, remote sensing, or any other useful space system. The invention also teaches several methods of operating a Statite including polar and near polar positioning; solar orbital positioning; and halo orbit positioning.

Solar Wind Effect

The solar wind has a density of around 5 ions/cm^3, moving at around 500 km/s; that would lead to an influx of 2.5e12 ions/m^2/s. This might appear large, but is actually a tiny amount, just 4e-12 mol (one gram of hydrogen is approximately one mol). Since the hydrogen could not naturally escape from the atmosphere it would gradually become more and more hydrogen rich, but it would take trillions of years before the effects became significant. The net force from the solar wind and the light pressure (which is larger than the solar wind pressure) is also minor compared to the attraction of the sun

It is possible to position a Statite directly over the North or South Pole of the spinning Earth. To an observer on the Earth, a Statite so positioned will appear fixed above the Pole, like the North Star, while the stars rotate around it. In this embodiment of the present invention, communication ground stations can use fixed mounted antennas and simple fixed gain, fixed frequency electronics similar to those used with satellites operating in equatorial geostationary orbit.

In another version of the present invention, the Statite may be offset from the Polar axis. It stays fixed above one point on the dark side of the Earth, while the Earth spins beneath it. In this embodiment the Statite does not have to be positioned directly opposite from the Sun. The Statite can be placed anywhere over a large area on the dark side of the Earth. This flexibility of position stands in strong contrast to the single linear arc of the equatorial geostationary orbit taught by the prior art.

From the standpoint of an observer on the rotating Earth, the embodiment of the Statite which is offset from the Polar axis appears to rotate around the Pole once every twenty-four (24) hours (a solar day). Thus, ground stations for communications with such communication Statites must have their antennas on a Polar mount and require a simple twenty-four (24) hour clock drive. Since the distance between the ground station and the Statite does not change significantly in magnitude, the doppler shifts for such a system are very low. Thus, the electronics needed for these versions of the invention are nearly as simple as those used with geostationary statites.

The Statite is an artificial space object comprising a useful payload attached to a solar light pressure propulsion system. The Statite is launched by a standard rocket launch vehicle either directly to the desired operating point in space where the light pressure propulsion system would be deployed, or to a lower position in space that is high enough that the light pressue propulsion system can be deployed and the Statite flown into position using light pressure. Since the final velocity of the Statite with respect to the center of the Earth during the operation of the Statite is essentially zero, the direct placement of the Statite into space involves a simple “popup” type launch with considerable savings in fuel over a launch to orbital velocity at the same altitude. The Statite uses the pressure of sunlight, not centrifugal acceleration provided by velocity, to counter the Earth’s gravitational attraction.

Once the Statite is near its desired position and the light pressure propulsion system is deployed, a sensing system could acquire three points, for example, the Sun, the Star Canopus, and the Earth. This data would allow an onboard computer to determine the Statite’s position with respect to the center of the Earth. The solar sail could then be trimmed to modulate the thrust level of the light pressure propulsion system and its direction of thrust to bring the Statite to the desired position in space and maintain it there.

The light pressure propulsion system could be combined with a solar power generation system that would enhance onboard electric power. Waste heat from the solar electric conversion system can provide a component of propulsive thrust. The availability of large amounts of onboard electrical power would be advantageous for Statites providing direct broadcast services.

A basic force diagram for the simplest version of the Statite system is shown in FIG. 1. For simplicity, the light pressure propulsion system is discussed as a flat solar sail ll In practice, however, it is likely that an improved light pressure propulsion system, such as the solar photon thruster, will be used.

In FIG. 1, the plane of the diagram is not normal to the plane of the ecliptic, but has been rotated about the Earth-sun line until the Polar axis is in the plane of the drawing. In FIG. 1, the North Pole 3, of the Earth 5, is tilted in an angle theta with respect to the terminator (the shadow line around the Earth), which is normal to the sunlight 7 falling on the Earth. The angle theta is due to the tilt of the Polar axis of the Earth with respect to the ecliptic and varies plus or minus 23.5 degrees during the year. ‘

Statite 9 is placed at a distance “R” from the Earth and at an angle beta with respect to the Earth’s Polar axis. For simplicity, FIG. 1 shows the Statite in the plane of the drawing. In general, this will not be the case. The Statite can be positioned nearly anywhere over the dark side of the Earth and does not have to be exactly opposite of the Pole from the Sun. The sail 1, of Statite 9 is then oriented tangent to the radial direction

Using this technology, the minimum distance from the earth for a Statite is sixty Earth radii or about nine times the geostationary orbit distance. Second and third generation sail and reflector technology could allow Statites to operate at lower altitudes and at angles closer to the Earth’s poles. FIG. 2 is a plot of Statite altitude given in Earth radii for several solar sail designs given in grams per square meter of sail area.

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