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Scientists take unusual approaches to SDI. Dozens of small-business contracts for SDI reveal the challenges of designing a shield against nuclear weapons. For instance, how can scientists supply the huge amounts of energy needed to power weapons platforms in space?

The Strategic Defense Initiative Organization is studying an orbiting antimissile gun that would use nuclear cartridges instead of gunpowder. It is funding SARAH, a space-based radar billed as able to see underground.

SDI contractors are researching satellites that would project 3-D images of themselves to fool attackers, and make forecasts of sunspot activity through the first decade of the 21st century.

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These exotic projects are among the dozens paid for by an SDI program that supports small business ideas. Documents describing this program's 1986 contracts have been made available to the Monitor.

Only small amounts of money are involved in these contracts. But their details provide an unprecedented glimpse into the SDI effort. They reveal the scope of problems that must be solved if defenses against nuclear weapons are to work, and the sometimes unusual ways in which American scientists are attacking those problems.

The nuclear cartridge, for instance, is an attempt to get around the problem of providing the huge amounts of energy that would be required by weapon platforms orbiting in space. Energy Applications & Systems Inc., of Corona Del Mar, Calif., is being paid approximately $90,000 to study the concept's feasibility.

The cartridge would be a container of radioactive material placed inside an orbiting nuclear reactor. When the reactor bombarded the cartridge with neutrons, it would shoot out a hot ionized gas capable of propelling a projectile at tremendous speeds. The cartridge would then be discarded, like a used shell casing.

``This is not a bomb,'' says S.Locke Bogart, president of Energy Applications.

Since SDI's inception Reagan administration officials have said that it is intended to be a nonnuclear defense system. Officals are thus sensitive to the attachment of the word ``nuclear'' to any SDI project. A summary list of SDI small-business contracts refers to the Energy Applications effort as only a cartridge concept.

Most people probably associate SDI research with development of weapons such as the space gun. In fact, much SDI work is focused on less flashy technologies. Simply finding targets would be one of the toughest jobs of a strategic defense. Many of the small-business contracts address this problem. Several, for instance, propose ways of keeping infrared sensors in space extremely cool, to increase their ability to detect the heat of man-made objects against the cold, dark background of space.

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A Texas firm, Space Services Inc., has a $50,000 SDI contract to work on another approach: tracking targets via SARAH (Synthetic Aperture Radar Addition Holography). A giant radar system mounted on two satellites, SARAH would be so powerful that US intelligence agencies could use its images to actually peer beneath the Earth's surface, claims William Grisham, Space Services' principal scientist

``It could help them do verification of arms treaties,'' says Mr. Grisham.

SARAH wouldn't be able to tell whether concrete silos contained missiles. But according to its inventor, SARAH would be able to gauge underground activity at missile sites - filling pipes with liquid, laying communications wires - to a depth of 30 feet.

NASA recently launched sensors able to ``see'' into sand. But radar able to penetrate denser soils has been thought by scientists to be not practical, given the state of current technology.

After launch, the ability of any SDI space platform such as SARAH or a space laser to do their jobs would be affected by something that seems a contradiction in terms: space weather. Two of the small-business contracts involve work on defining the environment of space and how it changes.

Both studies are being done by the same company: Nichols Research, of Huntsville, Ala. One is calculating how earthshine, or sunlight reflected by the Earth, bounces off objects floating just above the atmosphere. Space sensors would take knowledge of this phenomenon into account when trying to decide whether a particular object is a warhead, a decoy, or space junk.

The second aims at making forecasts of sunspot activity through the year 2010. Such solar activity affects solar wind, magnetic fields, density of space particles, and other aspects of the space environment.

``Skylab came down earlier than predicted because high sunspot activity made particle density in its orbit higher than predicted,'' points out Herbert Hunter, manager of the project for Nichols Research.

SDI space systems could also be affected by an adversary trying to shoot them down. Many critics say this would be the system's Achilles' heel, as satellites are large targets that travel predictable paths.

Among the more involved defensive measures being researched by small businesses is a holography projector for satellites, a project of UBC Inc. of Tampa, Fla. UBC officials declined to speak to a reporter. An SDI technical abstract of their concept says that satellites capable of projecting a 3-D holographic image of themselves in space could confuse hostile radars as to which is the real target.

Several other small firms are at work on holographic filters, which would in effect be armor plate against lasers.

By diffusing the beam of the incoming light, these filters could lessen its destructive power. Such armor could be used to protect space platforms against laser-carrying killer satellites. On the other hand, filters might be used to coat missile boosters, making them more resistant to the lasers of any strategic defense system.

Another approach to defense is suggested by Astro Innovations Inc., of Feasterville, Pa.: satellites able to squirt liquids into space. Running into a drop of liquid at orbital speeds would shatter any pursuers.

``Initially we thought of using water,'' says the firm's president, Joseph Bednarz. ``Now we think certain low-viscosity oils might be better.''