Nations keep an extra eye on each other. The heavens are crowded. Satellites can do everything from monitor fires to verify military intelligence. Spent rockets and other trash are an increasing threat. EYES IN THE SKY

Stand on your front porch at noon twelve years from now, and look north. Every three or four minutes, a satellite will be coming over the horizon. You may not see any - but they'll see you, says John McLucas, who ran the US Air Force's satellite program for five years. Seventeen countries plan to put 23 satellites into orbit to look down at Earth by the turn of the century, according to a report from an International Space Year conference this spring. And those are just the ones designed for scientific, not military uses.

Israel joined the spy satellite club last week when it launched Horizon I into orbit. (Israel calls it a ``technological satellite,'' but news reports claim it's a ``spysat'' or spy satellite.)

Will the world be a better place for all those eyes in the sky?

``On balance, it appears that every remote-sensing satellite adds to world security and stability,'' says Peter D. Zimmerman, a senior associate at the Carnegie Endowment for International Peace. And that's a good thing, says Ann Florini, a strategic-studies coordinator at the University of California, Los Angeles, because ``it's not something we could stop, even if we wanted to.''

Not every remote-sensing satellite can ``see'' well enough or provide data quickly enough to be of significant military value. The ability to see small objects depends on the fineness of a satellite's sensors and the height of its orbit. A satellite must be able to see objects the size of a station wagon (3 meters) to be of high military value.

But you don't always need such high resolution to pick out military targets. In a report published last year, a geoscientist and a civilian security analyst in Norway used 30-meter-resolution images from the US Landsat to find a secret Soviet airfield on the Kola Peninsula, as well as ``hardened'' aircraft hangars and hydroelectric plants.

France has been selling imagery on the open market from its 10-meter-resolution SPOT since 1986. Landsat images have been available since 1972. The Soviets now will sell you pictures from a five-meter satellite - but not if you want to see a socialist country. Both the United States and the Soviet Union have spy satellites with resolutions measured in tens of centimeters, and perhaps better than 10 cm (about 4 inches) for US models.

The proliferation of remote-sensing satellites over the next decade or so and the inevitable increase in their ability to see objects on the ground ``will make it harder for governments to lie to each other,'' says Ms. Florini. Satellites will certainly reduce the diplomatic leverage America now gains from sharing spy-satellite date with its allies, she says. They may also compromise US security in the event of military intervention in a crisis.

It's still relatively easy to dodge a spy satellite. US military personnel in Egypt for the attempted rescue of American hostages in Iran in 1980 simply stepped into an airplane hangar every time a Soviet satellite was overhead. More satellites, and more satellites with sophisticated sensors that can ``see'' through camouflage, means greater likelihood of discovery.

John McLucas foresees a gradual, but profound change as governments accommodate themselves to the new reality of continual surveillance. You can't keep running under cover and still get any work done, says Mr. Zimmerman. Such satellites ``will tend to inhibit construction of highly secret facilities,'' he says.

Zimmerman sees new opportunities for independent checking of arms agreements, as in Sweden's just-announced proposal to loft a reconnaissance satellite (see story, page 4). Canada is also studying a ``Paxsat'' for a similar purpose.

Zimmerman thinks overhead images might be used another way: to shame countries into changing behavior. For example, it is widely believed that forestry practices in India and Nepal contributed to the devastating floods in Bangladesh. Could satellite-picture proof help bring world opinion to bear?

A proposed five-meter resolution ``Mediasat,'' currently on hold, would give the public wider access to satellite images. Television news organizations - particularly ABC News - have been using SPOT and Landsat images more and more: The aftermath of the Chernobyl nuclear power station disaster was widely seen on TV. Photos of Yellowstone National Park shot from space before and after the fires now are being peddled by one independent satellite-image processors.