METEOROLOGISTS have been using weather satellites for nearly 40 years. A more recent development, however, has been the appearance of a new breed of "environmental satellites". They occupy a much lower orbit than the spacecraft normally used by meteorologists, and are equipped with instruments of the very highest sensitivity. These spacecraft like the European ERS-1, the American LANDSAT, and the one from France called SPOT, sometimes provide useful extra information for weather forecasters, but they have a wide range of applications in other fields as well.
A topical application is their use, currently under test in southern France, for the forecasting of forest fires. It is an easy thing to spot a forest fire once it has started, but the real trick is to anticipate when and where one is likely to occur. This can be done with the help of infra red images from the satellites those based, not on the optical variations of light and shade detected by a camera, but on the differences in temperature between one place and another within the satellite's field of view.
In normal circumstances, plants are kept cool by evapotranspiration, a process not too dissimilar to the human trait of sweating. When water is in short supply, however, evapotranspiration slows down, and the temperature of the plant increase a change that is detectable by the infrared sensors on some satellites. The immediate cause of forest fires, of course, is independent of the thermal state of the vegetation, but trees and shrubs that are stressed through lack of water are more vulnerable than others. If the relevant zones can be identified by satellite, groundbased surveillance can be stepped up, or perhaps even the public prohibited from the area in question, until the risk of fire begins to fall.
Another use of infrared sensors concerns infestations of Phylloxera vastatrix, a parasite that lives on grapevines, extracting the goodness from the roots. When a plant is attacked it takes about two years before the signs of sickness like yellowing leaves or falling fruit production - can be seen above the ground. By this time the parasite has infested all the vines in the vicinity and an extensive acreage has to be destroyed.
It has been found, that the temperature of a sick plant differs slightly at any given time from that of a normal healthy one. The difference between healthy and infected zones can therefore be detected by infrared sensors on a satellite long before any signs of infestation are noticeable to the human eye - and this allows the damage to be contained.