Radioactive sources are used in different industries for different purposes. To measure the moisture and density of the soil on which a road will be built, for example, construction workers use a special gauge that emits radiation. Some helicopters and planes use special detectors that can sense any ice accumulating on their wings. Older smoke detectors, too, use radioactive components. Even agricultural researchers use radioactive tracers: it’s how they find out how plants take in fertilizers. Radioactive sources are widely used in medicine to detect and treat disease and even for space travel.