A Penetrometer is a device to test the strength of soil. There are many different types of penetrometer. They are usually round or cone shaped. The penetrometer is dropped on soil or it is pressed against soil to measure how deep a hole it makes. This is a way to decide if soil is strong enough to build a road on. Scientists can use a penetrometer to measure how much moisture is in soil. Penetrometers are used on space probes such as the Cassini-Huygens probe, to measure the amount of moisture in soil on other planets.