A penetrometer is a device to test the strength of a material.
It is often used on soil. 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.
- University of Arizona document, "An impact penetrometer for a landing spacecraft". 
- A picture of a penetrometer from the European Space Agency (ESA)