From ROI Wiki

A Whisker is a small, short range, either FTL, or sublight spherical sensor probe sent out by a starship that is used to extend the sensor capabilities of the parent ship across a given area, or even help the parent ship maintain maximum stealth when such is needed. Each Whisker is compact, self contained sensor array with limited shields and no defensive capabilities of its own, having the singular purpose of being used for sensor enhancement for the parent vessel only. These can be deployed or retrieved as needed, and are powered by a small Quantum Power Core that makes them self sufficient for an indefinite period of time. These units are primary employed by Earthfleet starships, although there are several other races that also employ this kind of technology as well.

However, the use of Whiskers is generally rare within the galaxy. They're also good for remote surveillance, being able to be left behind in a system or area to collect data for long times before being retrieved again and their data downloaded for analysis. Some models of Whiskers also have stealth capabilities, and a limited number of larger, more high end Whiskers were even outfitted with FTL drives capable of taking them anywhere in the galaxy as needed. These units also usually include a specialized AI system to help with command decisions that the Whisker might need to make in order to complete its assigned mission and return intact and unharmed.