The payload defines the mission of the rocket. It can be anything from a science experiment and a communication device to the most valuable payload that has been carried by rocket into space, which is a human being.
The Payload sub-system is responsible for the implementation of a payload that will take advantage of the conditions that occur during flight (altitude change, temperature, acceleration, the composition of the air, etc.) in a way that might justify its inclusion inside the rocket. Of course, the payload can include more than one experiment, even if their scientific interests differ, as long as there is enough space to include them. These could range from biology experiments to the testing of new technology. The payload is usually housed in structures such as CubeSats or CanSats. Standard CubeSats are made up of 10x10x10 cm units. The smallest standard size is 1 Unit (1U) while the most used size in high-power rockets, such as SELENE, is a 3U CubeSat (30x10x10 cm).
Payloads could be deployable, which means it disconnects from the rest of the rocket at apogee and descents separately, or non-deployable. If the payload is deployable, it is essential to implement a Global Positioning System (GPS) (Read more about it here), as well as all electronics systems to acquire and store data. The data can be also viewed live via a telemetry system. Such systems are developed by the Avionics sub-system (Read more about that here).
For project SELENE, our latest rocket, the Payload sub-system chose to conduct research on environmental radioactivity. The objective of the experiment was to measure alterations in the concentration of Radon (Rn-222), a radioactive element, in the lower layers of the atmosphere.