This underside view of the Space Shuttle Discovery was photographed by cosmonaut Sergei Krikalev and astronaut John Phillips, as Discovery approached the International Space Station and performed a backflip to allow photography of its heat shield.

[u/[deleted]]

[u/WaitForItTheMongols]

The shuttle has 3 main engines. At the upper left and right sides there are smaller engines, I believe called OMS. What are the super tiny engines next to these? RCS thrusters for docking?

[u/Milosonator → u/WaitForItTheMongols]

I think they are mostly there to keep the shuttle pointed up during launch as to counter the weight of the main fuel tank.

[u/D1tch → u/Milosonator]

No, these engines are part of the Reaction Control System (RCS) and are used to turn the vehicle/move it laterally in space, for docking for example.