A Russian MIG21 chases a large cylinder UFO.
This is, by now, quite a famous sighting. Watch below.
The UFO was reported as being a cylinder in shape, around 80ft in length, accelerating to around mach 3 in 10 seconds.
Just to put that speed into some kind of context for you. An F22 Raptor has been estimated to have a top speed no faster than Mach 2.5.
One of the biggest problems with supersonic flight is with shock-waves interfering with engine operation. A turbojet or turbofan engine cannot use supersonic air. This is overcome in most supersonic aircraft with some form of variable inlet. Such an inlet will adjust to force shock-waves away from the inlet and slow the incoming air down to subsonic speeds. The higher the mach, the more difficult this becomes.