  It is widely accepted that the large obliquity of Uranus is the result of a
great tangential collision (GC) with an Earth size proto-planet at the end of
the accretion. We attempt to constraint the GC scenario as the cause of Uranus'
obliquity as well as on the mechanisms able to give origin to the Uranian
irregulars. Different capture mechanisms for irregulars operate at different
stages on the giant planets formation process. The mechanisms able to capture
the uranian irregulars before and after the GC are analysed. Assuming that they
were captured before the GC, we calculate the orbital transfer of the nine
irregulars by the impulse imparted by the GC. If their orbital transfer results
dynamically implausible, they should have originated after the GC. We
investigate and discuss the dissipative mechanisms able to operate later. In
particular Prospero could not exist at the time of the GC. Different capture
mechanisms for Prospero after the GC are investigated. Gas drag by
Uranus'envelope and pull-down capture are not plausible mechanisms. Capture of
Prospero through a collisionless interaction seems to be difficult. The GC
itself provides a mechanism of permanent capture. However, the capture of
Prospero by the GC is a low probable event. Catastrophic collisions could be a
possible mechanism for the birth of Prospero and the other irregulars after the
GC. Orbital and physical clusterings should then be expected. Either Prospero
had to originate after the GC or the GC did not occur. In the former case, the
mechanism for the origin of Prospero after the GC remains an open question. In
the latter case, another theory to account for Uranus' obliquity and the
formation of the Uranian regular satellites on the equatorial plane of the
planet would be needed.
