Deimos and Phobos Interior Explorer that the moons of Mars may have formed after a huge collision with a protoplanet one third the mass of Mars that formed a ring around Mars. A third hypothesis is that they may have co-accreted with the parent planet. DePhine mission would explore the origin and the evolution of the two Martian satellites, by focusing on their interior structures and diversity. The mission would attempt to determine if Phobos and Deimos true siblings, originating from the same source and sharing the same formation scenario. It would also determine if both natural satellites are rubble piles or solid bodies, and
Moons of Neptune formed "in situ". Triton and Nereid are unusual irregular satellites and are thus treated separately from the other five irregular Neptunian moons, which are more like the outer irregular satellites of the other outer planets. Firstly, they are the largest two known irregular moons in the Solar System, with Triton being almost an order of magnitude larger than all other known irregular moons. Secondly, they both have atypically small semi-major axes, with Triton's being over an order of magnitude smaller than those of all other known irregular moons. Thirdly, they both have unusual orbital eccentricities: Nereid has one of the