Several theories have been posited to explain the formation of binary-asteroid systems. Recent work suggests that most of them have a significant macro-porosity (a "rubble-pile" interior). The satellites orbiting large main-belt asteroids such as 22 Kalliope, 45 Eugenia or 87 Sylvia could have formed by disruption of a parent body after impact or fission after an oblique impact. Trans-Neptunian binaries may have formed during the formation of the Solar System by mutual capture or three-body interaction. Near-Earth asteroids, which orbit in the inner part of the Solar System, may have split by tidal disruption after a close encounter with a terrestrial planet. A possible explanation for the relatively greater occurrence of binary asteroids near or inside of Earth's orbit was described in the journal Nature (10 June 2008): this theory states that when solar energy (see YORP effect) spins a “rubble pile” asteroid to a sufficiently fast rate, material is thrown from the asteroid’s equator. This process also exposes fresh material at the poles of the asteroid.