▁occurs ▁when ▁the ▁moon ▁is ▁either ▁revol ving ▁around ▁the ▁primary ▁faster ▁than ▁the ▁primary ▁rot ates , ▁or ▁is ▁revol ving ▁in ▁the ▁direction ▁opposite ▁the ▁planet ' s ▁rotation . ▁In ▁these ▁cases , ▁the ▁tid al ▁bul ge ▁l ags ▁behind ▁the ▁moon ▁in ▁its ▁orbit . ▁In ▁the ▁former ▁case , ▁the ▁direction ▁of ▁angular ▁momentum ▁transfer ▁is ▁revers ed , ▁so ▁the ▁rotation ▁of ▁the ▁primary ▁spe eds ▁up ▁while ▁the ▁satellite ' s ▁orbit ▁shr inks . ▁In ▁the ▁latter ▁case , ▁the ▁angular ▁momentum ▁of ▁the ▁rotation ▁and ▁revolution ▁have ▁opposite ▁signs , ▁so ▁transfer ▁leads ▁to ▁decre ases ▁in ▁the ▁magnitude ▁of ▁each ▁( that ▁cancel ▁each ▁other ▁out ). ▁In ▁both ▁cases , ▁tid al ▁dec eler ation ▁causes ▁the ▁moon ▁to ▁spir al ▁in ▁towards ▁the ▁primary ▁until ▁it ▁either ▁is ▁torn ▁apart ▁by ▁tid al ▁str esses , ▁potentially ▁creating ▁a ▁planet ary ▁ring ▁system , ▁or ▁crashes ▁into ▁the ▁planet ' s ▁surface ▁or ▁atmosphere . ▁Such ▁a ▁fate ▁a wa its ▁the ▁mo ons ▁Ph ob os ▁of ▁Mars ▁( with in ▁ 3 0   to   5 0   mill ion ▁years ), ▁T rit on ▁of ▁Ne pt une ▁( in ▁ 3 . 6 ▁billion ▁years ), ▁and ▁at ▁least ▁ 1 6 ▁small ▁sat ell ites ▁of ▁U ran us ▁and ▁Ne pt une . ▁U ran us ' s ▁Des dem ona ▁may ▁even ▁col lide ▁with ▁one ▁of ▁its ▁neighbor ing ▁mo ons