index Unusable

What would cause the PK column index(on object_id) to become unusable
after the fllowing steps:  NO direct import or other such activites that
would def cause this....

alter table ship_objects add temp varchar2(500);

update ship_objects set temp = cmp_crc, cmp_crc = null;
commit;
alter table ship_objects modify (
        cmp_armor_hp_maximum varchar2(400),
        cmp_armor_hp_current varchar2(400),
        cmp_efficiency_general varchar2(400),
        cmp_efficiency_eng varchar2(400),
        cmp_eng_maintenance varchar2(400),
        cmp_mass varchar2(400),
        cmp_crc varchar2(500),
        cmp_hp_current varchar2(400),
        cmp_hp_maximum varchar2(400),
        cmp_flags varchar2(400),
        weapon_damage_maximum varchar2(400),
        weapon_damage_minimum varchar2(400),
        weapon_effectiveness_shields varchar2(400),
        weapon_effectiveness_armor varchar2(400),
        weapon_eng_per_shot varchar2(400),
        weapon_refire_rate varchar2(400),
        weapon_ammo_current varchar2(400),
        weapon_ammo_maximum varchar2(400),
        weapon_ammo_type varchar2(400),
        cmp_creators varchar2(400));
update ship_objects set cmp_crc = temp;
update version_number set version_number=185, min_version_number=185; commit;
alter table ship_objects drop column temp;
-- 
..
David


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