Undo segments doesnt shrink ..

 Hi,
Last time i had issue with undo segments going offline automatically. now i happen to see that my undo segments doesn't free the space , i'm unable to drop the undo segments as well ..  is there some thing any1 can help ..

here are the details

tablespace information
----------------------------------


Status    Tablespace Name TS Type      Ext. Mgt.  Seg. Mgt.    Tablespace Size    Used (in bytes) Pct. Used
--------- --------------- ------------ ---------- --------- ------------------ ------------------ ---------
ONLINE    RBS02           UNDO         LOCAL      MANUAL                10,000              9,683        97


undosegment information
------------------------------------

OWNER      SEGMENT_NAME     SEGMENT_TYPE    TABLESPACE_NAME  BYTES/1024/1024     BLOCKS    EXTENTS
---------- -------------- ------------------ ------------------------------ --------------- ---------- ----------

SYS        _SYSSMU1$        TYPE2 UNDO     RBS02                6                384         21