alter table / permissions

Howdy:

Running PostgreSQL 7.2.1 on RedHat Linux 7.2.

I want to change / alter someone else's table and add a column.
I think that I have to give myself permission for such things,
but when I try to make the modification, I get 'permission
denied'.

[snip example]

test_db=> \c bcn joe
You are now connected to database bcn as user joe.
test_db=> alter table tm_pg_hmp_num add column product text;
ERROR:  ALTER TABLE: permission denied
test_db=> \z tm_pg_hmp_num
       Access privileges for database "test_db"
       Table       |      Access privileges
-------------------+------------------------------
 tm_pg_hmp_num     | {=r,tom=arwdRxt,joe=arwdRxt}
 tm_pg_hmp_num_bak |
(2 rows)

[/snip example]

Since Tom is the creator of the table, is it only HE that
get's to make the modification?  I thought that we both
have the same rights and privilages.  Not true?

Thanks!

-X