Chemical Elements Poems 24: Three Isotopes, Twelve Lines

Two thirty eight has a half-life of most of a century.
It powers spacecraft distant from the Sun. 
Congress won't approve making more of the stuff, 
NASA will be up a tree when stockpile's done. 

Two thirty nine is fissile so it's been used a lot,
its history in reactors is sometimes rocky. 
It sustains chain reactions that are either fast or slow, 
to the lasting detriment of Nagasaki. 

Two forty four has been around since the world began
though the last of it will decay before long.  
It has no use in weapons, it has no use for heat.
It has no use at all, for right or wrong.