add some intro text to ch. 3.18
minor revisions, ch. 3.18
extend intro text to ch. 3.18
add note about NASA's Deep Space 1 mission to ch. 3.18 intro text
remove chapter notes on ch. 3.18
add quotation to ch. 3.18
minor revision to first paragraph, ch. 3.18 intro text
update status icon on ToC for ch. 3.18