Susan Helms is about to embark on “The Great Escape,” having severed almost all ties that bind her to planet Earth. Single and with no children, the veteran astronaut is scheduled to blast off this week aboard shuttle Discovery, setting sail on a four-month trip to the International Space Station. And for the past six weeks, Helms has been busy slashing earthly umbilicals. Like a soldier preparing for a lengthy tour abroad, the Air Force lieutenant colonel ended her lease at her apartment in Houston — cutting off telephone, cable television, Internet and utility services in the process.