A secure electronic ballot, generated by the Harris and Brazoria County Clerk's office, is uplinked by NASA's Johnson Space Center Mission Control Center. An e-mail with crew member-specific credentials is sent from the County Clerk to the crew member. These credentials allow the crew member to access the secure ballot.
The astronauts will cast their votes and a secure completed ballot is downlinked and delivered back to the County Clerk’s Office by e-mail to be officially recorded.
Almost all NASA astronauts live in Houston. Link -Thanks, Jerse!