Registration began Thursday at 10 a.m. EDT and will end at 5 p.m. Friday.
The Tweetup expected to culminate in the launch of the Mars Science Laboratory Curiosity rover.
Registration will at noon on Oct. 5 and closes at noon on Oct. 7. NASA will randomly select 150 participants from online registrations.
NASA said participants will be able to tour the Kennedy Space Center, speak with scientists and engineers, and, if all goes as scheduled, view the spacecraft launch.