Neil deGrasse Tyson speaks to a group of Tweetup participants at NASA Kennedy Space Center's NASA Causeway launch viewing site in Florida.

Astrophysicist Dr. Neil deGrasse Tyson with the American Museum of Natural History’s Hayden Planetarium in New York, speaks to a group of Tweetup participants at NASA Kennedy Space Center’s NASA Causeway launch viewing site in Florida during prelaunch activities for the agency’s Gravity Recovery and Interior Laboratory (GRAIL) mission. Participants toured the center and got a close-up view of Space Launch Complex 17B at Cape Canaveral Air Force Station. The tweeters will share their experiences with followers through the social networking site Twitter.GRAIL will fly twin spacecraft in tandem around the moon to precisely measure and map variations in the moon’s gravitational field. The mission will provide the most accurate global gravity field to date for any planet, including Earth. This detailed information will reveal differences in the density of the moon’s crust and mantle and will help answer fundamental questions about the moon’s internal structure, thermal evolution, and history of collisions with asteroids. The aim is to map the moon’s gravity field so completely that future lunar vehicles can safely navigate anywhere on the moon’s surface. Launch is scheduled for 8:37:06 a.m. EDT Sept. 8. For more information, visit http://www.nasa.gov/grail. Photo credit: NASA/Kim Shiflett