"It's pretty cool that we'll be standing just three miles away," Gadd said. "I expect it will be spectacular and grand and loud and I hope it goes on time."
Gadd will be blogging for Salt Lake City's Clark Planetarium during the Tweetup.
Rachel Donner, 37, a project manager at an advertising agency, does consider herself a space geek. Big time. She went to space camp twice.
"I've been super into astronomy and the space shuttle program for as long as I can remember it existing," she explained.
Donner (@rmiriam on Twitter) participated in the NASA Tweetup for Endeavor's blast-off in May, and a twist of fate (and family vacation) will put her back in Florida at just the right time. She will watch the launch from the Kennedy Space Center Visitors Center with her father, brother and 7-year-old nephew.
"It will be a great chance to introduce all of this to my nephew. I'm really excited for him to see it, " she said. "Dad is over the moon over this. He's seen some rocket launches, like in the '60s or '70s, but he's never seen a shuttle launch."
Both Utahns are wistful about witnessing the end of the space shuttle program.
"It's almost bittersweet," Gadd said. "My dad took me to Hill [Air Force Base] to see the shuttle in 1979 when they did the tests on the back of the plane. Here I am, 5 years old when I first saw it. I was 7 when my dad woke me up to watch the first launch on TV. I grew up with it it's all I know."
Donner is a bit more melancholy. "I'm really sad about it," she said. "Just because I think it's been such a great part of American scientific achievement. I'm disappointed there wasn't really an effort to create a replacement for it earlier, so that there wouldn't be this gap."
If the launch is delayed, both Utahns may miss it. Gadd has to be back at work July 10 and Donner's family vacation is over that same day.
Twitter hashtag: #NASATweetup or #STS135
Rachel Donner's blog: http://sts134.tumblr.com
Clark Planetarium blog: http://www.clarkplanetarium.org/blog