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Flies in space!

NASA is catching up with the schedule for the shuttle Discovery's launch preparations, making up for a delay that was sparked on Thursday by a lightning alert. This afternoon, the final experiments are being stowed in the orbiter's middeck area - including a collection of microbes that will be used to see how space travel affects mutation and DNA repair, and an intrepid platoon of fruit flies.

Fruit flies?

The fruit-fly experiment, like the microbial experiment, is designed to see how the space environment affects living things. As explained in this report, the flies will be carried in a sealed tray aboard Discovery, along with embryonic flies that should mature into adults during the flight, and samples of a fungus that would provoke an immune response in the flies.

After Discovery's mission, the flies would be exposed to the fungus back on Earth. The purpose of the experiment - known as Fungal Pathogenesis, Tumorigenesis and Effects of Host Immunity in Space, or FIT - is to see how the radiation and weightlessness of the space environment affects the immune system.

It's already well-known that spaceflight can suppress an astronaut's immune system, and that some bacteria may become more virulent in space. But scientists haven't been able to put their finger on the precise reasons why. FIT represents the best effort to date to unravel the genetic mystery.

The microbial experiment - titled Passive Observatories for Esxperimental Microbial Systems, or POEMS - will take another approach to the mystery, exposing various types of microbes to space on the shuttle as well as over the longer term on the international space station.

The flies didn't exactly volunteer for this duty, but if they did, they'd be rooting for Discovery to take off sooner rather than later.

"After two successive launch attempts, they will go in and change out the fruit flies. So we have another set of fruit flies that are in the Space Flight Sciences Laboratory that are ready if we stay at the pad a little longer," said payload manager Debbie Hahn.

The service structure that encloses the shuttle during processing should be rotated away from Discovery this evening, representing another step toward launch. But the weather prognosis is pretty much the same as it has been: 60 percent chance of unacceptable conditions on Saturday, Sunday and Monday, due to the potential for thunderstorms and lightning strikes from anvil clouds.

Even if there's no lightning, the presence of the particular types of clouds that can generate a spark within 20 nautical miles of NASA's Kennedy Space Center could lead launch controllers to call off the launch. "You could actually trigger a lightning strike with our launch," weather officer Kathy Winters observed.

That's happened before, most famously in 1969 during the launch of Apollo 12. No lasting damage was done that time, but it's just one more thing to be wary about as NASA gets ready for Discovery's fireworks show.

On the technical side of things, everything looks A-OK, said NASA test director Jeff Spaulding.

"While it's taken us nearly a year of hard work to get back to this point, I'm proud to announce that the vehicle, our launch team and our flight crew are ready to launch and all of us are ready kick off our nation's 230th birthday celebration a little early this weekend," he told reporters.

Check out our "Return to Flight" section for updates as the clock ticks toward Discovery's scheduled launch at 3:48 p.m. ET Saturday.