Little creatures can cause big problems.

SpaceX CRS-16, a joint rocket mission from the California-based company and NASA, is going to take off today from Cape Canaveral toward the International Space Station. There’s nothing particularly unique about SpaceX’s 16th cargo resupply mission to the ISS, except the fact that it was supposed to launch yesterday. The mission’s one-day delay was due to a curiosity—moldy mouse food.

Supply missions like CRS-16 often come with scientific payloads. CRS-15, for example, came with CubeSats from the Philippines and Boeing. CRS-16 is launching with 40 mice on board. Rodent Research-8 (RR-8) “examines the physiology of aging and the effect of age on disease progression using groups of young and old mice flown in space and kept on Earth,” according to a mission description.

However, as NASA was making its final inspections, something seemed off: mouse food. Unhealthy mold was confirmed on the food bars, forcing an extra day to find new ones.

Twenty young mice and just as many 0lder ones will reside in the ISS for 30- to 40-day or 60-day durations. After spending various amounts of time on the ISS, all of the mice will be euthanized. Then NASA will analyze their tissue samples, looking for ways to “evaluate muscle atrophy under microgravity conditions, and to identify molecular pathways and targets that could be used to develop novel therapeutics for muscle disease.”

NASA cites a “great demand” for these tissue samples, with interest ranging from the Department of Defense to the National Institutes of Health. It’s a study that they felt was worth delaying a rocket launch for, even if only a day.

You can watch the CRS-16 launch below.

Source: LiveScience