A “significant” hydrogen gas leak from part of the space shuttle Discovery has forced NASA to scrub the craft’s planned liftoff until at least the end of the month. Discovery’s crew will be accompanied by 16 mice that are part of a science experiment aimed at understanding why spaceflight makes humans more vulnerable to infections by viruses and bacteria. “We’re going to study the effects of spaceflight on the immune system, and in particular the innate immune system, which is responsible for first detecting pathogens in the body and reacting to them,” Dr. Roberto Garofalo, principal investigator of the Mouse Immunology-2 (MI2) experiment, told National Geographic News. Once Discovery returns to Earth, scientists will infect the animals with the respiratory syncytial virus and compare their health to a group of control mice that remained on Earth. The team chose the RSV virus because there is evidence that astronauts are more susceptible to certain respiratory infections, such as influenza, following spaceflight, said Garofalo, an immunologist at UTMB Health.