WASHINGTON — The launch of a SpaceX Dragon cargo spacecraft to the International Space Station will be delayed a day because a problem that may be a first in the history of spaceflight: contaminated rodent food.

NASA announced late Dec. 3 that the Falcon 9 launch of the Dragon spacecraft on a mission designated CRS-16 had been delayed from its planned Dec. 4 launch by a day. The delay is needed to replace food bars for a rodent investigation that had been contaminated with mold. The launch is now scheduled for 1:16 p.m. Eastern Dec. 5 from Space Launch Complex 40 at Cape Canaveral, Florida.

At a pre-launch press conference Dec. 3, agency officials had warned that this “small issue” could delay the launch, although at the time they were hopeful that they would be able to replace the food in time to allow launch preparations to remain on schedule.

“When we were in the process of preparing the rodents to be loaded on board the vehicle, we were looking at some of the food bars that is necessary for the rodents, and they were contaminated with some mold,” Joel Montalbano, deputy ISS program manager, said at the briefing. NASA decided to replace all the food bars for the experiment, including those that had been loaded on Dragon already.

The rodent experiment is part of “late load” cargo installed on the Dragon spacecraft less than 24 hours before launch. The replacement involved flying in unspecified hardware from the Ames Research Center in California that was not scheduled to arrive at the launch site until late Dec. 3, after the late load cargo would normally be installed. Montalbano said at the briefing that NASA was talking with SpaceX about how to modify the late load cargo schedule.

The rodent experiment is part of more than 1,000 kilograms of science investigations contained within the Dragon on this mission, as well as several hundred kilograms of crew supplies and vehicle hardware. The spacecraft is carrying nearly 1,000 kilograms of unpressurized cargo in the form of two experiments inside the spacecraft’s trunk section that will be mounted on the station’s exterior.

A one-day shift in the launch will also allow for improved weather for the launch. Forecasts had been predicting a 60 percent chance of acceptable weather, with concerns about clouds and lingering rain from a passing frontal system. The forecast for a Dec. 5 launch calls for a 90 percent chance of acceptable weather.

The delay, though, deprives SpaceX of the opportunity to perform two launches almost exactly 24 hours apart. The Dec. 4 launch window was at 1:38:51 p.m. Eastern, while another Falcon 9 lifted off from Vandenberg Air Force Base in California at 1:34 p.m. Dec. 3 on a mission to place 64 smallsats into orbit.

That 24-hour separation is about as close as SpaceX can currently perform two launches. “Twenty-four hours is, approximately, my comfort zone for reviewing data,” said Hans Koenigsmann, vice president of build and flight reliability at SpaceX, said at the briefing. Depending on the time of the day of the launches, he said it could be possible to compress the time between launches to as little as 13 to 14 hours.