On STS-51B, NASA flew 24 rats and 2 squirrel monkeys with Spacelab-3; one monkey showed space sickness while the rats returned with muscle loss

In April 1985, the Space Shuttle Challenger carried an unusual crew alongside its seven astronauts. Inside the Spacelab-3 module, sent up to test how living creatures cope with weightlessness, there were 24 rats and two squirrel monkeys. NASA’s official mission record for STS-51B shows that the shuttle lifted off on April 29, 1985, and spent seven days in orbit before landing at Edwards Air Force Base in California.

A flying lab built just for animals

The animals were not just caged and left alone. To keep them fed, monitored, and comfortable during the flight, NASA built a system just for them called the Research Animal Holding Facility, or RAHF. The facility used cameras, biotelemetry, and environmental monitoring equipment to observe the animals 24 hours a day. A NASA Ames Research Center report on the mission, titled 'Ames Research Center life sciences payload - Overview of results of a spaceflight of 24 rats and 2 monkeys,' says engineers had to design tightly fitted seals for the cages, because floating food and waste particles kept drifting into the Spacelab equipment. It's a small thing, but it speaks to a larger truth: Keeping animals healthy in microgravity is a lot more involved than most people think.


One monkey developed space sickness, similar to humans

The squirrel monkeys were partly chosen because they are easier to compare with humans in body and behavior than rats. One of the two monkeys aboard showed signs of what scientists call space adaptation syndrome, a kind of motion sickness that strikes many astronauts in the first few days of orbit. This monkey developed a lack of appetite and lethargy, similar to what was observed in the human crew members. A separate 1987 NASA conference paper, 'Animal Studies on Spacelab-3,' notes that this reaction indicated that squirrel monkeys might be useful stand-ins for studying the human body's response to weightlessness. The second monkey did not show the same symptoms, a reminder that space sickness affects different bodies differently, whether human or animal.

The rats told a very different story

Monkeys dealt with the journey in their own individual ways, but the rats showed more obvious physical deterioration. All 24 rats returned to Earth with reduced muscle tone and weakened immune function. The 1987 paper said the most dramatic change in the rats was a loss of muscle mass, and their long bones became more fragile than before the flight. Post-flight necropsies, the animal equivalent of an autopsy, carried out after the rats were euthanized for tissue study, as is standard for this kind of research, documented other physiological shifts, indicating changes in hormone activity and in how the rats' bodies processed fat and sugar. This doesn't mean the rats were in danger during the mission itself; in fact, the animals remained in good physical shape throughout the flight, and the changes only became evident when scientists examined their tissue after landing.

Why a 1985 experiment still matters

It is easy to dismiss the mission as a small footnote in space history, but Spacelab-3 gave researchers early evidence that muscles and bones weaken quickly in microgravity, which astronauts now counter with daily exercise on the International Space Station. The monkey's mild human-like illness helped confirm animal studies could provide real clues to how astronauts' bodies respond in the first days of a mission. The changes doctors later saw, less muscle, weaker bones, and one queasy monkey, gave scientists a clearer picture of what spaceflight does to a living body.

A small crew with a lasting legacy

Spacelab-3's main objective was to test a high-quality microgravity environment for materials processing and fluid experiments, rather than the animal research itself; the RAHF verification test was a secondary part of the flight, and in that sense, it did its job quietly and well. Its side findings, muscle loss in the rats and space sickness in one monkey, helped confirm that gravity, or the lack of it, changes the body faster than researchers had previously documented. Those lessons are still referenced today as NASA plans longer missions to the Moon and Mars.