Even well-trained astronauts can get sick in space. In their first two or three days, about 60 to 80 percent of them feel ill. They may lose their appetite or throw up.

Why? Deep inside the ear, there is a sensor that tells us which way is down. In space, this sensor gets confused, but the eyes still see the room. The brain gets mixed signals. Most astronauts feel better after a few days.

Astronauts go through hard training, but many still suffer from space motion sickness. According to a 2006 review, about 60 to 80 percent of astronauts experience it during their first two or three days in space. Symptoms include cold sweat, loss of appetite, nausea, and vomiting. The main cause is thought to be a conflict between the senses.

Inside the inner ear are organs that sense gravity to tell us which way is down. In weightlessness, they stop giving clear signals, while the eyes still see a "ceiling" and a "floor." This mismatch confuses the brain. Most astronauts adapt within a few days, but some feel sick again after returning to Earth.

Despite rigorous selection and training, astronauts are far from immune to motion sickness. A 2006 review reported that roughly 60 to 80 percent experience space motion sickness during their first two or three days in orbit, and a NASA medical brief cites figures of up to 73 percent. Symptoms range from pallor and cold sweats to loss of appetite, nausea, vomiting, and fatigue.

Because performance suffers, mission planners often avoid scheduling critical tasks for the first few days. The leading explanation is sensory conflict. The otolith organs in the inner ear normally detect gravity by sensing the pull on tiny crystals, telling the brain which way is down. In weightlessness, they no longer provide this reference, and head movements produce unfamiliar signals, while the eyes still interpret parts of the cabin as "up" and "down."

Researchers have noted that large head movements tend to worsen symptoms, so astronauts are advised to move their heads slowly. A second hypothesis points to fluid shifting toward the head, which may affect pressure in the inner ear. Susceptibility to motion sickness on Earth does not reliably predict who will be affected in space, and some astronauts experience similar symptoms again after returning to gravity.