On Friday evening (April 10) the Artemis II successfully splashed down in the Pacific Ocean, off the coast of San Diego, with experts calling the return to Earth ‘flawless’.
Reid Wiseman, Victor Glover, Christina Koch and Jeremy Hansen spent 10 days on a mission that took them around the Moon, pushing farther into deep space than any crewed flight has gone before.
As the capsule hit the water, the moment was widely praised. NASA commentator Rob Navias summed it up as ‘a perfect bull’s eye splashdown for Integrity and its four astronauts’.
Reaching space — and returning safely — puts the human body through a huge amount of stress, and scientists have long tracked how astronauts change physically when they leave Earth’s gravity behind.
Dr. John DeWitt, who served for two decades at NASA’s Johnson Space Center as Senior Biomechanist and Exercise Device Project Scientist in Astronaut Health and Performance, has explained how quickly the body recalibrates in a weightless environment.
He said: “On Earth, where we live in gravity 24 hours a day, our bodies are used to working against gravity when performing actions like pumping blood back from the legs to the heart and using our lower body and core muscles to maintain balance and posture. In space, the heart doesn’t have to work as hard because there is no gravity pulling blood towards our feet, and our posture muscles don’t have to work because astronauts just float. As a result, these systems start to weaken.”
As per Vice, NASA has documented a range of changes that can occur after time in orbit.
One issue the agency has highlighted is Spaceflight-Associated Neuro-ocular Syndrome, a condition linked to extended exposure to weightlessness.
NASA explain: “Chronic weightlessness can cause bodily fluids such as blood and cerebrospinal fluid to move toward the head, which can lead to optic nerve swelling, folds in the retina, flattening of the back of the eye, and swelling in the brain.”
Bone strength is another major concern, particularly on longer-duration flights. NASA estimates skeletal density can drop by between one and five percent per month while astronauts are in microgravity.
They explain: “In microgravity, weight-bearing bones like the spine and hips don’t need to be as strong to support the human body. Consequently, on average, bones lose between 1% and 1.5% of their density each month during four-to-six-month missions.
“… Exercise to reduce the deficits in postflight bone density and strengthen muscle force is an essential component of preventing declines in bone tissue.”
To counter muscle and bone loss, astronauts stick to strict daily training routines. Crew members typically exercise for around two hours per day to help stop their bodies from deconditioning.
“Each astronaut aboard the space station engages the muscles, bones, and other connective tissues that comprise their musculoskeletal systems using Earth-like exercise regimens,” says NASA.
“Astronauts have biked on stationary bicycles and run on treadmills in space for decades. One of the first missions on the space station flew TVIS, a treadmill with a harness to keep the user tethered to the machine and add some gravity-like force. A current piece of equipment called ARED allows astronauts to mimic weightlifting in microgravity.”
Extended missions have also been linked to a higher risk of anemia, which can happen because red blood cells are destroyed more quickly in space.
In Nature Medicine, researchers reported that astronauts’ bodies both create and break down red blood cells at a faster pace in orbit — about three million per second compared to two million per second on Earth.
Anemia is the term used when the body has too few red blood cells available to carry oxygen efficiently.
“It was first thought that red blood cell destruction occurred in the first days of space flight and then red blood cell control went back to its normal state,” explained principal investigator Guy Trudel at the University of Ottawa. “Now we know hemolysis, or red blood cell destruction, happens for as long as you are in space.”
There have also been reports over the years of astronauts developing blood clots while in space, prompting ongoing studies into what triggers them and how to reduce the risk.
While work continues, NASA says fluid shifts caused by microgravity are a likely contributor.
“In the absence of gravity, body fluids shift from the legs to the upper body and the head. This shift affects the flow of blood through the vessels in the head. Blood flow patterns are reported to change when astronauts are in space, and interestingly, the degree of change appears to differ between different astronauts,” they explain.
And it’s not just circulation and muscle that change — the immune system can behave differently off-planet too, raising questions like whether you can “catch a cold” in space.
NASA has noted that some viruses can reactivate while astronauts are away from Earth. One example is the Varicella Zoster Virus (VZV), responsible for chickenpox in childhood and shingles later in life.
“This reactivation is typically ‘subclinical’ in astronauts, meaning they are not truly sick and do not exhibit symptoms. However, the presence of the active viruses is a good indicator that those astronauts are experiencing reduced immunity during a space mission,” says NASA.
Radiation exposure is another key difference between Earth and space, because astronauts are less protected by the atmosphere and magnetic field—especially beyond low Earth orbit.
“Space radiation is made up of three kinds of radiation: particles trapped in the Earth’s magnetic field; particles shot into space during solar flares (solar particle events); and galactic cosmic rays, which are high-energy protons and heavy ions from outside our solar system,” says NASA.
“Beyond Low Earth Orbit, space radiation may place astronauts at significant risk for radiation sickness, and increased lifetime risk for cancer, central nervous system effects, and degenerative diseases. Research studies of exposure in various doses and strengths of radiation provide strong evidence that cancer and degenerative diseases are to be expected from exposures to galactic cosmic rays (GCR) or solar particle events (SPE).”