Stranded In Space: The Starliner Delay and Astronaut Health Risks

The Boeing Starliner spacecraft was launched on June 5, 2024, and docked at the International Space Station (ISS) the following day. The Starliner brought two NASA astronauts: Sunita Williams and Barry “Butch” Wilmore. Their mission was supposed to last for only eight days, but now they have been stuck on the ISS for two months. NASA has revealed that their return to Earth has been delayed to February 2025. 

The astronauts’ delayed return is attributed to the helium leaks discovered in the Starliner’s propulsion system. The leaks are caused by the degradation of seals on the thrusters due to heat from the thrusters and sliding surfaces. Thrust is the force that moves an object like a spacecraft through a fluid—which can be both gases and liquids—against friction with the fluid (drag). To keep a spacecraft moving, thrust is continuously produced by a propulsion system consisting of thrusters. 

The Starliner uses a cold gas propulsion system that utilizes the flow of pressurized gas like helium through a convergent-divergent nozzle. A convergent-divergent nozzle, also known as the de Laval nozzle, is an hourglass-shaped tube where gas from a storage tank enters through the convergent end, passes through the narrow “throat,” and exits through the divergent end. The nozzle allows high-pressure, low-velocity gas to enter and low-pressure, high-velocity gas to exit. Gas is compressed as it flows from the convergent section to the throat. Boyle’s law states that at a constant temperature, the pressure of a gas is inversely proportional to its volume. Thus, due to the smaller volume of the throat, gas pressure increases. When the gas exits through the wide divergent end, pressure dramatically decreases due to the sudden increase in volume. 

The dramatic decrease in pressure results in an equally dramatic increase in velocity due to Bernoulli’s principle. Bernoulli’s principle states that a decrease in fluid pressure corresponds to an increase in fluid velocity. This principle is derived from the conservation of energy in a fluid system. The decrease in pressure of the gas corresponds to a decrease in potential, or stored, energy. Due to the law of conservation of energy, there must be an increase in kinetic energy, or energy of motion, which corresponds to increased gas velocity. The velocity of gas expelled out from the thruster through the divergent end can reach the speed of sound. As a result, the force exerted by the high-velocity gas propels the spacecraft forward due to Newton’s third law of motion, which states that a force exerted on another object is paired with an equal and opposite force exerted by the second object. 

For a cold gas propulsion system, maximum thrust is dependent on the gas pressure inside the storage tank. Over time, when gas is used up, the pressure in the storage tank decreases, decreasing the thrust generated. Similarly, the Starliner’s helium leakage reduces the pressure in the storage tank, thereby reducing the thrust of the Starliner. This could disrupt the astronauts’ return to Earth if they rode the Starliner back to Earth. As a result, NASA determined that it was likely too dangerous for the astronauts to return to Earth in the Starliner spacecraft. The astronauts’ extended stay on the ISS provides scientists more time to improve thruster durability to prevent such helium leaks.

Besides technical problems presented by the Starliner delay, its astronauts may also face health issues resulting from their prolonged time in space. With Williams’s and Wilmore’s stay being extended to February 2025, they will be spending a total of eight months in space. This isn’t the first time someone has spent a long time in space; Russian cosmonaut Valeri Polyakov spent 437 consecutive days aboard the Mir space station. Long stays in space can have harmful effects on the human body. To combat this, scientists are researching methods to create artificial gravity.

Long durations in microgravity cause density loss of bone and muscle, known as bone and muscle atrophy. In space, weight-bearing bones become about 0.5%-1.5% less dense per month, as they no longer need to support the astronauts' full body weight. Furthermore, muscles don’t need to work as hard, leading to loss of muscle mass. While astronauts exercise regularly to help preserve bone and muscle mass, it was observed that nine crewmembers who regularly exercised lost 10%-15% of their calf muscle mass over the course of six months. This loss of muscle also led to a 32% decreased peak power measured in maximal muscle torque in nanometers. Bone and muscle atrophy has detrimental effects including severe falls and bone fragility, also known as osteoporosis.

On Earth, the heart must pump against gravity to get blood around the body and brain. The lack of gravity means less force is required to pump blood around the body and control blood flow, leading to the atrophy of the heart and blood vessels. Fluids in space are pushed up towards the top of the body, leading to a fluid imbalance with swelling in the face and the head, causing a decrease in blood volume. The accumulation of extra fluids in the skull increases brain pressure, resulting in hearing loss, cerebral edema, or swelling of the brain, and other issues. Additionally, a resultant increased pressure on the eyes can cause spaceflight associated neuro-ocular syndrome, which flattens the eyeballs, leading to farsightedness and blurry vision.

Astronauts primarily use exercise to maintain bone and muscle density, utilizing special treadmills and resistance devices that simulate gravity on their bodies. These devices use bungee cords and elastic bands to pull the astronaut down. This allows astronauts to maintain cardiovascular fitness and leg muscles. They also use Advanced Resistive Exercise Devices which use vacuum cylinders to create resistance, allowing astronauts to perform exercises like bench presses and deadlifts to maintain muscle mass. However, these machines are large and expensive, and as demonstrated by the health issues of current scientists, this approach is insufficient to fully prevent atrophy or counteract the effects of fluid redistribution in space. To address these challenges, researchers are exploring ways to produce artificial gravity which mimics gravitational force using an externally generated force. To create artificial gravity, constant force must be applied on the body at the acceleration of gravity, or approximately 9.81 meters per second squared. One potential method involves constantly accelerating the spacecraft, though this requires large amounts of fuel. A more feasible approach is to spin the spacecraft using centrifugal forces to create gravity. Centrifugal force is the force a person feels when they are pushed outwards from the center of a spinning object, like when a car takes a sharp turn. Implementing artificial gravity will improve astronaut safety and support their health on long-duration missions, allowing deep-space exploration and future human settlements.

Fortunately, Williams and Wilmore are safe on the ISS and are kept under close watch by NASA. NASA plans on undocking the empty Starliner from the ISS to return to Earth alone on September 6, 2024, at 6:00 p.m. EDT. NASA plans to have a Crew Dragon spacecraft, SpaceX’s reusable crew capsule, to bring the astronauts back to Earth in 2025. Despite being a setback, the astronauts’ extended stay in space serves as a learning experience for future spacecraft engineering and space health.