Artificial gravity

Artificial gravity is the creation of an inertial force that mimics the effects of gravity. In the vacuum of space, astronauts experience weightlessness, which can be very difficult for the human body to endure. Scientists can simulate gravity through rotation or through linear acceleration, such as the push from a rocket engine. Because of the equivalence principle, the feeling of linear acceleration is indistinguishable from the feeling of gravity. However, most research focuses on rotational gravity. This method uses a rotating frame of reference to create the appearance of a downward pull.

To understand how rotational gravity works, we must look at the relationship between two forces. When a spacecraft rotates, its hull provides a centripetal force. This is a force that pulls or pushes toward the center of the rotation to keep objects moving in a circle. In a rotating station, the hull pushes inward on the occupants. According to Newton's third law, this creates an equal and opposite force called centrifugal force. To the person inside the rotating station, this centrifugal force feels like it is pulling them "downward" toward the outer hull.

There are several physical differences between artificial gravity and the gravity we feel on Earth. First, centrifugal force is not constant; it varies based on your distance from the center. The force is directly proportional to the distance from the axis of rotation. This means if a spacecraft has a small radius, a person's feet might feel much heavier than their head. Second, moving objects inside the station experience the Coriolis effect. This is an apparent force that acts at right angles to the motion and the rotation axis. It tends to curve the motion of objects in the opposite direction of the station's spin.

These physical forces can cause significant health issues for astronauts. The Coriolis effect acts on the semicircular canals of the inner ear. This can lead to severe nausea, dizziness, and disorientation. To minimize these effects, engineers suggest a low spin rate. It is generally believed that a rotation of 2 rpm (revolutions per minute) or less will prevent these adverse effects. However, humans have shown the ability to adapt to much higher rates, as high as 23 rpm. Additionally, any movement of mass inside the station could cause a dangerous wobble. Engineers might use a liquid water supply as ballast to stabilize the axis of rotation.

History shows that we have been testing these concepts for decades. In 1966, the Gemini 11 mission attempted to create artificial gravity. The capsule was attached to an Agena Target Vehicle by a 36-meter tether. By using side thrusters, they rotated the two crafts together like a pair of bolas. The resulting gravity was only 0.00015 g, which was too small for astronauts to feel. However, researchers observed objects moving toward the floor of the capsule.

Later, in 1975 and 1977, the Bion 3 and Bion 4 satellites used on-board centrifuges to study specimens in artificial gravity.

The need for this technology is driven by the biological risks of weightlessness. In 1964, Soviet scientists feared humans could not survive more than 14 days in space. While we now know humans can stay in space for over 437 days, weightlessness still causes damage. A 1991 Spacelab experiment showed that muscle mass and white blood cell responses decrease in zero gravity. Furthermore, blood volume can drop by 10% within the first 24 hours. Long-term exposure can also cause brain swelling and eyesight problems. Artificial gravity could solve these issues by removing weightlessness entirely during long journeys, such as a six-month trip to Mars.

Many specific designs have been proposed to solve these engineering challenges. The 1989 NASA concept for a rotating Mars spacecraft is a well-known example. In 2011, NASA proposed the Multi-Mission Space Exploration Vehicle (MMSEV). This design included a torus-ring centrifuge that could provide between 0.11 and 0.69 g. Another concept, the Nautilus-X, was proposed as a centrifuge demonstration for the International Space Station.

Even the Mars Gravity Biosatellite was proposed to study the effects of 0.38 g on mice, though it was canceled in 2009 due to a lack of funding.

Ultimately, artificial gravity is a bridge to deep space exploration. While current spacecraft are too small or too expensive to provide useful gravity, the technology remains a major goal. Companies like Vast Space are even proposing the construction of the first artificial gravity space station. By mastering rotation and centripetal force, humans may one day travel to distant planets like Mars or Jupiter without the physical toll of weightlessness.