The interior of a free-falling system, if it is small enough, can be thought of as a “gravity-free space” in accordance with Einstein’s equivalence principle, meaning all gravity-related forces, such as weight and the buoyant force, disappear. The last force lifts the Cartesian diver up from the bottom when the external pressure on a stationary water-filled bottle is released. Nevertheless, if the external pressure is released and, simultaneously, the bottle with the diver at the bottom is allowed to fall, the Cartesian diver will stay on the bottle’s bottom. This behavior of the diver is due to the fact that the interior of a free-falling bottle can be thought of as a gravity-free space.

Abstract A number of studies have well described central cardiovascular changes caused by changing gravity levels as they occur e.g. during parabolic flight. However limited data exists describing the effect of microgravity on the cerebrovascular system and brain perfusion, which might heavily affect the intracranial pressure (ICP). In this study, performed on board the Airbus A310 ZeroG, cerebral blood flow velocity, diameter and cerebral blood volume were continuously monitored in the arteria carotis communis (ACC) for five consecutive parabolas by cardiac-gated B mode ultrasonography using a 7–12MHz linear transducer. Simultaneously heart rate (HR) was monitored. Data was assessed in the 1G condition pre parabola and at the start and end of each 1.8G and 0G phase. A lab-based study was administered where participants repeatedly changed between a seated and six-degree head down tilt (HDT) position for 20 seconds. During parabolic flight, HR increased during the two hypergravity phases and decreased back to 1G baseline in the weightlessness phase of the parabola. ACC diameter, blood flow velocity and flow volume showed a clear decrease in the first 1.8 hypergravity phase but returned to 1G baseline in the microgravity phase. Different results were obtained by changing from a seated to a 6°HDT position, resulting in an increase in ACC diameter, flow velocity and flow volume. Results suggest that the previously described changes within the middle cerebral artery (MCA) are rather caused by changes on the venous side but not an increased inflow during – at least – short term phases of microgravity.

occupants the sensation of weightlessness by following a parabolic flight path, which is the path that objects naturally follow while in free fall. The aircraft A reduced-gravity aircraft is a type of fixed-wing aircraft that provides brief near-weightless environments for training astronauts, conducting research, and making gravity-free movie shots. Versions of such airplanes were operated by the NASA Reduced Gravity Research Program, and one is currently operated by the Human Spaceflight and Robotic Exploration Programmes of the European Space Agency. T A reduced-gravity aircraft is a type of fixed-wing aircraft that provides brief near-weightless environments for training astronauts, conducting research, and making gravity-free movie shots. Versions of such airplanes were operated by the NASA Reduced Gravity Research Program, and one is currently operated by the Human Spaceflight and Robotic Exploration Programmes of the European Space Agency. The unofficial nickname "vomit comet" became popular among those who experienced their operation. NASA flew zero gravity flights on various aircraft for many years. In 1959 Project Mercury astronauts trained in a C-131 Samaritan aircraft dubbed the "vomit comet". Twin KC-135 Stratotankers were used until December 2004 and later retired. One, a KC-135A registered N930NA (also known as NASA 930, formerly USAF serial no. 59-1481), flew more than 58,000 parabolas after NASA acquired it in 1973, before being retired in 1995. It is now on display at Ellington Field, near the Johnson Space Center. The other (N931NA or NASA 931, formerly AF serial no. 63-7998) was also used by Universal Pictures and Imagine Entertainment for filming scenes involving weightlessness in the movie Apollo 13; it made its final flight on October 29, 2004, and is permanently stored in the Pima Air & Space Museum in Tucson, Arizona. In 2005 NASA replaced these aircraft with a McDonnell Douglas C-9B Skytrain II (N932NA) formerly owned by KLM Royal Dutch Airlines and the United States Navy. NASA concluded the Reduced Gravity Research Program and ended operations in July 2014, due to aircraft technical problems. It is replaced with private company contracts. As of 2015 NASA had a microgravity services contract with Zero Gravity Corporation (ZERO-G) and used its aircraft, G-FORCE ONE, a modified Boeing 727-200. Reduced…

ABSTRACT Despite extensive research on vestibular function in microgravity, particularly during orbital and parabolic flight exposure, several gaps remain regarding the spontaneous behavior of vestibular organs under non-terrestrial gravitoinertial conditions. In particular, semicircular canal function, typically assessed through vestibulo-ocular reflex (VOR) recordings, has yielded inconsistent findings, with reports describing either no effect or reduced performance in microgravity. Moreover, many of these studies are limited by methodological constraints that reduce the interpretability of their conclusions. To clarify these discrepancies, we evaluated horizontal and vertical VOR responses during parabolic flights to assess semicircular canal function under transient weightlessness. Participants were passively rotated at a constant frequency and amplitude during normogravity and microgravity phases, centered along the head’s vertical or inter-aural axis. Eye movements were recorded binocularly using infrared eye-tracking in darkness to eliminate visual influences, while participants were tightly restrained to minimize proprioceptive variability. Results show a reduction in VOR gain during microgravity in both axes, despite consistent rotational stimulation across gravity conditions. In addition, VOR gain remained reduced after parabolas in the horizontal plane, whereas vertical VOR performance was preserved. These are the first results to demonstrate an immediate alteration of semicircular canal function in weightlessness. Possible sources of the reduction in VOR performance in 0g are discussed. We also propose that the observed post-flight effects reflect a down-weighting of semicircular canal inputs during multisensory integration.