long periods of time. A variety of electric propulsion systems provide examples. Two examples of this long-duration, low-thrust, high-impulse propulsion that have either been practically used on spacecraft or are planned in for near-term in-space use are Hall effect thrusters and Variable Specific Impulse Magnetoplasma Rockets (VASIMR). Both provide very high specific impulse but relatively low thrust, compared to the more typical chemical reaction rockets. They are thus ideally suited for long-duration firings which would provide limited amounts of, but long-term, milli-g levels of artificial gravity in spacecraft. In a number of science fiction plots, acceleration is used to produce artificial gravity for interstellar spacecraft, propelled by as yet theoretical or hypothetical means. This effect of linear acceleration is well understood, and is routinely used for 0 g cryogenic fluid management for post-launch (subsequent) in-space firings of upper stage rockets. Roller coasters, especially launched roller coasters or those that rely on electromagnetic propulsion, can provide linear acceleration "gravity", and so can relatively high acceleration vehicles, such as sports cars. Linear acceleration can be used to provide air-time on roller coasters and other thrill rides. Simulating lunar gravity In January 2022, China was reported by the South China Morning Post to have built a small ( diameter) research facility to simulate low lunar gravity with the help of magnets. The facility was reportedly partly inspired by the work of Andre Geim (who later shared the 2010 Nobel Prize in Physics for his research on graphene) and Michael Berry, who both shared the Ig Nobel Prize in Physics in 2000 for the magnetic levitation of a frog. Simulating microgravity Parabolic flight Weightless Wonder is the nickname for the NASA aircraft that flies parabolic trajectories. Briefly, it provides a nearly weightless environment to train astronauts, conduct research, and film motion pictures. The parabolic trajectory creates a vertical linear acceleration that matches that of gravity, giving zero-g for a short time, usually 20–30 seconds, followed by approximately 1.8g for a similar period. The nickname Vomit Comet is also used to refer to motion sickness that aircraft passengers often experience during these parabolic trajectories. Such reduced gravity aircraft are nowadays operated by several organizations worldwide. Neutral buoyancy The Neutral Buoyancy Laboratory (NBL) is an astronaut training facility at the Sonny Carter Training Facility at the NASA Johnson Space Center in Houston, Texas. The NBL is a large indoor pool of water, the largest in the