for a period of time, rather than going up and immediately falling back to earth. This entails orbit, which is mostly a matter of velocity, not altitude, although that does not mean air friction and relevant altitudes in relation to that and orbit don't have to be taken into account. At much, much higher altitudes than many orbital ones maintained by satellites, altitude begins to become a larger and larger factor and speed a lesser one. At lower altitudes, due to the high speed required to remain in orbit, air friction is a very important consideration affecting satellites, much more than in the popular image of space. At even lower altitudes, balloons, with no forward velocity, can serve many of the roles satellites play. G-forces Many cargos, particularly humans have a limiting g-force that they can survive. For humans this is about 3-6 g. Some launchers such as gun launchers would give accelerations in the hundred or thousands of g and thus are completely unsuitable. Reliability Launchers vary with respect to their reliability for achieving the mission. Safety Safety is the probability of causing injury or loss of life. Unreliable launchers are not necessarily unsafe, whereas reliable launchers are usually, but not invariably safe. Apart from catastrophic failure of the launch vehicle itself other safety hazards include depressurisation, and the Van Allen radiation belts which preclude orbits which spend long periods within them. Trajectory optimization Carbon emissions Many rockets use fossil fuels. A SpaceX Falcon Heavy rocket for instance burns through 400 metric tons of kerosene and emits more carbon dioxide in a few minutes than an average car would in more than two centuries. As the number of rocket launches is expected to increase heavily in the coming years, the effect that launching into orbit has on Earth is expected to get much worse. Some rocket manufacturers (i.e. Orbex, ArianeGroup) are using different launch fuels (such as bio-propane,methane produced from biomass). Sustained spaceflight Suborbital launch Sub-orbital space flight is any space launch that reaches space without doing a full orbit around the planet, and requires a maximum speed of around 1 km/s just to reach space, and up to 7 km/s for longer distance such as an intercontinental space flight. An example of a sub-orbital flight would be a ballistic missile, or future tourist flight such as Virgin Galactic, or an intercontinental transport flight like SpaceLiner. Any space launch