Propulsion systems play an essential role in any mission’s success, enabling spacecraft probes to change their velocity in space and, if needed, escape Earth’s gravity.
Spacecraft thrust is determined by the rate of acceleration. Due to conservation of momentum, small accelerations applied over a longer time can generate equal force as greater forces applied quickly.
Launching from the Earth
Launching a spacecraft from Earth typically involves using a rocket or launch vehicle with chemical thrusters that burn fuel and oxidizer to generate hot exhaust gas that shoots out from its nozzle, where its equal-and-opposite reaction propels it in its intended path. This form of propulsion is currently the most frequently employed.
Space travel requires great velocity. Under the law of conservation of momentum, this requires vast quantities of energy – which must come from somewhere. One source is mass that is carried along and irretrievably burned up during propulsion – known as reaction mass. Rocket engines are specifically designed to alter this momentum by producing high specific impulse (change in momentum per unit mass consumed).
Launching from a planet
Launching a spacecraft from Earth requires using both rocket thrust and gravity to alter its orbital path; these processes are known as Hohmann transfer orbits. They are frequently employed when sending spacecraft on missions beyond our Solar System’s boundaries.
Although various propulsion methods can help accelerate a spacecraft, none can overcome gravity’s pull on it. To overcome this problem, an elliptical orbit must be created around its destination to propel it in an upward trajectory and avoid gravity’s pull.
Some spacecraft use electric propulsion to adjust their orbits, while others may enter planetary atmospheres or take advantage of gravitational slingshots. Other forms of propulsion systems are being explored as well, including nuclear pulse propulsion and electrodynamic tethers; these utilize nuclear power to heat an electric ion drive which then expels hydrogen or xenon out of an exhaust nozzle to provide thrust – these methods are much lighter and more energy-efficient than chemical rocket engines and will be tested out on Gateway (an upcoming international space station located in lunar orbit). One such system will be tested on Gateway; one such system will be tested again before 2024 on Gateway; another international space station scheduled to enter lunar orbit for testing will take place before 2024 on Gateway; another test site will take place 2024 on Gateway as one such testbed station which will also uses nuclear pulse propulsion technology.
Launching from a satellite
Spacecraft need to accelerate rapidly in order to navigate past Earth’s gravity in order to travel between planets, which requires electric propulsion systems that are both lightweight and more efficient than chemical propulsion methods. While conventional cars use fuel-oxygen mixture combustion chambers for thrust, while spacecraft use electric propulsion technology instead to accelerate propellant.
Modern lithium-ion batteries deliver enough power to run electric thrusters for hundreds of kilohours. This energy provides enough force for orbit-raising functions required to place satellites into their designated orbit, as well as most station-keeping operations performed by satellites.
ESA’s bepiColombo, GOCE and Psyche missions make use of electric propulsion thrusters utilizing electricity to accelerate ions instead of chemical rockets, thus requiring much less mass and reliability than chemical systems – this allows for precise trajectory reconstruction that helps make sense of scientific data returned by spacecraft missions.
Launching from a spacecraft
Launching a spacecraft from Earth involves an intricate blend of science, engineering and politics. Due to its expensive nature, governments often sponsor space missions which include astronauts or cosmonauts as participants.
Propulsion systems must provide enough acceleration to overcome gravity and leave Earth’s orbit, with force determined by Newton’s laws and mass as the primary factors dictating acceleration requirements.
Traditional chemical rocket engines offer relatively low specific impulse, prompting spacecraft designers to implement electric propulsion systems as an improvement to these systems. Electric propulsion uses electricity to accelerate propellant, which reduces mass and increases efficiency; plus they’re much safer than chemical propulsion methods. Electric propulsion could enable human missions to Mars or beyond by using solar sails or nuclear fission reactions as their power sources; for example ion thrusters use superconducting magnetic cells that heat hydrogen or xenon gas up to millions of degrees Celsius before expulsion through an expulsion nozzle to generate thrust – they create thrust.