As in throwing something, satellites are launched by rocket to enter their orbit around Earth, like any object being launched from the ground. But unlike its trajectory when being released directly, their paths follow curves rather than straight paths.
Satellites in high orbit offer scientists stunning pictures of our planet and solar system. At the same time, other satellites beam television signals and phone calls across the globe.
Spacecraft
Spacecraft are vehicles designed to travel and operate in outer space. This includes satellites that orbit Earth, robots sent to other planets and vehicles designed to transport astronauts.
Most spacecraft require rocket fuel to achieve orbit. Once there, however, they typically remain there until intervened by an outside force; changing one’s trajectory requires considerable fuel expenditure and requires either orbital insertion or removal from Earth’s plane.
Lagrange points (L-points), or special orbital locations that allow spacecraft to stay close to Earth without entering its orbit, such as Chandra X-ray Observatory and Hubble Space Telescope are two such examples; others from ESA such as SOHO, Herschel Planck Euclid Ariel JWST Athena exist within L points as well. Still others such as New Horizons and Voyager do not fall neatly into any one category above and stray off their intended trajectory altogether – but are beyond any category defined above!
Orbits
Just like different seats provide different perspectives of a performance, different orbits give satellites unique views of Earth. Some appear to linger over one spot for extended views; others whizz past several different sites each day.
Orbits tend to be elliptical in shape, with bodies (usually satellites or spacecraft) orbited being located at one of two foci. The distance between closest approach (periapsis or perifocus) to Earth and farthest point from it (apogee or apocentron) equals the semi-major axis of an ellipse.
Non-gravitational forces also impact orbits, including secular variations that gradually alter Keplerian elements over time, short-period variations that occur periodically and long-period variations with periods greater than an orbit’s period. Atmospheric drag is among these non-gravitational forces and its impact depends on atmospheric density.
Propulsion
Spacecraft use propulsion systems to move themselves through space, altering their position relative to Earth and other celestial bodies. Steering can be accomplished using reaction wheels or electrically driven attitude control systems.
Chemical propulsion is the foundation of all satellite propulsion, in which pressurized gases are stored in tanks and released when necessary to provide thrust. Launch vehicles usually employ this form of propulsion when entering and leaving Earth orbit, but most satellites also feature their own thrusters for fine adjustments and stationkeeping purposes.
Many return missions must address the challenge of slowing their orbital speeds without hitting ground (lithobraking) or burning up in atmosphere, necessitating an advanced on-orbit deceleration system like that used on ESA’s SMART-1 probe to the Moon or Japan’s Hayabusa1 and 2 sample return missions from asteroids belt.
Air-scooping electric propulsion (ASEP) seeks to lower this need by employing onboard thrusters that ’employ’ scarce air molecules from the upper atmosphere as propellant, significantly lowering launch costs while prolonging satellite lifetime by decreasing refueling requirements. This technology may prove advantageous both financially and for environmental considerations.
Control
As opposed to Earth, which remains constant over time due to gravitational forces, space undergoes constant fluctuations. Thus, its position control requires taking an orbital dynamics approach when making decisions for propellant subsystem commanding systems.
Most maneuvers in orbit involve changing a spacecraft trajectory, which results in velocity variations across its system. These command variations (Dv) are evaluated onboard by orbital control systems in order to calculate accelerations for propulsion subsystem control systems.
AOCS systems can also provide precise attitude control by employing devices like reaction wheels and control moment gyroscopes, which take advantage of conservation of angular momentum to nudge spacecraft into an acceptable range of attitude error. Mariner 10 utilized this technique and the Sun’s radiation pressure to rotate its Solar panels and high-gain antennas with minimal force exerted, though nonetheless significant.