An orbit around a body should be circular without propellant thrust

Cassini spacecraft is making its final 22 orbits around Saturn and is diving between the planet and its rings. Highly elliptical in trajectory, Cassini is said to be orbiting Saturn, but is it?


The answer is yes, but it must be noted that Cassini is travelling around Saturn with the help of timely thruster boost to help it negotiate the space between the planet and its rings. Without the help of propellant thrust, Cassini will orbit Saturn in a stable circular orbit, that with loss of kinetic energy due to gravitational pull, would result in progressive erosion of the orbit and final plunge of Cassini into Saturn or one of its moons.


Why should an orbit around a body be circular where there is no propellant thrust? The answer is governed by gravitational attraction between the body and the object, in this case, a satellite, since this is the most energy efficient trajectory for traversing around the body. Most importantly, the gravitational attraction between the body and the satellite is the same along the entire orbit, thereby, allowing the circular orbit to take shape. The last point is important, if the gravitational field between the body and the satellite is asymmetric, the resulting attraction between the body and the satellite would not be the same along the entire length of the orbit; thus, a slightly elliptical orbit will result.


On the other hand, propellant generates thrust that allows a spacecraft to traverse the space between itself and the planetary body in an orbit, which usually is not circular for the need to examine specific geological features on the planetary body from different distance perspectives.


Hence, gravitational attraction between a body and a satellite will ensure that the orbit around the body is circular if the gravitational field of attraction is symmetrical along the length of the orbit. If the body is asymmetric in mass, and thus, gravitational field, the resulting orbit will be slightly elliptical. Finally, with the help of thrust from burning propellant, a spacecraft would be able to take a highly elliptical orbit around the planet, such as Saturn, to examine specific details of the planet from different distances, that would help generate a global view of the phenomenon from afar, while a close flyby yields high resolution maps of topological details on the planet.


Category: space exploration, physics,

Tags: circular orbit, gravitational field, symmetric gravitational attraction, propellant thrust, planetary body, satellite,



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