When the shortest path is not a straight line
During decades humanity has dreamed about making journeys to our, sometimes, closer planetary friend: Mars. But, how such a journey would be like? And how much time could it take?
Well, the most common answer for many questions in science and engineering is “it depends”, and this time it will not be the exception. This article addresses an important route taken in space trajectories, such as for satellites or general space missions: the Hohmann Orbits.
What are Hohmann Orbits?
Hohmann orbits, often called Hohmann transfer orbits, are the most fuel effective way to travel between two circular orbits.
This is to say that for two planets following a circular orbit, for instance planet A and planet B, a rocket will depart from planet A and, rather than traveling to planet B in a straight trajectory, it will follow a curve and reach planet B on its own orbit later.
The rocket is launched and while it travels along the Hohmann orbit, both planets A and B (in this case, Earth and Mars respectively) continue their respective trajectories. Therefore, the rocket must reach Mars exactly when its orbit meets the Hohmann orbit, what happens at the transfer orbit furthest apside, the apoapsis.
What is an apside?
The apsides are the farthest and nearest points, respectively, of an orbit with respect to a reference central body.
What are the apoapsis and periapsis?
The two farthest and nearest point of an orbit to its center body are known as apoapsis and periapsis, respectively. In the Earth-Mars journey case, the reference body for the transfer (Hohmann) orbit is the Sun, similarly as the reference body for the Moon orbit is the Earth. So the transfer orbit periapsis is the own Earth and the apoapsis is the opposite point in the trajectory, that is when it encounters the Mars orbit.
We can also define the periapsis and apoapsis altitudes, which are the distance from the Sun to the Earth and Mars surfaces respectively.
Are Hohmann orbits fuel-efficient?
Absolutely, for fuel optimization Hohmann orbits are the way to go. Something important to keep in mind is that, due to the relative motion of the planets, the opportunity to make a launching followed by a transfer orbit from Earth to Mars with minimum energy takes place once in about every 25 months. On top of that, the time it takes for the whole journey between Earth and Mars due to their orbits being elliptical varies between 6 to 8 months. This is comparable with the time astronauts on the International Space Station stay, which is around 8 months.
Undoubtedly, a mission of astronomical efforts. We will need to keep looking closely to the Mars missions, whose objective is sending astronauts to our neighbor planet on the incoming years.
Meanwhile, thank you for reading, sharing, and keep in mind that your comments and suggestions are very well appreciated!
