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Earth to Mars Journey

Earth to Mars Journey – when the shortest path is not a straight line

During decades mankind 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. Nevertheless, in this article we will address what such dependencies are about and present an important route taken in space trajectories: the 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 travelling to planet B directly in a straight trajectory, it will follow a curve and reach planet B on its own orbit later. Seems it is not as simple as taking a interplanetary highway and parking on Mars!

Notice that the rocket is launched and while it travels along the Hohmann orbit, both planets A and B (in this case of study 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 aphelion or apoapsis. The apsides, namely aphelion (or apoapsis) and perihelion (or periapsis) are the farthest and nearest points, respectively, of an orbit with respect to a reference central body. In our case the reference body for the transfer orbit is the Sun (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.

Homman Transfer Orbit fom Earth to Mars. Source: NASA.

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 has place once in about every 25 months and the time it takes for the whole journey is about 7 months, however to be more precise, because Earth and Mars orbits are elliptical (not perfectly circular as it is usually illustrated) this time may vary from 6 to 8 months. This is comparable with the time astronauts on the International Space Station stay there, which is around 8 months.

We will need to keep looking closely to the Mars missions which have as an objective sending astronauts to our neighbour planet on the incoming years.

Meanwhile, thank you for reading and sharing, and keep in mind that from SpaceWayfinder we appreciate your comments and suggests!

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REFERENCES

NASA. Basics of Space Flight – Trajectories. Retrieved from https://solarsystem.nasa.gov/basics/chapter4-1
The Planetary Society. Homann Transfer Orbit Diagram. Retrieved from https://www.planetary.org/multimedia/space-images/charts/hohmann-transfer-orbit.html
MarsOne. How long does it take to travel to Mars? Retrieved from https://www.mars-one.com/faq/mission-to-mars/how-long-does-it-take-to-travel-to-mars