The Hohmann transfer orbit is a commonly used orbital maneuver in spaceflight that enables spacecraft to efficiently transfer from one circular orbit to another. It was first proposed by German engineer Walter Hohmann in 1925, and since then, it has been widely used in space missions, including interplanetary travels.
In essence, a Hohmann transfer orbit involves using the gravitational attraction of a planet or moon to change the velocity of a spacecraft and put it on a trajectory that intersects with the target circular orbit. This trajectory is an ellipse that is tangent to both the initial and final orbits, and it requires the least amount of propellant to achieve.
To understand how a Hohmann transfer orbit works, let’s consider a simple example of a spacecraft in a circular orbit around the Earth. Suppose the spacecraft needs to travel to a higher orbit to perform a mission, such as communication or scientific observation. Instead of simply firing its engines to increase its velocity and move to the higher orbit, the spacecraft can follow a Hohmann transfer orbit that involves two main maneuvers.
The first maneuver is called the transfer burn, which involves firing the spacecraft’s engines at a specific time to increase its velocity and put it on a highly elliptical orbit called the transfer orbit. This orbit has an apoapsis (the farthest point from the Earth) that is higher than the initial circular orbit and a periapsis (the closest point to the Earth) that is still in the initial circular orbit.
The second maneuver is called the capture burn, which occurs when the spacecraft reaches the apoapsis of the transfer orbit. At this point, the spacecraft fires its engines again to decrease its velocity and circularize its orbit at the target altitude. The result is a circular orbit at a higher altitude than the initial orbit, which is achieved using the least amount of propellant possible.
The reason a Hohmann transfer orbit is so efficient is that it takes advantage of the gravitational pull of the planet or moon to change the spacecraft’s velocity. By choosing the right timing and trajectory, the spacecraft can achieve the desired altitude with minimal propellant. This is especially important for interplanetary missions where propellant is scarce, and the cost of launching additional fuel is high.
To illustrate this point, let’s consider a mission to Mars. Suppose a spacecraft is in a circular orbit around the Sun that is slightly smaller than Mars’s orbit. If the spacecraft wants to reach Mars, it would need to fire its engines and change its velocity by a significant amount to intercept Mars’s orbit. This would require a lot of propellant, which would add weight to the spacecraft and increase the cost of the mission.
Instead, the spacecraft can use a Hohmann transfer orbit to reach Mars efficiently. It would fire its engines to put itself on an elliptical orbit that intersects with Mars’s orbit, and then use another burn to circularize its orbit around Mars. By doing so, the spacecraft can achieve its mission with less propellant, reducing the weight and cost of the mission.
Of course, there are limitations to the Hohmann transfer orbit. For instance, it assumes that the gravitational field of the planet or moon is constant, which is not entirely true. The gravity of the planet or moon changes slightly as the spacecraft approaches, which can affect the trajectory and require additional course corrections. Also, the Hohmann transfer orbit assumes that the spacecraft is in a circular orbit to begin with, which is not always the case.
Despite these limitations, the Hohmann transfer orbit remains one of the most efficient and widely used orbital maneuvers in spaceflight. It has been used for countless missions, from interplanetary exploration to satellite launches, and has enabled humanity to reach the farthest corners of the solar system.