Hohmann transfer orbit
An orbital maneuver using two burns to transfer between orbits.
The Hohmann transfer orbit is a method for moving a spacecraft between two orbits at different heights around the same central body. In the simplest version, both the starting and target orbits are circular and lie in the same plane. The spacecraft first fires its engine to enter an elliptical path that just touches both the lower and higher orbits. A second engine burn then adjusts the orbit to match the target. This technique typically uses the least possible propellant for the job, but the trip takes longer than a more powerful, direct transfer. If one orbit is much larger than the other, a bi-elliptic transfer can save even more fuel, though it takes even more time.
The maneuver is named after German scientist Walter Hohmann, who described it in his 1925 book *Die Erreichbarkeit der Himmelskörper*. He was inspired by Kurd Lasswitz’s 1897 science fiction novel *Two Planets*. When used between celestial bodies, the starting and destination points must be in the right positions relative to each other, creating a launch window. For Earth to Mars, these windows open every 26 months, and the journey takes about 9 months. Near planets with strong gravity, the Oberth effect can reduce the required delta-v.
In practice, the two burns are called periapsis and apoapsis burns (or perigee and apogee for Earth). The second burn is sometimes called a circularization burn. Real orbits may not be perfectly circular or coplanar, so the transfer may cover slightly less than 180° (Type I) or slightly more than 180° (Type II) around the primary. Multiple-revolution transfers—Type III (Type I plus 360°) and Type IV (Type II plus 360°)—are also possible.
The Hohmann transfer can also be used to bring an object, like an asteroid orbiting the Sun, into contact with Earth. The calculation of the transfer energy uses the sum of kinetic and potential energy, which equals half the potential at the semi-major axis.
- named_after
- Walter Hohmann
- field
- Astronautics
- known_for
- Hohmann transfer orbit
- publication
- Die Erreichbarkeit der Himmelskörper (The Attainability of Celestial Bodies)
Lore & Background
The Hohmann transfer orbit is an elliptical path used to move a spacecraft between two circular, coplanar orbits of different altitudes around a central body, such as raising a satellite from low Earth orbit to geostationary orbit. The maneuver requires two impulsive engine burns: the first accelerates the craft onto the elliptical transfer orbit, which is tangent to both the initial and target orbits, and the second adjusts the orbit to match the higher circular path. This method typically uses the lowest possible impulse (and thus propellant) for the transfer, but it requires a longer travel time compared to higher-impulse maneuvers. When one orbit is significantly larger than the other, a bi-elliptic transfer can use even less impulse at the cost of even greater travel time. The maneuver is named after German scientist Walter Hohmann, who described it in his 1925 book *Die Erreichbarkeit der Himmelskörper*, influenced by science fiction author Kurd Lasswitz. For interplanetary travel, the starting and destination points must be at specific relative positions, creating launch windows; for Earth to Mars, these windows occur every 26 months, with a travel time of about nine months. The transfer orbit traverses exactly 180 degrees around the primary in the ideal case, but real-world transfers may traverse slightly less (Type I) or slightly more (Type II), with multiple-revolution transfers labeled Type III and Type IV. The two burns are called periapsis and apoapsis burns, or perigee and apogee burns for Earth orbits, with the second often termed a circularization burn.
Reader's Guide
The Hohmann transfer orbit is a foundational concept in astronautics, enabling efficient transfers between circular, coplanar orbits around a central body. Its significance lies in its fuel efficiency, using the lowest possible impulse for such transfers, though it requires longer travel times. For missions like Earth to Mars, launch windows occur every 26 months with a travel time of about 9 months. The maneuver's reversibility allows it to be used for both raising and lowering orbits, with burns labeled periapsis and apoapsis burns. While highly efficient, low-energy transfers that account for real engine thrust limitations and gravity wells can be even more fuel efficient in some cases.
Did You Know?
- The Hohmann transfer orbit uses two impulsive engine burns: the first establishes the transfer orbit, and the second adjusts the orbit to match the target.
- For an Earth-Mars journey, the Hohmann transfer orbit requires a travel time of about 9 months.
- A Hohmann transfer orbit can be used to bring an asteroid orbiting the Sun into contact with Earth.
Frequently Asked Questions
What does the Hohmann transfer orbit actually do?
It moves a spacecraft from one circular orbit to another of a different altitude using exactly two impulsive engine burns. The craft rides an elliptical path that is tangent to both the starting and destination orbits.
How does the Hohmann transfer orbit sequence end?
The maneuver concludes with a second burn at the far end of the elliptical transfer path, circularizing the craft into the target orbit. At that point the spacecraft is fully committed to the new altitude and the transfer is complete.
Why is the Hohmann transfer orbit considered important in celestial mechanics?
It achieves the inter-orbit transfer with the lowest total impulse of any two-burn strategy, making it the most fuel-efficient standard option available. That efficiency is why it remains the default planning tool for mission designers.
What is the main drawback of using a Hohmann transfer orbit?
Because it trades impulse for time, the transfer lasts roughly half an orbital period of the ellipse, which can be considerably longer than a direct higher-thrust trajectory. Missions with tight timelines may therefore prefer a more energetic, multi-burn alternative.
More in Orbits And Celestial Mechanics 1-24
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