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Highly elliptical orbit

From Wikipedia, the free encyclopedia

Molniya orbit for the Northern hemisphere
Molniya orbit for the Northern hemisphere

A highly elliptical orbit (HEO) is an elliptic orbit with high eccentricity, usually referring to one around Earth. Examples of inclined HEO orbits include Molniya orbits, named after the Molniya Soviet communication satellites which used them, and Tundra orbits.

Such extremely elongated orbits have the advantage of long dwell times at a point in the sky during the approach to, and descent from, apogee. Bodies moving through the long apogee dwell appear to move slowly, and remain at high altitude over high-latitude ground sites for long periods of time. This makes these elliptical orbits useful for communications satellites. Geostationary orbits cannot serve high latitudes due to their altitude from ground sites being too low.[1]

Groundtrack of a Molniya orbit
Groundtrack of a Molniya orbit
The groundtrack of a QZSS orbit
The groundtrack of a QZSS orbit

Sirius Satellite Radio used inclined HEO orbits, specifically the Tundra orbits, to keep two satellites positioned above North America while another satellite quickly sweeps through the southern part of its 24-hour orbit. The longitude above which the satellites dwell at apogee in the small loop remains relatively constant as Earth rotates. The three separate orbits are spaced equally around the Earth, but share a common ground track.[2]

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In 1609, Johannes Kepler published Astronomia Nova, a book containing ten years of his efforts to understand the orbit of the planet Mars. He was using state-of-the-art astronomical observations from his mentor and employer, Tycho Brahe, who was famous for generating an enormous amount of high-quality data, and he needed to find the best explanation for the motions of Mars - a very tricky problem! There were three models of the solar system out there at the time, but none of them worked very well for Mars. First, the Ptolemaic system put the Earth at the center, with the Sun and planets orbiting it in perfect circles. There was also Copernicus’s heliocentric model, which set the Earth among the planets, revolving around the Sun. And finally, Tycho had his own system to propose, which combined aspects of both: he put the Earth at the center with the Sun and moon orbiting it, but let the other planets orbit the Sun. All three systems relied upon circular orbits, because the circle was accepted as an ideal shape. Copernicus, Tycho, and Galileo all believed that planets should travel along circular paths, but the data just didn’t fit. Instead, Kepler found that another shape, the ellipse, works a lot better. An ellipse is sort of like a flattened circle, and it has some special properties. You can draw one by taking a loose string... ...attaching both ends to the paper, and using a pencil to keep the string taught while moving all the way around the perimeter... The result is an ellipse! The length of the string never changed, meaning that the sum of the distances between each endpoint, or focus, and any point on the ellipse is constant. In Astronomia Nova, Kepler states that Mars travels in an elliptical orbit around the Sun, which is at one of the foci of the orbit. Later on, he expanded this first law to include all of the planets and demonstrated that this shape fit the available observations. The further apart the two foci are, the longer and skinnier the ellipse, and this “skinniness” parameter is called “eccentricity.” Comets can have very eccentric orbits, coming in quite close to the Sun before traveling back to the outer reaches of the solar system. On the other hand, In a perfect circle, the two foci would lie right on top of each other right at the center. The orbits of the planets in our solar system are not very eccentric at all. They’re really very close to circular, which is partly why perfectly round orbits seemed like a natural thing to expect in the first place. It wasn’t easy to abandon a central idea like that, but with his first law of planetary motion, Kepler rejected circular orbits and showed that an ellipse could better explain the observed motions of Mars. Generalized to all planets, it states that the orbit of a planet follows an ellipse with the Sun at one focus.


  1. ^ Fortescue, P.W.; Mottershead, L.J.; Swinerd, G.; Stark, J.P.W. (2003). "Section 5.7: highly elliptic orbits". Spacecraft Systems Engineering. John Wiley and Sons. ISBN 0-471-61951-5.
  2. ^ "The Tundra Orbit". Canadian Satellite Tracking and Orbit Research (CASTOR). 23 May 2010. Retrieved 2 October 2017.
This page was last edited on 9 February 2021, at 13:37
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