Halley's Comet has an eccentricity of 0.968 and an inclination of 162 degrees, according to NASA's Small-Body Database. Those two numbers say a lot. The comet swings in to 0.575 AU from the Sun, which is about 86 million km (53 million miles) and well inside the orbit of Venus. Then it flies out to 35.3 AU, about 5.3 billion km (3.3 billion miles). And because 162 is more than 90, it travels around the Sun in the opposite direction from the planets.
An AU, or astronomical unit, is the average distance between Earth and the Sun, roughly 150 million km (93 million miles). Every asteroid and comet in the database comes with a row of numbers like these. This guide explains what each one means, so that a page of orbital data reads like a short description instead of a puzzle.
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An orbit is an ellipse with the Sun at one focus
Kepler's laws, as NASA's Orbits and Kepler's Laws page states them, give the starting picture. Each orbit is an ellipse, which is a stretched circle, with the Sun at one of two special points called foci. The distance to the Sun changes during the orbit. A body moves fastest when it is closest to the Sun and slowest when it is farthest. The closest point is called perihelion, and the farthest point is aphelion. In JPL's glossary, perihelion is simply an orbit's closest point to the Sun.
Kepler's third law links time and size: the squares of the orbital periods are proportional to the cubes of the semi-major axes. For anything circling the Sun, with the axis measured in AU, the period in years is the axis raised to the power 1.5. A body with a semi-major axis of 4 AU takes 8 years. That shortcut works for asteroids, comets and planets alike.
The six elements and what they do
Six numbers fix an orbit completely. JPL's database lists them under the names below. The Eros values are the ones the database returned on November 11, 2026.
| Symbol | Name | What it tells you | Eros |
|---|---|---|---|
| a | Semi-major axis | The size of the orbit, as half of its long axis | 1.46 AU |
| e | Eccentricity | How stretched the ellipse is | 0.223 |
| i | Inclination | The tilt of the orbit against the plane of Earth's orbit | 10.8 degrees |
| om | Longitude of the ascending node | Which way the tilted orbit is turned around the Sun | 304 degrees |
| w | Argument of perihelion | Where in the orbit the closest point sits | 179 degrees |
| ma | Mean anomaly | Where the body is along its orbit at a stated moment | 62.5 degrees |
The last element depends on a date, called the epoch, which the database states with each record. Planets tug on small bodies, so the elements are always a fit to observations at a particular epoch and get refined when new data arrive. That is also why every number in this article carries a date. To see all six for any object yourself, the walkthrough of the JPL Small-Body Database shows where each one sits on the page.
Size and shape: a, e and the extremes
The semi-major axis sets the size. Ceres, in the main asteroid belt, has an axis of 2.77 AU, about 414 million km (257 million miles). It takes about 4.6 years to go around.
Eccentricity sets the shape. JPL's glossary defines it as the ratio of half the distance between the two foci to the semi-major axis. A circle has an eccentricity of 0, an ellipse has a value between 0 and 1, and a parabola has exactly 1. Ceres sits at 0.0797, nearly circular. Eros is 0.223, and Apophis is 0.191. Halley is 0.968, a long thin loop. Comet C/2020 F3 (NEOWISE) is listed at 0.999, almost a parabola.
The closest and farthest distances follow from the two numbers: perihelion is the axis times one minus the eccentricity, and aphelion is the axis times one plus the eccentricity. For Eros that gives 1.46 times 0.777, or about 1.13 AU (169 million km, 105 million miles), and 1.46 times 1.223, or about 1.78 AU (266 million km, 165 million miles). The database lists the same values. Perihelion distance even affects names: near-Earth asteroids with a perihelion under 1.15 AU must carry mythological names, as the guide to how asteroids and comets get their names explains.
Above 1, the orbit is no longer closed. The object 'Oumuamua, found in 2017, is listed with an eccentricity of 1.2 and a negative semi-major axis of -1.27 AU, which is how the database records a hyperbola. A hyperbolic object passes the Sun once and leaves for good.
Tilt and direction
Inclination, in JPL's words, is the angle between the orbit plane and the ecliptic plane, where the ecliptic is the plane of Earth's orbit. The asteroids in the table below sit between 3 and 11 degrees. Ceres is at 10.6 degrees and Apophis at 3.34 degrees. Anything above 90 degrees is retrograde, meaning it circles the Sun against the direction of the planets. Halley is at 162 degrees and Comet NEOWISE at 129 degrees.
The two angles om and w only matter once you want to draw the orbit. The first says where the tilted plane crosses the ecliptic, and the second turns the ellipse inside that plane. For a quick read, a, e and i are enough.
Reading real rows
| Object and class | a (AU) | e | i (degrees) | Period |
|---|---|---|---|---|
| 1 Ceres, main-belt asteroid | 2.77 | 0.0797 | 10.6 | 4.6 years |
| 433 Eros, Amor asteroid | 1.46 | 0.223 | 10.8 | 1.76 years |
| 99942 Apophis, Aten asteroid | 0.922 | 0.191 | 3.34 | 0.89 years |
| 1P/Halley, Halley-type comet | 17.9 | 0.968 | 162 | 76 years |
| C/2020 F3, comet | 358 | 0.999 | 129 | about 6,800 years |
Values from JPL's Small-Body Database, queried November 11, 2026. The class names come from the database. Apophis is an Aten, which means its orbit is mostly inside Earth's. The database notes that after its close approach to Earth on April 13, 2029, it will move to the Apollo class.
Checking the period with Kepler's rule works well. Eros has an axis of 1.46 AU, and 1.46 to the power 1.5 is about 1.76 years, which matches the database value of 643 days. For Halley, 17.9 to the power 1.5 is about 76 years, matching the 27,700 days in its record.
The long period of Comet NEOWISE deserves caution. The database fits it to 438 days of observations. A short arc of data cannot pin down a period that long, so treat that figure as an estimate. Eros, by comparison, has 9,130 observations spanning 46,582 days, about 127 years. How observations turn into an orbit explains why the length of that observed arc matters so much.
Halley's own orbit gets a closer look in the guide to Halley's Comet.
Common mistakes and myths
"A high eccentricity means a bigger orbit." Eccentricity measures shape only. Size is the semi-major axis. Two objects can share an eccentricity and differ enormously in size.
"Inclination is how tilted the object itself is." It is the tilt of the orbit plane against the ecliptic. The way the object spins is a separate property.
"Crossing Earth's orbit means a collision is coming." An orbit is a path, and collisions need both bodies in the same place at the same time. The database lists a minimum orbit intersection distance, which compares two paths. For Eros it is 0.149 AU, about 22 million km (14 million miles). That is a statement about orbit shapes, not about a future meeting.
"An orbit never changes." Planetary gravity shifts orbits over time, and new observations refine the fit. Elements are always tied to an epoch.
"Elements for comets are as precise as for numbered asteroids." They depend on how long the object was watched. A comet seen for one pass has far less data than Eros, which has been tracked since 1893.
Frequently asked questions
What does a semi-major axis of 1 AU mean?
It means the orbit is about the size of Earth's. For a nearly circular orbit, the object stays near 1 AU all the time. For an elongated one, the average of the closest and farthest distances is 1 AU.
Why do some objects have a negative semi-major axis?
The math of a hyperbola, which is an open path, gives a negative value. The database shows it for objects such as 'Oumuamua whose eccentricity is above 1.
What is the difference between perihelion and aphelion?
Perihelion is the closest point to the Sun and aphelion is the farthest. A body moves fastest at perihelion and slowest at aphelion.
Which numbers are the most useful for a beginner?
Semi-major axis, eccentricity and inclination. Together they tell you how big the orbit is, how stretched it is and whether it runs with or against the planets.
Sources
- Orbits and Kepler's Laws. NASA Science. https://science.nasa.gov/resource/orbits-and-keplers-laws/
- Eccentricity. JPL Solar System Dynamics glossary. https://ssd.jpl.nasa.gov/glossary/eccentricity.html
- Inclination. JPL Solar System Dynamics glossary. https://ssd.jpl.nasa.gov/glossary/inclination.html
- 99942 Apophis. NASA/JPL Small-Body Database API record, accessed November 11, 2026. https://ssd-api.jpl.nasa.gov/sbdb.api?sstr=99942&phys-par=1&discovery=1
- SBDB API documentation. NASA/JPL Solar System Dynamics. https://ssd-api.jpl.nasa.gov/doc/sbdb.html
- 433 Eros. NASA/JPL Small-Body Database API record, accessed November 11, 2026. https://ssd-api.jpl.nasa.gov/sbdb.api?sstr=433&phys-par=1&discovery=1
- 1P/Halley. NASA/JPL Small-Body Database API record, accessed November 11, 2026. https://ssd-api.jpl.nasa.gov/sbdb.api?sstr=1P&phys-par=1&discovery=1