Reading the Orbits: Kepler Elements Made Simple

Nearly every object in the Solar System travels on a slightly egg-shaped path called an ellipse. To fully describe that path, you need six numbers — the Kepler orbital elements. Once you understand them, the orbital table on each SpaceAtlas object page stops looking like a wall of symbols and starts reading like a story about the object's life.

The six elements

• Semi-major axis (a) — the “size” of the orbit. Think of it as the average distance from the Sun. A value near 2.7 AU puts an object in the main asteroid belt; beyond ~4.4 AU is the Kuiper Belt region.
• Eccentricity (e) — how stretched the ellipse is. 0 is a perfect circle; 0.9 is a very elongated path. Comets like Halley are famous for high eccentricity.
• Inclination (i) — how tilted the orbit is relative to the ecliptic (the plane Earth orbits in). Low values mean the object stays close to the plane; high values mean it dives steeply above and below.
• Longitude of the ascending node (Ω) — where the orbit crosses the ecliptic going upward, measured around the sky.
• Argument of perihelion (ω) — the angle to the closest approach point, measured within the orbital plane.
• Mean anomaly (M) — where the object actually is on its orbit right now, so we can predict its position at a given date.

Two values that matter for safety

Perihelion (q) is the closest the object ever comes to the Sun; aphelion is the farthest. Together with the orbit's period they tell you whether a near-Earth asteroid could ever drift close to our planet. SpaceAtlas computes the aphelion as a × (1 + e).

Read these six numbers together and you can answer three questions for any body: how far does it live, how tilted is its path, and how tightly bound is it to the Sun? That is the whole orbit, in one glance.