Which Orbit Has the Largest Semimajor Axis? Let's Break It Down
What do Elon Musk's Starship and NASA's Voyager probes have in common? Both are designed to travel on trajectories with the largest semimajor axis of any human-made object—though for very different reasons Still holds up..
But here's the thing: the semimajor axis isn't just a fancy physics term. It’s the key to understanding how big an orbit really is. And when it comes to comparing orbits, the one with the largest semimajor axis is the one that stretches the farthest from the body it’s orbiting The details matter here. Turns out it matters..
Short version: it depends. Long version — keep reading.
So which orbit has the largest semimajor axis? The answer depends on the
…on the central body being considered. In practice, in the Solar System, the semimajor axis grows dramatically as you move outward from the Sun, so the record‑holder among natural bodies is the orbit of the most distant known object: the distant trans‑Neptunian dwarf planet Eris. With a semimajor axis of roughly 68 AU (astronomical units), Eris’s orbit stretches far beyond Neptune’s 30‑AU path and even surpasses the scattered‑disk objects whose perihelia dip inward but whose aphelia can reach 100 AU or more Simple, but easy to overlook..
If we broaden the scope to include human‑made trajectories, the answer shifts. The Voyager 1 and Voyager 2 spacecraft, launched in 1977, are now on escape trajectories that will never return to the Sun. Their heliocentric orbits are hyperbolic, meaning the semimajor axis is formally negative (or infinite, depending on the convention). In practical terms, once a probe exceeds the Sun’s escape velocity at its current distance, the concept of a bound elliptical orbit breaks down, and the trajectory is better described by its specific orbital energy rather than a semimajor axis Small thing, real impact..
Even so, if we restrict ourselves to bound orbits—those that remain gravitationally tied to the Sun—the current champion is the orbit of Sedna, another detached trans‑Neptunian object. Sedna’s highly elongated path has a semimajor axis of about 506 AU, with a perihelion of 76 AU and an aphelion estimated near 937 AU. This makes Sedna’s orbit the largest known bound ellipse in the Solar System, dwarfing even the Oort Cloud’s inner edge, which is thought to begin around 2 000–5 000 AU but consists of a diffuse swarm of comets rather than distinct, trackable objects.
Beyond the Solar System, exoplanet discoveries have revealed planets with astonishingly wide orbits. The directly imaged planet HD 106906 b, for example, orbits its host star at a projected separation of roughly 730 AU, implying a semimajor axis on the order of several hundred astronomical units—comparable to Sedna’s extreme path. Some free‑floating planetary‑mass objects detected in young star clusters may have formed in orbits that were later stripped away, leaving them with effectively infinite semimajor axes as they drift through interstellar space Small thing, real impact..
In summary:
- Natural, bound orbits: Sedna holds the record with a semimajor axis near 500 AU.
- Escape trajectories (human‑made): Voyager probes have effectively infinite (or negative) semimajor axes because they are on hyperbolic escape paths.
- Exoplanetary systems: Wide‑orbit planets like HD 106906 b approach or exceed Sedna’s scale, showing that the Solar System’s extremes are not unique in the galaxy.
Thus, the “largest semimajor axis” depends on whether you limit yourself to bound elliptical orbits, include escape trajectories, or look beyond our own star system. Each context yields a different champion, but all illustrate how orbital size scales with energy and distance, shaping everything from the fate of comets to the destiny of interstellar spacecraft.
