As a follow-up to this question.

I have this JSON coming back from the ESA's DISCOSweb API:

"inc": 52.0,
"raan": 0.0,
"epoch": "1999-10-22",
"sma": 7791700.0,
"ecc": 0.00011,
"aPer": 0.0,
"mAno": null,
"frame": "J2000"

Which of these parameters (if any) aren't required to fully define an orbit?

I suspect the answer is epoch and mAno, but I'm not certain.


2 Answers 2


You need them all, but can omit Mean Anomaly at Epoch and Epoch if you don't care where the object is at any given time, and just care about the orbit itself.

Inclination and Right Ascension of the Ascending Node define the plane of the orbit.

Semi-Major Axis and Eccentricity define the size and shape of the orbit.

Argument of Periapsis defines the orientation of the orbit in its plane.

Mean Anomaly at Epoch defines how far along the orbit the object is at a specific time.

Epoch is that specific time.

The Frame is the coordinate frame, in this case the J2000 Earth-Centered Inertial Frame, that these orbital parameters are relevant to.


I'm not an expert, but iirc there are 6 main elements describing an orbit from an inertial frame: inclination, eccentricity, semi-major axis, true anomaly, longitude of the ascending node, and argument of periapsis.
True anomaly (an angle) at Epoch (a time) describes the current position of the satellite on the elliptical trajectory. Mean anomaly is the fraction of the orbital period the satellite has elapsed since periapsis.
Everything else describes the elliptical trajectory itself.


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