In orbital mechanics, which two properties determine an object's orbit around a planet?

In orbital mechanics, which two properties determine an object's orbit around a planet?

• A. Velocity and position
• B. Speed
• C. Mass and distance
• D. Direction alone

Answer: (A)

Knowing where you are and how fast you’re going in a planet’s gravitational field sets the entire orbit. In the two-body problem, the path is a conic section whose shape and orientation come from the initial position vector and velocity vector. These two pieces of information fix the specific orbital energy and angular momentum, which in turn determine whether the orbit is circular, elliptical, parabolic, or hyperbolic, and define the orbital plane and other elements like the semi-major axis and eccentricity. Speed alone doesn’t tell direction or curvature; position alone doesn’t tell how you’re moving. Mass and distance don’t specify the motion state either, and direction alone lacks the energy that shapes the trajectory. So the combination of position and velocity uniquely determines the orbit.

Knowing where you are and how fast you’re going in a planet’s gravitational field sets the entire orbit. In the two-body problem, the path is a conic section whose shape and orientation come from the initial position vector and velocity vector. These two pieces of information fix the specific orbital energy and angular momentum, which in turn determine whether the orbit is circular, elliptical, parabolic, or hyperbolic, and define the orbital plane and other elements like the semi-major axis and eccentricity. Speed alone doesn’t tell direction or curvature; position alone doesn’t tell how you’re moving. Mass and distance don’t specify the motion state either, and direction alone lacks the energy that shapes the trajectory. So the combination of position and velocity uniquely determines the orbit.