The key quantity allowing a satellite to sustain orbit without ongoing thrust is:

The correct option is Orbital velocity.

Step 1: Understanding Satellite Orbit Mechanics
When a satellite orbits around a planet (like Earth), it is constantly pulled toward the center of the planet by gravity. This gravitational force provides the necessary centripetal force required to keep the satellite in a circular or elliptical path.

Step 2: Role of Orbital Velocity
For the satellite to stay in orbit without falling straight down to the planet's surface or drifting away into space, it must travel forward horizontally with a specific tangential speed known as the orbital velocity.

Mathematically, setting the gravitational force equal to the centripetal force gives:

$\frac{GMm}{r^2}=\frac{m{v}^2}{r}$

Solving for the orbital velocity ($v$):

$v=\sqrt{\frac{GM}{r}}$

Where:
- $G$ is the universal gravitational constant.
- $M$ is the mass of the central celestial body (e.g., Earth).
- $r$ is the distance from the center of the celestial body to the satellite.

Step 3: Why Ongoing Thrust is Not Needed
In the vacuum of space, atmospheric drag is negligible at orbital altitudes. Because there is no significant friction to slow the satellite down, Newton's first law of motion dictates that the satellite will maintain its orbital velocity indefinitely without requiring any engine thrust or continuous propulsion.

Conclusion:
Therefore, the key quantity that allows a satellite to sustain its orbit continuously without ongoing thrust is its orbital velocity.