Orbital Speed

The orbital speed is the speed with which an object revolves around a larger object or orbits around the barycenter. If it is calculated during the revolution of a smaller body around a larger body, its speed is relative to the centre of mass of the larger body. This term denotes both the average speed over the entire orbit and the instantaneous speed of the object at any point in its orbit. The maximum speed in orbit is attained at the periapsis, whereas the minimum speed is achieved at the apoapsis. The condition for both the states is that the orbit must be entirely closed. A uniform circular motion takes place in the orbits.

What is orbital speed, and what are the necessary conditions related to it?

The orbital speed of any object in revolution around a larger body, like the orbital speed of Earth around the sun or the orbital speed of the moon around the Earth, is known as its speed of revolution in its orbit. The condition is that the orbit must not be open and must be perfectly closed. If a two-body system exists, the orbital speed can be easily calculated by calculating the distance from the centre of the larger object and the total energy of the object, which is also called the specific orbital energy. This energy stays constant irrespective of the positioning.

Radial Trajectories

Looking at the specific orbital energy, the trajectories get decided. The specific orbital energy is given by the expression – Ek − Ep. (kinetic energy − potential energy). The result for this can be positive, negative or zero. The trajectories depend on this value.

• If the specific orbital energy is calculated to be positive, the orbit will follow a hyperbola while keeping the larger body with a greater mass as the centre of the hyperbola. But, in such cases, the objects do not return.
• If the specific orbital energy is a negative quantity and follows the expression – Ek − Ep < 0. The orbit will be closed. There will be a radial elliptic trajectory with one focus on the other body being taken into account. The orbital speed of the moon and the orbital speed of the Earth are such that they form elliptical trajectories. 
• If the specific orbital energy is zero, then the trajectory is a parabola focusing on the other body, which is being taken into account. They are according to the expression (Ek = Ep) and also have open orbits.

Transverse, mean and instantaneous orbital speeds:

• The transverse orbital speed

This speed has a relationship of inverse proportionality with the distance to the central body or the body, which is larger in size and mass in accordance with the law of conservation of angular momentum.

• The mean orbital speed

The orbits with a small eccentricity can have their mean orbital speed calculated with the help of observations of the orbital period and even from the masses of the two respective bodies and the semimajor axis.

• The instantaneous orbital speed

The instantaneous speed of the body is the speed that is calculated by the binary pair of the velocity related to the propagated gravity.

The orbital speed of Earth and its satellites

All the objects, when revolving around a larger object in a two-body system, revolve at a particular speed to maintain the equilibrium of the system. Earth revolves around its respective orbit with a fixed speed of 29.78 km per sec. Similarly, the orbital speed of the moon is 1.022 km per sec. The orbital speed of Earth’s satellites depends primarily on their height from the Earth. The higher the orbit is, the longer the satellite can stay in it. This is because, at higher altitudes, the vacuum of space is complete.

Conclusion

The orbital speed of objects helps them maintain the balance of the system while constantly keeping them in motion around the larger body. There are three types of orbital speeds: transverse orbital speed, mean orbital speed, and instantaneous orbital speed. The orbital speed of the Earth is 29.78 km per sec in its orbit, and the orbital speed of the moon is 1.022 km per sec. The correct orbital speed is necessary as, without it, the object or the satellite may fly out of its orbit after moving too fast and collapse due to the high gravity of the larger body.