Question

Why are satellites normally sent into orbit by firing them in an easterly direction? Based on this, explain why the International Space Station (ISS) orbits the Earth at about 17,500 miles per hour at an altitude above Earth’s surface of about 230 miles. Would it be more efficient to have a higher or lower orbit for the ISS? Explain in detail why or why not.

Answer 1

When we launch a satellite into the specific orbit, it must end up spinning around the planet from which it has launched (Earth in our case). This spinning of the satellite prevents the satellite to fall back towards the Earth because of its gravity.

To launching a satellite into its orbit, massive rockets are used which provides enough thrust to satellite to achieve the escape velocity. Besides this, if the launching of the satellite taken place from the equator of the earth and also as the Earth itself rotates eastwards, this then gives an extra push for launch.

This technique is very useful to cut down the cost of the extra thrusters.

So, the eastward motion of the Earth provides the satellite an extra push to get it into the orbit.

That is why satellite is also fired up in the eastward direction to take advantage of the eastward rotation of the Earth.

The ISS travels at the speed of 17,500 miles per hour at the distance of 230 miles above the Earth’s surface. This particular value of velocity can be easily determined by using the approximation, i.e., let us assume the ISS revolves around the earth in a nearly circular orbit at the distance of (R_e+230 miles) away from the center of the Earth.

Here, R_e (3959 miles) is the Earth’s radius.

Now, as the ISS revolves in a circular orbit, so, the required centripetal force is provided by the Earth’s gravitation.

Which is a good approximation considering the fact that we assume circular orbit, but we know it revolves in an elliptical orbit.

So, moving with this high speed in the orbit, even at that altitude there is some particle that creates drag to the ship resulting in decreasing its altitude by a very small amount. To overcome this altitude loss, ISS fires its thrusters once every month to regain the loss.

As the ISS is a fully automated lab facility in the space, therefore, it requires supply and all from the earth and also it takes only 90 minutes to go one trip around the earth. Therefore, the Low Earth Orbit (LEO) is beneficial for such kind of operation.

And also, the LEO requires the lowest amount of energy for the replacement of the satellite and it provides low communication latency and high bandwidth.

The space stations and satellites in LEO are easily accessible for servicing and replacement too.

That is why LEO is more efficient for ISS.