# Question about sending satellite circular orbit

Gold Member
Is it possible to put in circular orbit an object from Earth's ground? My friend said yes and told me we were going to see how exactly in 2 years when we'll learn about a more formal mechanics.
But I've thought about it and I believe it's impossible. My thought is that the satellite will describe a spiral and not a circle. My friend says it will describe a spiral and then a circle... but how could that be? If it describes a spiral then it will always have some velocity in a direction that is not parallel to Earth's ground, so the circular motion is not possible. At best it could be very very close to it, but still it will never be a perfect circular path. (Unless the satellite has a little reactor, but we suppose it hasn't).
So who is wrong and why?
Thanks.

## Answers and Replies

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D H
Staff Emeritus
If your satellite doesn't have thrusters, no. It won't spiral, however. It's orbit will be in the shape of an ellipse. Since you don't know this, I'll assume you are fairly young. Have you been introduced to Kepler's Laws?

An elliptical orbit that starts from the surface of the Earth will hit the Earth sometime later. The only way to escape this is to escape the Earth's gravity field entirely -- or have the satellite perform an orbit insertion maneuver once it clears the atmosphere.

Mapes
Homework Helper
Gold Member
The engines are used again after launch to circularize the orbit. You might be interested in reading about the Hohmann transfer.

Gold Member
Since you don't know this, I'll assume you are fairly young. Have you been introduced to Kepler's Laws?
I think it's relative, but I'm not that young (21 years old), university freshman.
It's orbit will be in the shape of an ellipse.
I've been introduced to the laws of Kepler in about 5 minutes, so my professor didn't emphatize at all on them... and I have no exercise that is related to them. I'm surprised in a way that the path will be elliptic. This is quite complex to guess : Earth's rotation on itself (neglecting the influence of the Moon) makes things hard for me to guess.

And Mapes
The engines are used again after launch to circularize the orbit. You might be interested in reading about the Hohmann transfer.
this is exactly what I thought... Of course I'm interested in reading the "Hohmann transfer". I'll check it out, thanks.

Thanks to both!

D H
Staff Emeritus
I'm more than a bit surprised this never came up in your high school physics class, if you took one. Kepler's Laws are pretty standard fare for high school (or even lower level) physics.

Since you haven't even covered the rudimentary basics, jumping in to the concept of orbit transfers is not a particularly good idea. They will just look like a bunch of equations that have no rhyme or reason. You need to understand not only that orbits around a body with a spherical mass distribution are elliptical, but how and why they are elliptical before you can make sense out of transfer mechanisms. You have to learn how to walk before you learn how to run.

Gold Member
I'm more than a bit surprised this never came up in your high school physics class, if you took one. Kepler's Laws are pretty standard fare for high school (or even lower level) physics.

Since you haven't even covered the rudimentary basics, jumping in to the concept of orbit transfers is not a particularly good idea. They will just look like a bunch of equations that have no rhyme or reason. You need to understand not only that orbits around a body with a spherical mass distribution are elliptical, but how and why they are elliptical before you can make sense out of transfer mechanisms. You have to learn how to walk before you learn how to run.
In fact they teach that in high school but I missed a whole year. I had no chance with that... I turned 17 and due to family problems I had to move from Canada to France. In Canada I'd have to take 2 years before entering into University while in France only one year. So when I came to France the students already saw many concepts such as precalculus and a lot of physics/chemistry while I had no idea about what was going on. I had to pass the "bac" which is a big exam at the end of the year (I had homeschooled at that time because I couldn't understand ANYTHING in math, so I took particular classes with a tutor... but still, I couldn't do 2 years in 6 months) and failed badly. But at least I learned some basic stuff in math, so the year that followed I went to high school and was an average student overall. (but particularly weak at sciences courses.... which was my speciality). And finally I could get my diploma this time.
From there I moved to Argentina and study at University. Basically it restarted all precalculus and also restarted from 0 for physics, which helps me very very much.
I remember in high school when they were doing exercises about RLC circuits with plenty of differential equations... I had a vague idea about the meaning of everything... but now even if I didn't get RLC circuits (second year course), I can understand much more than before.
Just to say that Kepler's Laws were teached 2 years before entering University in France, so I totally missed that year.
So I more than agree when you say I need to understand well why orbits are elliptical before getting involved into something that already assume that!
Last word : from what I've learned till yet in Argentina in physics, I am just excited about what's coming up!

D H
Staff Emeritus