# Hi All, a quick question about gravity in outer space!!

Status
Not open for further replies.
F

#### floyd94

##### Guest
Imagine you’re an astronaut on a space walk, your at an altitude of about 150-175 miles and you want to get rid of something so you throw it strait down towards the earth as hard as you can. What will happen in about 90 minutes?<br />

T

#### tomnackid

##### Guest
I'm guessing you just read Clarke's short story "Jupiter V" <img src="/images/icons/wink.gif" />

F

#### floyd94

##### Guest
No, but I would like an answer. Its for a friend of mine.

K

#### krrr

##### Guest
"I think its orbit would get slightly more elliptic with the apogee increasing and the perigee decreasing."<br /><br />Right, I think. From he standpoint of the thrower, it will look like he's being orbited by the object.

S

#### spacester

##### Guest
<font color="yellow">I think its orbit would get slightly more elliptic with the apogee increasing and the perigee decreasing</font><br /><br />That is essentially correct. There are some implicit assumptions made to get there, but yeah.<br /><br />The tricky part is in what we mean by 'more elliptic'.<br /><br />Are we implying that the initial orbit is circular? In that case the apogee and the perigee suddenly become no longer equal, so the word 'more' is inoperative.<br /><br />If we account for the fact that there's no such thing as a perfectly circular orbit, we need to consider when the object is thrown. Orbital maneuvers are almost always done at apoapse or periapse, but did anybody tell the astronaut exactly when to throw it?<br /><br />If the object is thrown at perigee than the answer is exactly correct. If at apogee, the object's orbit becomes more circular. If not thrown at an apse, things get complicated, but it will be one or the other: more elliptical or more circular.<br /><br />In any case, both objects will still have essentially the same orbital period. So at the end of one orbit ("90 minutes later"), they would meet up again: the object would come rising back up to meet the astronaut, if nothing else changed. (Jo Astronaut doesn't fire her thrusters.)<br /><br />In reality, there would be some drag on both objects that would make a difference. But now I'm getting too carried away even for me. <img src="/images/icons/laugh.gif" /><br /><br />But what happens at 45 minutes after the throw? <div class="Discussion_UserSignature"> </div>

K

#### krrr

##### Guest
I didn't mean a gravity-induced orbit. However, at first the object, with its perigee lowered, will run faster (having the "inner lane"). After about 22.5 minutes, it will be at the same height as the observer, but before him. At apogee after 45 minutes, it will appear above the observer, then it will fall behind etc.

D

#### discovery_fan

##### Guest
You all will probably know it, but there is a great Nasa site dealing with such things:<br />http://www2.jpl.nasa.gov/basics/guide.html<br />Section 1, chapters 3, 4, and 5 explain the basics of orbits very nicely. Just in case someone has never seen it.

C

#### comga

##### Guest
Spacester's answer is correct, but he doesn't say what happens along the orbit. One can plot these things using some good approximations. I believe the equations are called the Clohesey-Wilshire equations (although my spelling is probably off. A good place to start is a book by Bate, Meuller, and White, with further apologies for spelling.)<br /><br />Assume the initial orbit is circular, although it doesn't matter much, and assume that the throw is at right angles to the astronaut's path. The projectile leaves his hand heading "down" towards the center of the earth. As said by others, going "on the inside track" it moves ahead, but eventually hits its new perigee. At that point it is moving in the same direction as the astronaut, but lower and going faster. Then the orbit radius increases until it is at the same altitude as the astronaut. Its at the same velocity, but out in front. It continues to rise towards its apogee, and slows down. Now its "above" the astronaut but going slower, so he's catching up. It then descends, gaining speed to macth that of the astronaut, but droping in altitude. After exactly one orbit it passes by the astronaut, heading down at the speed it was thrown, having made a pretty good approximation of a circle. <br /><br />Even if there was a measureable drag effect, it would pretty much effect both equally unless there was a dramatic difference in balistic coefficient, such as if the thrown object was a balloon. In that case, the circles would become loops, one per orbit, moving away from the astronaut in the direction of motion but losing altitude relative to the astronaut.

Status
Not open for further replies.

Replies
8
Views
452
Replies
8
Views
5K
Replies
2
Views
7K
Replies
1
Views
3K
Replies
7
Views
2K