Yes, and no. Simple answer is, no you can't. It's easy to get this wrong. You'd think that just push it and it is headed towards the Earth, no matter how slow it is, it would get there eventually.
But what you forget there is that since the ISS is orbiting the Earth, so is your tennis ball. And your throw only gives it a bit of extra velocity from the ISS.
Right now as I write this, the ISS is passing just South of Australia.
ISS Tracker
So if you throw the ball towards the Earth from the ISS you'd think it would fall into the Tasmanian sea, eventually.
But what you are forgetting there is that the tennis ball is also in orbit.
And as the ISS continues around the Earth, then 45 minutes later it will be traveling over the Atlantic just south of the UK.
And your throw towards the Tasmanian sea, now becomes a throw away from the Atlantic.
For instance if you throw it ten meters per second towards the Tasmanian sea relative to the ISS - it is now traveling 10 meters per second in that same direction - but because you are on the opposite side of the Earth now, that same direction now points out of the Atlantic back to the ISS.
If you could throw it with perfect precision towards the Tasmanian sea - and if the ISS was traveling in a perfectly circular orbit - then you would put it into a slightly elliptical orbit with the same semimajor axis (you haven't changed its period so its semimajor axis can't change).
So it's in a slightly elliptical orbit that would hit the ISS half an orbit later, gently at the same speed you threw it.
But in practice it's almost impossible that you can throw it as precisely as that - and as well as that, it would travel a slightly different path in the non uniform gravity of the Earth. So it will probably fall into an orbit with a slightly different period from the ISS. So would start falling behind or get ahead of the ISS, and so chance is low that it actually hits it. And then it has a different cross section to the ISS, and there's quite a lot of atmosphere there, so it would end up either falling towards the Earth more quickly or more slowly than the ISS. So by the time it phases around so that it is back in phase with the ISS it's either at a higher or a lower orbit.
And actually - the astronauts on board the ISS actually launch satellites from time to time, just by throwing them out of the ISS on a spacewalk. Just in the same way as this tennis ball except they don't through directly towards the Earth. I'm not sure how they are sure that the satellite won't hit the ISS again later on, but they must have worked that all out. I'll be interested to hear if anyone knows how they make sure they don't get hit by the satellite again later on in the ISS's orbit.
Here is a video of Cosmonaut Oleg Artemyev throwing a Peruvian science nanosatellite into a separate orbit from the ISS.
Spacewalking Cosmonaut Tosses Tiny Satellite Into Space for Peru (Video)
And here are some images of it, from Beautiful Images of Astronauts Releasing Nanosatellites Into Space
Note though that at the height the ISS orbits, then the tennis ball would eventually de-orbit through atmospheric drag. Which might take decades, depends on the cross section it presents to the atmosphere which for the tennis ball would be fairly small compared to its mass.
So strictly speaking, yes, you can de-orbit a tennis ball by hand.
That's because the entire ISS is also de-orbiting as well. So it will de-orbit because it is no longer attached to the ISS and doesn't have its own separate rockets to keep boosting it to a higher orbit.
If you went to a higher orbit, say to Geostationary Orbit - then you couldn't deorbit a tennis ball by hand, because there is no atmospheric drag to de-orbit it. Even though the satellite is poised over a single spot on the Earth.
To see that for GEO - suppose you throw the ball towards the Earth at midday with the sun behind you. Now at midnight, the ball is still traveling in that same absolute direction relative to you - away from the sun. But that direction now points from the Earth towards you because you are on the night side of the Earth. So just as for LEO, it will return to your hand (in idealized situation) half an orbit later.
But what you forget there is that since the ISS is orbiting the Earth, so is your tennis ball. And your throw only gives it a bit of extra velocity from the ISS.
Right now as I write this, the ISS is passing just South of Australia.
ISS Tracker
So if you throw the ball towards the Earth from the ISS you'd think it would fall into the Tasmanian sea, eventually.
But what you are forgetting there is that the tennis ball is also in orbit.
And as the ISS continues around the Earth, then 45 minutes later it will be traveling over the Atlantic just south of the UK.
And your throw towards the Tasmanian sea, now becomes a throw away from the Atlantic.
For instance if you throw it ten meters per second towards the Tasmanian sea relative to the ISS - it is now traveling 10 meters per second in that same direction - but because you are on the opposite side of the Earth now, that same direction now points out of the Atlantic back to the ISS.
If you could throw it with perfect precision towards the Tasmanian sea - and if the ISS was traveling in a perfectly circular orbit - then you would put it into a slightly elliptical orbit with the same semimajor axis (you haven't changed its period so its semimajor axis can't change).
So it's in a slightly elliptical orbit that would hit the ISS half an orbit later, gently at the same speed you threw it.
But in practice it's almost impossible that you can throw it as precisely as that - and as well as that, it would travel a slightly different path in the non uniform gravity of the Earth. So it will probably fall into an orbit with a slightly different period from the ISS. So would start falling behind or get ahead of the ISS, and so chance is low that it actually hits it. And then it has a different cross section to the ISS, and there's quite a lot of atmosphere there, so it would end up either falling towards the Earth more quickly or more slowly than the ISS. So by the time it phases around so that it is back in phase with the ISS it's either at a higher or a lower orbit.
And actually - the astronauts on board the ISS actually launch satellites from time to time, just by throwing them out of the ISS on a spacewalk. Just in the same way as this tennis ball except they don't through directly towards the Earth. I'm not sure how they are sure that the satellite won't hit the ISS again later on, but they must have worked that all out. I'll be interested to hear if anyone knows how they make sure they don't get hit by the satellite again later on in the ISS's orbit.
Here is a video of Cosmonaut Oleg Artemyev throwing a Peruvian science nanosatellite into a separate orbit from the ISS.
Spacewalking Cosmonaut Tosses Tiny Satellite Into Space for Peru (Video)
And here are some images of it, from Beautiful Images of Astronauts Releasing Nanosatellites Into Space
So strictly speaking, yes, you can de-orbit a tennis ball by hand.
That's because the entire ISS is also de-orbiting as well. So it will de-orbit because it is no longer attached to the ISS and doesn't have its own separate rockets to keep boosting it to a higher orbit.
If you went to a higher orbit, say to Geostationary Orbit - then you couldn't deorbit a tennis ball by hand, because there is no atmospheric drag to de-orbit it. Even though the satellite is poised over a single spot on the Earth.
To see that for GEO - suppose you throw the ball towards the Earth at midday with the sun behind you. Now at midnight, the ball is still traveling in that same absolute direction relative to you - away from the sun. But that direction now points from the Earth towards you because you are on the night side of the Earth. So just as for LEO, it will return to your hand (in idealized situation) half an orbit later.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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