Space balls from the ISS - Page 1
Forum section: electronics / general

author: ornea
author_rank: Unknown
timestamp: April 05, 2017, 02:42:09 pm
content: So Dave and Fran got me all interest in space and have been watching lots of youtube related video including an ISS space walk which got me wondering.Theoretically question: Assuming indestructible could an ISS space walker throw a cricket ball at the earth from the ISS and be sure it would hit earth.If so, how long would it take to hit.Would angle be critical.Could it be thrown in such a way as to bounce off the atmosphere to launch into space.They mentioned that a returning capsule's angle of re-entry is critical else it could bounce off the atmosphere, if so do they get another go or dont they have enough fuel to push them back.

author: Nusa
author_rank: Super Contributor
timestamp: April 05, 2017, 03:42:24 pm
content: Probably, but it would take a VERY long time. The amount of force in a human arm isn't terribly significant in this circumstance, but everything in orbit decays eventually, including the ISS itself.No, given indestructability, angle would cease to be important."bounce off the atmosphere" isn't something that happens literally. It simply means that descending at too high an angle means you won't ecounter enough of the earths atmosphere to slow you down enough to fall into the gravity enough to avoid exiting on the other side, at least on that pass. And yes, they'll get another go if they exit at less than escape velocity, but it'll take either time or fuel to do so. And time is limited by life support.On the other hand, if you enter too steeply, you encounter so much of the earths atmosphere at speed that the resulting heat from friction burns you to a crisp. Unless you're an invulnerable cricket ball, of course.

author: rdl
author_rank: Super Contributor
timestamp: April 05, 2017, 04:08:52 pm
content: Throwing the ball is all but irrelevant (delta V about 0.5% of total velocity). Just letting go of it would do as well.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 05, 2017, 04:45:14 pm
content: Note that you want to throw it "backwards" (reducing its tangential velocity), because that pushes it to a lower orbit. It will eventually decay (from solar radiation pressure and atmospheric drag) and reenter at a shallow angle. A cricket ball made of tungsten might survive reentry somewhat intact, but it certainly won't come down where you "threw" it.Tim

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 05, 2017, 05:15:03 pm
content: Putting some real numbers on it. The space station orbits at about 175 km altitude, where g has dropped off noticeably, to about 9 meters second. The altitude gives a radius for the orbit of about 6550 km. Plugging those numbers into a=v^2/r you get a speed of about 7675 m/s. If you are world class hurler you can throw at about 45 meters/sec. Assuming you can pull that off in a weightless environment while wearing a spacesuit (remember you are truly world class), you get that speed down to about 7630 m/s. Which then translates to a radius of about 6480 km, or an altitude of 100 km.That really does change the drag, so your ball will come down pretty quickly. Faster if you make it hollow for light weight. There are some other factors. This assumes you throw at exactly the right angle to end up in a circular orbit. Other angles could get an even lower perigee, but I am not an orbital mechanics whiz so you would have to go elsewhere for that. That dip into even denser atmosphere might bring you down faster yet.

author: rdl
author_rank: Super Contributor
timestamp: April 05, 2017, 06:48:50 pm
content: Of course, there's that non-intuitive "slow down to speed" up thing that causes orbital mechanics to induce headaches. You have to do your retro "throw" at the highest point in the orbit in order to lower the lowest point. Average speed increases. Or I could be all wrong, it's been quite a while since I played KSP.

author: Monkeh
author_rank: Super Contributor
timestamp: April 05, 2017, 06:53:14 pm
content: Quote from: CatalinaWOW on April 05, 2017, 05:15:03 pmErrrr, try 400.

author: Red Squirrel
author_rank: Super Contributor
timestamp: April 05, 2017, 07:17:27 pm
content: I think the best angle is to throw it retrograde which is the opposite direction the ISS is orbiting at. Might want to use a potato gun to be more effective. Since it's such a small object you don't really need as much delta V to change it's orbit, so I'm thinking one could perhaps deorbit something like that by hand though.Also want to double check the official game rule book, since if it lands in sea it's going to suck if you have to play it from there.

author: rdl
author_rank: Super Contributor
timestamp: April 05, 2017, 09:53:53 pm
content: If there were no atmosphere to take into account, it would take the same amount of delta V to de-orbit the space ball as it would for the entire ISS. However, what it would take to achieve that change in velocity would be much different between the two.

author: W9GFO
author_rank: Regular Contributor
timestamp: April 06, 2017, 12:14:08 am
content: Quote from: Red Squirrel on April 05, 2017, 07:17:27 pmDelta V is the same regardless of mass. Energy required to affect that delta V is what you don't need as much of for a smaller object.

author: Red Squirrel
author_rank: Super Contributor
timestamp: April 06, 2017, 12:59:07 am
content: Quote from: W9GFO on April 06, 2017, 12:14:08 amOh I see, I always thought it was affected by mass. But I guess that makes sense now that I think about it . Otherwise when you dock with something like the ISS it would need a boost right away.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 06, 2017, 01:34:20 am
content: Quote from: Monkeh on April 05, 2017, 06:53:14 pmYeah, I grabbed some numbers off the internet and they weren't all accurate. The net change in orbital altitude of 75 kilometers is still roughly correct. I was surprised you could make that much difference by throwing something.

author: W9GFO
author_rank: Regular Contributor
timestamp: April 06, 2017, 03:16:54 am
content: Quote from: CatalinaWOW on April 05, 2017, 05:15:03 pmYou cannot go from a circular orbit into another circular orbit with only one throw. At perigee you would need another "throw" to circularize the orbit.

author: ornea
author_rank: Unknown
timestamp: April 06, 2017, 05:16:52 am
content: Interpretting the comments above, throwing the ball aft (backwards) at 28000km/hr would cause the ball to become stationary relative to the ground and it would begin to fall. Taking about 1 hour to hit the ground.But thinking I could get it down quicker if angled toward the ground.So obviously there would be an optimum angle to take advantage of wind friction braking forces ...Lots to think about

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 06, 2017, 05:43:04 am
content: Quote from: W9GFO on April 06, 2017, 03:16:54 amTrue. So our imaginary space suited hurler tracks along in his jiffy George Jetson space scooter and does the second throw and takes another 40-50 meters/second off. That circular orbit will be lots lower. Or you can get 75 km altitude drop with two high school athlete throws.