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Physics blooper on the last Stargazing live - throw a ball towards the Earth from the ISS and it will return to you


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The ISS travels at about 40,000km/hr - so if the ISS crossed a rugby pitch lengthwise and you fired a typical bullet when the ISS passed the touchline at near ground level, the ISS would be at the other end of the pitch before the bullet travelled a few meters (about three). Of course, it would then smack into the stand and things would get a bit dangerous. Neglecting the almosphere for a moment,  if it were possible to slow something down from orbital velocity so it "falls" to earth by throwing it (the record is about 166km/hr), then you could speed it up to orbital velocity the same way (i.e. a tennis star would be able to serve a ball into orbit - apart from the fact that, putting the air back,  it would burst into flames, as would his/her racket and arm - and as would the ISS at Twickenham).

As noted above, the only way to hit the earth with a "falling" ball is to fire it back along the path of the ISS at orbital velocity so it is "at rest" relative to the earth - gravity then takes over. Getting something up to orbital velocity (above Mach 30) is quite hard (so rockets are big, compared to their payloads). Even a Firefox with strap-on boosters and Michael Gant/Clint flying it at full throttle couldn't do that -  I seem to rember the record for flight through the air is about Mach 10 - other than on the way to or back from orbit).

Going the other way is a bit different, in fact the delta-v needed to de-orbit the Shuttle from LEO, according to the Shuttle Crew Operations Manual, seem to be about about 200-500m/s (450-1000mph - easy for many aircraft - that's about a 1% velocity change, alarmingly low perhaps), and all it does is change the orbit so that it intersects the atmosphere, where drag gets rid of the rest of the velocity. That means most guns could de-orbit a bullet (even some .22 air-guns) but you couldn't do it with a golf-club, hockey-stick, squash raquet or other "human without explosive help" method (even a firework rocket isn't fast enough) - otherwise de-orbiting would be relatively cheap and we wouldn't have the problem of so much "junk" orbitiing in space - which the experts should really have noticed straight away. It also means that the shuttle astronauts couldn't amuse themselves by throwing down messages in suitably heat-proof bottles, or slinging their trash down to earth to burn-up

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To get the ball to fall most rapidly, throw it back along your orbit from where you've come. To make it climb to a higher orbit, throw it straight forwards along the line of your orbit.

It is its speed at right angles to Earth's gravity that counts.

This has very real implications for things like spacecraft who want to meet up and dock.

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What exactly did they say on Stargazing Live?

I'm totally baffled by the OP's logic, which seems to be saying that meteorites, space shuttles and debris could never reach Earth but would instead bounce back up.

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