Jon Steensen

Man-made satellites were put into orbit by the thrust of one or more rocket engines, which continuously changed their orbits gradually all the way up. It was not by a collision or quick change of orbit from a close fly by or a collision. In fact you cannot put an object into orbit with a quick impulse addition at surface level, as the object would return to the same point after one roundtrip around the earth. Unlike man-made satellites, most asteroids do not have that slow thrust over long periods, which slowly change their orbits, unless they thrust something into space, or are close to a massive object at nearly the same speed, following for longer periods. I know that celestrial objects can have an in build “rocket engine” e.g. do comets boil, when they come close to the sun, and Io has geysers that erupt into space due to tidal heating. Regarding the ejection of planets or moons from stable orbits it normally requires many interactions, so if that mechanism should work in reverse to catch objects, I would expect that it would require many interactions too. Most objects coming at a random speed from deep space I expect would most likely come in at such a speed and angle that they will only get one flyby, and get their orbit changed once e.g. by flying close to a moon and continue back into deep space. For example calculating the gravity assist manouvers to eject something from the solar system is very complicated, and it does not just happen by change that you are close enough to the planets for their gravity field to become significant. Likewise I think getting an object comming in at high speed from deep space and getting it to slow down enough to go into orbit, by it flying close to a planets moons, is very unlikely to happen by change. If the object got captured by interacting with something else, changing its path from a non-orbit to an orbit, I would expect to see orbits with a shape very similar to non-orbits. E.i. orbits that are far from circular, but instead reaches far into space where the object captured originally came from, and where it got its orbit influenced. I.e. If the orbit was changed by a collision in deep space, the conservation of energy dictates that orbiting object reaches the same point after one orbit. I know that many orbits exist, and non of them are especially magical, but I think it is much easier to achive a non-orbit, than for the speed distance relation to be such that an orbit is achived.

When watching documentaries, it happens quite often that some kind of scientist says that a planet might have captured one of its moon. I have never understood how that should be the case. When a moon orbits a planet, we can predict where it will be in future according to Newton's laws of gravity. The result will be that it is the same orbit as it is currently in. Likewise we can let time “run backwards” and calculate where the moon must have been in the past. We get that it must have been in the same orbit as it is currently in. As we do not see moons suddenly leave their orbits and fly into space, I would not expect them to suddenly fly in from space and start orbiting a planet. Either they are in orbit, always has been and always will be, or they are not in orbit and will just fly by once (maybe more if they orbit the same star as the planet). The only thing that can change an orbit is some kind of interaction with another celestrial body, e.g. colliding with an asteriode. In order for that to change the moon-to-be’s orbit so it orbits a planet, it must enter a collision of just the right mass/velocity combination at just the right time near a planet. As space I mostly empty, especially around the planets (as part of the planet definition is that they have cleared their surrounding area of debris), I find this to be a very unlikely explanation of the capturing mechanism. So how does it work?

I apologize if a similar topic has been answered somewhere else in this forum. I could not find the answer after a quick search, here or on google. If it is the case that I ask a question that has been answered before, please point me in the right direction. I have a project where I consider electroplating a metal object with another metal lower in the reactivity series, but I am not completely sure I understand how it would work and which voltage would be required. E.g. plating a piece of copper wire with aluminium, using an electrolyte of an Aluminium salt of some kind in water solution. In the beginning this requires a voltage difference (which can be found from the reativity series), as a less noble metal is coated onto a more noble metal, that part is easy. I was wandering what will happen after the first thin layer of Aluminium has been applied? Will it then be like trying to electroplate aluminium onto aluminium, which in theory could be done without any voltage drop? (when the internal resistance in the solution is neglected. Even with a nonzero resistance in theory the smallest voltage drop would then be able to drive a very small current, which, given enough time, could do the plating. Albeit it will take a very long time with a near zero voltage). How thick has the plated layer have to be in order for the kathode to act as if it was made from Aluminium? Just thick enough to be waterproof? Alternatively, will the copper in the core always make it act as if it was completely made of copper? Say there is a voltage drop when the part has been coated, then what will happen when more electrolyte is added, and the part of the cathode above the electrolyte level becomes exposed to it? Will the required voltage go up again? Or will the Alumnium just be deposited on the part that is already coated, when if the voltage is reduced below the critical limit for alu-plating copper?