The Most Confusing Things About Spacecraft Orbits

hello its Scott Manley here now a couple of months ago I crunched the numbers on whether a sufficiently athletic astronaut could throw a ball back to Earth from the space station and while I compared the speed of the projectiles to the retrograde velocity required from D orbiting a spacecraft many many people couldn't see a problem the natural way people think about is that in zero-g in the absence of drag and with the force of Earth's gravity any object propelled straight down towards Earth would have nothing to stop and would quickly reach the Earth's atmosphere in D orbit and that's a fine example of how orbital mechanics can defy our normal expectations of motion so let's do a demo in trouble space program if I did actually project an object straight down from this space station then indeed it would initially appear to move down towards the planet below but as time went on it would start to move laterally it would actually start to move ahead of the space station in its orbit and about 1/4 of a way around the orbit the object would stop getting lower towards the planet and it would actually start rising again I don't know about a nut half Arbit later it would actually be once again at the same altitude as a space station although it would be ahead of us fast forward this more it actually rises up above the space station and a full orbit later if you've thrown it just right it could actually come back and hit you I think one of the things that really confuses people is that there's actually two coordinate frames the space station keeps a constant orientation relative to the earth day it always has the same down and the space station has to keep this orientation by rotating slowly as it moves around the earth now as soon as you throw the ball it actually is no longer being controlled by this rotation so its initial velocity vector carrying it away from the space station doesn't rotate with the space station so that means a bit 23 minutes later as the has moved 1/4 of a way around its orbit guess what it's now going sideways it's actually going along the Space Station's orbit and actually going slightly faster so it then starts to rise up and of course 45 minutes later halfway around the orbit the velocity vector is pointing straight out and bringing it across the space station in the opposite direction and if we follow that for even longer it will again slow down and start coming back down what's happened is that the orbit has become slightly elliptic and instead of the object orbiting at the same altitude it now oscillates between a low point and a high point or as astronomers and Kerbal space program players say perigee and Apogee it was entirely possible to drop the perigee far enough that it ends up inside the atmosphere but it has to do that inside this first quarter orbit before it starts going up and turns out if you do the math you need about four times as much velocity to do this compared to just throwing it out backwards when you ruin orbit you're already moving around the earth a bit seven and a half kilometers per second so if you throw an object away you're changing the total velocity it's not a simple arithmetic addition it's a vector addition so when you throw a ball downwards that's at 90 degrees to that setting and half kilometer per second velocity if you throw it down it like 100 meters per second you do the math it turns out that you only change the total magnitude of the velocity by about two-thirds of a meter per second and because the velocity is practically unchanged and the altitude is practically unchanged the energy of the orbit really doesn't change by very much so it's a very inefficient way of changing their orbital energy if you want to go up or down you want to maximize the change to your velocity magnitude so you want to project the object either forwards to increase the energy or backwards to decrease the energy and this explains why most maneuvers performs in Kerbal space program are either pro-grade or retrograde burns to adjust the size of the orbit now that's for dropping an object to bank from law Earth orbit where the radius of the orbit is very close to the radius of the planet but what about dropping something see from the earth into the Sun save some of that radioactive waste that people talk about throwing into the Sun well the Sun may be huge but it's radius is actually really small relative to the radius of Earth's orbit so dropping an object into the Sun requires a much bigger kick and we now know that applying that kick retrograde is the most efficient way to drop your perihelion down except when you want to get more than about 40% to the weight of the Sun at that point the most efficient way to fire your engines literally turns around and is better to accelerate forwards increasing your orbital velocity lifting your spacecraft higher and higher and now as you cost outwards your velocity drops and as your velocity drops it gets very very small and from there it only takes a very small kick to drop your perihelion down into the Sun now the crossover point actually caught a corresponds to the escape velocity this is a critical point where a spacecraft has enough energy to escape the gravity of the central body it's orbiting of course this has the downside of taking significantly more time to reach your goal because you have to go a long way out before your velocity drops sufficiently and no real spacecraft would ever use this trick because it's possible to plan gravity assists from other planets for example the park or Solar Probe is going to launch this year and it's getting it closer to the Sun than anything else but it is gonna do this using gravity assists from Venus for another example of orbits being banked front let's talk about constellations of cube sets the Dove satellites are basically Earth Observation cube sets they are fit into a 3u form factor which means they are ten centimeter by 10 centimeter by about 30 centimeter about that size right they're tiny they're built around a a camera with a hundred millimeter lens and their earth observation satellites know several often get launched at the same time on spacecraft so they will all end up at roughly the same point in the same orbit a little clump of them floating in space but to make them useful they want to spread these things out along the orbit so that they can more evenly cover the earth the spacecraft are small so they don't have rocket engines but they do have something called magnet or kurz that they can use to control their attitude relative to the earth they also have solar panels that are used to provide power and can be used to apply drag so it seems easy enough if two spacecraft want to separate then one of them goes into a low drag configuration turning at solar panels edge on to the Earth's atmosphere the other goes into a high drag configuration trying to grab as much atmosphere and slowing it down more so that it falls behind it's low drag faster twin and this sounds very convincing but it's not true the bizarre thing is that that spacecraft with the high drag configuration that is attempting best to slow down in fact speeds up because as it catches the Earth's atmosphere what that does is it slows it down a little initially but that means that its orbit drops and as its orbit drops that means that it has to move faster because it's in a lower orbit so paradoxically the ones that are designed to be super streamlined end up going slower than the ones that are designed to drag experience as much drag as possible so paradoxically yet if you want to go faster in orbit you have to hit the brakes if you want to go slower than or but you need to accelerate so if you are in a spacecraft and you're looking out for the space station and it's behind you you want to fire your thrusters to take you away from the space station similarly if you're behind the space station you want to fire your thrusters to you know slow yourself down a little so that you fall down and catch up on the space station they like yes this is kind of crazy and don't be sad if you find this really hard to follow because even astronauts have been confused by this particular aspect of orbital mechanics on Gemini 4 crewed by James McDivitt and a Dwight their main mission plan was to perform the first spacewalk by a u.s. astronaut but late in the planning it was decided that they would attempt orbital rendezvous with the Titan rockets upper stage now because the rendezvous was a late addition to the schedule there wasn't really any special training added to prepare the crew for the ideal maneuvers and having achieved orbit the pilot turned the spacecraft around and began to accelerate towards the target to bring them closer together and as you can imagine if this didn't work as time went on they didn't really seem to get any closer and the more and more maneuvers were attempted to correct this but the crew two of the us's best pilots were unable to close the distance and eventually they abandoned the attempt after using about half the spacecraft's maneuvering fuel but I'm gonna say to be fair to the crew of Gemini 4 situation wasn't ideal the booster wasn't a perfect target because it would still be venting fuel and gases at this time which was probably changing its orbit also at this point in the Gemini program they didn't have a rendezvous radar so they had a hard time estimating the distance to the target but I'm gonna also point out that Gemini 12 its radar changed and the crew was able to navigate in using a sextant and paper but I will also mention that the navigator the copilot on that was none other than Buzz Aldrin who literally wrote the book on orbital rendezvous I mean literally that was his PhD thesis and I guess that's my whole point here is that sometimes I will say things and do things and it doesn't make sense until you really think about it a little more so I hope you've learned a little here I'm Scott Manley fly safe [Music] [Music]

Channel: Scott Manley

Views: 805,098

Rating: 4.9581017 out of 5

Keywords: orbit, orbital mechanics, science, rockets, gemini, buzz aldrin, rendezvous, spaceship

Id: i5XPFjqPLik

Length: 11min 8sec (668 seconds)

Published: Fri May 11 2018

Reddit Comments

[u/JoshuaACNewman]

Weirder, it won’t hit you. It will wind up in front and ahead of you. To make it go down, throw it backward and it will wind up in front of you, too! If you throw it ahead of you, it will wind up falling behind you.

Who else here plays Kerbal Space Program?

[u/LucyEleanor]

It would hit the surface if it was thrown fast enough. The speed component towards the earth would have to be enough so hit the surface before speed component to the side moves it out of the path on the earth and into orbit.