Nooooooooooo. You totally stuffed up the key bit of the explanation! It's not the body's rotation that actually matters, in fact the moon or Earth could be spinning the totally opposite way and you'd still aim for the same side of the moon (but then it would be the trailing edge obviously.) What really matters is the way the moon is moving in its orbit. If you aim for just behind it you tend to get towed along by it, which can launch you off into interplanetary space- which is bad. But if you go past ahead of it in its orbit it tends to slow you down and drop you back down towards the Earth, which is much safer.
I was wondering about that too. Thanks for the (real) explanation. If you look at the very last part at 5:37, the space craft's approach is opposite what the logic of the 'spinning' argument would suggest. That is, it is coming in 'with the spin' and should speed up more, and that is not what you want when trying to land on earth. It is the relative motion of the bodies and their masses that matters, not their spin. There is a small relativistic effect, but it is far to small to be detected without sensitive specialized equipment. The Gravity Probe B satellite was launched to investigate this (predicted) phenomenon. It took a year in orbit, and a lot of data analysis to demonstrate anything. I think this may be where the confusion in this video is coming from. The effect is called 'frame dragging' and it is not a factor (in fact not even detectable) in normal orbital motion, hence the need for the Gravity Probe B mission to investigate it.
Thanks BooBaddyBig. A much better explanation in a lot less time. One thing I learned though is that the spacecraft was in an elliptical orbit with apogee near the moon. I always wondered how the moon had enough gravity, but the craft is moving much slower there.
Aerospace engineer chiming in here. The explanation is correct in spirit but mixes up one thing: the gravitational assist (or slow down) has nothing to do with the rotation of the moon, but rather the orbit of the moon: do a fly-by in the 'same' direction as the orbit, you get a gravity assist; do it the other way and you get slowed down (and the planet/moon speeds up)
It has nothing to do with the spin of the planet (or moon), it has to do with the orbit. If the moon was rotating the other way, it still would be best to fly in a “figure-8”. (It isn’t actually a figure-8 because the moon moves during the orbit.)
I think she said that. But its because the moon rotates so slowly that the rotation doesn't matter, and the orbital direction takes precedence. If the moon rotated in only 10 hours, then the surface velocity would be more significant compared to the orbital velocity, and the direction you orbit would matter for landing and takeoff. It's because the rotation is slow that the rotation doesn't matter, and the transfer delta-V cost becomes more important.
It does due to relativistic framedragging. not significant, not even a meter/s but does exist. the prograde/retrograde pull of the flyby would be the important bit tho.
I agree with a few other comments saying this explanation is scientifically innaccurate, which is unusual for Amy. The conclusions about fuel savings and safety are correct, but the error is that "leading/trailing hemisphere" of earth's or moon's rotation has been confused with "leading/trailing hemisphere" of orbital path. In this case the moon's orbital path defines one hemisphere (not fixed to the rock) as leading in its orbit, and another trailing. If the moon rotated the opposite direction yet orbited the same, the figure eight orbit would have the same benefits.
Yup the spin of the moon had nothing to do with it. You want to go past the leading edge of the moon (whatever direction it is spinning) to avoid gaining speed from gravity assist. Gravity has nothing to do with spin.
Yes I was going to comment on this also. The gravity seen by another object nearby has no connection with the first or second object's spin. That is assuming the bodies are not asymetrical in a major way. Think about it this way: how would just spinning a symetrical object on its axis produce any different gravitational field if in fact the object is symetrical?
It rotates Whitt, albeit very slowly compared to earth. The Earth completes one rotation approximately every 24 hours, giving us a rate of around 15 arc seconds per second. The moon rotates once per orbital period, due to it being tidally locked. This means it rotates much slower, around one arc second for every two real seconds.
I haven't researched this or anything, but it seems to me that it is not the direction of the moon's rotation but the direction of the moon's orbit around earth that determines the amount of momentum transferred in a gravity assist. Maybe I'm wrong, but I don't see how the moon's rotation should effect this. After all the spin doesn't effect the center of gravity of the moon and therefore not the gravitational force either.
Can you fix this by adding captions to explain that it's the leading/trailing edge of the ORBIT around the earth that matters; NOT the edge of the rotation like you said?
Uhm, are you sure? My experiments with Kerbal space program tells me it's not about how the moon rotates, but how it revolves around the earth. Also the rotation of the moon being not just not to scale, but also showing it's back to the earth was... Not optimal. The rotation of the object is never mentioned in any description I've seen, only it's orbit.
I thought the same, I would’ve thought it was because when entering with the leading hemisphere the spacecraft will take a longer and sharper curve, spending more time where the gravitational field is stronger. That makes more sense than that it is the moon’s rotation that is the reason.
Woah yes, i never had a video leaving me this confused. All i could think about was the fact that the moon is tidally locked. took me a good five minutes that the moon rotates in RELATION to the spacecraft... damnit.
Hi mate I really like "vintage space" so far you are the only person that has put in depth & explained how the Saturn 5 works "WOW, total of 83 motors & rockets used" and breaking down the tasks of the Apollo missions. Looking forward to learn more. Only watching the first episode of "vintage space" is one small step, but one giant leap of knowing more.
Amy I love your vids and i hope you publish a correction. You've probably learnt this by now but the change in momentum from a lunar flyby is different, depending on the side you pass by because of the direction of orbital travel not rotation. The reason is a bit like how you gain more speed surfing with a wave than if you go towards one (you spend more time falling into the moon's gravity well). Naturally you would want to land and take off in the rotational direction if possible for the savings you mention (they apply to landing as well); but it is impossible to get a free return going the other way, because you gain energy from assist.
Thanks for clearing that up, it didn't make any sense to me at all that the "spin" of the Moon should have a thing at all to do with gravity effects on the Apollo.
for all intents and purposes, a spinning planet is a perfect sphere, and it makes no difference whether you orbit it "clockwise" or "anticlocwise". What matters is the orbital direction of that planet relative to your motion. That is what makes slingshot maneuvers work, and not from which side of the planet you approach it.
It really shouldn't be that complicated for a "space enthusiast" to grasp. It shows how she and other people just never thought about it, only tried to ingest bits and pieces of information about orbital mechanics. It's about how much time you spend being accelerated or decelerated. The latter being favorable in this case. In this case it's even more like an orbital transfer. If your apogee is inside the moon's orbit and a little ahead of it, when you reach it your velocity is minimal and it's like using L1 or being in a pseudo orbit around the Earth and the moon.
The "side" she's talking about is the orbital direction of the planet/moon. Not the type of hemisphere we normally associate with one continent or another, but the side that is facing in the direction of orbital movement, or away from it. (leading or trailing) For a fast-rotating planet like Earth, the clockwise/anticlockwise orbit does make a difference during ascent and landing, because the surface velocity is about 1000MPH, which reduces the delta-V you need if you orbit the same direction. But the moon moves so slowly, that it is irrelevant compared to the transfer cost.
Great video Amy. This is an area with a lot of confusion. You do not so much gain momentum in a gravity assist (unless there is a burn involved). What actually happens is that your angle of exit from the body you are passing by (in this case the moon) is changed relative to that body. You can demo that really easily in KSP (a Tylo example is a really good one due to the massive gravity assist possible). Because the vehicle changes direction, the relative velocity from the parent body (in this case Earth) is changed.
Gravity assists do change the momentum of space crafts (both speed and direction). It steals the momentum of the body. Jupiter's orbit is changed (by an amount too small to measure) when space probes use it for gravity assists. I think Amy butchered the explanation leading edge/ trailing edge in the video. Others have pointed her mistake in this matter.
Yup. It changes the momentum relative to Earth. But not the moon. The speed you enter into the Moons Sphere of Influence is the exact speed you exit the Moons Sphere of influence. You can easily try this in game. The momentum change relative to the Earth is due to the angle change from the gravity assist. Energy is indeed stolen from the Moon to achieve this.
I'm pretty sure your explanation is wrong or at least wrongly animated. The crux is not the rotation of the moon itself. It's the rotation on the orbit around the earth. Going with the orbit means you will get dragged along longer. The actual rotation of the moon has no way of transferring any energy to a orbiting craft. Ignoring the fact that the moon doesn't rotate much as it is tidally locked with the earth.
You’re right on that. Sorry Amy, this one isn’t correct. It’s the direction of the moon’s orbit around the Earth that makes the difference. Not the spinning of the Moon.
Yes, I was also confused by the point that rotation transfers momentum to the spacecraft. I also think it is because if spacecraft goes in the same direction as moon orbit around the earth, the moon can drag the spacecraft to a higher speed and the spacecraft can miss the moon.
A big +1 for How Apollo Flew to the Moon. Great book. I read it several years ago and have been reading it again recently. I've just got past the bit about how they crap in space.
Vintage Space, I'm sorry, but I think you are incorrect for the reason why they arrived on that side of the moon. The reason is not because the moon rotates around it's axis. The reason is because of the moon's orbit around the earth. In the picture, the moon's orbital velocity actually decelerates the craft in relation to the earth, meaning it will return to earth. If the craft arrived on the other side of the moon. the part of the moon velocity around the earth would be added to the craft. Since the moon is so massive, the change of the moon's velocity would be imperceptible.
Oh no you are one of them morons who thinks he knows more. Sorry she is correct here. She does her research and I am sure she has people she talks to that knows their stuff too. It seems you lack the understanding of basic physics. Please do us all a favor and do research before you post misinformation. Also it makes you look like a total jackass.
I believe Joe is right. The moon rotates about once every 28 days, that's why essentially the same side always faced the earth. It's the approach to the leading edge of the moon that slows down the spacecraft.
Anyone who's played KSP knows Joe is right and also knows these drawings are a strange perception of a complex flight path (where the loop around the Moon is in relation to the Moon, not to the Earth, and the loop around the Earth is in relation to the Earth).
RetroGuy123 - No, Joe is correct. The Moon rotating on its axis has nothing to do with it. I think she just worded it poorly. They approach the leading hemisphere because that is the side facing the direction that the moon is orbiting around (kinda like rotating) the Earth. Something else to keep in mind, everyone, and I mean everyone makes errors. RU-vid presenters are no different. If you can sift through the garbage in the comments you can find people who appreciate accuracy attempting to bring those errors to light. Joe did so in a respectful manner, he does not deserve to be called a moron.
I'm like 90% sure gravity assists have nothing to do with rotation and everything to do with orbits... The moon just happens to rotate in the same direction that it orbits but if it had a retrograde rotation this launch profile wouldn't change.
It has everything to do with mass, and the trajectory of the object in question...what is the orbit of the moon, at the time? And so on. Orbit is not consequential.
Mike Parker It depends on which direction the body is orbiting and where you are relative to that orbital motion, so the orbit is not insignificant. If you're ahead of a body in it's orbit you are slowed down relative to that orbit, if you're being the body in it's orbit you get a boost.
I agree but, just to make it clear, Earth's rotation (relative to Moon's orbit) is still relevant to get a figure 8 in a context of maximum fuel savings.
A. V. Sure, obviously a retrograde launch off of Earth would be ridiculously fuel heavy, and a retrograde landing would either be fuel intensive or have a really hard re-entry.
Very interesting Amy. I was just having a similar conversation today, but in relation to the sun...I’m not sure we got to the bottom of things. Anyway, there’s one thing you touched on that I’m wondering about: I don’t believe that the rotation of the moon about its axis has an impact of the energy gain or loss. With no atmosphere, the moon’s rotation doesn’t change the orbit of the spacecraft (to first order at least). I believe it has more to do with the direction the moon orbits around the Earth. If flying by the trailing edge then the moon is in front of the s/c on approach giving the s/c a pull in the direction of its motion, speeding it up. If flying by the leading edge then the gravity tug is pulling the s/c back, away from the direction of motion, slowing it down. I can only think of three ways in which the rotation of a body could affect a s/c orbit in this scenario: 1. Atmospheric drag. 2. Non uniform mass distribution. 3. General relativity effects. I think 2 and 3 are in the noise for a typical body and 1 only matters if there is a rotating atmosphere of sufficient density and height. What do you and others think? Cheers.
Your take seems right to me, I would only have added an explanation as to why 'speeding it up / slowing it down' would make a difference. I think the size of the entry window for a 'free return trajectory' would be shrink when sped up, and be larger when slowed down. While a correction burn might make the difference negligible NASA might have been addressing possible 'failure modes' for minimal thrust free return. As my math is no longer up to disproving my own speculation I'll accept someone else's opinion. And especially; Thanks for all the videos.
As other people are pointing out, the majority of the assist comes from the body's orbital velocity and alteration of exit trajectory for the object, not the body's rotation. For rotation to be a factor, it would have to be a tidal effect, which would be small in comparison.
I agree with some of the comments. As much as I appreciate Amy and her programs, she is mistaken in her explanation, which is a rare case for her. The direction of spin of the moon on its axis is of no importance in celestial mechanics; rather, the direction that the moon is moving in relation to earth is what is important.
This should be fun! I can visualize her right now, a phone in one hand and open books everywhere. I sense a part two of this video coming soon. Either way, I enjoy each and every vid she posts.
Got some funky synchronicity there. You've upped your game on the graphics for sure! To quote a album lyric... "There is no dark side of the moon, actually it is all dark." Crazy 8's!
Awesome, well explained videos. You have a good gig going with this RU-vid channel. Keep up the hard work ... With Space X growing as fast as they are you’ll have a career at doing this.
Sorry Amy, but it is a mistake to say that this is because of rotation. Clearly the ship doesn't "sense" the body's rotation. Others have explained it one way. Here's another way to look at it. Energy can be neither created nor destroyed. The total energy must remain constant, therefore if the planetary body loses energy, then the ship or probe must gain energy, and vice versa. So if the ship passes behind the orbital motion of the body, then body is moving away from the ship, therefore gravity must slow the body down. So if the body loses kinetic energy the ship must gain an equal amount. Now that is an equal amount of energy, not an equal amount of speed. The same amount of energy gained by the ship would make it go much faster, while the change to the speed of the body is negligible. In the case of Voyager passing behind Jupiter, the orbit of Jupiter slowed about 1 inch in a billion years as I recall. So if you intend to enter orbit and not get a slingshot effect, then you pass in front of the moving body. The body accelerates and the ship slows, so the ship's highly eccentric orbit becomes more circular.
After some reading of comments and thinking, you can't land on the moon from the trailing side (behind in orbital terms), as you would have to burn precious fuel to "catch" the moon, which is constantly receding from you. It would be a nightmare. From the leading side (ahead of the moon), it catches up with the spacecraft with no expenditure of energy on the spacecraft's part, except to slow enough to drop into orbit. So the figure 8 is an absolute must, regardless of the moon's rotation. However, if Gravity Probe B means anything, there might be a 0.0000000000000001% advantage to taking the leading hemisphere side, but that had nothing to do with Apollo's trajectory. Love your vids, Amy!
I'm sorry,but I think that the moon's far side can never be seen from Earth, thus the figure 8 has almost no relation to the lunar axis of rotation. In addition to all that, the vehicle can still use the oval type trajectory as it is still quite efficient though. It is like aircraft, when taking off, they fly along with the wind, helping with acceleration instead of slowing down and being less efficient, which is only used for landings. One thing I am unsure of is why the Apollo Direct Ascent used a different trajectory though. Just hoping you could clarify. Thank you. Appreciate if you can give a reply.
If it is about spin then surely they would have returned to Earth on the Earth's trailing hemisphere, not the leading hemisphere that the "figure 8" diagram shows? This gives more credence to the many comments that it is not the spin of the large bodies that determined which direction they orbited them, but actually it was the orbital trajectory of the moon (and, upon return, possibly earth) that determined where they directed their craft.
Yep. The only way rotation plays in is with a massive, quickly rotating body (a black hole or neutron star, e.g.) by which one might make use of the "frame dragging" phenomenon. That said, Earth isn't nearly massive enough for that to make a difference, let alone the Moon. =)
As others have pointed out, the gravity assist (whether used to speed up or slow down) has nothing to do with a gravitational body's direction of rotation, and everything to do with the direction it orbits its parent. But there is another related correction. The "leading hemisphere" is the half of the body on the side of the body's own orbital direction around its parent. Trailing hemisphere is the other side away from direction of travel. The energy extracted from the gravitational body is the same whether it passes behind the trailing hemisphere or in front of the leading hemisphere, but the direction the spacecraft is traveling after its orbit bends around the body either makes it go faster relative to it's original earth to moon trans orbital path, or slower relative to the same path. In both cases the velocity relative to the moon is high enough to exceed the moon's escape velocity, but one direction is faster in the earth's frame of reference, the other slower. It's also possible to produce a retro gravitational boost by going around the trailing hemisphere side, but it requires getting much closer to the gravitational body, such that the spacecraft makes a roughly 270 degree turn (accelerating the whole time) before it's ejected in a retro direction. In this case it might lose so much velocity relative to earth that it's new highly elliptical orbit intersects earth itself. This is how you could basically drop something straight "down" toward earth. BTW, you can simulate the slingshot effect with a couple disc magnets on a flat surface (oriented so they want to stick edge to edge). Quickly slide one magnet past the other at a distance where it's visibly attracted to, but moving fast enough that it won't quite have time to stick. If you make a very fast curved path around the initially stationary magnet, you may be able to slingshot it quite some distance. Using a couple of rare earth magnets, I can make a fast pass, and fling the other magnet over a meter across the floor, without making any contact.
Guys, seeing lots of people with varying levels of confusion and issues with this video. Perhaps the companion blog post will clear it up. Namely: It's direction of the Moon's travel that's important here. I tried to show it with animation but I think my trying to keep the hemispheres/edges clear for you guys referencing rotation too much made it more confusing. This should help! blogs.discovermagazine.com/vintagespace/2018/04/21/why-apollo-flew-in-a-figure-8/#.WtuEmC-ZNTY
There is a really nice mobile game that could help on that one. It's called spaceflight simulator. It is basically a (2D) KSP for broke people like myself. I never thought on which relative side of the moon it would save more fuel, but I guess getting in front of the moon's path would certainly help
I'm sorry but the rotation of the Moon had nothing to do with gravity assist, only the direction of orbit matters. It just so happened that the rotation of the Moon is in such a way that passing today the leading hemisphere would mean going against the orbit of the moon around Earth, which is what we want in order for the spacecraft to lose momentum with respect to Earth.
You said - "(To clarify because I didn’t make it totally clear in the video above: it’s the spin and the direction of travel that matters here.)", again you are referencing "spin" as being an important factor in the gravity assist. It is not. The spin of the Earth is important because rockets launched in the direction of spin get some free velocity and therefore determine which way the orbit goes, but the spin of the Moon is completely irrelevant. If the Moon were to spin the opposite direction nothing about the figure 8 "free return trajectory" would change.
Yeah but Apollo 8 and others ignited the SPS on the far side in order to drop out of the free return trajectory and enter lunar orbit, so the only craft which really stayed on the free return trajectory the whole was the apollo 13 odyssey-aquarius.
AbleArcher Not quite... They (Apollo 13) went off the free return (for Fra Mauro) before the SM exploded. The first DPS engine burn was *to put them back on a free return* trajectory. Thus, 13 was the only one to slingshot around the Moon without going into lunar orbit.
I’m reading Apollo 13 (by Jim Lovell) and I’ve watched the 13 press conference and in the book and conference, he says they burned off of a free return trajectory. In the book, he specified that it would be necessary because of the shadows to make decent safer as terrain hazards would be more distinguishable. Also, in the book he said that only 11 and 12 were set up in a free return trajectory as they were still the missions NASA was playing it safe with.
Good video as always, but one thing sort of caught my attention. Isn't the Moon tidally locked with the Earth, therefore not spinning? I thought the same side always faces us hence the "far side of the moon" was largely unexplored.
Schuffert Family Life Anything tidally locked will rotate compared to a fixed frame of reference, but it does not rotate when compared to the reference frame of the Earth as illustrated in the animation.
Yes. This video is well presented, but kind of bogus in it's explanation. The moon IS tidally locked to the earth so it's rotation is not important in the way she described it. When a spacecraft approaches on the 'leading edge', in front of the moon - the moons gravity draws it backward. It slows the object down. If the spacecraft approaches the moon from behind the moons path then it falls towards the moon while the moon is moving away from it, and it can slingshot. The rotation has nothing to do with it. I might be wrong.. but I think she is talking crap on this one.
not to mention that you are "mapping" (representing) three MOVING objects in FOUR dimensions, on a two dimensional plane and (as mentioned by Amy) factoring in rotation and gravity "The shortest distance between two points is a straight line" only works on a flat plane in two dimentions
I thought that you receive a gravity assist by aiming to pass the body at the trailing edge because you have more time to be accelerated by it as you approach, as you spend more time in its gravitational field during approach this way, and less time to be decelerated by it as you depart. The opposite effect happens if you aim for the leading edge. This was my understanding of gravity assist or braking.
i don't know if you got the explanation part correct, but the animation made it pretty clear (for me at least). it seems pretty self-explanatory to me that going against the moon's rotation will cause the "figure-8" motion providing a free return path to earth. going the other direction, the craft would get the same delta v, but added to the velocity, instead of deduced from it, causing a bigger elliptical orbit around the earth. the delta v from gravity assist isn't of huge importance here, it's about the two orbits resulting from the two different flight paths. i think that's what most people aren't happy about in the comments.
Another advantage of the trajectory if you're actually going to land. By going in a "retrograde" orbit around the Moon, it means when you land, the sun is at your back. With the low sun angle, it makes the shadows sharp and easy to see, but it also means you're not trying to land while looking into the sun.
As a "science educator" accuracy should be the most important. Please remove this video, it is wrong and gives misleading information as several of the comments have pointed out.
Great video. Just got my Patreon package. Love the channel. Can you possibly answer what tasks or why chimps were used for tests? Were they required to do anything in flight?
At around 2:30 the physics in this video is wrong. What matters is if the spacecraft approaches the moon from behind as the moon moves on its orbit around the earth, in which case it gets a gravity assist and accelerates, or if it comes out in front of the moon as Apollo did in which case it decelerates making the moon landing or return to earth easier. Ether way the effect is due to the relative positions of the spaceship and the moon as they move around the earth. The spin of the moon around its own axis is irrelevant. Your explanatory diagram should show how the spaceship approaches a moon that moves, not a moon that spins.
Why does it matter which side of the moon the spacecraft approaches? I would've thought mass is mass and gravity is gravity. If the craft isn't in contact, why does it slow down more one way vs the other? Using the trampoline example that so many use to visualize gravity, I can see why it might matter the path taken through the gravitational field but what does the spin of the moon have to do with it?
It doesn't. The moon's rotation has little to no effect. What does have an effect is the motion of the moon itself as it's in orbit around the Earth. For this discussion, I'll use the terms "leading edge" to represent the part of the moon in front as the moon goes around its orbit and trailing edge as that part on the opposite side of the moon. So, we have the spacecraft approaching the moon and due to gravitational acceleration, the spacecraft increases its speed relative to the moon. How does this increase in speed relative to the moon affect the spacecraft speed relative to the Earth? The first thing to take note of is that no matter how the spacecraft is approaching the moon, as long as the trajectory doesn't impact the moon, the spacecraft will NOT go into orbit unless a burn is performed to adjust its velocity. Now if the approach is across the trailing side of the moon, the acceleration caused by the gravity will cause the spacecraft to go faster relative to the earth. Think of it as the moon "dragging" the spacecraft along behind it. If however, the approach is across the leading side of the moon, the acceleration caused by gravity will cause the spacecraft to go slower relative to the Earth. After all, the path the spacecraft is going is in the opposite direction as the path the moon is going, and the moon will attempt to drag the spacecraft slowing it down relative to the Earth. If you're really interested in understanding a lot about orbital mechanics such as this, you could do far worse than purchasing Kerbal Space program and try a few landings. Trust me, you'll quickly see what's going on.
The way I understand it, if you aim to intersect the moons orbit at the trailing edge, you will have a greater interaction with the moons gravitational field as you approach, because you spend more time in it's field prior to intersecting its orbit. Once you pass its orbit at the trailing edge, the moon is moving away from you so you spend less time in its gravitational field and there is less time to be decelerated by it. Due to this, you have a higher velocity after your interaction with the moons gravity than you did before. Vice versa for aiming at its leading edge during approach.
Chris Kostokanellis There's an actual exchange of momentum. Spacecraft can steal momentum from planets and moons but approaching the body's trailing side. The planet/ moon slows down and the spacecraft speeds up. Amy got this one wrong.
I just love that I understand a lot more about trajectories and acceleration from KSP and The Expanse. Hey Amy, you might have said in another video, but do you watch The Expanse? And if so, what is your opinion of it?
fun fact: Apollo 13 was not aiming for a free return trajectory because of the landing site requirements and photography of the dark side of the moon, so when things go haywire they have to correct their trajectory into a free return one
I think you might slightly misunderstand what is actually going on here. Prior to Apollo 12, lunar missions (including orbit only missions) were put onto a circumlunar free return trajectory on the translunar injection burn. In other words, immediately upon leaving Earth for the moon, the trajectory after that burn would circle around the moon and return them to Earth without any further maneuvers. This method actually restricted where they could land on the moon. You say Apollo 13 was not put on a free return because of landing restrictions, but in reality - the trajectory they were put on was to AVOID the landing restrictions that came with a circumlunar free return trajectory. Apollo 12 and 13 were still put on a free return trajectory, but just a different type. Instead of being put on a trajectory that circled the moon and then return to Earth. They were put on a highly elliptal Earth orbit that came JUST short of reaching the point where the moon's gravity would take over. If they did nothing else, they would simply follow that highly elliptical orbit and return to Earth - having never quite reached the moon. After systems checks and jettisoning the S-IVB and docking/extracting the lunar module and moving safely away from the S-IVB, a midcourse maneuver was performed to put them on a translunar trajectory that was not a free return. So, it was sort of a hybrid. They were free return until safety checks, then they were put on course for a landing site that would have not been available on a regular circumlunar free return. Obviously the explosion on Apollo 13 occurred after the midcourse burn taking them off the elliptical Earth orbit and into a non-free return translunar one, so they did indeed have to be put back onto a free return.
I haven't watched the video yet, but the first few seconds have Luna's 'dark side' spinning toward then away from Earth. I already feel I'm missing something.
The animation of the moon spinning is highly exaggerated and technically incorrect , a little more accuracy by Amy in that depiction would have made more sense . The moon does spin on its axis though - orbits earth every 27.22 days and rotates on its axis approx. every 27 days , however observed from Earth it does not appear to be turning due to being locked in a synchronous orbit , so hence we only see one side of the moon from Earth ,never the dark side 🙂
It’s the direction of the orbit not the rotation. That’s why Voyager 2 was still able to use a gravity assist off of Uranus even though it has retrograde rotation and a nearly perpendicular axis of rotation. The only time the rotation of the body matters is when launching from or landing on it. Since both launch and landing takes place against the moon’s rotation, they would require more fuel than going with the rotation. But the very slow rate of rotation, the light mass of the lunar module, and the weaknesses of the moons gravity made it inconsequential in comparison to the fuel required to get the CSM and LM together into a lunar orbit from the other direction.
Great video Amy, got an Apollo based question, all footage of Apollo and later EVA suites have the helmet staying in one position with the head moving inside, however looking at footage from Gemini missions and even Ed White's first EVA show the helmet freely moving, even looking at the camera, surely this is a design that seems sound so how, why did they change the suites for a ridged design?
This seems to imply that an orbit in a direction opposite to planetary (or lunar) rotation is impossible because going against the rotation would always slow down the vehicle. This implies gravity rotates. Is that true?
I can't believe the number of comments from viewers who want to dispute her premise. She is absolutely correct. The direction of spin has an effect whether the acceleration is positive or negative. Grab a pencil and work it out. I did many years ago in college. It was difficult at first, but easy once you grasp the mechanics of navigating space. Retrograde orbits are rare and this is why.
Earth's rotational motion certainly does give a big boost to rockets launching from our rotating planet to the east (thus the rarity of retrograde orbits), but that is not the same thing as a gravitational assist maneuver which instead has everything to do with a spacefcraft's trajectory with respect to a body's orbital motion (its revolution), not its rotational motion. Conflating the two may have been the source of Ms. Teitel confused explanation.
It's amazing how many people don't have a solid grasp of physics and think they can do some sideways math and their head to assume they can disprove actual physics.. like I'm such a percent sure. Really means you don't know but you think you do. It's a good thing all these people that are know-it-alls about nothing aren't actually in charge of anything important.
You are totally awesome. I'm surprised you didn't feature the Apollo 8 mission patch. Do you like Scotch? And isn't the Cosmosphere just the best museum ever? We should do dinner sometime, in a parallel universe where I'm the elderly sage teaching a young apprentice: instead, it would be an eager old man prompting, *what else can I learn from you??* So far: *LOTS!* I've commented before about some of the weirdness surrounding Apollo 13, but on older posts that you may not have personally seen. I'd love to see your 'put' on the NASA pickup truck that burned to the ground after the Wet Countdown Demonstration Test about April 9, 1970 or so. And the two lucky NASA security guards who *weren't* in the truck when a gentle breeze wafted ashore...
I can't believe anyone trying to post educational space videos would leave this up. At 1:00 you show moon rotating so as to allow the spin to be seen from earth. Yet--- we always see the same side! I know why, DO YOU??? And then you jabber about the moons rotation affecting orbital entrance of spacecraft and of course everyone is correct in telling you the rotation is irrelevant. Maybe you are confusing entering an orbit as opposed to launching from earth? The rotation of earth DOES effect the energy required to reach orbit. Launching WITH rotation adds to your speed so escape velocity is reached easier, launching AGAINST rotation would mean you have to first overcome the rotational speed. Assuming you have only grossly over stated moons rotation-- I guess the same effect means the LEM returning to Command Module from lunar surface means they are using slightly more energy to achieve orbit by launching against rotation.
The only reason why it appears like a figure 8 is because you are drawing it on a piece of paper where the moon doesn't move... In reality, the trajectory doesn't look like a figure 8 at all, it's a long eleptical oval that goes slightly higher than the moon, and while the spacecraft gets close to the top (farther than the moon) it slows down slower than the moon's speed in its orbit around the Earth... Letting the moon pass "under" it while continuing on it's oval eleptical trajectory back to Earth... You can learn about orbital mechanics by playing a great little game app called "Simple Rockets"
