Space station gives physics a boost

By Phil Plait | November 10, 2011 6:29 am

This is one of the coolest videos I’ve seen in a while: during a routine reboost of the International Space Station to a higher orbit, the astronauts on board show that the station tries to leave them behind!

What a fantastic example of Newtons’s First law: an object in motion tends to stay in motion unless acted upon by an outside force. As the ISS circles the Earth, all the forces on it are balanced. You can think of it this way: the force of gravity pulling it toward the Earth is balanced by the centrifugal force (or the centripetal acceleration, which is equivalent*) outward. Because there are no leftover forces on the ISS, it feels like it’s in free fall, what some people call weightlessness. No force means no acceleration which means no weight.

However, that’s not always the case. Even a few hundred kilometers up, there’s air. It’s thin, but over time it robs energy from the ISS, dropping it lower in its orbit. This is called drag, and it’s a very tiny force (too small to feel on board the ISS), but it adds up over time. To prevent the station from falling too far and burning up, every now and again low thrust rockets are used to push it up into a higher orbit.

But that applies a force that is not balanced! While the rocket is firing, the ISS feels a force upwards. The astronauts inside, though, aren’t connected to the station unless they hold on. So they don’t feel that force. The station moves upward while their bodies stay on their individual orbits around the Earth. It’s only when they hit the back wall (or grab onto a handhold) that they feel the force from the rocket.

The camera is mounted on a bulkhead at the end of the ISS opposite the rocket, looking back "down" toward it. When the rocket fires, from the camera’s point of view the astronauts move away, "downward". When they reach the end of that corridor, they grab on, climb back up, and fall again.

Think of it like this: imagine you’re in a car, with a camera on the dashboard facing you. When you hit the gas and accelerate forward, you feel like you’re being pushed back in your seat. What’s really happening is that the engine is pushing the car forward, leaving you behind. The seat then pushes on you, accelerating you along with the car. If the seat weren’t there, you’d fall to the back of the car — just like the astronauts on the ISS!

On the ISS, a rocket does the accelerating. On Earth, gravity does that job when you drop something. But on Earth, it would fall much more rapidly — 5 meters (about 16 feet) in the first second, 15 meters in the second, 25 in the third, and so on. But the rocket is far more gentle than Earth’s gravity; judging by the video, I initially guessed it was less than 1% of Earth’s gravity. I think that’s about right: if this was the reboost they did on October 26, then they accelerated at about 0.02 meters per second per second, which is 1/500 of Earth’s gravity. Not much, but enough to see!

And enough to move the station. The thrust was held for nearly two minutes, which was enough to boost the ISS about 3.2 km (2 miles) higher than it was. Not a huge amount, but enough to extend its life for quite some time.

Physics! It’s not only cool, it can also save your $100 billion science project.

Tip o’ spacesuit visor the Burt Humburg. Image and video credit: NASA.


* Yes, they are equivalent. Just to be sure, I’ll link to this again. If you don’t read that before leaving a comment telling me there’s no such thing as centrifugal force, then you are granting me tacit permission to make fun of you.


For you math and physics dorks out there, they changed their velocity by about 1.8 meters per second over 114 seconds. That’s an acceleration of 1.8/114 = 0.016 m/sec/sec. Earth’s gravity is an acceleration of 9.8 m/sec/sec, so the ratio is about 0.0016. I rounded a bit in the numbers in the main text.

CATEGORIZED UNDER: Cool stuff, NASA, Science

Comments (62)

  1. Rick

    They look like they’re having so much fun! Big kids in a $100 billion dollar tree house in space.

  2. Chris P

    “Whatever your gravity is when you get to the door, remember — the enemy’s gate is down.”

  3. Chris

    To see how often and the amount of boost they need look here

  4. Timmy

    It would be a lot cheaper if they just dangled the ISS from a rope off the back of the giant space turtle. Then we’d at least know what sex the turle is.

  5. Timmy

    @Chris P: Ender’s Game?

    We’re a literate bunch, aren’t we! :)

  6. Dr.Sid

    I wonder .. how do they change height with only one burn ? IMHO they should use two .. one in perigee (or anywhere if their orbit is circular) to make orbit elliptical, with apogee at desired height .. and second burn in apogee, to make orbit circular again.

  7. Ross Cunniff

    It demonstrates one of the principles of relativity as well – they feel like *they* are accelerating to the other end as the ISS is boosted (the ISS being Einstein’s “space elevator” in his thought experiment) – http://en.wikipedia.org/wiki/General_relativity#Geometry_of_Newtonian_gravity

  8. ragnar

    ” On Earth, gravity does that job when you drop something. But on Earth, it would fall much more rapidly — 5 meters (about 16 feet) in the first second, 15 meters in the second, 25 in the third, and so on.”

    Did I miss the biggest science story of the century? When did acceleration due to gravity on Earth change from 9.8 m/sec to 5 m/sec? Was it global warming that did it?

  9. Kevin

    Falling really slowly is probably an even stranger sensation than weightlessness. Cool video!

  10. I wonder if they can propel themselves by blowing.

  11. The Raptor

    So, long ago I had a teacher pose a question to the class. What would happen to a fly in a moving car? Why don’t they get slammed into the window at 70 mph? The answer was the fly is already accelerating at 70 mph. Which is why if you jump or drop something in a car, you don’t end up somewhere else.

    However… with what you say, that if we were floating in our car, and the direction or speed of travel changed… then we would get tossed to the back. But would we? Or are there other forces at work that would over come that and would keep us from ending up smashed on the window?

    This is something I’ve never seen posed as a question. Probably because the math on it would be.. interesting.. but just as a doing experiment, it would be pretty simple.

    But instead of jumping, use an RC helicopter. Would it get smashed into the glass at every turn, or would it remain in place? (I also understand the air acts as a media for flying objects and because its touching the car, it gets turned and not slammed into a wall.) Or even drop something while turning that has some weight to it. Does it end up somewhere else or did it hit the ground right where you dropped it.

    I do notice that when I’m in a car and make a turn, that the direction of flow of the AC changes ever so slightly.

    So… I suppose the question becomes. What would happen if you jumped while in a fast moving, but enclosed container that was on the ground… as it made a turn. Would you get moved to the side? And why not if not? My guess is that gravity comes into play because you’re on the ground… something being negated on the space station.

    (Should the answer to the above be, you wouldn’t get moved… then it makes me wonder what would happen if the container was traveling close to the speed of light.. and you jumped, while it turned… would that be enough speed let the container pass under you?)

  12. ozprof

    @8

    Noel

    Maybe that is why no beans on the ISS!

  13. astronomynut

    @ Raptor: an interesting experiment for you to try is to have a helium filled balloon in your car at some speed. The balloon will remain straight up and down until the car changes speed. If the car slows down quickly, the balloon will travel towards the back of the car because the air inside the cabin does not feel the deceleration and continues forward. As the air in the cabin “piles up” at the front windshield, the balloon will travel back.

  14. Henry

    I’m afraid I don’t like your explanation of weightlessness very much!
    If a spaceship was just falling towards the Earth (no “centrifugal force”) the occupants would still be in free fall, and would be just as weightless as the occupants of an orbiting spacecraft.

  15. tjm220

    “But on Earth, it would fall much more rapidly — 5 meters (about 16 feet) in the first second, 15 meters in the second, 25 in the third, and so on.”

    Wouldn’t it fall 9.8m in the first second?

  16. Blargh

    tjm220:

    Wouldn’t it fall 9.8m in the first second?

    The acceleration is 9.8 m/s^2 (i.e. you gain 9.8 meters/second per second). But that doesn’t mean that you fall 9.8 m in the first second. Now why is that?

    I’m sure we can agree that to move 9.8 meters in one second, you need to have an average speed of 9.8 m/s, right? (That’s the definition: distance traveled per time taken)
    But remember: you start off at 0 m/s! So it takes you one full second to reach that speed – the whole time before that second is up, you’re traveling at a slower speed – so during that second, you’ll travel a distance less than 9.8 m.

    (You can try doing the math yourself – in the end, you should end up with the equation for a falling body from rest: distance traveled, d = 1/2 * g * t^2)

  17. drow

    @13 nope, S = 1/2 a t^2, so at t = 1 second, it’s velocity may be 9.8 m/s, but it’s only travelled half that, because it’s accelerating smoothly from a dead stop, not moving at a constant velocity over that 1 second.

  18. Bob

    I want their job. You get to float and don’t have to wear shoes. Awesome.

  19. Tim G

    Here’s another video of a boost. The rocket(s) fire soon after the 3:20 mark.

    I’ve seen a similar video that takes place inside a space shuttle and the acceleration was noticably greater. I’m trying to find that video now.

  20. VMA131Marine

    @ Dr Sid

    The orbital maneuver you describe is a classic Hohman transfer, which assumes that the duration of the burns is << than the total orbit transfer time. This solution doesn't work for low thrust applications when thrust is applied throughout the transfer. The shape of the final orbit must be adjusted by varying the direction of the thrust vector during the transfer.

  21. Renee

    Obviously, their inertial dampeners are on the fritz.

  22. Dragonchild

    “When you hit the gas and accelerate forward, you feel like you’re being pushed back in your seat. What’s really happening is that the engine is pushing the car forward, leaving you behind. The seat then pushes on you, accelerating you along with the car. If the seat weren’t there, you’d fall to the back of the car — just like the astronauts on the ISS!”

    IF the seat weren’t there, my gravity-bound arse! This actually happened to me. I was flying into a convention to work as a translator, but the driver picking me up at the airport was running behind on a VIP pick-up. He was a friend of mine, so as soon as he pulled up he wasn’t shy about telling me to get the ____ in and let’s go. I threw my stuff in and quickly sat. . . on the back of a minivan seat that happened to be folded down. The driver floored it, and with the seat back UNDER me instead of behind I literally went tumbling, heels over head, to the back of the van.

  23. Smitty

    @ Chris P

    Darn it, you beat me to it. As they started to “drift backwards” relative to the POV of the camera, that’s the first thing that came to mind.

    I wonder what fraction of percent of G acceleration that is.

    I was also reminded of a learning experiment that was filmed on the old Skylab where the Astronaut placed a bar magnet to float in front of him to demonstrate how it re oriented itself to the Earth’s magnetic field.

  24. ND

    “The answer was the fly is already accelerating at 70 mph.”

    70mph is the speed not acceleration. I assume you’re saying the car is moving at a steady 70mph and not accelerating.

    For your first scenario with the car and the fly, what’s more interesting is what happens to the fly when it’s in the air in the car at full stop, and the car takes off. What happens to the fly when it’s not physically connected to the car (not resting on the dashboard for example), and the car starts accelerating?

    About your second to last paragraph, it’s not very clear, but gravity is always at play whether you’re’ in a turning car, on an RC copter, or in a space station orbiting the Earth.

    One thing to take into account in the car situation, to make the car turn, the front wheels impart some amount of force at angle to the car’s forward moving direction. When you drop something in the middle of that turn, that ball you were holding no longer feels the force pushing the car in a new direction. It will continue direction it was moving at the moment it lost contact with your hand. We’re assuming the air can be ignored here because the object is a dense and with some mass to it.

    Also think about the rotating, circular space station from 2001 Space Odyssey.

  25. 7. ragnar:

    Don’t be so cheeky. It’s 9.8 m/s/s, so you’re not going to have moved 9.8 M after the first second, but rather will be traveling at 9.8 m/s after the first second.

  26. BTW, #2 Chris P is now my favorite person.

  27. Chris

    tjm220 “Wouldn’t it fall 9.8m in the first second?”

    No. It’s starting from zero and it takes time to reach 9.8 m/s2 (1 second in fact) so in that first second the drop is just under 5 meters.

  28. The Raptor

    astronomynut : Yeah, balloons. I did think about it, but removed it in my post, cause it got too long into the point. I’m more curious what would happen with a heavier object… but the balloon still shows some interesting physics going on… yet, it doesn’t end up slamming into the side of the car at 70 mph. I’m suspect it doesn’t because of the same situation with the fly. It’s sitting in a media and its weight isn’t all that great, but it has a lot of air grabbing it, keeping it mostly still.

    ND: As for speed vs acceleration.. yes. Speed is steady. I should have said the fly is already traveling at that speed. Though, when small insects are in the car, I do notice that they don’t appear to be having problems with acceleration (and I mean acceleration this time).. thus the whole, the air acting as a media that’s allowing them to accelerate/turn with the car. As above with the balloon.

    Yes, gravity is in play, but in low earth orbit, you’re in free fall, negating the obvious effects of gravity. Sorry to have written that in a confusing way… Let me try again. What I was trying to say was that, does the fact that gravity is acting on the car, the air, the person in a way it can be felt.. while in space, you’re in free fall.. have anything to do with why the people in the space station where able to experience the movement of the station? Vs here on the ground where gravity forces us to be firmly planted in the car..

    So yeap, that’s what I’m curious about is a heavy object. What happens to it really? I’ve dropped stuff in the car before and had it roll to other places because of the turn acting on it once it hit the floorboard… but I never noticed if the drop was straight.. or angled away from the direction of the turn.

    Hmm… Now I want a high speed camera to observe the true path of a dropped ball the car turned a corner. Or an RC helicopter and a large van. So I can fly it in the van. That would be neat.

  29. If this were a movie, a the end of that long “fall” someone would have sat on the ISS’s self-destruct button.

    I had a nice demonstration of this principle once when I was rear-ended at a traffic light. All the junk tucked into places on and around my dashboard – change, cassette tapes (this was in 1994 or so), and other stuff – came flying out towards me after I was hit. That wasn’t what happened, of course: it was staying more-or-less in the same place (dragged forward a bit by friction) while the car was pushed forward around it. I was strapped into my seat, so the seat pushed forward against me, but I was held in place relative to the seat (and the steering wheel, and the windshield.)

  30. DigitalAxis

    @11 Raptor:

    I would imagine you could test this… Standing in the aisle of a bus or a subway train as it’s accelerating forwards, you should be able to jump a lot farther backwards, than forwards. Why? Because in the brief time you’re airborne, you are not accelerating to match the speed of the bus/train.

    If you jump straight up, you’ll land behind where you were standing because the bus accelerated and moved underneath you.

    If you jump forward, it won’t look terribly impressive- your forward velocity is competing against the bus’s acceleration forwards.

    If you jump BACKWARDS, you’ll go a lot farther than you normally would, because the bus is moving the other way while you jump.

    Actually… I see this effect all the time on my commute in on the subway. It’s a lot harder to walk in the direction the train is moving than to walk against the train’s motion. It feels like walking forward is ‘uphill’ and backward is ‘downhill’. I try to not do that, because when the train stops accelerating, I lose my balance- walking forward no longer requires the same amount of effort.

  31. Infinitewell

    @Henry

    Excellent point. All true projectiles feel weightless, even the ones without any horizontal velocity!

    I don’t buy the need for a perfectly-matched second force, either. Someone will have to convince me, because with an elliptical orbit I’m pretty sure the station undergoes constant speed changes.

  32. Keydar

    But the forces aren’t balanced. In orbit you experience just one force, which is gravity pulling you inward. The only reason you feel weightless is because there is no contact force on you, counteracting gravity. There is no contact force because both you and the station are not accelerating relative to one another. No relative acceleration means no relative force.

    Centrifugal force is only a perceived force, there isn’t anything pushing you outward. Gravity is the inward centripetal force, which is why your tangential velocity is constantly bent toward earth (you are accelerating inward).

  33. Dr.Sid

    VMA131Marine: they say the burn lastes 114s .. that is a lot less then orbit time. Also in those 114s, while speed might be changed, height would not change any dramatically. Sure, if the burn lasted till you get to new desired altitude, vectoring should work. But it does not seem to be the case here.

  34. Cindy

    Phil,

    I am going to nitpick but it’s not the centripetal/centrifugal. It’s that you imply that force = acceleration. I know you know, but there are readers of your blog that may not fully understand Newton’s second law. Sorry, but I was just grading a bunch of physics quizzes where some of my students had force = acceleration which messed up their calculations.

    If you had written that centrifugal force and centripetal FORCE are equivalent, then I wouldn’t nitpick.

  35. Chris P

    @Timmy (#5)
    @ Smitty (#24)

    Yep, Ender’s Game. First thing I thought of too :D

  36. tony

    Very cool vid. Thanks Phil! I’ll have to show that to my Physical Science class. We’re covering Newton’s 3 laws next week.

    As a rough guess, it looks like it is about 6 meters from the starting point to the “door”. I timed that it took about 15 seconds for the astronaut to fall this distance. a=2deltaY/t^2 = 0.05 m/s^2, in rough agreement with the other accelerations mentioned here.

    Just a nitpic, but the ISS would not feel a force “upward”. They would be firing the engines to accelerate the ISS prograde, or parallel to the ground in the direction of their travel. Firing the engine for an “upward” force would only increase their eccentricity, not their semi-major axis. Their apogee would be raised and their perigee would be lowered, defeating the purpose of the burn. A strong enough upward burn might even put their perigee dangerously low.

  37. Tom Callahan

    @Chris P: Just finished listening to the audio book a few weeks ago, and while reading the comments I automatically read your comment in my head in the voice used in the audiobook!

  38. CR

    Re: Ender’s Game: I read the book years ago (and enjoyed it)… is the audio book worth a listen? Who reads it?

    Back on the ISS motion topic, I remember a few years ago seeing a video of a space shuttle doing some maneuver like that, only I think it was sideways. Direction aside, the crew floated in place while the shuttle moved around them until they were bumped by the cabin wall. (I had to re-type that last bit, thanks to my Eathbound frame of reference: I originally typed “…until they bumped into the cabin wall.” Heh, even though I knew better, I still referred to how it LOOKED relative to the fixed camera.)

  39. RobT

    @ The Raptor -

    There is an easy way to test inertia in your car. Put an object that slides along the dash easily then make a sudden change in direction or speed. The object will move in the opposite direction of the change.

    Also, every car passenger has experienced it whenever the car undergoes heavy breaking. The main thing keeping in your seat then is the seat belt.

    To see about floating objects, tie a heavy object to a string and hang it from the rear view mirror. Change direction or speed again and the object will swing in the opposite direction. Either way it is inertia at work – the object will want to stay moving in the direction it was.

    Another thing affecting how a balloon might behave in a moving car is the air resistance acting on the balloon. A balloon has quite a low density due to its large surface area. Since inertia is dependent on mass the low density of the balloon requires less of a force to change direction so air resistance becomes more of a factor.

  40. The article is great, the video sounds awesome; but too bad it is in some format (probably Flash) that doesn’t play on my computer. If the video is on YouTube and you include a link, I might have been able to watch it there, because chances are it is on youtube in HTML5 – which *does* play on everything (as does OGG-Theora)

    Good article anyway.

  41. Chris P

    @Tom Callahan

    wonderful!

  42. molybdenumfist

    Would the rocket be firing upwards in the same sense we on the ground think of it (away from the centre of the Earth), or forwards – increasing the speed of the ISS to raise its orbit?

  43. I make it 4.9m in the first second, 19.6 in the second and 44.1 in the third, or, using the rough rounding that Phil uses, 5, 20 and 44.

  44. Tim G

    molybdenumfist,

    Thrust should be forward ( in the direction of the motion of the ISS ), not up ( away from the Earth ).

    If the ISS were in a circular orbit and you want to raise it to a higher circular orbit, then ideally, you would give it two separate “kicks”. The first kick would raise the high point of the orbit while keeping the low point of the orbit the same as the height of the original circular orbit. The high point of the orbit ( apogee ) would be opposite the location of the first kick, now the low point of the orbit ( perigee ). When the ISS reaches apogee of the new elliptical orbit, the second kick comes in to raise the perigee of the orbit to be equal to the high point.

    However, the orbit may not be very circular and the thrust perhaps isn’t enough to provide a kick and must be sustained for a significant amount of time.

  45. Fun fact:

    The orbital maneuver Tim G just described is called a “Hohmann Transfer.”

  46. TheRaptor

    DigitalAxis – very interesting. There are no trains/subways/buses where I live.. why I asked here to see what other people have experienced. What experience I do have is limited and brief.

    RobT – which is totally, totally cool. And very correct. Which is also why the things I drop end up under the seat. Now that I’m awake, I can say we know the consequences of this.. a sudden stop sends things out windows (people included). Why seat belts are a good idea.

    Tying an object off by a string would be fun. Good idea!

    I would think all of this also mean that in any sci-fi/fantasy movie where they have the object floating in the middle of the space craft because it levitates/flies. Or the area in the middle of the ship where gravity doesn’t work and they go float around in it… reality dictates that things should really be in the wall any time the ship turns or accelerates. Unless it has a counter force to it, like being tied off or firmly bolted into the wall like at the space station. (or, inertial dampeners).

    That has some implications.

    Ty guys!

  47. I always wondered what astronauts did when they reboosted the ISS, whether they had to strap down or what.

  48. Great clip – educational and fun. :-)

    Reminds me somewhat of the “feather and hammer drop” Galilean experiment performed on the Moon by the Apollo 15 Commander David Scott.

    (Click on my name to see that.)

    @49. TheRaptor : I believ the Mythbusters did an episode on unsecured items in cars and the havoc they can wreck too! ;-)

  49. TheRaptor

    Ahh, I haven’t seen it yet. I will have to look for it!

  50. Tony, even a radial burn adds energy to the system, so the period (not only the eccentricity) must increase.

  51. Peter B

    Anton Sherwood @ #53 said: “…even a radial burn adds energy to the system, so the period (not only the eccentricity) must increase.”

    Are you sure? It’s just that a prograde burn would add energy and raise apogee, and therefore a retrograde burn would do the opposite. I assumed a radial burn would be half way between the two, and thus have no effect on total energy (and thus period).

    Fortunately, I don’t do this for my job. ;-)

  52. Peter B

    Anyway, one thing about the burn which fascinates me is that it’s an example of a form of propulsion which doesn’t really work on the surface of the Earth – low thrust used to move large mass. It works in space due to the lack of friction.

  53. Peter B

    RobT @ #42 said: “There is an easy way to test inertia in your car. Put an object that slides along the dash easily then make a sudden change in direction or speed. The object will move in the opposite direction of the change.”

    There’s an ad which appears occasionally on TV which I suspect is from the UK (in earlier screenings of the ad the two women spoke with English accents, but more recently their accents were Aussie) which illustrates this. Two women are in a car, and there’s an open packet of spherical chocolates (ah, can’t remember their name – I suppose it doesn’t matter) on the dashboard. The passenger is navigating for the driver, and constantly getting her to turn right. Each time the car turns another chocolate rolls out of the packet and down to the passenger…

  54. ASFalcon13

    Phil, I know you’re smarter than this.

    “As the ISS circles the Earth, all the forces on it are balanced.”

    Umm…no, they’re not. Holy Haleakalā man, this is Physics 101. If they were balanced, then the station would be traveling in a straight line. This is a direct consequence of Newton’s laws: and object will maintain the same momentum unless acted upon by a net external force. Since the station’s trajectory is curved into an orbit (and, thus, its momentum is changing continuously), it’s clear that the forces aren’t balanced in any way, shape, or form – there’s undoubtedly a net external force at work.

    “But,” you say, “centrifugal force is opposing gravity!” To which I say (and I know you’re going to kick and scream about this) there is no such thing as centrifugal “force”. It’s an artifact of choosing a non-inertial reference frame (in other words, a frame that’s accelerating and rotating along with the station). Sure, non-inertial frames are nice sometimes – in this case, the ISS is in a constant position in its LVLH reference frame, because it’s defined as such – but Newton’s laws don’t apply in a non-inertial frame. Because of this, you can’t choose to work in a non-inertial reference frame but then turn around and try to use Newton’s 1st and 2nd laws (F=dp/dt) in that frame to invent forces out of thin air…they don’t apply, remember?

    To actually be able to do physics in a non-inertial reference frame, you have to do some work to figure out what the laws are that actually do apply to that frame…but, to do that, you end up having to apply Newton’s laws to figure out how the frame itself is rotating and accelerating relative to an inertial frame! “Centrifugal force” is a mathematical artifact of this process, nothing more.

    Want to convince yourself that centrifugal force doesn’t exist? Newton’s third law, buddy: if centrifugal force is acting on the station, then the station must be exerting an equal and opposite force on something else. Where is that equal and opposite force?

    “…every now and again low thrust rockets are used to push it up into a higher orbit. But that applies a force that is not balanced! ”

    No, this isn’t it at all. We’ve already established that the forces were unbalanced before the rocket ever fired. The rocket isn’t suddenly “unbalancing” anything.

    To figure out what’s actually going on here, let’s think about what forces are working on each object…

    “Because there are no leftover forces on the ISS, it feels like it’s in free fall, what some people call weightlessness.”

    I’m not even sure what the heck “leftover forces” is even supposed to mean, but, before the rockets fire, the ISS doesn’t just feel like it’s in free fall, it ***is*** in free fall…and so are the astronauts. Gravity works on all of them, and accelerates all of them equally (think back to the hammer and feather experiment on Apollo 15). There are other forces working on the station only – drag, solar pressure, etc. – and, while these forces are significant in the long term (as you’ve pointed out with the need for reboost), they’re insignificant enough compared to gravity in the short term that we can ignore them for this analysis. So, before the rocket fires, gravity is working on both the ISS and astronaut, so the astronaut experiences no acceleration relative to the ISS. Check.

    Then the rocket fires. In this case, if the astronaut isn’t hanging on to the station, the rocket is applying a force to the ISS only. The astronaut is still in free fall, but the ISS is also being accelerated by the rocket, so the astronaut accelerates relative to the station…in this case, out of the Harmony node, through Destiny and Unity, and eventually hitting the slanted wall of PMA1.

    So, that’s what this is illustrating. Gravity is working on everything, but the rocket is accelerating only the ISS and anything attached to it. Simple enough.

  55. Smitty

    @Messier Tidy Upper & @TheRaptor

    I think you are referring to the ‘Killer Tissue Box’ myth. In that a tissue box in an accident can fly forward with sufficient force to kill you. It was busted. :-) (2005, S03E12)

    My favorites that they have touched on lately have been taking the physics thought experiments and actually trying them out IRL to put them to the test. Like the bullet drop (2009, S07E12) and the moving vehicle ball drop. (2010, S08E04)

  56. @ASFalcon13 (comment 57)
    Well said! You took the words right out of my mouth! The concept of centrifugal force works for a ball being swung on the end of a string because the “equal and opposite force” of the tension in the string keeps the ball from flying off and hitting your friend in the head. This is NOT a good analog for orbital mechanics and to present it as such is confusing to someone who doesn’t already understand the physics. I won’t bother going into this anymore, as I had planned to, because Falcon did a fine job.

    I do have one other area I would like to nitpick:

    “While the rocket is firing, the ISS feels a force upwards.”

    Firing a rocket “up” is generally a waste of energy and there are no applications I can think of at the moment in which you would ever deliberately design an orbit transfer that way. Raising an orbit requires an increase in VELOCITY. Since your velocity in orbit is already thousands of miles per hour in the “forward” direction, adding a couple of meters per second in the “upward” direction is not going to change that much.

    You can show this with simple trigonometry.

    Let’s say the ISS velocity is 8,000 meters per second and we’re going to do a burn with a delta-v (engineer speak for change in velocity) of 2 meters per second. If you burn the rocket in the direction of travel, your orbital velocity is simply now 8,002 meters per second. But if you were to do a radial burn, which Phil implies in his post above, your new velocity would be the hypotenus, or in trig terms that’s velocity = sqrt(8000^2+2^2) = 8,000.00025. I have thrown away that 2 m/s of rocket fuel.

    this is why when the ISS does a reboost, we (generally) keep the pointy end forward, as they say, and fire the Service Module main engines tangential to the orbit. This clears up what some of the other commenters were asking about (@54 Peter B). A radial burn of course adds energy but its mostly wasted fuel.

    The confusing thing about an orbital maneuver is that when you get to your new higher orbit, you are actually moving SLOWER than you were at the lower orbit… this orbital mechanics thing can be very confusing and is hard to simplify, as you can tell.

    @6 Dr Sid
    There are several reasons we don’t do a two burn maneuver (ie, a hohmann transfer) when reboosting the ISS orbit. Firstly, as others have pointed out, the ISS orbit is never perfectly circular. I am not in the trajectory design office, but my understanding is that we actually do our maneuvers at APOGEE to simply raise the low side of the orbit, making the orbit more circular. By doing the burn at apogee we in effect pull perigee out of the denser atmosphere, reducing the overall drag on the orbit and raising the average altitude of ISS. When you are only increasing your orbital velocity by less than a thousandth of a percent, a maneuver like a hohmann transfer becomes somewhat meaningless, from an efficiency perspective.

    We do often do two reboosts within a few days or a week or two of each other. The logistics associated with planning and executing a reboost make it easier to do one reboost at a time, and do another one on another day. A hohmann transfer would mean doing a burn 90 minutes after the first burn which means the ISS systems would have to stay configured for the reboost for an additional 90 minutes or more. Part of configuring for reboosts includes parking the large solar arrays which may lead to degraded power generations, which can be hard to deal with depending on our position in the orbit.

    The last thing to consider is the very difficult task that the TOPOs have (TOPO is the name for our trajectory officer) of planning our orbit to avoid orbital debris and other satellites as well as to keep us in the right location to link up with future Progress or Soyuz visiting spacecraft. I imagine there is something about that planning that leads them to prefer a single burn boost.

    Hope that was enlightening!
    Ad astra,
    - Ben H.
    Mission Control, Houston, TX

  57. Penny

    Whats a jist, physicist?

  58. larry

    Centrifugal force does not play any part in the orbit of the space station.

    The enormous velocity of the space station in orbit in a horizontal path can be thought of as a distance over ground velocity. Each second the space station will travel some amount of distance over ground, and each second gravity will bend that line vertically downward. So there is a horizontal speed per second and a vertical (falling) speed per second. The curve of the earth over the distance that the space station travels each second falls away the exact same distance that the space station falls in it’s vertical velocity (gravity).

    This is why objects in orbit are considered to be constantly falling.

    Larry

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