# Can somebody explain these space travel physics to me? **URL:** \ **Category:** Factual Questions\ **Created:** [November 5, 2019, 6:51pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047 "2019-11-05T18:51:06Z")\ **Posts on this page:** 20\ **Page:** 4
**Author:** ![Folly](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/folly/32/3219_2.png) [@Folly](https://boards.straightdope.com/u/Folly)\ **Post date:** [November 6, 2019, 9:26pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/61 "2019-11-06T21:26:51Z")
> [@Great\_Antibob](#): > > It’s possible, given sci-fi level rocketry technology. > > You can have it burn fuel throughout its flight. Or launch with a truly atrocious velocity. Or use that nifty warp technology they seem to have. Except, as I recall that scene, it’s not even possible with sci-fi level technology. The missile reaches the sun in about 10 seconds and the effect is instantaneous. ---
**Author:** ![Gray\_Ghost](https://avatars.discourse-cdn.com/v4/letter/g/c0e974/32.png) [@Gray\_Ghost](https://boards.straightdope.com/u/Gray_Ghost)\ **Post date:** [November 6, 2019, 9:31pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/62 "2019-11-06T21:31:08Z")
> [@Bear\_Nenno](#): > > Crazy, right!? Trying to push yourself “away” from the Earth can actually make you crash into it. “[East takes you Out, Out takes you West, West takes you In, In takes you East. Port and Starboard bring you back](https://en.wikipedia.org/wiki/The_Smoke_Ring_(novel)).” It doesn’t make intuitive sense to me either. It’s still yet another sci-fi property I’d kill to see on the big screen. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [November 6, 2019, 9:40pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/63 "2019-11-06T21:40:02Z")
> [@Gorsnak](#): > > You could just link to a Scott Manley KSP tutorial video on Youtube. 🙂 That’s what I was going to say… someone really should make a video titled “Everything I ever needed to know about orbital mechanics, I learned playing KSP”. But… to answer the OP in a super-simplified fashion, there are three parts to it. 1. When something’s in orbit around something else, they’re being pulled toward it by gravity, but they’re ALSO going fast enough to, in essence, fall PAST the item continually. That’s why getting into space is more a matter of going fast enough, and not as much about going high enough. Another corollary is that since you’re being pulled at a constant rate, adding speed means you orbit wider, and removing speed means you orbit closer. So what happens when a space capsule or whatever decides to re-enter, is that they turn around and burn their engines exactly in the opposite direction that they’re traveling, slowing themselves down, and shrinking their orbit until it intersects with the Earth. 2. Everything in space is (for the most part) in orbit around something. We’re in orbit around the Sun. The moon’s in orbit around the Earth. 3. What Munroe is getting at is that the best way to get an object out of Earth orbit, and into a solar orbit that won’t have enough speed to continue falling “past” the sun might well be to fire a rocket away from the Sun, because you could get just far enough away so that you’re no longer in Earth orbit, but are instead in Solar orbit, but without much velocity, and you’ll likely impact the Sun at some point in that orbit. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [November 6, 2019, 9:49pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/64 "2019-11-06T21:49:20Z")
> [@dtilque](#): > > Assuming you launch from the equator, it’ll stay in geosynchonous orbit. This is one case where you don’t have to give it sideways thrust to stay in orbit. The satellite has sideways velocity inherited from being on Earth, which is rotating once per day. That’s exactly how much speed it needs to stay in orbit at geosynchronous altitude. The circumference of the Earth is much, much less than the circumference of a geosynchronous orbit (GSO). One must travel a hell of a lot faster to circumnavigate that orbit in one day than to circumnavigate the Earth in the same period. > [@](#): > > Now if you launch from somewhere else, it’ll have less velocity inherited from Earth, so the satellite won’t stay in a geosynchronous orbit. Depending on the launch site latitude, it could either go into a different orbit or fall back to Earth. Even if launched at the equator, it inherits roughly 1600kph of tangential velocity from the Earth. But the velocity required for GSO is close to 11,000kph. Launching straight up will result in the satellite falling straight back down. Well, not _straight_ down, but generally straight, and regardless, it crashes back into the Earth. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [November 7, 2019, 3:01am UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/65 "2019-11-07T03:01:04Z")
> [@Qadgop\_the\_Mercotan](#): > > The course included orbital mechanics and the like, and I remember my mind being completely blown over and over again by the seeming contradictions of it. To go up, you must go down. To slow down, you must speed up. I think it is the way you are recalling it that makes it sound confusing. Like, what does “to slow down, you must speed up” mean? It could mean something like that you will move more slowly if you accelerate into a higher orbit. But what they always teach people at least sounds straightforward and not contradictory, e.g., an object sweeps out equal areas in equal times, energy is conserved, angular momentum is conserved, square of the orbit time is proportional to the cube of the semi-major axis. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 7, 2019, 3:31am UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/66 "2019-11-07T03:31:53Z")
> [@](#): > > Quoth **Dinsdale** : > > Don’t even TRY to tell me about the pixies inside my computer Of course not. It’s not pixies; it’s leprechauns. Japanese leprechauns. And when the repairman comes, all he’s really doing is slipping the little guy some sushi and soda bread. Or, for the really expensive problems, Guinness and saki. At least, that’s the way my mom explained it to me, and she wouldn’t lie. ---
**Author:** ![Limmin](https://avatars.discourse-cdn.com/v4/letter/l/f04885/32.png) [@Limmin](https://boards.straightdope.com/u/Limmin)\ **Post date:** [November 7, 2019, 7:18pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/67 "2019-11-07T19:18:30Z")
Cool thread folks. Lots of basic questions here about “if I do this in orbit, what happens?” I suggest to the curious: Get a copy of the game “Kerbal Space Program” (look it up online). It’s a great place to learn orbital mechanics and play around with rockets and spacecraft. Circling back to OP’s mention of Randall Munroe, here is his comic where he claims that Kerbal Space Program really helped his understanding of orbital mechanics: [https://xkcd.com/1356/](https://xkcd.com/1356/) ---
**Author:** ![Limmin](https://avatars.discourse-cdn.com/v4/letter/l/f04885/32.png) [@Limmin](https://boards.straightdope.com/u/Limmin)\ **Post date:** [November 7, 2019, 7:20pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/68 "2019-11-07T19:20:52Z")
> [@DPRK](#): > > I think it is the way you are recalling it that makes it sound confusing. Like, what does “to slow down, you must speed up” mean? It could mean something like that you will move more slowly if you accelerate into a higher orbit. But what they always teach people at least sounds straightforward and not contradictory, e.g., an object sweeps out equal areas in equal times, energy is conserved, angular momentum is conserved, square of the orbit time is proportional to the cube of the semi-major axis. This was mind-blowing to NASA and the early astronaut corps too. They had to wrestle with these strange ideas, so different from flying an aircraft, in their first efforts at rendezvous and docking in orbit. Here’s a pretty nice video documentary of the whole idea: [https://www.youtube.com/watch?v=RFmS5DL-SSM](https://www.youtube.com/watch?v=RFmS5DL-SSM) ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [November 7, 2019, 8:24pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/69 "2019-11-07T20:24:22Z")
> [@Qadgop\_the\_Mercotan](#): > > I actually took an undergrad Astrophysics course in college, taught by one of the Professors who was putting together instruments and experiments for the Hubble Space Telescope (whose operational center is conveniently right on campus). The course included orbital mechanics and the like, and I remember my mind being completely blown over and over again by the seeming contradictions of it. To go up, you must go down. To slow down, you must speed up. > > But every once in a while, I would be able to conceptualize why it had to be that way; those were Golden Moments when my brain briefly achieved a slightly higher level of functioning. Sadly they never lasted for more than 8 seconds before I lost the insight. But I remember I had it once, briefly. A tiny tiny bit of it, anyway. > > QtM, no rocket scientist for sure. > > ETA; Damn, that was a fun course though. I was a history major. I didn’t take those pre-med science courses. I had to learn my astrophysics on the street. ---
**Author:** ![Great\_Antibob](https://avatars.discourse-cdn.com/v4/letter/g/e47c2d/32.png) [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)\ **Post date:** [November 7, 2019, 9:23pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/70 "2019-11-07T21:23:28Z")
> [@Little\_Nemo](#): > > I had to learn my astrophysics on the street. On the mean streets of Hell’s Observatory? ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [November 9, 2019, 1:35pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/71 "2019-11-09T13:35:39Z")
I need to make a correction to my posts. If orbiting clockwise at the 3 o’clock position, aiming “straight up” and firing your rockets will raise your orbit _at the 6 o’clock_ and lower it at the _12 o’clock_. My previous posts stated that it was the 3 and the 9 respectively. I didn’t think that through carefully before posting, and I apologize. ---
**Author:** ![Johanna](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johanna/32/8318_2.png) [@Johanna](https://boards.straightdope.com/u/Johanna)\ **Post date:** [November 9, 2019, 5:59pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/72 "2019-11-09T17:59:33Z")
“[Set the controls for the orbit of Neptune, then set the controls for a -13.1 km/h retrograde burn, then shut the controls off and wait](https://youtu.be/3zuEfmmCA5s)” ---
**Author:** ![Johanna](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johanna/32/8318_2.png) [@Johanna](https://boards.straightdope.com/u/Johanna)\ **Post date:** [November 9, 2019, 6:09pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/73 "2019-11-09T18:09:57Z")
\*13.1 km/s that is ---
**Author:** ![vislor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/vislor/32/5933_2.png) [@vislor](https://boards.straightdope.com/u/vislor)\ **Post date:** [November 11, 2019, 4:25pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/74 "2019-11-11T16:25:25Z")
Thanks for the interesting discussion! Like Little Nemo, I’m a history major and then worked in software development. Reading this thread made me want to understand this better, so I went out and bought Kerbel Space Program to try. Those poor Kerbels. (Think Galaxy Quest. “Those poor people.”) I returned it to save as many Kerbels as I could. ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [November 13, 2019, 1:33am UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/75 "2019-11-13T01:33:31Z")
I remember a scene in a book I once read - my gut says _Space_ by **James Mitchener** , but my memory does fail me sometimes. I would have read it in my teens, which was some many years ago. The astronauts in training (hotshot pilots all) are taken to a racetrack where there are two rigged jeeps. The jeeps are set to go round in a circular path at a constant speed, but if the accelerator is pressed, it turns momentarily inward, and if the brake is pressed, it turns momentarily outward. The jeeps are set 180 degrees apart, and the astronauts told to use the brake and accelerator pedals to bring the two jeeps together, one behind the other. As pilots, they expect this to be an easy task - of course, it is almost impossible. This is deliberate, and is presented as a way to teach the astronauts that manually flying a spacecraft is completely counterintuitive for these pilots. Once in space, they have to rely on ground control to make the burn calculations for orbital control. ---
**Author:** ![Limmin](https://avatars.discourse-cdn.com/v4/letter/l/f04885/32.png) [@Limmin](https://boards.straightdope.com/u/Limmin)\ **Post date:** [January 17, 2020, 9:50pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/76 "2020-01-17T21:50:05Z")
> [@Bear\_Nenno](#): > > I need to make a correction to my posts. > > If orbiting clockwise at the 3 o’clock position, aiming “straight up” and firing your rockets will raise your orbit _at the 6 o’clock_ and lower it at the _12 o’clock_. My previous posts stated that it was the 3 and the 9 respectively. I didn’t think that through carefully before posting, and I apologize. Wait…that doesn’t seem correct either. Maybe show us a diagram of what you mean? If you’re orbiting clockwise at the 3 o’clock position and pointing straight up and thrusting, then you are thrusting retrograde, that is, opposite the direction of motion. When you slow yourself down while in circular orbit, then the altitude of your orbit on the opposite side from where you are now, will drop. Why is this? Conservation of angular momentum, in other words, the product of your orbit radius (from Earth center) and tangential velocity (along the orbit, perpendicular to your orbit radius) will be the same. So slowing down at your current altitude results in a lower altitude (and higher speed) at the opposite end of your orbit from where you made the maneuver. So… you’ll be lowering your orbit at the 9 o’clock position, directly opposite 3 o’clock. At 6 and 12 o’clock, you’ll be at a lower altitude than 3 o’clock. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [January 17, 2020, 10:12pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/77 "2020-01-17T22:12:35Z")
> [@si\_blakely](#): > > I remember a scene in a book I once read - my gut says _Space_ by **James Mitchener** … > The astronauts in training (hotshot pilots all) are taken to a racetrack where there are two rigged jeeps… That was in “The Right Stuff” by Tom Wolfe. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [January 17, 2020, 11:20pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/78 "2020-01-17T23:20:03Z")
> [@Limmin](#): > > Wait…that doesn’t seem correct either. Maybe show us a diagram of what you mean? > > If you’re orbiting clockwise at the 3 o’clock position and pointing straight up and thrusting, then you are thrusting retrograde, that is, opposite the direction of motion. I think by “up”, Bear\_Nenno meant “away from the body being orbited”. That is, in a direction opposite to the local gravitational force. So looking at the diagram on a piece of paper, “up” at 3 o’clock would be to the right. ---
**Author:** ![Limmin](https://avatars.discourse-cdn.com/v4/letter/l/f04885/32.png) [@Limmin](https://boards.straightdope.com/u/Limmin)\ **Post date:** [January 23, 2020, 4:22pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/79 "2020-01-23T16:22:45Z")
> [@markn\_1](#): > > I think by “up”, Bear\_Nenno meant “away from the body being orbited”. That is, in a direction opposite to the local gravitational force. So looking at the diagram on a piece of paper, “up” at 3 o’clock would be to the right. Got it. Yes, clarifying your definitions is the first step in solving a problem. In this case, a diagram would have helped! ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [January 23, 2020, 7:07pm UTC](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047/80 "2020-01-23T19:07:20Z")
> [@CalMeacham](#): > > You don’t have to completely negate your angular momentum to get to the sun – you could make a Hohmann orbit in which your inner radius is about at the surface of the sun, which is effectively getting you to the sun. But it still requires more delta v than going out of the solar system and falling back. But it’d be a lot quicker than going all the way out and coming back. Actually, if you’re satisfied with just getting to the orbit of Mercury (which is plenty hot enough for most purposes), it’s both less “expensive” in terms of energy and much faster to just Hohmann transfer inwards. Hohmann orbit simply means "orbit around sun at lowest level possible? [Previous page](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047.md?page=3) [Next page](https://boards.straightdope.com/t/can-somebody-explain-these-space-travel-physics-to-me/843047.md?page=5)