Can somebody explain these space travel physics to me?

Factual Questions
21

Orbits don’t spiral. once you’ve finished your burn, you have established your new orbit. If you are travelling away from the central mass (the sun) faster than escape velocity, it is a hyperbolic orbit and you never come back. If you are travelling less than escape velocity, it is an elliptical orbit. Your current position and velocity vector determine the path of the orbit. If the path of the ellipse takes you too close to the surface of the sun, you will not describe a complete ellipse…

But absent the slingshot effect - you spend 9km/s, then need to spend a bit more (guessing about 4km/s) to circularize the orbit there, then you need to shed 13km/s to fall into the sun. The only reason Jupiter is efficient is the slingshot effect, where you an Jupiter trade momentum vectors so that you are going in a more desirable direction, rather than spend all that Delta-V.

If as the OP suggests, you go to the edge of the solar system (but presumably, not beyond) then you will still have sufficient angular velocity and direction to swing around the sun like a comet instead of falling directly into it.

22

When you think about it, the only way to fall directly into the sun (on a purely radial path) is to have an angular velocity of zero relative to it. Everything else is an elliptical orbit.

23

That seems to be pessimistic. A transfer orbit from the distance of Earth out to the distance of Jupiter only has an orbital speed at apogee of 7.7 km/s. That’s what you would have to kill. So going out to the distance of Jupiter, then killing the orbital speed takes 16.7 km/s.

Going out to the distance of Saturn would take 10.3 + 4.0 = 14.3 km/s

Neptune: 11.7 + 1.4 = 13.1 km/s

Diminishing returns, but large savings over the 30 km/s of going direct into the Sun.

ETA: Sorry misread scr4’s post, he said it’s less than Jupiter’s orbital speed. I got my orbital speed numbers from Orbital Velocity Calculator

24

You do not seem to be missing that for the basic manoeuvres you want to point your rocket “prograde” or “retrograde”, that is, in the direction of your orbit or opposite, in order to raise or lower your orbit. You would not be able to get to the sun by starting in orbit around it and firing your motor while pointing directly at the sun; think about it!

Next, objects really far from the sun orbit more slowly than objects close to the sun. So you might imagine that if you were hypothetically really out there, you would be barely moving, and it would not take much fuel to arrest any residual velocity to ensure it sucks you back in for a collision. So as an initial exercise you could try ignoring extra planets and approximately calculate how much fuel it would take to launch away from the Earth and out of the Solar system.

This is all pretty theoretical; the video tells us that the actual satellite is supposed to swing by Earth and Venus (not Jupiter) a few times in order to get to the Sun.

25

Circularizing your orbit, once you’re out at apohelion, would be counter-productive. You’d be boosting forward a bit, then boosting backwards that bit and more. You’d only do that if you wanted to hang out in the vicinity of Jupiter for a long time before continuing on to the Sun.

26

Note to self: ANOTHER topic to ask my rocket scientist son about when he visits over turkey day, because I just can’t get my head around this.

I often wonder if I am lacking the education/vocabulary, whether something about me makes it impossible for me to grasp things that seem counter-intuitive (well, wouldn’t the sun’s gravity pull it in if you launched TOWARDS the sun?), or if somehow I’m willingly allowing myself to not understand it/not putting the mental effort in. I enjoy reading threads like this, but I consistently have a hard time understanding them. And I do not generally consider myself a stupid person.

27

Start with this: Near Earth we have the space station. Why does the Earth’s gravity not “pull it in”?

Your explanation of this question will help others structure their sun-launch explanation to you so that you can understand it.

28

Would it be possible to use a solar sail to approximate this kind of drag? If the object had a sail which was at a 45 degree angle to the sun, would the energy from the sun pushing it out and back eventually slow it completely down?

29

So … if there were really such a craft as the Starship Enterprise, or the Orville or whatever … and that craft had “magic” propulsion and no real fuel concerns:

Said craft launches from Earth, with an aim to land neatly on the Sun’s surface. The crew doesn’t even give the slightest rip about conserving fuel, gravity assists, or any of that. So, the entire way to the Sun, is the crew constantly having to course-correct to actively prevent from going into orbit (even a really elongated one)? Does making the trip a presumably straight shot in fact require constant “steering”?

30

“Completely” stop relative to the sun? Not sure. But you can use a solar sail to get in closer to the sun for sure. Might not even be necessary as long as you reduced your orbital velocity enough for gravity to do it’s work.

It’s possible to “tack” (or something like it) using the solar wind. You don’t have to go straight out. The Japanese Ikaros probe tested solar sail technology to get to Venus.

Basically, yes, you’ll need constant thrust. Though the definition of ‘straight’ gets a bit wonky with all these celestial bodies in relative motion to each other and gravity ‘curves’ local spacetime.

31

(As the creaky gears try to turn…) Because it is falling, right? :dubious: Which pretty much strains my brain to the breaking point.

And which pretty much represents my miles-wide-but-inches-deep, superficial familiarity w/ so many scientific principles that really interest me. With other liberal arts bullshitters, I’m capable of spouting a factoid or name or 2, sufficient to convey the impression that I actually know something. When I read something, or someone intelligent explains it to me (another kid and her SO are molecular biologists! :smack:), it makes sense to me in the moment, but just doesn’t stick w/ me, and doesn’t scale/transfer to other situations.

Same way I tried to read “A Brief History” several times, and can point out the exact page where it exceeded my ability (willingness?) to understand.

I think that now, in my late 50s, I’m reaping the results of having studiously avoided putting in the effort to study math and sciences in high school and college. My brain is warped toward bullshit rather than deep understanding of facts.

But I’ll keep reading these threads, and reading books and watching programs that interest me but somehow don’t seem to stick.

32

That might be one of the harder things to grok about orbital mechanics: A destination like the Sun is not actually stationary.

In the everyday earthbound human experience, the Sun is essentially reckoned as a never-changing point in space. Something stationary that you can aim at upon launching and just “stay the course” and make it there. An apparent “straight shot”, as it were.

But in fact, the Sun is ALWAYS moving in relation to countless other objects in space – The center of the Milky Way. I guess its own planets. Etc.

…

Thought experiment:

If we had a “magic tape measure” (or rope, string, whatever) that could instantaneously stretch from the center of the Earth to the center of the Sun … and that tape measure could be stretched taut between the two objects … the tape measure would not follow a straight line, but some kind of curve. Even when pulled taut. Right?

33

If you had constant thrust, from a solar sail or an ion engine or similar, then you can indeed spiral down into the Sun. Essentially, every moment your thrust is moving you into a slightly lower orbit.

34

I thought they did. Like Skylab. I figured that you had to occasionally give them a burst of new thrust in order to keep them in orbit.

35

While that is true, it’s a very minor consideration for things we’ve been discussing in this thread.

The most important effect is, when you start from Earth orbit, you are already moving at a significant speed, about 30 km/s. And there’s no friction in space to slow you down. If you used a massive engine to come to a complete stop, you would fall straight down to the Sun. But that requires launching something at 30 km/s relative to the Earth. The fastest spacecraft ever to leave the Earth was the New Horizons, which left Earth at a speed of 16.3 km/s, so a little more than half the speed you need to fall down to the Sun. And this required launching this 1000-lb probe on top of an Atlas-V with 5 strap-on boosters. That’s a rocket that can put 41,000 lb into low earth orbit.

I think it would be straight. Every part of this “tape measure” is in a circular path around the Sun, but not moving fast enough to stay in that path without external force directly away from the Sun. That force is provided by the tension of the “tape measure”. (Though the fact that the Earth’s orbit is slightly elliptical may change this slightly.)

36

Only because the space stations are in low earth orbit, where the atmospheric drag is not negligible. For an object orbiting the Sun and not near any planet, the drag is not even measurable. (Unless it happens to collide with an asteroid.)

37

Skylab was in an unstable low earth orbit. I can’t explain what you’re asking in general but he wants you to consider that anything launched from the earth has earth’s orbital velocity and will tend to stay in earth’s orbit around the sun. Aiming directly at the sun doesn’t help to reduce that orbital velocity. To get to the sun you have to change the orbit to intersect the sun or get very close. It sounds like using the gravity of other planets to alter the orbit is the most efficient way to do that, and that means increasing the radius of the orbit initially. Or else I have no more understanding than you about this, or maybe less.

38

Nah, you can still have ‘straight’ lines. This thought experiment gets a bit hairy, though, because you can’t actually measure both ends simultaneously in the real world - there’s enough distance that relativity is an actual concern.

But never mind that. The issue is that the Earth is in orbit around the sun. If you start from Earth orbit, you already have significant velocity relative to the sun. So, boosting ‘straight’ towards the sun is not efficient.

It’s the old saying that you don’t shoot where something is, you shoot where it’s going to be.

Yeah, that wasn’t the greatest example. Even the ISS is constantly having to get little nudges to maintain its orbit. It’s got a low orbit and is skimming the atmosphere constantly.

Satellites in geosynchronous orbit are probably a better example. There are several that can conceivably be up there for thousands of years in stable orbits if otherwise left alone.

39

Yes, they are falling. All objects in earth orbit are constantly accelerating towards earth due to the earth’s gravity, which in low earth orbit (LEO) isn’t much less than the 9.8m/s[sup]2[/sup] we experience on the surface. At 400km up, you’re looking at 8.7m/s[sup]2[/sup]. However, as an object in LEO falls towards the earth, it’s also moving sideways. Quite rapidly. And as it moves sideways, the surface of the earth gets further away, because the earth is not flat. If an object in orbit were not subject to earth’s gravity it would instead travel in a straight line. Imagine drawing a straight line next to a circle, and then measuring the “altitude” of points on the line as it moves away from the circle - that altitude increases. But of course objects in LEO are subject to gravity, so their trajectory is continually bent towards the surface of the earth - it’s just never bent towards the surface faster than the surface bends away beneath.

So that’s the basics of why things in orbit don’t fall down, but it doesn’t get into the orbital mechanics of raising and lowering orbits. Orbital mechanics aren’t hard to understand, but they are very, very counter-intuitive if you haven’t grasped how orbits work. To go “down” in an orbit, you actually need to slow down your sideways motion. If you’re not moving sideways as fast, then the surface below you isn’t bending away as fast, while you’re still being accelerated towards it at the same rate. However, just slowing down a bit won’t make your whole orbit lower. As you get lower, you’ll pick up speed (like you’re a marble rolling down a hill), and assuming you didn’t slow down so much that your orbit now intersects the atmosphere, that speed will eventually become enough that you’ll start going back up, and you’ll end up at exactly the same place you were when you slowed down in the first place. If you want your whole orbit to be lower, you need to slow down your sideways motion a second time when you reach the lowest point. To go up you do the opposite - first you increase your sideways motion. This makes the surface curve away from you faster so that your altitude will increase. As your altitude increases, your speed will decrease until you start losing altitude again and you end up exactly where you were when you first sped up. To stay in a higher orbit, you have to increase your sideways speed a second time when you’re at the high point of your orbit.

These pairs of accelerations are called Hohman transfers, and the orbit you’re on in between them is called a transfer orbit. If you watch SpaceX launches, you’ve seen the first half of this when they launch satellites to “geostationary transfer orbits.” First they have the initial low earth orbit insertion - the long initial burn of the second stage. Then they have the “coast phase” which is actually just waiting until the rocket is over the equator. They wait till they’re over the equator because, if you haven’t noticed by now, in orbit you always loop back around to where you were when you made your maneuver. So since these satellites are supposed to end up over the equator, you want to make your maneuver over the equator. Then the second burn of the second stage is picking up a bunch more speed so that the satellite will coast up to the altitude of geostationary orbits (36,000km or so). If nothing further happened, the satellite would then come falling back down to LEO altitude, and then coast back up to GSO altitude, and back down, and back up. What actually happens is that the satellite uses a maneuvering thruster to complete the second half of the Hohman transfer (also called “circularizing” as it turns the more elliptical transfer orbit into a nearly perfect circle). This of course happens long after the SpaceX broadcast ends, and may not be all done at once.

So all that said, why is it easier to get to the sun by initially moving out rather than in? Well, in some cases rather than using a Hohman transfer to raise or lower your orbit it’s more efficient to use something called a bi-elliptical transfer instead. It’s more efficient because when you’re at the high point of a very elliptical orbit you’re moving very slowly, and when you’re moving very slowly you can change your orbit more with less delta-V. So to get to the sun, you’re starting out by increasing your sideways speed relative to the sun so that you’ll gain altitude (relative to the sun now, not earth) and as you gain altitude you’ll slow down till you’re moving much slower than the 30km/s that the earth is. You can then slow down that remaining sideways motion with less effort. And that’s all before considering any planetary fly-bys to do gravitational slingshots.

40

What happens if instead of a thruster burn (forwards) to slow down, you do a thruster burn ‘up’ (away from the Earth)? Thanks!