Why is that - wouldn’t you be working against solar gravity? Like you don’t have to get them there quickly, just launch them in some orbit that will decay and be taken in?
Because the Earth is really cookin’, and anything anyone you hurl toward the sun will inherit that orbital velocity as well, meaning that they’ll actually end up going around the sun, instead of into it. And due to the speed it would pick up on its way in, it would basically take up a highly-eccentric yet stable elliptical orbit.
“Well, what if we throw them in the other direction, to make up for it?” That’s called retrograde, and that’s basically exactly what you’d have to do: cancel out the Earth’s entire orbital velocity. Which would take a lot of energy, plus a couple of really exacting gravity assists from planets on the way in.
(Edit to add: I may have explained this poorly. Basically, if you don’t change your orbital speed at all, any movement you make toward or away from the host body means you just end up in an orbit of the same average distance, but in a more eccentric [elliptical] shape.)
By contrast, even though the escape velocity from the solar system is no slouch (42 km/s), you get to start with the Earth’s orbital velocity (30 km/s)–meaning you’re already a little under 3/4 of the way there. Plus, if you can make it to Jupiter and Saturn, you can get a significant gravity assist, and they’re much bigger targets for such a maneuver than Mercury or Venus are.
So, yeah, bottom line: you only need a delta-V of about 12 km/s to get out of the solar system, but a delta-V of 30 km/s to get to the sun without going into orbit.
So, yeah, bottom line: you only need a delta-V of about 12 km/s to get out of the solar system, but a delta-V of 30 km/s to get to the sun without going into orbit.
This is true, but the possibility of gravity assists mostly nullifies the difference. If you can get out to Jupiter you can basically choose: either let it sling you out of the system, or let it cancel out all your orbital velocity so you fall into the sun.
Good question, but if you cancel out only a little bit of orbital velocity, you just orbit in a little bit closer. Without any appreciable drag acting on you, there’s nothing that will keep your orbit decaying. You’ll just be in a smaller, perhaps slightly more eccentric orbit.
Yeah, orbital mechanics gets a little bit mind-bendy sometimes. If you’re in a stable circular orbit, accelerating in the direction you’re traveling will actually result in you traveling more slowly because you have moved to a higher orbit, and firing engines to slow down will actually speed you up because you move in closer to the host body and take up a faster orbit.
This is actually a problem spacecraft deal with regularly. If a Dragon capsule is behind the ISS and wants to dock, using its thrusters to accelerate toward the ISS will actually result in it falling further behind. Decelerating will get it closer, though it will then be in a lower orbit. Orbital rendezvous is tough.
You can just change the shape of your orbit (but not your orbital energy) with the help of a sufficient gravity well from solar orbit, so it intersects with the Sun. Drag (aerobraking!) within the Sun will slow whatever is left of you enough to sap your orbital energy
The curvature of spacetime does wild shit to how you would expect physics to work. If you want to fall into a gravity well, you have to slow down or you’ll just slingshot past it.
Picture going for a very tight periapse in a highly elliptical orbit. Now make the periapse lower. Lower still, within the atmosphere or below the surface of the thing you’re trying to hit. If you don’t plan on arriving alive it’s much cheaper to arrive like a meteor
The reason you need to slow down is because you’re starting on Earth, which means you’re moving fast enough parallel to the sun’s surface that for every foot you fall downwards toward the sun, the sun’s surface curves away by 1 foot. This results in the nearly circular orbit around the sun we exist in.
If you start speeding up, the orbit becomes more elliptical, except your aphelion starts raising away from the sun because now you’re moving fast enough that you’ve moved more than 1 foot sideways in the time you’ve fallen 1 foot downwards.
Slowing down has the opposite effect. If you get your speed down to 0, you’ll fall straight down toward the sun as normal with gravity. But you don’t need to go all the way down to 0 velocity to enter the sun, you just need to slow down until your elliptical orbit brushes up against the sun’s surface. If you then want to speed back up to avoid falling into the sun, you need to do it parallel to the sun’s surface. At this point, speeding up toward the sun will actually make you fall into the sun faster.
So basically the problem isn’t that you’re moving too fast to fall into the sun. By virtue of Earth’s orbit, you’re moving too fast in a direction away from hitting the sun’s surface.
So you have ~30km/s in a near circular orbit. You interact with a gravity well to point your vector at the Sun (a highly elliptical orbit). Sure you’re carrying enough energy to come out of that with a very high aposol, but with the perisol within the Sun that energy will convert to heat
You don’t need to kill all your earth orbit speed to hit Earth, just enough to aerobrake
You don’t need to kill all your lunar orbital energy to hit the moon if you’re happy to lithobrake
No one is talking about reaching the surface of the sun alive
That’s the thing - in space, orbits don’t decay. Orbital decay only happens if there’s dust or atmosphere that you bump into along your orbit to slow you down. But in interplanetary space, there’s no dust or atmosphere, and certainly not enough to decay your orbit fast enough to achieve results (otherwise, the Earth would have already decayed and melted in the Sun)
You need to spend fuel to lower your orbit to hit the Sun, and you need to spend fuel to raise your orbit to escape the solar system. It turns out to be really freaking difficult to hit the sun because it simply requires so much fuel to lower your orbit enough to hit the Sun.
Orbital decay isn’t just friction from particles, you also have imperfections in the orbit and other objects influencing the eccentricity over time. The moon has gravity too for instance.
Y’all need to pay some Kerbal Space Program. It’ll teach you more about orbital mechanics than a physics degree and a job at NASA (according to XKCD). The only problem is, once you have this knowledge, a lot of sci fi becomes annoying.
Why would an orbit decay without something to slow the spacecraft down like an atmosphere? The problem is that any object we launch from earth has a lot of orbital velocity, which makes it almost impossible to hit the sun directly, you would have to use a lot of complex gravity assists from the inner planets to take away enough momentum. Using gravity assists to accelerate outwards is much easier
To escape a body of mass you need to have enogh velocity (kinetic energy) to overcome the gravitational pull of that body. You can imagine it like a ball sitting in a bowl. With little velocity it will just roll back and forth but if it’s fast enough it can roll out of the bowl and escape it’s influence.
That critical speed is called “escape velocity” and it depends on mass and distance from a body. The escape velocity of earth (from the surface) is about 11.2 km/s and the sun’s escape velocity (from earth orbit) is about 42.1 km/s. Earth orbits around the sun at about 29.8 km/s. If you launch in the direction of Earth’s orbit, you will orbit the sun already at about 41 km/s, so you “only” need 1.1 km/s more to escape the sun, too.
If you tried to reach the sun, you could launch in the opposite direction leaving you orbiting the sun at about 18.6 km/s. Since there is almost nothing in space you won’t slow down from friction and the orbit won’t decay. Instead you’d have to accelerate opposite the direction you’re traveling. Now, calculating exactly how much you’d need to decelerate isn’t trivial since you don’t want a stable orbit but an elliptical orbit that just touches the sun at the closest point (perihel). I don’t know how much deceleration that takes, but it’s propable that it’s easier than accelerating by 1.1 km/s to escape the sun.
Why is that - wouldn’t you be working against solar gravity? Like you don’t have to get them there quickly, just launch them in some orbit that will decay and be taken in?
Because the Earth is really cookin’, and
anythinganyone you hurl toward the sun will inherit that orbital velocity as well, meaning that they’ll actually end up going around the sun, instead of into it. And due to the speed it would pick up on its way in, it would basically take up a highly-eccentric yet stable elliptical orbit.“Well, what if we throw them in the other direction, to make up for it?” That’s called retrograde, and that’s basically exactly what you’d have to do: cancel out the Earth’s entire orbital velocity. Which would take a lot of energy, plus a couple of really exacting gravity assists from planets on the way in.
(Edit to add: I may have explained this poorly. Basically, if you don’t change your orbital speed at all, any movement you make toward or away from the host body means you just end up in an orbit of the same average distance, but in a more eccentric [elliptical] shape.)
By contrast, even though the escape velocity from the solar system is no slouch (42 km/s), you get to start with the Earth’s orbital velocity (30 km/s)–meaning you’re already a little under 3/4 of the way there. Plus, if you can make it to Jupiter and Saturn, you can get a significant gravity assist, and they’re much bigger targets for such a maneuver than Mercury or Venus are.
So, yeah, bottom line: you only need a delta-V of about 12 km/s to get out of the solar system, but a delta-V of 30 km/s to get to the sun without going into orbit.
That’s a great explanation, thanks! 🙏
This is true, but the possibility of gravity assists mostly nullifies the difference. If you can get out to Jupiter you can basically choose: either let it sling you out of the system, or let it cancel out all your orbital velocity so you fall into the sun.
I feel like that might be difficult to do without just falling into Jupiter, but I am no rocket scientist.
They would still be destroyed in a hot crucial, so it still works.
Why would you need to entirely cancel the earths orbital velocity, surely you just need to cancel a
tinybit of orbital velocity?Edit: https://space.stackexchange.com/questions/43913/do-you-need-0-km-s-velocity-to-crash-into-the-sun
Canceling out only a tiny bit puts you on an orbit similar to earth’s. You need to kill basically all of your momentum.
Good question, but if you cancel out only a little bit of orbital velocity, you just orbit in a little bit closer. Without any appreciable drag acting on you, there’s nothing that will keep your orbit decaying. You’ll just be in a smaller, perhaps slightly more eccentric orbit.
But you’d need a higher velocity to orbit closer…
Yeah, orbital mechanics gets a little bit mind-bendy sometimes. If you’re in a stable circular orbit, accelerating in the direction you’re traveling will actually result in you traveling more slowly because you have moved to a higher orbit, and firing engines to slow down will actually speed you up because you move in closer to the host body and take up a faster orbit.
This is actually a problem spacecraft deal with regularly. If a Dragon capsule is behind the ISS and wants to dock, using its thrusters to accelerate toward the ISS will actually result in it falling further behind. Decelerating will get it closer, though it will then be in a lower orbit. Orbital rendezvous is tough.
You can just change the shape of your orbit (but not your orbital energy) with the help of a sufficient gravity well from solar orbit, so it intersects with the Sun. Drag (aerobraking!) within the Sun will slow whatever is left of you enough to sap your orbital energy
Yeah, gravity assists are a cheat code here, but the delta-V is still being changed—just by stealing velocity from elsewhere.
the issue is not counteracting gravity, the issue is decelerating enough to hit the sun
What’s wrong with them striking the sun at full speed?
The curvature of spacetime does wild shit to how you would expect physics to work. If you want to fall into a gravity well, you have to slow down or you’ll just slingshot past it.
Picture going for a very tight periapse in a highly elliptical orbit. Now make the periapse lower. Lower still, within the atmosphere or below the surface of the thing you’re trying to hit. If you don’t plan on arriving alive it’s much cheaper to arrive like a meteor
This sounds an awful lot like the the idea that you can never actually catch up to anything because all you can ever do is close the distance by half.
This sounds an awful lot like the the idea that you can never actually catch up to anything because all you can ever do is close the distance by half.
The reason you need to slow down is because you’re starting on Earth, which means you’re moving fast enough parallel to the sun’s surface that for every foot you fall downwards toward the sun, the sun’s surface curves away by 1 foot. This results in the nearly circular orbit around the sun we exist in.
If you start speeding up, the orbit becomes more elliptical, except your aphelion starts raising away from the sun because now you’re moving fast enough that you’ve moved more than 1 foot sideways in the time you’ve fallen 1 foot downwards.
Slowing down has the opposite effect. If you get your speed down to 0, you’ll fall straight down toward the sun as normal with gravity. But you don’t need to go all the way down to 0 velocity to enter the sun, you just need to slow down until your elliptical orbit brushes up against the sun’s surface. If you then want to speed back up to avoid falling into the sun, you need to do it parallel to the sun’s surface. At this point, speeding up toward the sun will actually make you fall into the sun faster.
So basically the problem isn’t that you’re moving too fast to fall into the sun. By virtue of Earth’s orbit, you’re moving too fast in a direction away from hitting the sun’s surface.
That’s a very good explanation.
So you have ~30km/s in a near circular orbit. You interact with a gravity well to point your vector at the Sun (a highly elliptical orbit). Sure you’re carrying enough energy to come out of that with a very high aposol, but with the perisol within the Sun that energy will convert to heat
You don’t need to kill all your earth orbit speed to hit Earth, just enough to aerobrake
You don’t need to kill all your lunar orbital energy to hit the moon if you’re happy to lithobrake
No one is talking about reaching the surface of the sun alive
The problem is, you have so much speed that you keep missing.
That’s the thing - in space, orbits don’t decay. Orbital decay only happens if there’s dust or atmosphere that you bump into along your orbit to slow you down. But in interplanetary space, there’s no dust or atmosphere, and certainly not enough to decay your orbit fast enough to achieve results (otherwise, the Earth would have already decayed and melted in the Sun)
You need to spend fuel to lower your orbit to hit the Sun, and you need to spend fuel to raise your orbit to escape the solar system. It turns out to be really freaking difficult to hit the sun because it simply requires so much fuel to lower your orbit enough to hit the Sun.
Sir Isaac Newton is the deadliest son of a bitch in space
Orbital decay isn’t just friction from particles, you also have imperfections in the orbit and other objects influencing the eccentricity over time. The moon has gravity too for instance.
Y’all need to pay some Kerbal Space Program. It’ll teach you more about orbital mechanics than a physics degree and a job at NASA (according to XKCD). The only problem is, once you have this knowledge, a lot of sci fi becomes annoying.
You’re starting with the speed of the earth
Ah… centrifugal force, ofc!:-)
Why would an orbit decay without something to slow the spacecraft down like an atmosphere? The problem is that any object we launch from earth has a lot of orbital velocity, which makes it almost impossible to hit the sun directly, you would have to use a lot of complex gravity assists from the inner planets to take away enough momentum. Using gravity assists to accelerate outwards is much easier
I remember watching a video about that. The gist is that you have to leave earth orbit or something idk.
You leave earth orbit into a solar orbit that is slightly shifted depending on which direction you were facing when you left earth’s orbit
Short answer; the earth is orbiting really fast around the sun.
To escape a body of mass you need to have enogh velocity (kinetic energy) to overcome the gravitational pull of that body. You can imagine it like a ball sitting in a bowl. With little velocity it will just roll back and forth but if it’s fast enough it can roll out of the bowl and escape it’s influence.
That critical speed is called “escape velocity” and it depends on mass and distance from a body. The escape velocity of earth (from the surface) is about 11.2 km/s and the sun’s escape velocity (from earth orbit) is about 42.1 km/s. Earth orbits around the sun at about 29.8 km/s. If you launch in the direction of Earth’s orbit, you will orbit the sun already at about 41 km/s, so you “only” need 1.1 km/s more to escape the sun, too.
If you tried to reach the sun, you could launch in the opposite direction leaving you orbiting the sun at about 18.6 km/s. Since there is almost nothing in space you won’t slow down from friction and the orbit won’t decay. Instead you’d have to accelerate opposite the direction you’re traveling. Now, calculating exactly how much you’d need to decelerate isn’t trivial since you don’t want a stable orbit but an elliptical orbit that just touches the sun at the closest point (perihel). I don’t know how much deceleration that takes, but it’s propable that it’s easier than accelerating by 1.1 km/s to escape the sun.