4 ms·

Nope. It takes far more delta-V to get to the Sun than to get to Mars. To get to Mars you need to change your solar orbit from that of Earth to match that of M

Nope. It takes far more delta-V to get to the Sun than to get to Mars. To get to Mars you need to change your solar orbit from that of Earth to match that of Mars. The difference is a small fraction of Earth's orbital velocity. By comparison, you only need to add about 41% of Earth's velocity to escape the solar system entirely. But to get to the Sun, you need to subtract 100% of Earth's orbital velocity. It's a colossal amount of delta-V.

benj111 8y ago
On the other hand where do you ultimately end up if you only add 40% delta V ?
irrational 8y ago
That's so counter-intuitive. It feels like the sun is a giant gravity well and we just have to give something a bit of a shove in the right direction to fall down into the well. It's not like its rocket science.
- mkl 8y ago
  That might work if it was starting stationary, but it's not. It's starting from Earth, which has enormous orbital velocity perpendicular to the way to the sun. Everything on Earth already has that velocity too, and there's no way to get rid of it except cancel it out by burning a lot of fuel to basically go just as fast in the opposite direction.
  - Klathmon 8y ago
    I always hated using terms like "orbital velocity" trying to explain things to people who don't know what it means. The analogy I use is imagine you have a ball on a string. If you spin that ball around your head, so the string is tight, how hard do you think it would be to hit a friend with it? Now how hard would it be to hit yourself with it? A friend, you just let go! The ball is already spinning and WANTS to go flying out further away. But to hit yourself? You are going to have to first stop the spinning, then pull it toward yourself, or just pull repeatedly on the string harder and harder until it spins faster and faster until it hits you. It's a lot more effort! It may not make sense at first, but it's easy to forget that earth is spinning around the sun pretty damn fast, and to get near it you need to first stop (or GREATLY reduce) that spin, that orbit, and only then can you start to approach it.
    - benj111 8y ago
      Surely in your analogy you would need a trained hamster sitting on the ball of wool, spinning a ball of cotton, trying to hit you, rather than you trying to hit yourself? I suspect the analogy might break down before we got anywhere useful though?
    - Sean1708 8y ago
      > A friend, you just let go! I'm kind of in two minds about this analogy because on the one hand it does correctly explain why it's difficult to get to the sun from the earth, but on the other hand we don't get to mars by turning the sun's gravity off.
      - Klathmon 8y ago
        That's a fair critique, but most people (that I've spoken to about this anyway) understand that a rocket needs to "push against gravity" to go places, they just forget or don't know that orbital velocity is a thing and what it means (or that you need to cancel that orbital velocity out before you can start using gravity, and that gravity is ultimately pretty weak) The analogy is meant to try and explain why going toward the sun might be harder, especially when it is so counter-intuitive if you've never thought about it, and it does so by taking some liberties with parts of orbital mechanics that people already instinctively understand.
        Sean1708 8y ago
        I think on balance it's a definitely a decent analogy, this was just something that jumped out at me as being a possible source of confusion. If this is an analogy that you've used successfully without this confusion coming up then it's probably not too big a deal.
  - irrational 8y ago
    I get that. I understand why it is this way. But it is still counter-intuitive if you don't understand the physics (or your not a rocket scientist ;-)
ordu 8y ago
> But to get to the Sun, you need to subtract 100% of Earth's orbital velocity. Yeah, one would need to substract 100% of velocity if it wanted to land on a surface of the Sun safely, bringing the ship to a full stop. But why might one bother with the slowing down, if ship will just get melted by the heat of the Sun? You need just to rotate a vector of velocity by some clever gravity assist maneuver.
- cecilpl2 8y ago
  Not true. To even touch the surface of the Sun you need to subtract nearly all of the Earth's orbital velocity. Parker Solar Probe, for example, did this with a series of gravity assists off of Venus. https://www.youtube.com/watch?v=cMNQeCWT09A https://www.youtube.com/watch?v=cMNQeCWT09A
ricardobeat 8y ago
This is really interesting. Is there a connection between this fact and the universe being continuously expanding?

benj111 8y ago

On the other hand where do you ultimately end up if you only add 40% delta V ?

irrational 8y ago

That's so counter-intuitive. It feels like the sun is a giant gravity well and we just have to give something a bit of a shove in the right direction to fall down into the well. It's not like its rocket science.

mkl 8y ago
That might work if it was starting stationary, but it's not. It's starting from Earth, which has enormous orbital velocity perpendicular to the way to the sun. Everything on Earth already has that velocity too, and there's no way to get rid of it except cancel it out by burning a lot of fuel to basically go just as fast in the opposite direction.
- Klathmon 8y ago
  I always hated using terms like "orbital velocity" trying to explain things to people who don't know what it means. The analogy I use is imagine you have a ball on a string. If you spin that ball around your head, so the string is tight, how hard do you think it would be to hit a friend with it? Now how hard would it be to hit yourself with it? A friend, you just let go! The ball is already spinning and WANTS to go flying out further away. But to hit yourself? You are going to have to first stop the spinning, then pull it toward yourself, or just pull repeatedly on the string harder and harder until it spins faster and faster until it hits you. It's a lot more effort! It may not make sense at first, but it's easy to forget that earth is spinning around the sun pretty damn fast, and to get near it you need to first stop (or GREATLY reduce) that spin, that orbit, and only then can you start to approach it.
  - benj111 8y ago
    Surely in your analogy you would need a trained hamster sitting on the ball of wool, spinning a ball of cotton, trying to hit you, rather than you trying to hit yourself? I suspect the analogy might break down before we got anywhere useful though?
  - Sean1708 8y ago
    > A friend, you just let go! I'm kind of in two minds about this analogy because on the one hand it does correctly explain why it's difficult to get to the sun from the earth, but on the other hand we don't get to mars by turning the sun's gravity off.
    - Klathmon 8y ago
      That's a fair critique, but most people (that I've spoken to about this anyway) understand that a rocket needs to "push against gravity" to go places, they just forget or don't know that orbital velocity is a thing and what it means (or that you need to cancel that orbital velocity out before you can start using gravity, and that gravity is ultimately pretty weak) The analogy is meant to try and explain why going toward the sun might be harder, especially when it is so counter-intuitive if you've never thought about it, and it does so by taking some liberties with parts of orbital mechanics that people already instinctively understand.
      - Sean1708 8y ago
        I think on balance it's a definitely a decent analogy, this was just something that jumped out at me as being a possible source of confusion. If this is an analogy that you've used successfully without this confusion coming up then it's probably not too big a deal.
- irrational 8y ago
  I get that. I understand why it is this way. But it is still counter-intuitive if you don't understand the physics (or your not a rocket scientist ;-)

ordu 8y ago

> But to get to the Sun, you need to subtract 100% of Earth's orbital velocity. Yeah, one would need to substract 100% of velocity if it wanted to land on a surface of the Sun safely, bringing the ship to a full stop. But why might one bother with the slowing down, if ship will just get melted by the heat of the Sun? You need just to rotate a vector of velocity by some clever gravity assist maneuver.

cecilpl2 8y ago
Not true. To even touch the surface of the Sun you need to subtract nearly all of the Earth's orbital velocity. Parker Solar Probe, for example, did this with a series of gravity assists off of Venus. https://www.youtube.com/watch?v=cMNQeCWT09A https://www.youtube.com/watch?v=cMNQeCWT09A

ricardobeat 8y ago

This is really interesting. Is there a connection between this fact and the universe being continuously expanding?