When wanting to exit our solar system, do we have to always go outward from the sun in the direction of the other planets? Or can we simply travel “north”?
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What you’re referring to is called “the plane of the ecliptic”, basically the flat plane in which more or less the whole solar system orbits.
And the short answer is we don’t *have* to, but it’s way more efficient. It’s hardly impossible, just costly… and we don’t really have a good reason to do it.
Rocket science all comes down to fuel efficiency. Acceleration is expensive, and you pretty much have to carry the mass of all the fuel you’re going to use from the very beginning of your trip. Fighting gravity is expensive.
Advantages of travelling in the plane of the ecliptic:
* You get to use your existing orbital velocity (Earth’s speed as it travels around the sun) as a starting point for the escape velocity you need to climb out of the sun’s gravity well.
* You get to use the gravity of planets and other massive bodies further out to help “slingshot” you further and faster.
Problems with leaving the plane of the ecliptic:
* Orbital velocity is in the wrong direction, so it doesn’t really help you much. You could try to find a way to slingshot around the south pole of another massive body, like maybe Venus, to translate that momentum into a different axis, but that requires a bunch of extra maneuvering, and maneuvering costs fuel.
* Instead of being helped along by gravity wells further out, you basically have to hit escape velocity sufficient to overcome the *collective gravity well of the entire solar system’s mass*. You’re a long way from most of it, but it still matters.
Finally, basically everything we want to get to is in the plane of the ecliptic.
* Nearby rocky planets like Venus and Mars that might be candidates for colonization.
* Asteroids and the asteroid belt, which might have resources we want, and might need to be studied closer in light of possible future collision hazards.
* Gas giants that bear closer study, like Jupiter and Saturn.
* Jovian/Saturnian moons, which might have resources or even extra-terrestrial life (Titan, Enceladus, and Europa, especially).
* Cometary bodies/the Oort cloud, if we want to send a probe *way* out there.
Up the Z axis is basically empty. Even if you were trying to get to Alpha Centauri, ~ 4 lightyears away, you’d be better off heading along the plane of the ecliptic initially and then slingshotting off Jupiter to get you off the plane at the right angle.
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