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While looking from Earth's surface it gives the impression that all planets, stars, sun etc are above our heads. When a rocket travels from earth to outer space, it takes a trajectory and gives the feeling that in order to explore space one always have to travel horizontally regardless of interplanetary journey or out of solar system.

Is it true? I mean if one has to get out of solar system he will still have to travel horizontally instead of going up, up and up? Kind of gives impression that our universe is a long rectangle.

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    $\begingroup$ Virtually all rocket launches go up a short distance to get out of most of the atmosphere, then fly towards a more horizontal path to build up orbital velocity. Have a look at this answer for more. $\endgroup$ – Andy Oct 18 '16 at 9:01
  • $\begingroup$ Thinking of horizontal and vertical is just going to confuse you. Would you consider a rocket launched in Australia to be "down"? $\endgroup$ – pjc50 Oct 18 '16 at 12:21
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    $\begingroup$ I suggest checking out Why does the SpaceX Falcon 9 rocket do a 180 flip for reentry?, particularly my answer which (somewhat inadvertantly) turned into a bit of a space travel primer. You may also be interested in xkcd what-if: Orbital Speed. $\endgroup$ – a CVn Oct 18 '16 at 15:18
  • $\begingroup$ @pjc50 what do you mean by "down" and Australia here? $\endgroup$ – Volatil3 Oct 18 '16 at 17:03
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    $\begingroup$ @Volatil3 I'm questioning what you mean by "vertical". Take a globe. Place a finger on Florida, imagine that's a rocket taking off from Kennedy. You've called that direction "vertically" or "up". Now without rotating the globe, put another finger on Australia. Imagine that's a rocket taking off from Woomera. Note that these are different directions. Are they both "up"? $\endgroup$ – pjc50 Oct 18 '16 at 17:26
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This is the result of two physical facts: that gravity, pointing toward the center of the Earth, is the only way to define "down", and that most of the Solar System is quite close to a single geometric plane, the ecliptic, which sort of is like the rectangle you describe. It's also partly because most of our communications are essentially two-dimensional, so depicting the actual three-dimensional nature of space is rather difficult.

The first of those physical facts is pretty straightforward. Because "down" is always toward the Earth's center, to go anywhere away from the Earth we have to go "up". That's really all there is to it. (Although, in practice, most spacecraft have to achieve orbit before leaving Earth's vicinity, and orbit is moving sideways at very high speeds while falling, such that the inertia of that high speed counters the acceleration of falling. So while spacecraft do need to go up, they also need to go sideways comparatively much faster until they're quite far out from Earth: tens if not hundreds of thousands of kilometers.)

The second of these is a bit more involved. The orbit of a body always lies in a single plane (discounting the various perturbing effects that can shift it slightly), so if the Sun had a single planet and nothing else it would, by definition, be a system with the "long rectangle" effect you describe. But in fact our actual Solar System is very close to being that same "long rectangle", despite the numerous bodies in it: the planets orbit within just a few degrees of the same plane, called the ecliptic (which is defined as the plane that the Earth orbits in), and many of their moons aren't much farther off.

Since it's so close to the truth most of the time to depict a trajectory from "above" the ecliptic without worrying about it going a little up or down, and since it's so much more difficult to print or display a full three-dimensional orbital track, most publications simplify it the way you've seen. But the actual inclination of orbits does need to be tracked for interplanetary probes, of course, any of which would miss essentially any target by millions of kilometers if not more if they simply assumed everything was in the ecliptic.

In the case of interstellar missions, things get more complicated. Stars aren't distributed very tidily in relation to the ecliptic. However, practical interstellar exploration, if we ever have the technology to do it, will still follow a single plane for most of the mission, or as close as makes no matter, and since the distance between stars is vastly larger than the entire Solar System, diagrams can just represent it as a dot on one side of the page and show a nice tidy long slightly curved line to a dot on the other side. The actual trajectory from Earth might look like a series of turns around the Earth and Sun before a slowly straightening path off toward the destination, but you can understand why the groundside view is not very practical to put in a single picture — you can't see the spacecraft most of the time, and when you can, you can't really see what it's doing.

If we ever get to the point of scheduling multiple star system stops in different Sun-relative inclinations in the same trip, then things will finally need to be represented more accurately as a routine. But that's so far in the unforeseeable future there's no real point worrying about it.

It's fair to mention, though, that the Voyager spacecraft in particular have rather high inclinations at present; they used their last gravity assists (from Saturn and Neptune respectively) to shoot off at a fairly sharp angle of 36° and 79° respectively relative to the ecliptic. Prior to that their inclinations varied within the normal few degrees of the ecliptic. Accurately showing the Voyager 1/2 trajectory from 9.5 AU (Saturn) or 30 AU (Neptune) to their current position (which, at least in Voyager 1's case, is well over 100 AU from the Sun) would still mostly focus on the post-ecliptic phase, but there's enough beforehand that a good diagram might really need to work on showing the transition three-dimensionally.

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A free program to visualize the solar system may help you; try Celestia, it's quite good. Turn on the visualizations of the orbits and you will see how they are nearly all in the same plane. Since there's no up or down in space, it's meaningless to talk about horizontal or vertical. Since the solar system is very close to a plane we could call that flat, horizontal, aka the ecliptic, whatever, but it's an arbitrary thing.

One sort of up and down in space is in terms of gravity, and we can climb up and down the "gravity well" by traveling faster or slower. Imagine space as a stretched rubber sheet, with each planet a weight on the sheet, causing it to bend downwards. To leave a planet's surface, it doesn't matter what direction you'd go, it's uphill in every direction. Space is sort of the same. With a lot of energy you can get up out of the planet's gravity well, and move across the sheet and eventually fall into a different depression.

If you wanted to leave the solar system, and have a lot of energy (like a massive rocket) you could do it in any direction you want, aiming at any star. You just need enough energy to get up out of the gravity well. But in practice we don't have that kind of energy, so we need to have space probes swing by a planet or two in a slingshot maneuver, which gives them a boost of speed. This means they often travel in the ecliptic "horizontal" plane to start with, since that's where the planets are.

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