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Let's take a look at the trajectory of variety probe missions.

Variety Probe Mission Trajectory New Horizons and Ultima Thule will be 4.1 billion miles away when it visits the Kuiper Belt object. This chart shows the path of New Horizons compared to other probes that have left the solar system. Credit: NASA/Johns Hopkins University Applied Physics Laboratory [1]

I wonder, why all these probes tend to explore outer system were launched to go outwards of the ecliptic plane instead of go upwards or downwards? What I'm talking about here is go upwards or downwards that is considerably closer to 90 degrees. I was told the ecliptic plane of any solar system tends to stay relatively uniform, with only Kuiper Belt Objects (KBOs) showing bizarre inclinations. The closest thing we'd find "below" the Earth would be an Oort Cloud object or outer star system, is it accurate to say so? What would we find if we go straight upwards and downwards?


Point of interest

1 Nola Taylor Redd, Space.com Contributor, January 2 2019, NASA's New Horizons Just Made the Most Distant Flyby in Space History. So, What's Next?

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    $\begingroup$ The interesting things are in the ecliptic plane more or less. $\endgroup$ – Organic Marble Jan 3 at 14:21
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    $\begingroup$ Why would we want to? What goal are you intending to achieve by doing so? What do you consider the primary purpose of such a mission? What do you consider are/were the primary purposes of the missions you are asking about? How would those have been met by using a trajectory that wasn't in the ecliptic? $\endgroup$ – Makyen Jan 3 at 19:01
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    $\begingroup$ @Dave No, that's not as easy a question, not even close. We don't have unlimited resources. The number of missions isn't unlimited. There must be a reason to do something, and that reason must be more important than all the other projects that are competing for the limited resources available.. $\endgroup$ – Makyen Jan 3 at 22:53
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    $\begingroup$ You'd find a bunch of nothing, and then have to spend your entire second stage's budget (probably more) on a 90 degree inclination change, or continue to find nothing that Voyager 1 hasn't already. Launched at 90 degrees inclination, the only thing you can do is keep going that way. $\endgroup$ – Mazura Jan 4 at 0:21
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    $\begingroup$ Probes don't explore. They're sent to do closeups. $\endgroup$ – Agent_L Jan 4 at 10:54
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Starting out from Earth, you have the free 30 km/s from Earth's movement around the Sun, which is in the plane of the ecliptic. To get far out of the plane you either have to boost a similar amount "up" or "down" (which is beyond the capability of current rockets) or go via one of the gas giants, and use its gravity to change course. So at least to start with you don't really have a choice.

One example of using a gas giant to change plane was the Ulysses probe, which, although it only barely got into the Outer Solar System, did orbit at almost 90 degrees to the ecliptic (80.2 degrees, in fact) in order to get a look at the North and South poles of the sun and the radiation and magnetic fields coming from them.

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We've had 5 flyby missions to the outer solar system so far. All of them had primary missions at one or more planets. That set the main constraints for their trajectories. Anything after the last planetary encounter was secondary.

For Voyager 2, for instance, the Neptune flyby was aimed at a close encounter with Triton, which reduced the possible exit trajectories:

enter image description here

You also see Voyager 2 now has a significant angle relative to the ecliptic.

The others:

  • Voyager 1 had moons to visit at Saturn
  • for the Pioneers, I don't think an interstellar mission was considered at all
  • New Horizons had to fly by Pluto, so it was limited to Pluto's plane (Pluto is not large enough to change NH's course significantly)

What would we find if we go straight upwards and downwards?

There are almost no targets of interest outside the ecliptic. No planets or other bodies we can visit until you get to the Oort cloud, which is 1000 AU out (10x further than the Voyagers are now after 40+ years of traveling). The only reason to choose a trajectory perpendicular to the ecliptic, is to have another measurement point for the solar wind. But spending an entire mission to 100 AU on just that would be expensive for very limited return.

A study for an interstellar mission is underway, but that too is planned to include a KBO visit so it'll be constrained to the ecliptic.

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  • $\begingroup$ Is it really just objects of interest? Would it not be more difficult to send a probe along the axis perpendicular to the elliptical plane? $\endgroup$ – Ellesedil Jan 3 at 17:47
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    $\begingroup$ A bit, but not much, I'd expect. You do a gravity assist at Jupiter, then use Saturn to aim the probe out of the ecliptic. $\endgroup$ – Hobbes Jan 3 at 20:18
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    $\begingroup$ One very interesting target outside the ecliptic is ʻOumuamua. $\endgroup$ – gerrit Jan 4 at 17:39
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    $\begingroup$ at 26 km/s, Oumuamua is going to be difficult to catch up with current technology. $\endgroup$ – Hobbes Jan 4 at 17:41
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    $\begingroup$ Indeed challenging! A paper on chasing 'Oumuamua. Notice the the extreme manoeuvering on page 9. Out to Jupiter, swing around to basically fall into the Sun, make extra close flyby and light a heavy booster at perihelion. Then it's outwards and "upwards" to get the cigar and possibly collapsed alien light sail. (Although it it really is a light sail, it would be the dropped deceleration sail and the real fun will start in 100+ y or so when the actual payload arrives. Where is Larry Niven?) $\endgroup$ – David Tonhofer Jan 5 at 12:04
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It is important to realize that space probes aren't really useful for finding objects in deep space. Space is so empty that a probe sent in a random "exploratory" direction would have a negligible chance of detecting an object orbiting the sun. The best way to find objects outside the ecliptic is to look for them using really large Earth-based or orbital telescopes. And there doesn't appear to be much out there.

All of the probes on their way out of the solar system were meant to explore current or former planets, which are all close to the plane of the ecliptic. That really is the reason the probes are close to that plane.

It would be of interest to sample the far reaches of the Sun's magnetosphere in different directions, but it hasn't been judged worth the expense.

Finally, it is easier to send probes out within the plane of the ecliptic because we can use the motions of the planets to give a speed boost. The Earth's motion of about 30 km/s is a nice start, but it's useless if you are heading directly towards the ecliptic north pole. And you can't use the outer planets as gravitational slingshots if you are going perpendicularly away from them. (You can use one to steer your craft up out of the plane, as @Organic Marble pointed out was done by Ulysses, but that sacrifices the speed boost you could have gotten from that planet.)

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    $\begingroup$ Your last statement is wrong. The Ulysses mission used Jupiter to change the plane of its orbit out of the ecliptic. en.wikipedia.org/wiki/Ulysses_(spacecraft)#Jupiter_swing-by $\endgroup$ – Organic Marble Jan 3 at 20:31
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    $\begingroup$ I think the last statement is about the assist,, which is like cos theta of the inclination change: plus or minus a maximum in plane, dropping to zero when completely out of plane. $\endgroup$ – Bob Jacobsen Jan 3 at 22:58
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    $\begingroup$ +1: Probes aren't sent to look for, they're sent to look at. $\endgroup$ – chrylis Jan 4 at 18:07
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Staying in the same plane as the planets of the solar system allows us to use them for propulsion by means of a slingshot maneuver. Actually this Wikipedia page even has gifs showing the Voyagers as an example, so I guess it doesn't make much sense for me to go into detail here: https://en.wikipedia.org/wiki/Gravity_assist

But in essence there is no reason to not use such maneuvers for additional acceleration and as mentioned before going perpendicular to the plane would require propulsion systems we don't have (yet). Even if we had such technology, there is still no reason to not use that free energy anyway, since these probes have no specific destination.

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A separate point - there's a degree of circular reasoning.

Humanity can't see much of anything interesting outside the plane yet near enough to visit, so we don't go there when there are far bigger and more exciting things in/near the ecliptic plane.

So the upshot is we haven't had a closer look. Probes cost money and without a defined set of mission goals that show a useful purpose, then the money goes to the more interesting proposals.

Who knows what science might learn by shooting a generic probe straight up or down?

Answer funding money comes easier when there's an interesting purpose.

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  • $\begingroup$ Up/down could be a valid use for a solar-sail equipped probe that doesn't have to be anywhere on-time. $\endgroup$ – Criggie Jan 4 at 21:37
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    $\begingroup$ We've gotten really good at detecting objects in the solar system. We can see objects 30 km across at 40 AU. If there were any objects out of the ecliptic plane (other than a few outlier KBOs) we'd have found them already. $\endgroup$ – Hobbes Jan 5 at 9:02
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    $\begingroup$ When Planet Nine (or is that "Planet? Nein!") becomes a known quantity, there will be something to go "up and far". $\endgroup$ – David Tonhofer Jan 5 at 12:48
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Mostly because launching upwards would be useless to us, as we have extremely limited deep space travel capabilities.

Besides that; all rockets are launched along our equator anyways, the pull and movement of the Earth allows for an easier launch, and if we launch along our familiar plain we are able to utilize gravity assists from other celestial objects.

Gravity assists are a big reason, they give a huge boost to the rocket being launched and they give a flyby opportunity (great for research).

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  • $\begingroup$ We do have satellites in retrograde orbits and in polar orbits, so clearly some launches are not entirely prograde equatorial. It depends on what the mission objectives are. If the mission objectives are such that there is benefit from a non-prograde-equatorial launch, or from transitioning out of the ecliptic, that'll be another factor that enters into "should we do this, or can possibly multiple alternative missions give us more science for the same amount of money?" discussions. $\endgroup$ – a CVn Jan 6 at 17:44
  • $\begingroup$ @aCVn Right, thank you. I was aware of the retrograde and polar orbiting satellites, I guess I was just talking about most missions the majority are in equatorial orbits. There are plenty of reasons to launch into different orbits even if they are just changing the orbital plane by a few degrees. Take GPS Navstar satellites for example, they all have different orbital planes. Thanks for pointing that out. $\endgroup$ – PolyversialMind Jan 6 at 18:06

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