Expert: Courtney Seligman - 3/14/2009

Question
Why can't the Cassini–Huygens space craft travel above or below the Asteroid Belt and the rings of Saturn?  Why does it have to go through them.  I would think it would be easier and safer to just go above or below these things.  It seems like if the space craft has to travel through these things it's like there's a "roof" and "floor" in space that you have to stay between. Is there something out there that prohibits us from going up or down in space?  I hope this makes sense.  Thank you.

Answer
There isn't any restriction on our ability to go above or below the plane of the Ecliptic. There are practical reasons why non-Ecliptic orbits aren't generally used, but they can be achieved, if there is sufficient reason to do so.

At launch from the Earth, spacecraft have two velocity components -- the one they shared with the Earth at launch, which is exactly in the plane of the Ecliptic, and any additional velocity provided by launch fuel in excess of that required to merely 'escape' the Earth's gravity. The latter is always a small fraction of the Earth's orbital velocity, and if we wish to go well outward or inward from the orbit of the Earth, its vertical component must be even smaller. So at launch, orbits are invariably in nearly the same plane as ours.

However, we can alter the orbit by 'throwing' the spacecraft above or below another planet (preferably Jupiter, as the most massive planet, and the closest of the more massive ones). The interaction of Jupiter's gravity with the spacecraft's existing motion can substantially alter its motion, and has been used to change the orbit of every spacecraft sent to the outer solar system. In particular, the Ulysses spacecraft, which is in a polar solar orbit (so that it can look down at the Sun's north pole, and on the other side of its orbit, up at the Sun's south pole), was placed in its highly inclined orbit by a very close approach to Jupiter.

So, as far as orbits in general are concerned, we can go up or down, more or less as high or low as desired, but only by using a gravitational assist from a planet. The question is, should we bother with this, for a given spacecraft? For Ulysses, it was a necessity. But for other spacecraft, it would usually be counterproductive.

For the Cassini spacecraft, there was no practical reason to avoid the Ecliptic plane in its flight to Saturn (though see the note about the asteroid belt at the end). In fact, since Saturn is in that plane, putting the spacecraft in a non-Ecliptic orbit would have made it far harder to go into orbit around the planet, as its approach velocity relative to the planet would have been higher if it hadn't been moving in nearly the same plane as the planet.

Once in orbit around Saturn, there was a need to alter the plane of its motion relative to the plane of the planet's orbit, because Saturn has an axial tilt of 27 degrees, and almost all its moons have orbits in its equatorial plane, rather than its orbital plane. I believe (though I'd have to refer to mission manuals to be sure) that the decision to go 'through' the rings was made to allow as close an approach to the planet as possible, without actually running into anything. If the spacecraft had kept well outside the rings, it would have needed more fuel to alter its orbit. So although there was some minor risk of hitting something while passing through a gap in the rings, it was felt an 'acceptable' risk, given the much larger gravitational assist available by passing closer to the planet.

Note: As far as the asteroid belt is concerned, although there are many asteroids, they are so small and so far apart that in that portion of the flight which went through the asteroid belt, it would have been unusual to get close enough to see one, let alone worry about running into one. As a result, no effort was made to avoid that region.