Expert: Tom Whiting - 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
Kelly,
Excellent question....I was asking the exact same question about
50 years ago concerning the asteroid belt.

With the rings of Saturn, the Cassini spacecraft DOES go over and
under the ring structure, but mainly stays outside their reach to begin with.  In the Rings, the particle count (density) is extensive, and a high speed trip through the rings would be suicidial, except in the gap regions, and even then you would be taking a big chance on impacting a 3 inch chuck of water-ice or frozen gas.  I believe one of Cassini's final mission requirements, or I should say, options....is a plunge down into the ring structure to end the mission in 3 or 4 more years from now.

The asteroid belt is an entirely different story, density wise.
It does not pose the danger that sci-fi writers envisioned, so there
is no need to divert above, or below the Belt....which you could do, but we found it's not necessary.  The chuck density is so low, that even if you WANT to encounter or photograph an asteroid, a spacecraft
moving in that region actually has to go out of their way to
encounter or photograph an asteroid. So, the Main asteroid belt
with it's some 200,000 bodies from hundreds of miles wide down
to 1/2 mile wide, the volume of space between Mars and Jupiter is
so very huge, as to not constitute a problem or danger for spacecraft.

Its kind of like you flying in a commercial jet aircraft out over
the Pacific Ocean...your plane and perhaps at any one time, 4000
other jet aircraft are over the Pacific Ocean, and yet.... during your 12 hour flight, you're lucky if you happen to spot even one other aircraft in flight, because the volume expanse is so huge.
I don't believe we've ever had a mid-air collision out over any
ocean surface with 2 commercial jet aircraft.  Those usually happen
near airports, where the aircraft density is super-high.

Now multiply that Pacific Ocean volume expanse by a hundred billion, and you get the idea of the volume of the Asteroid Belt region between Mars and Jupiter. So the asteroid belt is just a no problem area, like we once thought it was. So there is no need to 'waste'
extra fuel, going over or under, which we could do....it's just..
not necessary.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA

FOLLOW UP:
There is no "roof or floor" but space is 3 - dimensional.  BUT we
all do exist on a plane...one plane called the plane of the ecliptic.
When you draw out all the planets and asteroid belt on a sheet of
paper, they are all basically on that sheet of paper....there's no planets going 90 degrees to that piece of paper, as we're all on it.

So it's very desirable (from a fuel standpoint) to stay on that
plane when traveling to the other planets. Once you deviate your
spacecraft off the plane, then you are not traveling in exactly the
correct direction, and you'd have to use even more fuel to get back on that plane of the ecliptic, if you want to travel to a Solar System planet or asteroid.  So our spacecraft want to stay on that plane if at all possible...for time and fuel savings.
Hope this helps,
Clear Skies,
Tom