QUESTION:
Is it possible to make vehicles hover on Mars?  How?  What about airplanes
and helicopters?

ANSWER from Chris Salvo on February 28, 1997:
There are many ways that we know about to make things hover (and there 
are some ways that only work in science fiction, but we won't talk 
about those).  Helicopters, Harrier jets and the V-22 Osprey all hover 
by pushing air downward with enough force so that the equal and 
opposite reaction is a force large enough to hold them up against the 
pull of gravity.  Hovercraft work similarly by pushing air into a 
region underneath them causing a pocket of high pressure air to exist 
between them and the ground (or water or whatever they happen to be 
over).  Another way to hover is by using magnetic forces like some 
high speed trains do.  A magnet is placed in the bottom of the train 
and another one of the same polarity in the track.  The two magnets 
push against each other and cause the train to hover.  Airships and 
balloons also hover by displacing the surrounding air with a large 
volume of another gas that weighs less (like helium or air that is 
hotter than the surrounding air).
     
Any of these methods could work on Mars, but there are some 
differences between Mars and Earth that will make some of them work 
better than others.  First, the gravitational force on Mars is 0.38 
that of Earth.  This means that something that weighs 100 pounds on 
Earth will only weigh 38 pounds on Mars.  This makes the problem of 
hovering quite a bit easier because a smaller force must be generated 
to oppose the force of gravity.  Second, the atmosphere on Mars is 
much thinner than on Earth.  This makes it much harder to get a lot of 
force out of rotors or to trap air under a hovercraft, but it could 
probably be done.  Jets engines could be designed to work at the low 
pressures of the Martian atmosphere, but there is very little oxygen 
in the Martian atmosphere.  In fact, the atmosphere is mostly carbon 
dioxide which would not burn in a conventional jet engine.  
     
The best bet may be a magnetic system like some high speed trains on 
Earth use.  The problem is that it requires a track to be built 
wherever the craft will go.  This limits its use a great deal.  If 
your hovering vehicle must be able to go all over regardless of 
whether or not there is a track built then the magnetic system will 
not work.  
     
Another way that might work well would be an airship (like the 
Goodyear blimp).  If your vehicle did not need to be very fast, an 
airship filled with helium could be made to work quite nicely.  It 
would have to be quite large to provide enough lift, but with modern 
lightweight materials it might work very well.
     
I hope this helps, but please don't limit yourself to my thoughts.  
I'm sure there are a lot of other ideas that would work that I have 
not thought about.  The best choice will depend upon exactly what you 
want this system to do.  Does it have to be fast?  Does it have to go 
far?  Does it have to go anywhere (like a plane) or only from one 
specific place to another specific place (like a train)?  What will it 
carry?  Ask yourselves lots of questions like these and the answer 
with become more clear.
     
Chris Salvo

ANSWER from Mark Adler on November 25, 1997:
The key difficulty is that Mars' atmosphere is radically thinner than
Earth's, requiring any device that relies on air pressure for lift to be
much larger, yet no more massive than a similar device on Earth.  This
applies to hovercraft as well as airplanes.