I'm just reading Kim S. Robinson's Mars Trilogy (Actually I wanted to read it for quite some time, but only got around to ordering it with a bunch of other scifi classics in October) and I noted something...

In Red Mars, the settlers ride in blimps over the planet. They get into a dust storm, yet manage to land and even walk around in it. This is explained by the thinner atmosphere, and thus less total pressure.
However, if that is so, how is the blimp supposed to fly? I remember from a recent trip to the Zeppelin museum in Friedrichshafen that the payload a blimp can carry is extremely small compared to its total size. If the atmosphere is that much thinner, could a blimp even rise, even though Mars gravity is much lower that Earth gravity?

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Si tacuisses, philosophus mansisses.

"Half of what I say is meaningless, but I say it so that the other half may reach you."

I don't know if this could completely justify it, but keep in mind that the pressure inside the blimp will be just as much lower. Basically, assuming similar average molecular mass for two atmospheres, the same mass of helium (or hydrogen, or whatever) can lift the same mass of cargo. It'll just need a lot more volume to hold the same mass in a thinner atmosphere. And of course, with more volume comes more mass in the gas envelope... that's where I'm not sure if it would work or not.

Atmospheric pressure on Mars is less than 1% earth, but the air is colder and the molecular weight is higher and the gravity is lower. Atmospheric density also varies widely on Mars, so some blimps might be limited to just the lowlands.

Anyway, I'm guessing your blimp would have to be about 50x the size on Mars for the same mass of payload and structure. The envelope of course is bigger, so that's going to also have to be compensated for. Maybe roughly 4x the size in every dimension to carry the same payload as a similar sized blimp on Earth (that might still be underestimating. The props for Mars will have to be larger, and the engines will likely be atomic, which may also require more weight. Possibly you'd use sand for ballast; almost certainly you'd use hydrogen for the lift gas).

Thing is, while the envelope would indeed have to be larger, that doesn't mean that it's not possible. Especially since with the lower gravity, you wouldn't need so much lift to get a set mass to rise....

Besides, it's not actually KSR's idea, there have actually been robotic probes based on blimps sent to venus already (the Vega probes, two blimps. First one folded after less than an hour IIRC, but the second lasted a few days). There was another due for lanch on the russian Mars98 probe, but it was delayed and dropped from the probe in the end. JPL has designs for future Mars blimp probes (they call them aerobots) and for Titan.

So it does work, the scales are just different. Which, as KSR will point out as you continue to read the trilogy, will probably be the main thing we'd see from a nisei generation of settlers on Mars - the ability to think on a scale that we currently don't.

Sparks, I agree with you on Venus, since that planet has a rather thick atmosphere iirc...
Do you have any links for the Mars project, though? I would really be interested in the technology the russians and/or JPL used...

__________________
Si tacuisses, philosophus mansisses.

"Half of what I say is meaningless, but I say it so that the other half may reach you."

Okay, here's a link to a PDF on the Titan project:
http://www.lpi.usra.edu/meetings/out...1/pdf/4096.pdf

A quick survey of robotic blimp probes:
http://web.nps.navy.mil/~ksford/plan/node1.html

A spaceref article on the aerorover idea:
http://www.spaceref.com/news/viewpr.html?pid=5176

And the JPL page on aerobots/aerorovers:
http://www.jpl.nasa.gov/adv_tech/balloons/summary.htm

Point of interst:

If you like the series, there's a fourth book, The Martians, that provides backstory and documents referred to in the series. (I won't tell you which documents, incase you aren't that far along yet)

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