Re: A case for the eradication of unmanned Mars missions

Doug Jones (
Wed, 08 Dec 1999 18:58:23 -0800

"Robert J. Bradbury" wrote:
> On Wed, 8 Dec 1999, Brian D Williams wrote:
> > The notion of "Cheap Power from space" has been thoroughly
> > discredited, see Robert Zubrins "Entering Space" for details.
> >
> I strongly doubt that Zubrin understands either self-replication
> or nanotechnology sufficiently to *discredit* "Cheap Power from
> Space". All he can possibly do is point out that using current
> technologies it isn't going to be "cheap".

Entirely true, especially since *peak* power can be sold to the utilities at higher prices- simple earthbased photovoltaic without storage is very closee to being profitable even now, while O'Neill-type space systems would require huge risky investments with poor ROI.

> I mean really now,
> quoting Zubrin as a source on this list? For heaven's sake
> this man wants to *colonize* something at the bottom of a
> gravity well. That means you are going to have to pay
> through the nose every time you want to go someplace else
> or ship something. Give me exclusivity on the Mars to Earth
> FedEx route and I'll show you a rich man.

Actually, with direct entry into the Mars atmosphere, Mars is easier to get to than the moon, in terms of delta-V. Granted, you need a larger crew vehicle for the longer mission duration, but if it's a colonization mission, you need the living quarters anyway. It has advantages for near-term colonization in that volatiles can certainly be extracted from the atmosphere. As for Mars to Earth transportation, the propulsion can be supplied by a single stage *pressure fed* LOX-methane rocket, about the dumbest of dumb boosters.

> The idea of "colonizing" a planet has been discredited since
> O'Neill showed it was entirely unnecessary. I think Greg
> is right, that the Mars folks are a bunch of romantic
> idealists in love with their pipe dreams.

A dispassionate examination of the detailed trades seems not to favor either approach. I've been a senior assoiciate of SSI *and* a Mars Undergrounder, so I've looked at both pretty closely, and it seems a wash.

> Since you have to have a completely sealed environment anyway
> (otherwise your atmosphere leaks out) you are never going to
> lose any H2O. So you can get buy with a lot less than you
> need in an Earth based environment. With colonization you
> are going to need power, lots of it, and for that the moon
> is better.

Habitats leak, and badly. Current state of the art, given the trades between fabrication costs, mass per unit volume, etc, yield habitats with nearly 1%/day overboard leakage. Every joint, hatch, power feedthrough, and window mount _leaks_, and substantial amounts of makeup gases and water will be needed.

Power is the weakness of Mars bases, volatiles *and* power (2 week nights) are both a problem for lunar bases. An appropriate asteroid may allow good access to sunlight for power as well as containing adequate volatiles, but if the delta-V requiremenat are low, the launch windows are rare, a catch-22 of the synodic period.


Attribute         Moon               Mars                NEO
Launch window     Daily              22 months           2+ years
Flight time       3 days             9 months            3-6 months
Stay time         indefinite         16 months           years at minimum
Volatiles         Polar ice/concrete Atmosphere          Chondrites?
Delta-V from LEO  6 km/s             3.5 km/s            3.5 to 6 km/s
Delta-V return    2.5 km/s           6 km/s              0.5 to 2 km/s
Sunlight          1350 W/m2          470-690 W/m2        <1350 W/m2
Nights            350 hr             12 hr               avoidable

The tradeoffs are not immediately obvious. I think the moon's south pole might be best *if* volatiles can be extracted, since it would have frequent launch windows, short travel times, and reliable power (if only in limited geographical locations). While the delta-V for landing is high, the short trip time allows use of high performance cryo stages.

Without lunar volatiles, I feel Mars would be a better choice, and random NEOs might have advantages at particular times, but at the cost of very long manned missions.

> > >You're still dead if there's a decompression. You still leak stuff
> > >outside and have to continuously replace it. But you cannot launch
> > >anything with a mass driver from the surface, with the possible
> > >exception of the top of Olympus Mons.
> Eugene, I don't think thats accurate. If you've got calculations
> I'd like to see them. NASA is studying mass drivers to assist
> launching on Earth, so I would think they have to work on Mars.
> The question comes down to whether the friction limits launch
> speed to sub-orbital rates, in which case you need rocket assists.

You don't need mass drivers on Mars- a skyhook lowered from Phobos can troll through the upper atmosphere at about mach 1 (250 m/s in cold CO2), with a good taper ratio. Indeed, Phobos & Deimos are wonderful resources for O'Neill development in the Mars system, far better than our own moon.

> > >Mars should be a second
> > >step. If you have the Moon, Mars is easy. Not vice versa. And you
> > >can't do both, it's too expensive.
> If you did self-replication here on Earth first, you could do them
> both very quickly and very cheaply...

Yeah, if we had some ham, we could have ham & eggs, if we had some eggs...

Doug Jones
Rocket Plumber, XCOR Aerospace