[Beowulf] Re: blackbox on Mars?

Robert G. Brown rgb at phy.duke.edu
Fri Oct 20 16:21:31 EDT 2006


On Fri, 20 Oct 2006, Jim Lux wrote:

>>   What in the world would it do that
>> couldn't be done far cheaper back on earth, or in orbit?
>
> Anything that requires realtime control with a time scale smaller than the 
> propagation delay of the comm path.

Yeah, but as I was saying that would be ``nothing'' with comm latencies
of 0.01 seconds or better from geosync Mars orbit.  I can't do payload
cost in my head for a Mars run, but I'm going to guess that whatever it
is, it will cost several orders of magnitude more per kilogram to
deliver anything at all gently to the surface of Mars than it would to
deliver it to a geosync Mars orbit.

So any SANE mission to Mars, rather than a silly one, would be a two
step process -- build and deliver an orbital platform and maybe two more
small relay satellites (the latter don't need to be there for the first
ground mission).  On the orbital platform put your cluster (if one is
needed) and high powered earthlink antennae and radio booster.  Solar
power abundant and not attenuated by Mars' atmosphere or inconvenient
things like "nighttime", plenty of 3 degree Kelvin sky for blackbody
cooling if you shield the cooling coils with power collection panels on
the other side and/or reflective foil.  No attenuation of the Earthlink
signal.  Point a nice reflection dish antenna down to the ground, match
it with a nice uplink antenna on the ground.  No need to worry about FCC
regs, use the entire bloody spectrum if you like, maybe use a visible
laser or a maser to encode the channel, should be able to get GHz per
channel on hundreds of channels at a guess or at LEAST GHz+ of net
bandwidth on the uplink.

By putting your high powered and heavy things in orbit, you save
hundreds or thousands of times their weight-cost in delivering those
same things to the Mars-ground.  The economy of scale is obvious, and
was used before on all of the moon missions for more or less the same
reasons although they didn't take along clusters and had much less
gravity to contend with on the landing.  Leave anything behind in orbit
that you won't need on the ground.

Personally, I'd have put up a minimum of three orbital platforms up
ANYWAY before even attempting what they've already attempted in terms of
landing mars rovers.  Geosync on Mars is a lot closer (in km) than
geosync on Earth, and if the military and weather programs have given us
nothing else, they've given us simply gangbusters orbital cameras.  I
mean who on this list hasn't gone on Google Earth yet?  Through EARTH's
atmosphere and from EARTH orbit you can see my house here:

   35^\circ 59' 17.10"N
   78^\circ 58' 50.22"W

At 512 feet you can clearly make out my Ford Excursion in the driveway,
see the row of cypruses along the back, see the white table on my back
deck that is about 4' across, see an azalea bush out front (next to the
sidewalk) that is about 1.5' across.  And this isn't the world's best
camera or as high resolution as they could manage, even through Earth's
soupy atmosphere.

(Google Earth is the world's greatest advantage to fisherhumans as well
-- check out:

   44 04 22.63N
   82 58 03.95W

at an eye altitude of around 12000 ft.  That's where I go fish for
smallmouth bass and walleye every summer.  I know parts of those reefs
where I wade-fish like the back of my hand, other parts I know well from
trolling them and fishing them for almost 30 years, but it is SO COOL to
be able to look down and see just exactly where this dip or that dip in
the reef is compared to what I remember, to look for holes that I might
have missed.  Alas, they don't take pictures of the water at as high a
resolution or out as far as I'd like, so I can't make out certain
individual boulders or underwater rocks that I'd really like to be able
to see from the sky...;-)

So long before the next Mars Rover type mission, they need to give us
"Google Mars", but from much closer in and with the higher resolution
better optics and less atmosphere can yield.  I would make the whole
damn planet available online -- I'm not kidding about Google Mars -- and
update the images and views available agressively (adding time as a
dimension as well as aspect/angle of view where possible).  That would
make 90% of the work required for a navigation rover-driver
precomputable, as it would permit the mapping out of the terrain to be
traversed down to the maybe ten centimeter scale.  As in individual
rocks, and not terribly small rocks at that.  You can almost make that
out on Google Earth already under much worse circumstances.

> Things like real time image processing for navigation (although that's not 
> cluster scale, yet.. they do it on a much slower machine, but, on the other 
> hand, they don't drive all that fast 10cm/sec).  Several DARPA grand

Right.

> challenge contestants had small clusters in their vehicle for this kind of 
> thing though.

Yeah.  Immediate grounds for elimination, IMO.  There are better things
to do with the weight.

> Signal processing for seismic or subsurface radar is another possibility.

No.  Not on the ground.  Not with at least 1 GHz uplink bw, definitely
not with multiple GHz.  The main point is that I can't see much reason
that one wouldn't be able to establish as much bw as you like to a
station only a few thousand km away directly overhead with a thin, quite
transparent atmosphere to punch through.  Certainly as much bw as one is
likely to have to or within a local cluster, and latency is completely
unimportant (and way under a second anyway).

The only thing that would justify a local cluster is a LOW LATENCY
requirement, where "low" is "microsecond" vs "millisecond" and where the
cost of the lowered latency is likely to be absolutely astronomical in
terms of e.g. other scientific apparatus that is displaced to provide
it not to mention dollars.

>>  There are damn
>> few things that I can think of that might even NEED a cluster's worth of
>> compute power on that end of things -- maybe, just maybe, running AI
>> robots or helping to preprocess images -- but the former SEEMS like it
>> could be managed with a very small cluster -- 4-8 CPUS tops, mostly for
>> failsafe -- and the latter could be done anywhere.
>> 
>> Moving data is cheap.
>
>
> Not from Mars, and particularly not from the surface of Mars. You'd have to 
> trade the cost of processing on the surface (which would allow a smaller, 
> lighter, lower powered radio) against the cost of processing elsewhere 
> (cheaper) but that would require a bigger heavier radio with more power.
>
>>  In fact, it is "free" -- available at opportunity
>> cost on a communication channel that has to exist no matter what, and
>> bandwidth in that channel is similarly cheap, all the way to Earth.
>
> That channel is not cheap, nor is it available 24/7. For instance, you might 
> be on the far side of Mars, with no direct path to Earth.  There are a few 
> relay satellites available (most with very skinny pipes) and for the current 
> ones at least, the comm pass lasts <20minutes.  Putting a 24/7 communications 
> infrastructure in place around the moon or Mars is a pretty expensive 
> proposition (many billions of dollars).  It might well be that spending a

As I said, that channel has to exist no matter what, so its cost delta
is irrelevant.  Also, the advantages of putting as serious network of
satellites around Mars are far greater than just establishing a high bw
channel from surface to orbit and orbit to Earth, although those are
definitely advantages -- as noted before with Google Mars.  Finally,
you've got the arithmetic exactly backwards -- it almost certainly costs
ten times more to lower mass gently to the ground on Mars than it does
to put it into Mars orbit.  You at the very least have to carry the
reaction mass required for entry and landing, and I while (as I said) I
can't do orbital payload computations in my head I'm guesstimating that
it is perhaps 30:1 for Mars -- on TOP of well over 100:1 just to get out
of the Earth's gravity well.  So one is paying perhaps 5000:1 to get to
Mars surface vs maybe 150:1 to get to Mars orbit.  Fuel isn't the only
expense, true, but if it is as little as 50% of the total cost the
marginal difference is astronomical.  So no, I don't think that there is
any credible way that it isn't cheaper to build orbital infrastructure
first and THEN go down than to try to just go down.  The latter may sell
to the public better, of course, but hey...

> billion dollars on a 1000kg cluster on Mars is cheaper than 10 billion 
> dollars to create the broadband communications infrastructure needed to put 
> it elsewhere. Part of the problem is the way in which money for such things 
> is doled out.. generally by the missions, which are science driven.  Human 
> spaceflight is a notable exception for the money distribution, and 
> infrastructure (if needed for the human flight) can get funded.  It also 
> helps that human flight is, overall, a bigger chunk of change than robotic 
> flight (robotic exploration missions run in the $100M to $750M range, and 
> tend to stand alone, although part of an overall program)

I believe that this is a big part of the problem.  Putting a bunch of
really well equipped satellites in orbit around Mars isn't nearly as
sexy as putting down a single rover, even if it actually costs less and
teaches us more and gives us the eventual chance of putting down a rover
that can leave most of its "brains" up in the sky, leaving more room for
interesting payload.

> You'd be surprised... for batch processing type applications, where it's ok 
> to wait til the next satellite pass for the next data bolus, the trade is 
> easy.

What next satellite pass?  Equatorial geosync, please, right over or in
easy line of sight with the rover site.  Mars' atmosphere is so thin
that cutting through it on a tangent is still easier than cutting
straight out of Earth's, so this really should work all the way to the
poles.  Three such satellites really ought to span the globe as far as
communications is concerned, although of course more would be better.  A
handful of satellites in polar orbits for doing mapping wouldn't hurt as
well.

Really, one could send up just ONE big payload to Mars orbit and ship
out to different end-stage orbits along the way (where it is cheaper) --
this isn't about sending five or ten flights, just one big flight with
five or ten payloads.  If one is really clever, the empty payload
container (which contains e.g. your cluster and primary communications
and power system) arrives with a nice, empty cargo bay, all ready for a
future flight which transforms it into an instant human-occupiable space
station for a real mars visit.  THAT would sell, surely...;-)

> We prefer the term failure tolerant. (fail proof is hard to come by) But 
> there is a fair amount of work on how to do just that, and clusters are 
> actually a good way to have the *system* reliability be good, even if the 
> *component* reliability isn't. (barring aging and systemic failure 
> mechanisms.. total dose damage, for instance)
>
> As above, there is a tradeoff

Ya, what was the phrase used elsewhere in this thread?  "Incontestable
proof"?  Oxymoron city, at least in science...;-)

> SO far, no patron has stepped up with $200M to send something to Mars, yet.

Well there are those hardy optimists that have dropped $200M trying to
build a commercially viable low earth orbit transportation company.  The
problem is that even if one drops the cost per kg to the $20 or so that
they somehow envision (a figure that isn't crazy different from the cost
per kg to fly somebody around the world now with a corporate profit
margin left over, which alone tells me to blow a juicy razzberry:-) what
they hell is there to DO in space besides achieve military dominance,
predict the weather, let antiterrorist units track individual cars
carrying nuclear bombs or nerve gas on 24 (with wholly incredible
resolution and never any possibility of rain)?

In the research, communications, weather, observation, and military
domain low earth orbit is a very useful place to be.  As far as ANYTHING
ELSE -- e.g. manufacturing something of actual value, I have yet to see
credible evidence that orbit is good for much.  The moon ditto, unless
it proves to have some raw materials in abundance that are scarce on the
Earth.  Mars triple-ditto.  And I say this with great regret, as a
reader of nearly every decent SF novel ever written on the subject, many
of them many times over...;-)

That doesn't mean that I think we should abandon these projects -- au
contraire!  It does mean that I think that there needs to be a long term
plan in place, that it needs to be recognized at the outset that the
investment required is for pure science with no expected payoff (however
often pure science has created UNexpected payoffs beyond our wildest
dreams), and that in the long run the projects DO need to break even or
win a bit in return on investment compared to, say, ending world hunger
or eliminating war or achieving universal literacy.

    rgb

>
>
>
> James Lux, P.E.
> Spacecraft Radio Frequency Subsystems Group
> Flight Communications Systems Section
> Jet Propulsion Laboratory, Mail Stop 161-213
> 4800 Oak Grove Drive
> Pasadena CA 91109
> tel: (818)354-2075
> fax: (818)393-6875 
>

-- 
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu


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