Cost to build Gerard K. O'Neill's "Island Three"

On Oct 2, 10:27*pm, Carey wrote:

This seems like the way to go if deorbiting Phobos is in fact a
desirable event.

I don't like the notion of de-orbiting Phobos.

Some Mars enthusiasts like to day dream of a martian beanstalk being
our gateway to the resources in the main belt. They correctly point
out that Mars has an angular velocity similar to earth (in other words
an ~24 hour day) and less mass. Mars synchronous is about 17,000
kilometers above Mars surface which is less than half earth's ~36,000
kilometers geosynch altitude. They correctly point out that since
stress is much less, less exotic materials can be used. No carbon
nanotubes needed, T-1000 or even spun basalt does the job.

While they demonstrate a Mars elevator is much less implausible than
an Earth elevator, they still don't demonstrate a Mars elevator is
doable in our world where financial realities impose limits.

To hurl stuff to earth, Ceres or beyond, a Mars beanstalk would have
to greatly exceed 17,000 kilometers in length. Building even a
primitive industrial infrastructure on Mars is a stretch. Such a huge
mega project as a beanstalk on Mars I don't see for centuries if not
millenia.

Now take another look at this Phobos tether: http://clowder.net/hop/TMI/PhobosTether.jpg

About 14,000 km in length, less than the distance to Mars synchronous
orbit. It offers trans earth insertion as well as trans Ceres
insertion. It offers most the powers a martian beanstalk confers with
a small fraction of the length and material. The chief difference is
that from Mars' surface a small suborbital hop is needed to dock with
the tether foot. A 300 km, .6 km/sec hop.

This Phobos stalk is shorter and suffers less stress than a Martian
beanstalk. So spun basalt is also a possible material and an even
smaller taper does the job.

Phobos could not only be our beach head to Mars -- It could be the
beach head to the solar system.

But even a Phobos tether is a stretch. Sometimes humanity moving
beyond LEO seems implausible. But it's fun to daydream.

On 10/3/2010 12:20 PM, Hop wrote:
On Oct 2, 10:27 pm, wrote:

This seems like the way to go if deorbiting Phobos is in fact a
desirable event.

I don't like the notion of de-orbiting Phobos.

Some Mars enthusiasts like to day dream of a martian beanstalk being
our gateway to the resources in the main belt. They correctly point
out that Mars has an angular velocity similar to earth (in other words
an ~24 hour day) and less mass. Mars synchronous is about 17,000
kilometers above Mars surface which is less than half earth's ~36,000
kilometers geosynch altitude. They correctly point out that since
stress is much less, less exotic materials can be used. No carbon
nanotubes needed, T-1000 or even spun basalt does the job.

But we don't need all of Phobos there to make that idea work; a chunk of
it a few thousand feet in diameter would be plenty big to do that
concept with.
For that matter, there's always Deimos to do it with also.
Being in a higher orbit, Deimos is even more favorably situated to toss
things around the solar system with due to having less gravity well to
climb out of.

Pat

On Oct 3, 6:00*pm, Pat Flannery wrote:
For that matter, there's always Deimos to do it with also.
Being in a higher orbit, Deimos is even more favorably situated to toss
things around the solar system with due to having less gravity well to
climb out of.

Pat

To fling stuff from Mars, you need so called centrifugal force to
greatly exceed Mars gravity.

Centrifugal force is w^2 r, where w is angular velocity in radians and
r is distance from Mars' center.

Phobos makes a rotation every 7 hours 40 minutes, Deimos 30 hours 20
minutes. Phobos' angular velocity is about 4 times that of Deimos,
therefore w^2 is almost 16 fold difference.

Gravity gradient is steeper deep in Mars' gravity well. Gravity is
falling off much more gradually by the time you reach Deimos' 20,000
km altitude.

For trans earth insertion, the Oberth effect is more helpful deeper in
Mars gravity well.

A tether extending ~28,000 km from Deimos could hurl stuff to earth.
In contrast it takes only 6,155 km of tether from Phobos for trans
Earth insertion.

On Oct 4, 12:14*am, Pat Flannery wrote:

Okay, I understand the concept now, but why do you need all of Phobos
there to do this?
Even 1/10,000 of its total mass would be more than enough to give you a
very massive and stable anchor to attach your tether to.

Pat

That would be 1/10,000 of the tether's momentum bank. If it enjoys a
lot of use, that may not be enough.

That's also all but 1/10,000 of the resources sent to the bottom of a
deep gravity well.

Why are you so fired up to send Phobos down?

Phobos' low density indicates its a rubble pile with lots of voids and/
or volatile ices. Well before it reached Mars, it'd exceed its Roche
limit and smear into a string of pearls like Shoemaker Levy-9. You
can't use pusher plates to send down an elongated dust cloud. Nor can
you drag down a dust cloud with a tether.

This would be an orbital debris nightmare that would pretty much make
Mars off limits.

If you were able to send down a fraction of this dust cloud, you
wouldn't have a cannonball plunging into the north pole, but a dust
storm burning up in the upper atmosphere.

Even if you could send down a cannonball I haven't seen any convincing
models that this would add substantially to Mars atmosphere.

On 10/3/2010 6:43 PM, Hop wrote:

To fling stuff from Mars, you need so called centrifugal force to
greatly exceed Mars gravity.

Centrifugal force is w^2 r, where w is angular velocity in radians and
r is distance from Mars' center.

Phobos makes a rotation every 7 hours 40 minutes, Deimos 30 hours 20
minutes. Phobos' angular velocity is about 4 times that of Deimos,
therefore w^2 is almost 16 fold difference.

Gravity gradient is steeper deep in Mars' gravity well. Gravity is
falling off much more gradually by the time you reach Deimos' 20,000
km altitude.

For trans earth insertion, the Oberth effect is more helpful deeper in
Mars gravity well.

A tether extending ~28,000 km from Deimos could hurl stuff to earth.
In contrast it takes only 6,155 km of tether from Phobos for trans
Earth insertion.

Okay, I understand the concept now, but why do you need all of Phobos
there to do this?
Even 1/10,000 of its total mass would be more than enough to give you a
very massive and stable anchor to attach your tether to.

Pat

On 10/3/2010 9:01 PM, Hop wrote:
On Oct 4, 12:14 am, Pat wrote:

Okay, I understand the concept now, but why do you need all of Phobos
there to do this?
Even 1/10,000 of its total mass would be more than enough to give you a
very massive and stable anchor to attach your tether to.

Pat

That would be 1/10,000 of the tether's momentum bank. If it enjoys a
lot of use, that may not be enough.

That's also all but 1/10,000 of the resources sent to the bottom of a
deep gravity well.

Why are you so fired up to send Phobos down?

Because I can melt a whole pile of subsurface ice on the surface, create
water clouds, and increase the surface atmospheric pressure - making it
a lot easier to colonize the place.
Considering the havoc a three-mile wide asteroid wreaked on Earth when
it hit Yucatan at the end of the Cretaceous age (like creating tidal
waves in Hudson's Bay and Hawaii and blowing most of the water out of
the Gulf of Mexico) dropping something far larger on a far smaller
planet should really create some impressive effects in regards to
changing its climate.


Phobos' low density indicates its a rubble pile with lots of voids and/
or volatile ices. Well before it reached Mars, it'd exceed its Roche
limit and smear into a string of pearls like Shoemaker Levy-9. You
can't use pusher plates to send down an elongated dust cloud. Nor can
you drag down a dust cloud with a tether.

It's probably indeed a rubble pile, but not a dustball.
For starters, if it was all dust or gravel, even its low gravity would
have formed it into a sphere, not the oblong shape it is.
So there are some honking big chunks of rock in it, and those will do
nicely as they descend under it's orbital path as it breaks up under
tidal stresses at low orbital altitude. In fact, having its mass hit all
over the place in a ring around the planet is better than having it hit
all in one place as far as total atmospheric heating goes.


This would be an orbital debris nightmare that would pretty much make
Mars off limits.


Only at that altitude; they now think Phobos may be a result of a major
asteroid impact on Mars throwing debris into orbit, like a small scale
version of what created Earth's moon:
http://www.msnbc.msn.com/id/39399235...science-space/
Even if it did turn into a dust ring of some sort, any spacecraft
encountering it in Mars orbit would be moving at the same relative speed
it would be, and given its low orbital altitude, could easily avoid it
by orbiting at a higher altitude and different orbital inclination.


If you were able to send down a fraction of this dust cloud, you
wouldn't have a cannonball plunging into the north pole, but a dust
storm burning up in the upper atmosphere.


And you start figuring out how much atmospheric heating that amount of
dust hitting the atmosphere and burning up would generate, because it's
going to get mighty toasty down there if it comes in in a fairly short
period of time.


Even if you could send down a cannonball I haven't seen any convincing
models that this would add substantially to Mars atmosphere.


Well then, you may think there's lots of water ice on the Moon, but none
on Mars, although NASA is very keen on pointing out that there are vast
frozen water deposits lurking just under the Martian surface. ;-)
You melt a lot that into water vapor and yes, the atmospheric pressure
rises, just like fog is more dense than air over deserts.

Pat

Pat Flannery wrote:
Because I can melt a whole pile of subsurface ice on the surface, create
water clouds, and increase the surface atmospheric pressure - making it
a lot easier to colonize the place.

Assuming the subsurface ice even exists... And even if it does, given
the lack of thermal input to keep it liquid, it'll start freezing out
soon enough.

Not that your "whole pile" amounts to that much water on a planetary
scale in the first place.

Considering the havoc a three-mile wide asteroid wreaked on Earth when
it hit Yucatan at the end of the Cretaceous age (like creating tidal
waves in Hudson's Bay and Hawaii and blowing most of the water out of
the Gulf of Mexico) dropping something far larger on a far smaller
planet should really create some impressive effects in regards to
changing its climate.

Yeah, what that impactor did was *cool* the Earth - exactly the
opposite of what you'd want at Mars.

And you start figuring out how much atmospheric heating that amount of
dust hitting the atmosphere and burning up would generate, because it's
going to get mighty toasty down there if it comes in in a fairly short
period of time.

Um, no. There is neither enough mass nor orbital energy in Phobos to
accomplish much in that department.

Well then, you may think there's lots of water ice on the Moon, but none
on Mars, although NASA is very keen on pointing out that there are vast
frozen water deposits lurking just under the Martian surface. ;-)
You melt a lot that into water vapor and yes, the atmospheric pressure
rises, just like fog is more dense than air over deserts.

But what doesn't rise is the partial pressure of 02. Nor does the
minor (and temporary) rise in pressure give any especial advantage, in
fact it makes things like suit design even harder.

D.
--
Touch-twice life. Eat. Drink. Laugh.

http://derekl1963.livejournal.com/

-Resolved: To be more temperate in my postings.
Oct 5th, 2004 JDL

On Oct 4, 3:49*am, Pat Flannery wrote:

Because I can melt a whole pile of subsurface ice on the surface, create
water clouds, and increase the surface atmospheric pressure - making it
a lot easier to colonize the place.

IF you vaporize enough dry ice and water ice, you might trigger a
runaway green house effect. Is there enough Martian CO2 and water ice
to do this? Not demonstrated.

IF you were successful in triggering a runaway green house effect,
would it result in an atmosphere thick enough you wouldn't have to
wear pressure suits? Highly doubtful.
For starters, if it was all dust or gravel, even its low gravity would
have formed it into a sphere, not the oblong shape it is.

Ummmm.... No.

Try Googling "tidal elongation".

This would be an orbital debris nightmare that would pretty much make
Mars off limits.

Only at that altitude;

Pulling Phobos down to Mars would scatter debris throughout Phobos'
orbital plane from ~6000 km up (Phobos' present altitude) to Mars
upper atmosphere.

Even if it did turn into a dust ring of some sort, any spacecraft
encountering it in Mars orbit would be moving at the same relative speed

Something incoming from earth would be following a hyperbola with
regard to Mars.

It would NOT be moving at the same relative speed as the debris it
encounters.

it would be, and given its low orbital altitude, could easily avoid it
by orbiting at a higher altitude and different orbital inclination.

Mars' equatorial plane would be filled with debris from roughly 6000
km altitude to the top of Mars' atmosphere. An inclined orbit would
pass through the debris plane at its descending or ascending node.


And you start figuring out how much atmospheric heating that amount of
dust hitting the atmosphere and burning up would generate,

Current guesstimates place Phobos' Roche limit at 5000 km altitude.
This is when tidal forces would start breaking it apart. Nukes or
trying to drag it down would probably start breaking it apart earlier.
Most of it would remain in orbit about Mars as a plane of orbital
debris.

Even if you could send down a cannonball I haven't seen any convincing
models that this would add substantially to Mars atmosphere.

Well then, you may think there's lots of water ice on the Moon, but none
on Mars,

Um... No. That's not what I may think.

There is good evidence of Martian water.

Is there enough to trigger runaway greenhouse? Is there enough to
substantially raise Mars' atmospheric pressure?

Could de orbiting Phobos vaporize enough ice to do this?

This you still haven't demonstrated.

When I ask for a convincing model, I'm hoping for some math and
physics.