Building the TMT

So how are we going to build the Thousand Mile Telescope?  Once we get to the Kuiper Cliff, is there anything we can use?

We will do the usual thing, the same kind of procedure that we will use to build the rest of Solar Civilization: arrive with several general-purpose 'factories'.  Each factory is a system of machines that can:

• mine asteroidal materials
• refine the materials into useful forms
• make solar arrays for power
• reproduce themselves on a larger scale
• build cramped, cold, uncomfortable habitats for the grad students
• make the machines that will make the big mirrors.

 For the mirrors themselves, I propose using only three elements: silicon, oxygen, and aluminum.  The silicon and oxygen to be combined into SiO2 -- quartz -- that gets foamed into a volume much larger than it would be if solid, but still very rigid.  Probably a fractal kind of structure, like the inside of bones.  It it easy to foam stuff in zero-G because the bubbles don't try to rise.





Once you make the big low-mass, rigid disks, you use a polishing process that simultaneously creates a very slight paraboloid, and fuses the surface smooth.  Use a directed-energy thing, like those Martian death-rays.  The green ones.

To get optical smoothness you have large polishing machines crawling around the surface for a year or so, doing final polishing.  And finally the biggest vacuum-deposition gadgets ever made -- no need for a "chamber" -- to apply the aluminum coating over the entire surface of each disk, resulting in an optically-smooth aluminum coating a thousand Angstroms thick or so.





OK, maybe I don't know every last detail, but one must leave something for the engineers to figure out or they will become irritable and despondent.

One note about cleanliness.  The entire mining, refining, and manufacturing process must be kept clean.  Any waste that cannot be used should be at least well controlled, for example by packing it into containers.  The last thing we want is a cloud of unused gas and dust floating around the mirrors, and us hoping that it eventually blows away in the practically nonexistent light pressure.  We are making telescopes here, let's not simultaneously make smog around them.






How much material do we need?  Will there be enough?

Let's use enough SiO2 that, if it were solid, it would be a meter thick.  (It will actually be probably at least ten times thicker than that after foaming, but for those bubbles we will use whatever waste gas is handy.  Probably oxygen, there's gobs of that stuff.

So how much SIO2 do we need?

The total area of all the dishes is the same as a single 1000 mile dish, one meter thick.  500 mile radius is 804.5 Km radius, so the volume is

pi * ( 804500 m )^2 * 1 m 

== 2e12 m3

== 2e18 cm3

* 2.6 g cm-3 ~= 5e18 grams

We need 5e18 grams of SiO2.  Can we get it?

I think the Kuiper Belt probably has two distinct populations of objects in it: the fluffy ones and the stony ones.  The good ones for us are stony, like Pluto, which is maybe 70% stony stuff like rocky asteroids, and 30% water-ice.

Let's assume we can find more things like Pluto.  (Because I don't want to rip apart Pluto, just for old time's sake.)  What kind of composition will we find?

Silicon is always the limiting factor, because there's so much oxygen.  And that's without counting the water ice!  Silicon is about half of the mass that we need, so let's be generous and call it 3e18 grams.  And silicon is about one-fifth of the mass of the stony planetesimals in the Kuiper Belt, so we will need to process a total of about 15e18 grams of material total, assuming the worst case that it's all mixed together evenly.

How much rock is that?  Assuming (conservatively) a density of 2 grams per cm3, we need to process 8e18 cm3.  Or 8e12 m3.  Or 8e3 Km3.

That's a sphere 25 kilometers across.

That is not a very big rock.

It's smaller than the Earth.

( In each of these pictures, the two objects are shown in proper scale with each other.)

Earth and Moon

It's smaller than the moon.

Moon and Pluto

It's smaller than Pluto.

Pluto and Vesta

It's even smaller than Vesta.

Vesta and the TMT rock.

That's it -- the little thing to the right of Vesta.  That's the size of the rock we need to build the Thousand Mile Telescope.  It would just about make one of the larger craters on Vesta (which is the third-largest main-belt asteroid.)

In the Kuiper Belt, we are going to be able to find a zillion rocks this large.

And each time we find one, our machines and graduate students will go to work on it.

Mining a little rock.

And we'll start making dish-arrays out of it.

A Dish Array of the Thousand Mile Telescope

And I suppose we could make some habitats too.  Even grad students need someplace to live.