[Server-sky] A kind of thinsat to help deorbiting small debris

[Keith Lofstrom]

When people talk about thousands or millions of potential debris
objects, I'm not impressed.  The sphere below GEO is 300 billion
trillion cubic meters, and almost all of those cubic meters are empty. 
When I walk to the library, I navigate through a few thousand cubic
meters of space filled with hundreds of unpredictable potential
colliders, mis-operated by idiots yakking on their cell phones,
rather than passive objects following Newton's laws to eight decimal
places.  Accurately track the debris objects, and space becomes
far safer than a crosswalk.


>   http://www.bloomberg.com/news/2013-11-14/real-life-gravity-space-debris-spells-business-for-astrium.html#!
...

Check out Star Inc's electrodynamic tether debris collector:
http://www.star-tech-inc.com/id121.html

Some space advocates talk about extracting resources from the moon. 
Jerome Pearson points out that there are thousands of tons of high
grade aluminum in orbit already - we call it debris because we
don't know how to harvest it and re-use it (yet). 

Much of the unharvested aluminum is in highly eccentric, highly
inclined orbits (Delta, GEO, and Molniya upper stages, mostly) and
hard to bring to space processing facilities to melt and recast. 
However, there are two good uses as-is for cylinders of 5mm to 10mm
aluminum-lithium tankage:  armor for ISS, and ballast for launch-weight
reduced server sky thinsats.  

Jerome posits twelve 100 kg EDDE systems to capture all the larger
low earth orbit objects in 7 years.  But electrodynamic tethers
don't work very well above 2000 km or so.  That is the /lower/
practical altitude for light sails.  Light sails won't be effective
moving a large rocket body, but they could move small bits of one
if we laser cut pieces off of it (hovering a few millimeters away,
not from thousands of kilometers, definitely not through a
dispersive atmosphere).  One gram chunks would make dandy
ballast for ultralight thinsats - less to launch from earth.

But worst case, if we choose to reenter that valuable aluminum,
remember that powdered aluminum is the main fuel component of
solid fuel rockets.  Rendezvous some oxidizer and the appropriate
combustor and nozzle with that aluminum, and it can make its own
delta V.  It is far more difficult to deliver that kind of energy
from the ground and through an atmosphere, because 99% of the
time, any given LEO object is over the horizon from a ground laser.
When and if an object appears, it isn't in view for very long. 

Whatever you plan for dealing with an orbital object, you must
still find it, very accurately.  Radar sensitivity decreases as
the inverse fourth power of distance in vacuum; atmosphere adds
more inaccuracies.  The reason we don't "just get out of the way"
of more space debris is that the tracking error for passive
objects in high LEO is one kilometer.  If we knew where the stuff
was to 10 meter accuracy, very little delta V would be needed to
get out of the way of it, and we could confidently ignore a much
larger quantity of objects in the short term.  With high precision
tracking and orbit computation, we could also prioritize the
likely colliders that need orbit change first.

Server sky arrays make dandy illuminators for look-down synthetic
aperture radar.  We can find small objects, and characterize their
position and velocity to millimeters and to micrometers per second. 
You can't do that from the ground, or with small aperture, low
power, lower frequency orbiting radars.  You also need a lot of
computing to correlate and characterize the reflected energy. 
That is a similar calculation to sorting out uplink from millions
of terrestrial internet customers.  Server sky will do radar until
the internet business becomes profitable.

We can already do that kind of hyper-accurate tracking of objects
if they are designed to be found.  The LAGEOS laser geodesy
satellites are tracked accurately enough to measure continental
continental drift, with millimeter-per-year velocity accuracy. 
Most rocket bodies and fragments are NOT designed to be found;
that could be fixed by pasting light weight, solar powered, RF
fabric on inner and outer surfaces.  In other words, the solar
cell and radio portion of thinsats.

BTW, the LAGEOS satellites the nearest neighbors to the M288
orbits, and the only valuable assets we need worry about
colliding with.  It's lonely up in the van Allen belt gap.

Most "pollution" is valuable material we are too stupid to use.
We drilled wells into oil formations for hundreds of years,
hoping to find brine for making salt, before Thorla and McKee
started selling the gunk that came out of their brine well as 
medicinal "Seneca Oil" in 1814. 

We will learn to do the same with space "debris".  The companies
that get paid to remove it will profit handsomely from reselling
it.  Stake your claims now - the Russians would probably be glad
to sell the headache and responsibility for their orbiting
derelicts for a little hard currency.  

Keith

-- 
Keith Lofstrom          keithl at keithl.com