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Subject: SPACE Digest V11 #202

SPACE Digest                                     Volume 11 : Issue 202

Today's Topics:
          Space-tech excerpt: Orbital debris
                  Progress-M
              Alan Shepherd in Australia
----------------------------------------------------------------------

Date: Fri, 30 Mar 1990 16:57-EST
From: [e-mail]
Subject: Space-tech excerpt: Orbital debris


I recently went through and excerpted some of the most informative
discussions from the space-tech mailing list, which I run.  Here's one of
them; expect a couple more over the next week or so.

Space-tech is a mailing list for discussing concepts for space development,
with emphasis on the technical problems and and how to solve them.  To join,
send mail to [e-mail].  Bring a pencil!

   ///////////////////////////////////////////////////////////////////////
  /// Marc Ringuette /// Carnegie Mellon University, Comp. Sci. Dept. ///
 /// [e-mail] /// Pittsburgh, PA 15213.  Phone [phone](w) ///
///////////////////////////////////////////////////////////////////////



Space-tech excerpt: Orbital debris    [270 lines, fall '89]

The topic:  how to reduce or eliminate orbital debris, which may become
a serious practial problem in low orbit?


------------------------------

From: Steven Deterling <[e-mail]>

I am working on a project here at A&M this semester and am wondering
if this group has any suggestions or information to offer.  We are trying
to design a mission (or set of mission) for the purpose of removal of orbital
debris.  Any ideas would be greatly appreciated.

------------------------------

From: [e-mail]

My first idea is to have a very-large-surface-area setup where you try to
vaporize small bits of debris by putting an obstacle in front of it.  For
instance, a mylar sheet or some sort of ultra-light foam.  The critical
factor for that kind of scheme would be whether a thin film would do
something useful with the debris - presumably you'd like to vaporize it so it
wouldn't have a destructive impact if it hit somebody.  Maybe vaporizing it
would also decrease its decay time.

Actually, I have a better idea than a single film:  two or three mylar films
spaced far enough apart that when debris strikes one, if it breaks up at all,
the fanned-out secondary debris hits the second sheet, and again for the
third.  It may allow you to destroy much larger chunks of stuff because it
forces each object to come into contact with a larger surface area of the
sheet (as opposed to just punching a tiny little hole).  This is assuming
objects break up, as opposed to just lose a few molecules off the surface.

I wonder if you take your average small object and run it into a film at a
few km/s, what happens?  What's a representative sample of debris?  (My guess:
chips of paint, metal shavings, bits of rubber, metal bolts, entire assemblies;
probably biased toward the really small stuff, but you care more about the
big stuff).

===

Or maybe you were thinking of bigger pieces of debris that you can track and
predict the orbit of.  One option would be to chase the debris with a
low-acceleration tug and put it in your garbage bag.  But this is probably
prohibitively expensive except for the very largest objects - it may take
weeks or months per object, and each craft needs propulsion, power, and
communications.

A second tactic: put something in the object's way that vaporizes it.  I
wonder how much foam you'd have to put in the way of a chunk of metal before
it was destroyed?  The better you can predict the orbit, the more stuff you
can stack in its path.  If you can't get the orbit down pat, maybe you can
use on-board radar and shoot a gun at it - a small solid projectile may
be able to break up a large object; I wonder if this isn't worse than nothing.

===

Another line of thought: how do you cheaply contain the debris before it is
generated?  For instance, some sort of foam or glue or something that
prevents an object from breaking up before it burns up completely.  You carry
a small amount of this gunk along on your mission, and wire it so that when
you eject something, it foams up.

But probably most debris is generated by accident, in small quantities.  Or
is it?  Do you have figures?

How about putting a plastic bag around the vehicle after it injects into
orbit?   :-}

------------------------------

From: Steven Deterling <[e-mail]>

We also are basically keen on the idea of vaporizing debris.  I do not
know enough about hypervelocity collisions to say whether or not a small
piece of junk running into a mylar sheet will generate enough heat to
be destroyed.

We have considered some form of encapsulation for bigger pieces of junk.
Not too sure about what to yet, however.  The scenario we are starting to
look at real closely is to have some sort of craft with an engine on the
back and a high power laser of some sort attached to vaporize small
particles.  The front of the craft would be a collector for larger,
"non-vaporizeable" particles.  When the collector was full, it could
be started on a trajectory toward the Sun and the rear of the craft could
be separated for re-use.  We are looking at possible using an ion engine
for our craft. We are not too concerned with the time span, a mission of
5 years or so would not be bad.  As long as we are decreasing the amount
of debris in orbit, we are being beneficial.  Comments on this scheme from
everyone would really be helpful.

------------------------------

From: Joe Beckenbach <[e-mail]>

As for encapsulation:  for anything too big to vaporize but too small to grab
easily, perhaps spray on some foam concrete or let it plow through layers of
foam metal, something either to increase its size or to slow it down so it
will either embed or decay.  I think this is the basic idea that everyone's
been trying to figure out how to do.

------------------------------

From: [e-mail] (Kevin Ryan)

Cleaning orbital junk, eh?  If only the atmosphere did a better job of
slowing them down, at least for a little while...

   Enter the little men in white shirts and wire-frames, with little
plastic pocket protectors.  They smile at the brass hats.  "We have a
solution to the orbital debris problem.  Detonate a large, clean
(relatively clean, of course - all things are relative) thermonuclear
device in the upper atmosphere.  It will cause a large and temporary
'hump' in the atmosphere, thus greatly slowing the orbital junk, which
will soon reenter.  After the 'hump' subsides, relaunch the satellites
of your choice.  This cleans out debris in _all_ orbits which intersect
this rather large 'hump.'  If you don't get them all, use a larger
device, or do it repeatedly."  Slowly, the brass hats start to smile.
This would mean getting to use some of their BIG toys...

   I wish I was spinning this out of imaginary cloth.  I have seen
serious (!) suggestions for creating such an atmospheric 'hump' to
slow/divert/destroy ICBM's - and if it works for high suborbital
missiles, it should work for LEO debris, which should be at the very
least less aerodynamic.

   Before I'm flamed, let me emphasize - "Not on MY planet, monkey boy!"
Just thought I'd chuck it in for amusement...

------------------------------

From:    [e-mail] (Henry Spencer)

Jordin Kare has observed that the kind of 1MW laser that would be used
as a feasibility-test system for a laser launcher could also be quite
useful in sweeping up debris.  It could vaporize very small pieces,
and could de-orbit larger ones by blowing pulses of gas off their
leading surfaces (a laser retrorocket).

------------------------------

From: [e-mail] (Christopher Neufeld)

   Over the past couple of weeks we've seen a few ways to clean dust and
grit out of low earth orbit, where it could damage satellites, shuttles, or
the space station. Two of the more memorable ones were the ice cube in an
opposing orbit, and the giant flypaper. I submit that there is an easier
and more selective way to do the same thing.
   According to some calculations I made this afternoon, and which I'm
still having trouble believing, it's very easy, assuming that most of the
grit is going spinward, in the direction of most satellite launches. This
grit goes from west to east as seen by an observer on the ground. A mirror
is placed in the sunlight in the east as seen by a terrestrial observer.
The mirror reflects sunlight across the sky, from east to west, so that it
is shining directly into the path of the orbiting grit.
   The scenario I used was a mettalic flake 1mm in diameter, and 0.1mm
thick, in a circular orbit 300km above the surface of the earth. It turns
out that the photon pressure on the flakes lowers the perigee of the orbit
to 100km, at which time it can be said to be braking in the atmosphere and
out of our way, in only 50 hours of exposure. If we have 5% coverage, this
is 1000 hours real time, or roughly six weeks.
   The advantage to this approach is that it works best on small objects. A
communication satellite would suffer a delta-v of only about 1m/s, which I
presume is within the tolerance of the onboard thrusters to compensate.
   An alternative solution is to put a giant sunshade which blocks light
reaching orbit as they cross from day to night, while still letting the
particles get the sun in their faces as they go from night to day. I favor
the first approach because it is easier to stabilize the mirror than a
sunscreen, since solar pressure on the mirror acts to oppose the earth's
gravity, while solar pressure on the sunshade adds to the earth's gravity.
Also, a mirror can be easily aimed to sweep different orbits, while a
sunshade or a retrograde ice cube would require a lot of effort and time to
do the same.

   Here are the calculations:

   I used the following parameters for the solution of the great cosmic
vacuum cleaner:

   Particle is a cylinder: 1 mm in diameter
                           0.1 mm thick
   Particle's specific gravity: exactly 7x10^3 kg/m^3
   Particle orbiting at exactly 300 km in a circular orbit
   Mass of the earth: exactly 6x10^24 kg
   Earth has no higher order gravitational moments.
   Gravitational constant: exactly 6.67x10^-11 N m^2/kg^2
   Gravitational acceleration at the earth's surface: exactly 9.81 m/s^2
   Radius of the earth: 6.387x10^6 m
   Radius of the orbit: 6.687x10^6 m
   Orbital velocity: 7.736x10^3 m/s

For purposes of momentum transfer from the particle: I used the effective
area of the particle as 1/2 the area of an end cap, and assumed that all
radiation incident on the (tumbling) flake was absorbed. This is actually a
conservative estimate, since the actual figure goes from 1/2 for a perfectly
absorbing slab to 2/3 for a perfectly reflecting slab. This under-estimation
of the area will absorb any inefficiencies in the mirror, since I am still
using a power flux at the particle of 1.4 kW/m^2, the solar flux in space
at one astronomical unit.
   Force on the particle is Psolar/(speed of light) * area of particle.
This gives an acceleration of 3.333x10^-4 m/s^2 for as long as the particle
is in the beam.

   Now, it is necessary to find the delta-v on a particle orbiting at
300 km to drop the perigee to 100 km. This turns out to be about 60 m/s. See
the note at the end of this article for the math behind this calculation.
The acceleration will provide this impulse in only 50 hours. If we have 5%
coverage, this is 1000 hours real time, or roughly six weeks.
   Now, I have to justify my assumption that hitting the particle several
times will result in the lowering of the perigee, but will not change the
apogee, which will stay at 300 km. Assume that the orbit is initially
circular. I hit it with the beam as it traverses some 15 degrees of its
orbit. The particle slows down by some small amount, then continues in its
orbit as a free particle. From classical mechanics, a gravitational orbit
is closed (no precession). So, the particle must return to the point at
which it received the initial impulse. This argument then repeats for each
orbit. So, after giving it a delta-v of 60 m/s, the apogee is at 300 km
while the perigee is at 100 km. It is now hitting atmosphere, and will
quickly be removed from worry.

   For a Clarke orbit, the delta-v is 1500 m/s, which takes quite a bit
longer, but the algebra is essentially the same. In this case, though, the
mirror has to rotate to track the sun as it moves relative to the orbit
over a period of one year. The mirror must shine into the orbits always at
apogee to get the efficiency I've postulated, and apogee will precess with
respect to the earth and sun, since it will always point to the same fixed
stars.

------------------------------

From: [e-mail]

I like this approach a lot!  However, I'm concerned about two questions:

  1. How big a mirror do we need?  What is the size of the cross section
     through which most of the grit goes?

  2. What are the magnitudes of the forces involved on the mirror itself,
     and how much of its time can be spent usefully?

My guess is that the mirror would have to be very large and that it
would have somewhat less than a 25% duty cycle because it would probably
want to remain in a single orientation throughout its orbit.  However,
I don't really trust my guesses on this at all.

------------------------------

From: Christopher Neufeld <[e-mail]>

   I'm still working out the orbital dynamics for the mirror, but I usually
have a pretty good feel for the orbits without doing the math (that's why I
suspected my initial erroneous results). The situation I'm looking at is a
dynamically unstable SOLAR orbit leading the earth by a bit. I would choose
the position of the mirror and its angle so that the light pressure from
the reflection exactly balances the earth's pull. If it drifted away from
the earth a bit, the pull would be weakened, and it would tend to drift
further, so the mirror would have to be furled slightly to lower the
outward solar pressure and bring it back into line. If it drifted toward
the earth, the pull would be strengthened, and extra mirror kept in reserve
for that eventuality would be unfurled until it is back where it belongs.
The feedback scheme shouldn't be impossible.
   Anyway, in a while I'll work out the details of the sail: its mass
per unit area, position with respect to the earth, and a typical size.
I expect that the mirror can be shining in a useful direction at least 90%
of the time. More details as they become available.

------------------------------
[ End of excerpt ]

------------------------------

Date: 22 Mar 90 06:37:58 GMT
From: munnari.oz.au!csc!ccadfa!usage!metro!ipso![e-mail]  (Mathew Lowry)
Subject: Progress-M

Can anyone tell me about the Progress-M, launched on August 23 by the USSR?

International number 1989-66-A, the only ref I've got is

"First of a new series of automatic carge spacecraft"
        Perigee 191 km, Apogee 235 km
        Period 88.5 minutes
        Inclination 51.6 degrees

(Information from Telecommunication Journal, Satellite Launchings section)


Probably best to email, and I'll summarise,

                        Thanks in advance
                                Mathew

------------------------------

Date: 23 Mar 90 00:34:21 GMT
From: munnari.oz.au!csc!ccadfa!usage!metro!ipso!stcns3![e-mail]  (Dave Horsfall)
Subject: Alan Shepherd in Australia

Just heard an interview on the radio this morning with Rear Admiral
Alan Shepherd Jr, first American in space.  [ Actually, I could have
sworn the interviewer said "first man in space" at least once, but
I digress... ]

Anyway, apparently he is a director of Kwik Copy Corporation (sp?),
and is in Sydney for a company convention.

--
Dave Horsfall (VK2KFU)  Alcatel STC Australia  [e-mail]
[e-mail]  ...munnari!stcns3.stc.oz.AU!dave

------------------------------

End of SPACE Digest V11 #202
*******************