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Date: 15 Sep 88 17:33:43 GMT
From: attcan!utzoo!  (Henry Spencer)
Subject: Re: SDI and Orbital Debris

In article <6445@dasys1.UUCP> axelson@dasys1.UUCP (Kevin Axelson) writes:
>If an SDI system is successfully used against say 500 missiles, would
>the resulting space debris be signfigant?  Would we be grounded?  Would
>the SDI equipment be damaged by this material?

Probably none of the above.  Based on the evidence of the Delta experiment
a year or two ago, a hypervelocity collision between an ICBM and a space-
launched interceptor will produce two debris clouds, each roughly following
the original path of one of the incoming objects.  (The first instant
of contact produces a shock wave which pulverizes both objects; the
debris clouds then pass through each other without interacting much.)
The ICBM debris will hit atmosphere roughly where the ICBM was aimed at
the moment of destruction; the interceptor has probably accelerated
enough since it was launched that its debris will be in either an escape
trajectory or a reentry trajectory.  The situation for things like beam
weapons would be similar except for no interceptor debris.

So this, so to speak, "main-line scenario" does not produce orbital debris.
There are, however, some secondary possibilities.  Fractional Orbit
Bombardment Systems (FOBS) involve actually putting warheads in orbit very
temporarily, so intercepting them could leave debris in orbit.  One might
expect that occasionally an ordinary ICBM intercept, especially at very
close range, would leave interceptor debris that didn't quite manage to
reenter or escape, and ended up in orbit.  And one should beware of SDI
systems that leave orbiting debris behind them; for example, many missiles
that are fired from sealed tubes simply punch out through the seal, and
this might leave small amounts of seal debris in orbit.  My guess is that
none of these things would be too significant, compared to the volume of
trash we already have in orbit.
NASA is into artificial        |     Henry Spencer at U of Toronto Zoology
stupidity.  - Jerry Pournelle  | uunet!attcan!utzoo!henry

Subject: Re: Trash in Space
From: (Henry Spencer) 
Date: Aug 01 1995

In article <> (Pete Jennings) writes:
>As to the legal ramifications, I suspect that possession will be nine tenths
>of the law in this case.  Precedents from naval salvage will be applied...

Unfortunately, the current legal situation is that the nautical precedents
are irrelevant, because the rules are different in space.  Under current
international treaties, there is no such thing as abandoning a space object.
It remains the responsibility of the launching state, period.  Anything you
do to it which could conceivably cause it to reenter early or in a more 
expensive place will have the lawyers on your doorstep.

These rules certainly need changing, but until they do, it will be difficult
to get financial backing for any scheme which runs afoul of them.

>Any organization desiring to profit from space salvage should build and
>launch a fleet of automated, lightweight conducting tethers.  Each would 
>consist of a bus with solar arrays, some grappling mechanism, and tethers
>to deploy above and below the orbit altitude.

Bear in mind that there's a fair likelihood of the tethers being broken
by debris hits.

>...Now, the scheme may be limited in altitude
>due to the need to complete the electrical circuit in the ionosphere.  
>However, it is the low orbits which most need policing, as they will be
>home to the first space colonies and factories.

Actually, the low orbits aren't that much of a problem, because debris
left there tends to come down due to air drag.  The worst problem areas
right now are at slightly higher altitudes; at 900km or so, it's quite
possible that the debris reproduction rate (from collisions) already
exceeds the removal rate.

(I'll repeat a previous recommendation:  "Orbital Debris, A Technical
Assessment", by the National Research Council, National Academy Press
1995, ISBN 0-309-05125-8, ordering details at  This
book should be required reading for anyone new to the subject.)
The problem is, every time something goes wrong,   |       Henry Spencer
the paperwork is found in order... -Walker on NASA |

Date: 19 Nov 88 23:29:46 GMT
From:!  (Phil Karn)
Subject: Re: Oxygen from the moon

>There is already quite a bit of metal oxide in Earth orbit,
>because many existing solid-rocket motors use a bit of aluminum for extra

Yes, aluminum is a standard component of solid rocket fuel. However, I find
the assertion about Al2O3 in orbit somewhat hard to believe. First of all,
the majority of solid rocket fuel is burned by lower stage boosters which
are nowhere near orbital velocity.

This leaves upper stages, particularly perigee and apogee kick motors.  A
typical exhaust velocity for a medium sized vacuum-fired solid motor is
(very roughly) 2500 m/sec.  Since the thrust vector used to boost a
spacecraft up from a low circular orbit has the motor nozzle pointing
against the velocity vector, particulate exhaust from a perigee kick motor
in low earth orbit is effectively de-orbited. (Examples of such motors are
the PAMs used on Delta and Shuttle, and the first stage of IUS.  Perigee
kick motors are unnecessary with Ariane, which has a liquid-fueled third
stage that places a geostationary payload directly in the transfer orbit.)

The exhaust from solid fueled apogee kick motors also de-orbits, although
why this is so is less obvious.  At the apogee of a geostationary transfer
orbit where the kick motors are fired, spacecraft inertial velocity is about
1.6 km/sec.  This is barely enough to stay in orbit (it corresponds to a
perigee of 240 km, given by Ariane to AMSAT Oscar-13).

Exhaust fired at 2.5 km/sec in a roughly opposite direction will have a net
inertial velocity of -0.9 km/sec, insufficient to stay in a retrograde
orbit. As the spacecraft picks up velocity during the burn, the inertial
velocity of the exhaust decreases through zero and becomes positive again,
but never large enough to stay in orbit.

Of course, I suppose it's possible that some tiny fraction of the exhaust
manages to come out at small relative velocity, depending on the nozzle

About the only solid fuel exhaust that is likely to remain in orbit for any
length of time is that from RETRO rockets like those used to retrieve
Mercury, Gemini and probably their Soviet equivalents.  (Yet another good
argument against manned space flight, no?  :-)) Even there, however, I
suspect the particles are so small that solar photon pressure perturbs them
into decaying quickly, if the air drag at perigee doesn't do it first.

I am much more concerned about larger (millimeter and centimeter sized) bits
of orbiting debris such as those produced by inadvertent or deliberate
explosions of or collisions between orbiting objects. These are too small
and numerous to be tracked easily, yet large enough to do substantial
damage. They are also too large to be affected significantly by solar
radiation pressure.


From: Henry Spencer <>
Subject: Re: Debris in GEO
Date: Fri, 29 Dec 1995 03:43:27 GMT

In article <> (Paul F Austin) writes:
>It seems to me (being no orbital mechanic) that all debris in GEO orbits in 
>the same plane, in the same direction and at the same group velocity. Because 
>of this, the delta V between objects in GEO should be rather small and 
>certainly not the km/s range that you get from random intersecting orbits.
>Is debris a major threat in GEO?

Unfortunately, could be.  GEO is not entirely stable, which is why the
life of comsats (etc) is limited by their supply of station-keeping fuel. 
Notably, lunar and solar perturbations tend to change the inclination of
the orbit; the result is a need for "north-south" stationkeeping, which
is the major fuel consumer.  (Lesser perturbations, from things like the
Earth's noncircular equator, require "east-west" stationkeeping, but this
is small by comparison.)

Without stationkeeping, the orbital inclination of an object initially in
GEO will change in a complex pattern, and other elements of the orbit
will also change slightly.  The result is an orbit that will often be
passing through GEO at significant relative velocity twice a day.

Mind you, "significant" relative velocity here is rather smaller than in
LEO, because all the orbital velocities are slower up there, and because
even the perturbed orbits don't cross each other at quite the sharp angles
found in LEO.  GEO is also a lot larger than any particular low orbit; if
the object being abandoned is inert -- no fuel, no stored energy -- the
risks of abandoning it are actually fairly small at present.

On the other hand, air drag is effectively zero up there, so there is
nothing that naturally cleans debris out.  Worse, it's nearly impossible
to detect and track uncooperative debris at such high altitudes.

There is considerable, although not universal, sentiment that shifting
dying GEO birds to a graveyard orbit a few hundred km higher is a good
Look, look, see Windows 95.  Buy, lemmings, buy!   |       Henry Spencer
Pay no attention to that cliff ahead...            |

From: Henry Spencer <>
Subject: Re: Debris in GEO
Date: Mon, 1 Jan 1996 21:19:10 GMT

In article <4c6rbu$> George Paine <> writes:
>> There is considerable, although not universal, sentiment that
>> shifting dying GEO birds to a graveyard orbit a few hundred km
>> higher is a good thing.
>It's SOP for us to kick spacecraft out of the way at end-of-life...

To elaborate slightly on my earlier comment...  The reasons why there is
some doubt about graveyard orbits for GEO birds boil down to:

- It's not really permanent.  But it does defer the problem for (probably)
tens of thousands of years. 

- The collision risk is still there, in the graveyard orbit, where
energetic events could send debris into orbits intersecting GEO.  But if
you put the graveyard orbit high enough (300km above GEO is the current
consensus minimum, but the issue is still being studied), and make sure
to empty tanks and discharge batteries, the chances of this are small.

- The collision risks produced by an abandoned satellite in GEO are 
currently very small, although they may become more significant in future.

- It's expensive in lost earning potential, because the fuel reserve
needed for disposal cuts into stationkeeping fuel... especially since the
quantity of remaining fuel generally is not known precisely, so a safety
margin adds to the penalty.  (A recent ESA study of satellite servicing
identified towing services -- moving dying GEO birds to graveyard orbit --
as the single most promising near-term market, because eliminating that
fuel reserve is a clear and immediate benefit worth quite a bit of money.)  
But doing anything else about the problem is even worse; in particular, 
deorbiting or boosting to escape, which are more permanent solutions, 
are impossibly costly.

The bottom line is that, despite some doubts -- in particular about just
how much altitude you need to add -- it's becoming accepted practice.
Look, look, see Windows 95.  Buy, lemmings, buy!   |       Henry Spencer
Pay no attention to that cliff ahead...            |

From: Henry Spencer <>
Subject: Re: SSTO size (was Re: landings)
Date: Mon, 29 Jan 1996 20:14:10 GMT

In article <4dfjsu$> (RHA) writes:
>>  >> ...A pressurized assembly hangar would be
>>  >> immensely valuable and not all that difficult to build. 
> From time to time I wondered about this approach myself, I also
> wonder whether this can really work today. My concern is due to all
> the debris in orbit. How long could a balloon last? And what would
> the effect be of even a chip of paint (such as the one which hit 
> the shuttle windshield, forcing replacement) passing through the
> balloon at hypersonic velocities, less than 100 feet from a worker?

It wouldn't pass through the balloon; you're committing a common error,
extrapolating from low-velocity experience to realms in which it does
not apply.  The paint chip *explodes* the instant it hits the balloon.
The concern is over the damage done by the explosion:  shock wave,
possible shrapnel (if the debris particle is rather larger than a
paint chip), and how big a hole it makes.

The orthodox solution to this works just fine:  put a thin outer shell
outside the balloon, so the explosion takes place at the outer shell
rather than the pressure wall.  With a substantial gap in between, the
fireball and debris spread out enough before reaching the balloon that
they do no damage to it.  If you're still concerned, add a tough
protective layer just outside the balloon, to soak up the impact. 
Further refinements are possible.

You will, incidentally, want at least one outer shell anyway, for thermal
insulation.  While the vision of a transparent balloon with people and
hardware inside and space outside is sort of cute, it's unrealistic.

Any large object in orbit has to worry about debris impacts, but with
a bit of straightforward engineering the problem can be made manageable.
The Earth is our mother.                           |       Henry Spencer
Our nine months are up...                          |

From: (Henry Spencer)
Subject: Re: Bring Hubble Back
Date: Thu, 13 Jan 2000 19:25:24 GMT

In article <01bf5ca2$3d010e80$LocalHost@user>,
Bruce Grant <bgrant@furhold.ifb."nospam"> wrote:
>> That was the Long Duration Exposure Facility...
>Speaking of which, I can't remember hearing of the results beyond (I
>think) a few reports on the size distribution of micro-meteor impact
>craters. Was anything particularly interesting, unexpected, or
>now-how-did-*that*-happen discovered?

There were some surprises, but nothing vast and startling.  There have
been several conferences on LDEF results.  The two interesting tidbits I
remember are that there were a significant number of debris hits on the
*rear* side (that is, there is some noticeable amount of debris in highly
elliptical orbits, so at LEO altitudes it is moving substantially faster
than a LEO satellite and could produce a damaging hit despite having to
overtake the LEO bird from behind) and that the distribution of surface
damage suggests that X-rays from solar flares are significant in the
deterioration of exposed materials (areas that got equal sun exposure
nevertheless had quite different levels of deterioration, and it happened
that the major flares all happened when the worst-rotted side was facing
the Sun, so they're the prime suspect).
The space program reminds me        |  Henry Spencer
of a government agency.  -Jim Baen  |      (aka

From: Henry Spencer <>
Subject: debris vs ISS (was Re: chicken cannon)
Date: Thu, 26 Mar 1998 03:45:21 GMT

In article <>,
Grungy <> wrote:
>> ...there is
>> an uncomfortable gap between the biggest incoming debris that ISS's walls
>> can handle without serious trouble (circa 1cm) and the smallest debris
>> that can be tracked readily (circa 10cm)...
>At what relative velocity can the ISS's walls survive a hit from a 1cm object?
>I suppose it depends upon what the object is.

I think that's at worst-case debris arrival velocity, with the debris
assumed to be solid aluminum (more or less the worst case).

It turns out, by the way, that the worst-case velocity is not the maximum
possible velocity.  Really slow debris carries little energy and so can do
little damage, while really fast debris fragments more readily on hitting
the outer wall.  There's an intermediate velocity where the energy is
enough to be a problem but not enough to reliably break the debris up into
really small pieces, and debris arriving at around that velocity hits the
inner wall hardest for a given debris size.

>Does the ISS have a double-walled structure?

Yes.  In fact, its shielding could be made significantly more effective if
the outer wall was deployable -- it turns out that increasing the spacing
between the walls is one of the best things you can do, and it costs very
little mass -- but NASA distrusts the extra complexity, especially after
what happened with Skylab's deployable bumper.

>What about the "windows"?

They have a sacrificial outer pane which is expected to be replaced now
and then (or was, when the station was planned for a long life -- don't
know whether window replacements are still provided for).

>The Crew Systems Division (later Crew & Thermal SD) did some rail gun
>tests on the Space Suit Assembly in the early '80s.  The projectile of
>choice was a tiny pellet of nylon...  Launched at 7kps, in a vacuum, at
>a one-foot-square layup of SSA materials, they produced a 1cm hole -
>through all of the layers...

An interesting sidelight on this, though, is that apparently nobody has
ever tested the effect of even a rather smaller hit on a suit pressurized
with oxygen.  There is real concern that the edges of a small hole -- too
small to badly injure the occupant or immediately depressurize the suit --
might ignite... but a lot of the testing is done with light-gas guns,
which throw hydrogen gas around, and so the safety people don't like the
idea of oxygen-filled targets.
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         |

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