Mopping up Space Junk

I have a few ideas to throw into the global pot for consideration.

After their useful lives, some satellites can deorbit with chemical or
ion rockets or deploy vary large sails to increase their drag
coefficient or be perturbed by the solar wind. Another method could be
to extend long cables to use the Earth’s magnetic field to induce
drag.
However, all these require active systems installed on the junk and be
commanded to deploy.

The only methods for getting rid of space debris is to either go up
there and get it, or wait a very long time for it to decay out or hit
something else and hope that the resulting debris ends up with lower
velocities that will decay quickly and not contribute to the cascade
effect.

Matching an orbit to be able to retrieve something is horrendously
costly in fuel and really not practical except for large or valuable
pieces in really inconvenient orbits.

My idea would be to scale-up Stardust's aerogel capture system and
deploy kilometre-sized megafoams in orbits designed to intersect as
many debris fields as possible. While probably not able to capture
most debris, one or more collisions could reduce their kinetic energy
substantially to aid quicker reentry. Any foam shrapnel produced
should be relatively harmless and decay very quickly. I can't imagine
any process that could create kilometre-scale aerogels on Earth, let
alone in-situ!

A few tons of foaming agents could be shipped up in batches and then
mixed together when the right quantities are reached. Foams would
expand massively in a vacuum if the bubbles in the material could
remain gastight under the extreme stretching. Space would be a very
harsh environment for foam, as they would deform a lot depending on
temperature so some pretty amazing materials would need to be used.

Many shapes are possible - either big amorphous blobs, tentacular
tangles or more structured lattices.
If foams are extruded while being formed they could make long
continuous tentacles.
These could also be used instead of sails as aerobrakes for
deorbiting.
I imagine enormous structures could be made in space with just a few
tons of materials.
They would have to start off in high orbits because their own orbital
decay would be substantial. Maintaining altitude and manoeuverability
would be difficult using rockets, especially after absorbing unknown
amounts of angular kinetic energy from the collisions it is looking
for!

If the foaming material has a high enough albedo or aluminized, they
could provide usable night illumination too.
If we make thousands of them, perhaps they could also reduce
insolation by a useful amount as well.

The foam would have to be environmentally benign for reentry. If it
didn't burn up it would shrink massively with increasing atmospheric
pressure. If enough survives, it might even preserve anything it
caught.

NASA know an awful lot about foam now, anyone from NASA like to
comment on feasibility?

How easy would it be to make foams in a vacuum?

Andy Lee Robinson

Andy wrote:
My idea would be to scale-up Stardust's aerogel capture system and
deploy kilometre-sized megafoams in orbits designed to intersect as
many debris fields as possible. While probably not able to capture
most debris, one or more collisions could reduce their kinetic energy
substantially to aid quicker reentry. Any foam shrapnel produced
should be relatively harmless and decay very quickly.
[[...]]
A few tons of foaming agents could be shipped up in batches and then
mixed together when the right quantities are reached. Foams would
expand massively in a vacuum if the bubbles in the material could
remain gastight under the extreme stretching.

This is a clever idea. Let's crank a few numbers to see how it works...

In article (20 Aug 2002)
John Stockton discussed a similarly clever idea, having sounding
rockets dump a gas cloud into space (without orbital velocity) just
ahead of a debris chunk, so the debris would pass through the gas
cloud before the gas cloud dispersed. As John wrote then
To drop the perigee of an object in circular long-term orbit by a truly
significant amount (so that it becomes no more than a short-term object)
will require a change of velocity of the order of, say, 80 m/s, or 1%.

To obtain such a change of velocity by hitting what would be an
effectively stationary gas cloud would require the object to hit gas
totalling around 1% of its own mass.

In other words, if the aerogel is too tenuous, then debris won't be
decelerated enough by passing through it.

What matters here is basically the areal density of the absorber,
in grams/cm^2. That is, if you cut a debris-fragment-sized hole through
the absorber, the mass of absorber in that hole, m, is given by the
product of this areal density, and the hole's area. If a debris-fragment
of mass M passes through the absorber, it is then looses a fraction
m/M of its orbital velocity. We want this fraction to be at least
f=0.01.

[In my simplistic analysis, it doesn't matter whether
a given areal density comes from a very low-density
aerogel that's quite thick, or from a higher-density
aerogel that's a bit thinner.]

A line or two of algebra reveals that this means the absorber areal
density must be at least (4/3)*f*r*rho, where r is the typical radius
of a debris fragment and rho is the typical mass density of a debris
fragment. Setting rho = 3 grams/cm^3 gives f (in grams/cm^2) =
0.04 * r (in cm). So to deorbit 1cm-and-smaller diameter debris
fragments (a 1cm-diameter fragment has r = 0.5cm, and in this crude
masses around 0.4 grams) needs an absorber with an areal density of
0.04 grams/cm^2. That's 400 grams/m^2, so an absorber 100 meters
on a side has a mass of 4 tonnes. So far, so good.


Unfortunately, near-Earth space is *big*, so it takes a lot of
absorber area to sweep it in any reasonable time. As Henry Spencer
put it in another old post to this newsgroup, (article
on 6 Mar 2006),
An absorber 100 meters on a side --
a *big* thing to build, especially since it needs a fair bit of mass per
unit area -- sweeps out about 7000 cubic kilometers a day. Assuming you
put it into a slightly elliptical orbit, with apogee 100_km higher than
perigee so that it sweeps out a range of altitudes rather than just one,
it's trying to sweep a shell with a volume of 53,000,000,000 cubic km.
In 20 years it sweeps about 0.1% of that volume.

In other words, our 100-meter absorber just isn't big enough to do
the job in any reasonable time. If we want our system to sweep, say,
10% of that volume in 20 years (which is still not really "doing the
job in a reasonable time"), then it needs to be 100 times bigger,
i.e., 1 kilometer on a side. But now it masses 400 tonnes, so
launching it isn't quite such a simple job.

Making the system another factor of 10 larger in area, so it nominally
sweeps all the target volume in 20 years (i.e., so it's actually a
"solution" to the orbital-debris problem, at least in this 100km-wide
range of orbital altitudes) means it now has a mass of 4000 tonnes.


In other words, while the aergel system can be made to work, it's not
going to be small, fast, or cheap.


There's also another rather serious issue to worry about, namely the
fate of all the non-debris satellites in that range of orbital altitudes.
The aerogel probably has to have a substantial delta-V capability (i.e.,
it has to carry a rockets and (given its huge mass) a *lot* of fuel)
to be able to maneuver so as to avoid hitting any known satellites.

In fact, it's probably better to divide up that 4000 tonnes and 10 km^2
absorber area into 1000 100-meter-on-a-side 4-tonne absorbers, each
with its own maneuvering capability. This way "only" 4 tonnes of
absorber (+ rockets + fuel + control/structures/communication/etc)
has to be maneuvered each time there's a satellite in the way.

[The requirement to maneuver the absorber also means
that the whole thing (aerogel + any supporting structure)
has to be structurally strong enough to handle the
maneuvering g-forces. We probably want to make the
maneuvers as gentle as possible consistent with not
hitting other satellites.]

Someone has probably done a more careful analysis of such a
debris-sweeping constellation, but I've never seen it...

--
-- "Jonathan Thornburg [remove -animal to reply]"
Dept of Astronomy, Indiana University, Bloomington, Indiana, USA
"Space travel is utter bilge" -- common misquote of UK Astronomer Royal
Richard Woolley's remarks of 1956
"All this writing about space travel is utter bilge. To go to the
moon would cost as much as a major war." -- what he actually said

On May 6, 7:30 am, Andy wrote:
My idea would be to scale-up Stardust's aerogel capture system and
deploy kilometre-sized megafoams in orbits designed to intersect as
many debris fields as possible.

Variations on the theme have been suggested before, like large ceramic
fabric disks.

While probably not able to capture
most debris, one or more collisions could reduce their kinetic energy
substantially to aid quicker reentry. Any foam shrapnel produced
should be relatively harmless and decay very quickly.

I'm not sure I'd trust the foam to be "harmless." The shuttle has had
some divots blown out of its windows by paint chips.

On the other hand, if there's any solid substance with a chance of
being
harmless in orbital impacts, it's aerogel. So if it passes muster in
simulations and tests, it'd be the material for these debris sweepers.

I can't imagine any process that could create kilometre-scale
aerogels on Earth, let alone in-situ!

Probably something involving an inflateable mold.
http://en.wikipedia.org/wiki/Echo_satellite

Mike Miller

On May 6, 7:30 am, Andy wrote:

My idea would be to scale-up Stardust's aerogel capture system and
deploy kilometre-sized megafoams in orbits designed to intersect as
many debris fields as possible. While probably not able to capture
most debris, one or more collisions could reduce their kinetic energy
substantially to aid quicker reentry. Any foam shrapnel produced
should be relatively harmless and decay very quickly. I can't imagine
any process that could create kilometre-scale aerogels on Earth, let
alone in-situ!

Too late, that was already suggested in a story published in Analog
Science Fiction & Science Fact.

Earl Colby Pottinger

On May 11, 6:07 pm, Hipupchuck wrote:
Snip.

All you need is a satellite with a targeting laser blaster on it. Have
it seek and disintegrate space junk or slow it down so it falls to earth.

Nice idea.. but difficult.. molten space junk is still space junk, and
fragments of space junk is yet more space junk, and may even take
longer to decay if cross-sectional area decreases, eg, a breaking up a
flat panel into tiny fragments.
If a laser could vapourise or plasmarise a target, then it could be
useful, but have watch out for anything else in its path. Boiling
random patches of ocean might not be popular!
Perhaps solar storms could be seeded to increase atmospheric drag or
solar pressure?

Cheers,
Andy.

Andy Lee Robinson ) writes:

On May 6, 7:30 am, Andy wrote:

My idea would be to scale-up Stardust's aerogel capture system and
deploy kilometre-sized megafoams in orbits designed to intersect as
many debris fields as possible. While probably not able to capture
most debris, one or more collisions could reduce their kinetic energy
substantially to aid quicker reentry. Any foam shrapnel produced
should be relatively harmless and decay very quickly. I can't imagine
any process that could create kilometre-scale aerogels on Earth, let
alone in-situ!

[...]
However good an idea it seemed at the time, letting loose hundreds of
millions of fragments was really irresponsible.
In such a high orbit with the copper's high density, they could be
there for a very long time.

However very large, luminous, negligible density megafoam structures
are complely different. Their orbits would decay quickly, and more so
after absorbing debrital energy with the average resultant against the
direction of travel.
Just hope they could mop up a useful amount of debris before reentry.

Actually I'm under no illusions about the vastness of the volume
involved...
Even a 1km blob in a 1000km orbit would occupy such a small volume,
that 3 million of them would be required to cover just 1% of sky at
that altitude, but would be likely to mop up all intersecting debris
within a few hundred orbits.

[...]

So I concede tha the only viable solution is to not pollute in future
and to track and avoid for a few thousand years and pray.

As impractical solutions go, why not use a cloud of finely divided ice? At
least, if it doesn't leave orbit, it will eventially evaporate.

--John Park


======================================= MODERATOR'S COMMENT:
Please consider trimming your quotes in the future.

In sci.space.tech message e741088d-ad41-4d99-868a-4a2dbbcd431b@a7g2000y
qk.googlegroups.com, Wed, 3 Jun 2009 00:00:49, Andy Lee Robinson
posted:

...

Actually I'm under no illusions about the vastness of the volume
involved...
Even a 1km blob in a 1000km orbit would occupy such a small volume,
that 3 million of them would be required to cover just 1% of sky at
that altitude, but would be likely to mop up all intersecting debris
within a few hundred orbits.

...

Firstly, there is the small but important set-of-cases involving impact
between two objects of which both are trackable and at least one is
manoeuvrable. In that case, dodging is effective.


Otherwise, up there we have a large number of useful, easily-damaged
objects, going in many directions some more popular than others, and
with drifting orbital parameters. And we have a much larger number of
useless objects, some shatterable but mostly hard to destroy, going in a
similar spread of directions.

Collision rate, per pair, is roughly proportional to their combined
cross-sectional area and their relative speed. Collisions with useful
objects are bad, on the whole.

Send up a quantity of unguided "brooms", of any type. Each will have a
certain total collision rate with junk; the best that can happen is that
the junk is taken aboard, disintegrated into bits with less damage
potential, or made to re-enter atmosphere sooner. The worst is that
some of the broom adds to the junk. The average will be in between.

Each broom will also have a certain collision rate with useful, un-
manoeuvrable stuff; most such collisions will be harmful to the useful
stuff, and many will create more numerous junk.

Brooms will also collide with each other, unless very carefully orbited.

Consider all those area-dependent collision rates, and their effects.
ISTM inevitable that launching unguided brooms will be of negative
benefit - by hitting useful stuff, or by creating more harmful junk.

The case is, of course, different for anything that can discriminate
between junk and useful stuff, and act on the difference.

--
(c) John Stockton, nr London, UK. Turnpike v6.05 MIME.
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

Andy Lee Robinson wrote:
On May 11, 6:07 pm, Hipupchuck wrote:

Snip.

All you need is a satellite with a targeting laser blaster on it. Have
it seek and disintegrate space junk or slow it down so it falls to earth.


Nice idea.. but difficult.. molten space junk is still space junk, and
fragments of space junk is yet more space junk, and may even take
longer to decay if cross-sectional area decreases, eg, a breaking up a
flat panel into tiny fragments.

Breaking up a flat panel into tiny fragments won't prolong the orbital
life of the pieces. You could have part of the space junk stay up longer
by cutting off pieces with lower mass to cross-sectional area ratio. For
instance if you cut off solar arrays to a satellite, the solar arrays
will de-orbit relatively fast and the rest of the satellite will be up
there longer. It isn't obvious to me what is best, having a smaller
satellite (e.g. satellite without solar arrays) staying up there longer
or a larger satellite coming down faster, but I would think that in most
cases you are better off leaving the solar arrays on the satellite so
it deorbits faster.

For "normal" objects, you can't prolong the life of a satellite by
cutting it into pieces. If you increase the mass to cross-sectional area
ratio by cutting the thing you didn't do it by increasing the mass, so
you have to do it by reducing the cross-sectional area. Normally, you
don't reduce the cross-sectional area by cutting something up, unless
you can somehow stack the pieces. That could happen for some objects
for instance if you have an umbrella which is open but spring loaded to
close, if you cut of a piece that lets it close, you can reduce the
cross-sectional area.


Alain Fournier