It would be useful to be able to more easily recover satellites or their payloads from orbit.
Currently, there are two main ways of getting something back to Earth (or onto the surface
of another rocky planet with an atmosphere) after it's been to space:
1) give it equipment to survive atmospheric
entry and land on its own [1], or
2) give it equipment to survive atmospheric entry but not landing, including a parachute, and
use an aircraft to catch it as it's descending under the parachute [2].
Method 1 is mainly suitable for large objects such as crewed spacecraft and rovers, but has
been used to return samples to Earth. Before looking into this stuff, I only knew of one
time this was done deliberately, by Stardust [3]. However, it seems the Soviet Zenit spy
satellites' camera-and-film capsules landed on their own [4], and the American Corona
satellites' film capsules, which normally used method 2, were also capable of landing on their
own [5].
Method 2 is only suitable for small things like sample capsules. This is cheaper than
something that could land on its own to build and launch (because it has less
mass), but recovery is more difficult because you have to intercept the descending payload
with an aircraft. It's also unsuitable for landing on planets other than
Earth, because we don't yet have suitable aircraft there.
Both of these methods have a big disadvantage: The vehicle must be designed for reentry
from the start.
Sometimes, it would be nice to recover intact a spacecraft that was not designed for reentry.
Perhaps some unexplained event (an unusual failure, growth of
some form of life, etc.) occurred and we'd like to bring it into a lab to investigate. Perhaps
it's at the end of its useful life, but it's carrying some incredibly
valuable piece of equipment. Perhaps it's an enemy spy satellite that we'd like to reverse
engineer. Sometimes it isn't even a spacecraft, and wasn't designed at
all—wouldn't it be wonderful to be able to gently capture an asteroid or a would-be
meteor(ite) for study?
This used to be possible using the Space Shuttle, but that is no longer in operation, and
SpaceX hasn't developed a recovery system yet (though I have an idea on
that that I should post).
Another thing that would be useful is cleaning up debris in orbit, which we don't yet have any
practical solution for.
I have the solution!
Have a big tube. The diameter must be larger than the satellite or other object you want to
capture, and the length is probably measured in kilometers. It
doesn't have to be solid—it can be inflatable, or a tensegrity structure, or something like
that. It doesn't have to be all that strong, but it does have to be stiff.
The cross-section can have any shape, but I'll assume here that it's circular.
Along the side of it, there is a spline-like bulge all the way along. In operation, the tube has
its axis horizontal, with this bulge downward. In the bulge (which is
really a channel, from the point of view of the inside of the tube) there is a solution of soap
and water. There are also lots of loops of string spaced along the
inside of this soapy water channel, maybe one every 100 mm to 10 m or so. Each loop is
attached to a winch at the bottom of the channel, so it can be
constricted and pulled completely below the surface of the water. Each loop also has several
cords attached to it at intervals, which go up to pulleys spaced
around the circumference of the tube. Each of these cords, after passing over its pulley,
comes back down inside the tube's wall, or on the outside of the tube,
and ends up on the same winch, but wound the other way, and probably winding onto a part
of the drum with a different circumference, to accommodate
different distances they need to travel. In this way, one winch can be used both to pull in the
loop and submerge it in the soapy water, and to pull the loop out
of the soapy water and stretch it out to encompass most of the width of the tube.
The purpose of this is to slow satellites gently, by having them break through soap bubbles.
Each loop of string creates a two-dimensional soap bubble when
expanded. Each bubble popped by the satellite takes some energy from it, reducing its
speed. This slows the satellite down very gently and saves it from colliding
too hard with the atmosphere and burning up. Some of the bubbles won't form properly or
will pop prematurely, but that's not a big problem, because there are all the others still.
The advantage of using soap bubbles over a tube full of air is that they're gentler. Air gets
pressurized in front of the satellite, which causes it to heat up. Soap
bubbles won't do that. Also, each soap bubble dissipates energy in popping, while air needs
to flow out of the way after absorbing energy. (Popping releases the
bubble's own potential energy, but it takes an activation energy to pop a bubble that's not
ready to pop yet, and that energy comes from the satellite's kinetic
energy.) Sheets of plastic film might also work, but they're not reusable without being
remanufactured.
There are three ways to use such a machine to recover satellites.
First, you can launch it into orbit so that it's a satellite itself. It orbits at a specific altitude
and can be used for recovery of many satellites at this altitude. It
adjusts its orbit to intercept a given satellite (or piece of space junk). When it's nearly at the
intercept, it submerges and then expands all of its loops, making
many soap bubbles. The satellite passes through, is slowed, and is thereby deorbited. Due to
conservation of momentum, the tube will be pushed in the direction
of travel of the satellite it's deorbiting; with clever planning, this can probably be used to
maintain its own orbit. It doesn't need to slow the being-deorbited
satellite to a stop, just enough that its perigee is within the atmosphere. This role would be
best served by a sealed tube (probably inflatable for launchability)
with doors on the ends, to avoid excessive evaporation of the liquid. It will also need a liquid
that can stay in place in the channel without gravity, or to rotate
about its axis to keep the liquid in place centrifugally.
Second, you can launch it suborbitally to deorbit a single satellite. You would have to launch
it very precisely to make the intercept, but if you time it right, you
could get two potential intercepts with one launch (on the way up and on the way down,
with the satellite making one more orbit between the opportunities).
This role needs an expandable version, so that it fits into the fairing of the launch vehicle.
Third, you can use it within the upper atmosphere, possibly carried by balloons, to more
gently slow a satellite that has been deorbited by one of the previous
methods or by its own propulsion, and therefore is going to fall in a known location. In this
role, it would be on an angle, so it would need a pump to pump the
liquid from the bottom of the channel to the top, to keep a roughly uniform depth along the
length, or, more practically, each loop of string could have its own
reservoir. At the end of the tube, there could be a detachable soft bag with a parachute
attached, possibly filled with plastic foam pellets or something like that,
which will carry the satellite softly to the ground or to midair retrieval.
N/A [2018-04-24]