ISDC2013ss2

Garbage to Gold - Locating, Capturing, and Re-using Space Debris

Server sky arrays contain massive amounts of computation and huge numbers of agile radios. Although intended for internet data service to the ground, the arrays may also be reprogrammed to act as radars.

Locating and Tracking Space Debris

Large server sky arrays can focus kilowatts of millimeter wave radio energy into precisely aimed pencil-thin beams. Beams from many arrays can overlap in one small region of space, creating high power standing waves. When a debris object crosses though these regions, it will encounter peaks and zeros, and its radar reflections will have a millisecond-scale amplitude variation unique to its specific trajectory, position, and velocity.

Server sky arrays can also operate as narrow bandwidth receivers, capturing the narrowband return energy and correlating against expected signatures with massive amounts of array calculation. This will result in the location of objects down to centimeter scale. Trajectories can be precisely computed to centimeter position accuracy and micrometer per second velocity accuracy.

Since one launch can put hundreds of high power arrays into orbit at once, we can search many regions of space in parallel. A collection of arrays can look down upon billions of cubic kilometers of space, and switch between target regions in microseconds. There is no atmospheric distortion or path delay in space, and we can locate the arrays to a few micrometers, so we can greatly outperform ground radar systems, in frequency bands that do not penetrate the atmosphere.

Light Sail Orbit Instability

A very light object such as a server sky thinsat is significantly perturbed by light pressure. If a thinsat is too light, its orbit will drift out of its assigned slot, creating a hazard to other satellites.

Increasing velocity at one side of an orbit increases the altitude at the other side of the orbit. Light pressure increases thinsat orbital velocity as it moves away from the sun, pushing the orbit outwards as it moves towards the sun half an orbit later. Similarly, light pressure decreases orbital velocity moving towards the sun, pushing the orbit inwards as it moves away from the sun.

If the sun was always in the same direction from the earth, the result would be a slow increase in orbit eccentricity, moving perigee down and apogee up, until perigee intercepts the earth, or the thinsat strays into the path of another satellite. Fortunately, the sun and earth rotate once a year in relation to the fixed sky, so the orbital changes add up to zero over the course of a year. Starting with an elliptical orbit pointing away from the sun, that orbit can be made to precess once per year, matching the apparent position of the sun.

Unfortunately, as thinsats get lighter, this "stable" orbit must become increasingly eccentric. The stable orbit perigee may drop or the apogee climb into the path of other satellites. If there are billions of thinsats in orbit, collisions may become difficult to avoid, even if the arrays are highly maneuverable and the other satellites are precisely tracked. This problem gets worse as orbit radii increase.

The only way to restrict the eccentricity of the stable orbits is to restrict the maximum area-to-mass ratio of thinsats to about 10m²/kg at 6400km orbital altitude, perhasp 2m²/kg at GEO altitudes, even less at the laGrange positions. While this is still far lighter than current "aircraft-style" satellites, it places a lower limit on launch cost.

We would prefer to make thinsats lighter and lighter. We can reasonably expect to learn to manufacture thinsats 5 microns thick, with area to weigh ratios of 100m²/kg or more. If we can achieve 15% solar cell efficiency, and $2000/kg launch, that would result in 10 watts in orbit per dollar of launch cost. But such ultra-thinsats would not be stable.

What if we could launch these very thin satellites, and attach ballast weight in orbit?

Reusing Space Debris

Instead of mere avoidance, we can collect the derelict objects, then recycle or re-enter them. NORAD tracks 1500+ spent upper stages, with acres of aluminum tank.

Specialized satellites with high $I_{SP}$ VASIMR thrusters and powerful lasers can cut the skins and tanks into penny-sized ballast mass for future ultra-thinsats. The ballasts are ferried to M288 to attach to new ultralight thinsats. Every kilogram delivered to M288 with fuel-efficient space tugs enables an extra kilowatt of ultra-thinsat.

Many of those rocket bodies are far from M288, in LEO and MEO orbits accessible to electrodynamic tether EDDE capture systems. Those objects can be collected into "junkyards" in low orbit for other re-uses, or de-orbited and re-entered. Accurate server sky radar will help mission planning for EDDE as well as detect and characterize potential tether-cutting colliders.

In the long term, we will run out of usable space debris. Continuing cost reduction requires more efficient launch systems such as the launch loop, or another source of ballast mass.

Far less energy is needed to launch mass from the moon down to lower orbits. While space manufacturing is very difficult, attaching penny-sized lumps from regolith to thinsats in microgravity is about as easy as manufacturing can get.