ISDC2013ss2
Garbage to Gold - Locating and Re-using Space Debris with Server Sky
Keith Lofstrom / Server Sky PO Box 289 Beaverton Oregon 97075 / Email: keithl@server-sky.com
Abstract
Server Sky thin satellite arrays make excellent radar transmitters and receivers, and will be able to track very small objects in space with very high accuracy.
Thinsats have a minimum mass-to-area ratio limited by orbit stability in solar light pressure. We can reduce launch mass by another order of magnitude by adding simple mass ballast after launch and before deployment. The skins and propellant tanks of more than 1600 derelict upper stages in orbit can be cut into penny-sized ballast weights with lasers, and attached to ultra-thinsats. After the space debris is consumed, lunar regolith can be launched from the moon and formed into ballast, another small step towards space manufacturing.
Locating and Tracking Space Debris
Server sky arrays contain millions of radios and computers. Although intended for internet data service to the ground, the arrays may also be used as radars.
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 an object crosses though these regions, it encounters radio peaks and zeros, and its radar reflections will have a millisecond-scale amplitude and phase variation unique to its specific trajectory, position, and velocity.
Server sky arrays are also receivers, capturing the narrowband return signal and correlating it against expected signatures with massive amounts of array calculation. This can locate objects down to centimeter scale. Trajectories can be precisely computed to centimeter position accuracy and micrometer per second velocity accuracy.
One launch can put hundreds of high power arrays into orbit, which can search many regions of space in parallel. Groups of array can look down upon billions of cubic kilometers of space, and select different target regions in microseconds. There is no atmospheric distortion in space. Array can locate the arrays to a few micrometers, greatly outperforming ground radar systems, in frequency bands that do not penetrate the atmosphere.
Light Sail Orbit Instability
Server sky thinsats are light sails, perturbed by light pressure. Light pressure modifies orbits, moving sunward thinsats away from the Earth and starward thinsats inwards. Starting with a starwards, sufficiently eccentric orbit, the light pressure orbit modification can track the relative position of the sun. Insufficiently massive thinsats may drift out of assigned orbits, creating a hazard to satellites in other orbits. If there are thousands of thinsat arrays 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 keep arrays in stable low-eccentricity orbits is to restrict the maximum area-to-mass ratio of thinsats to about 10m²/kg at 6400km orbital altitude, and perhaps 2m²/kg at GEO. While this is still far lighter than current "aircraft-style" satellites, it places a lower limit on launch cost.
If we can someday thinsats 5 microns thick, with area to weigh ratios of 100m²/kg, 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, and recycle many of them. NORAD tracks 1500+ spent upper stages, with acres of aluminum skin and 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 ultra-thinsat cost reduction will need another source of ballast mass. Far less energy is needed to launch mass from the moon down to lower orbits. While sophisticated space manufacturing will be difficult, attaching penny-sized lumps from regolith to thinsats in microgravity is an easy manufacturing process.
Conclusion
Besides server sky's main mission of providing orbiting internet access and communication, replacing terrestrial power generation and infrastructure, it can also be used for high precision radar, protecting billions of dollars worth of other satellites. As server sky evolves, it will create a high value market for recycled space debris, and later for regolith from the moon. Server sky may be the first step to a space-based economy.
Note to reviewers: ISDC2013ss1 describes Server Sky and applications such as computation and communication for the developing world. This talk should follow that one.