Sunday, October 27, 2013

Space Debris, an Interesting Number

"On-orbit, predicted conjunctions vary based on the debris density at the altitude of the vehicle. For altitudes of 350-400 km, approximately 3 maneuvers would need to be made annually." from the FAA's Draft Established Practices for Human Space Flight Occupant Safety.

This gives one an idea of the impact space debris is having right now: an average of three maneuvers per spacecraft per year to avoid collision. As space debris is expected to increase, the costs associated with dealing with space debris will undoubtedly rise. We can also expect the destruction of more operational satellites, which has happened twice so far.

The most dangerous space debris is the thousands of large pieces, mostly Russian upper stages, in polar orbits. When these have collisions, tens of thousands of pieces of debris are created. Indeed, we may be currently in a very slow motion chain reaction of collisions creating debris that in turn creates collisions which create debris ... The debris is moving at very high speeds, so even a small piece can destroy a satellite. The film 'Gravity,' while not particularly accurate technically, highlighted this quite effectively.

What to do: the single easiest and most effective act would be to get rid of the large pieces. It's been estimated that removing 10 per year would be sufficient, statistically, to start reducing the total amount of debris in LEO assuming other activities don't add much. We should get started on serious debris reduction. Otherwise, given enough time we could easily pollute Earth orbits with enough debris to end the space age.

1 comment:

  1. Minor comments: 3 maneuvers per year per spacecraft is for human-rated, where the maneuver threshold is at 1e-4 probability of collision. Uncrewed assets - even high value assets like Hubble - accept a higher threshold in the conjunction analysis before triggering a collision-avoidance maneuver, and so they are maneuvered less frequently. Most satellites - even large ones - in congested orbits (basically sun-sync) are maneuvered less than once per year for debris-avoidance. Quite often, these maneuvers are accomplished with station-keeping which has to be done anyway There is risk in all maneuvers: particular thruster-stuck-on failures which can lose a mission.

    The most important thing for space collision avoidance right now is better data.

    1) The DOD needs to open up their high accuracy catalog. The low-accuracy data they currently distribute is not good enough to justify the risk of a collision maneuver. They JSPOC says "there's a 1e-4 probability of collision for you at 9:00AM wednesday. We suggest you do a maneuver. Email us your planned maneuver and we will get back to you within six hours and let you know if it decreases the probability of collision. Not efficient. Not open.

    2) The S-band fence needs to be completed on an accelerated pace. Fixed price contract with construction bond. No more studies, no more overruns.

    3) The current space fence must not be decommissioned until the new one is ready. No more using the fence as a sacrificial pawn in the sequester games. It provides most of the space debris tracking data for the US SSN.

    4) Other countries must open up their space surveillance networks' data. In spite of the foot-dragging, lack of transparency, and political games being played by the US SSN commanders and contractors, at least the US releases daily updates on all tracked objects, (not "high accuracy" data, but a lot better than nothing). The rest of the world (Russia, Europe, China, Japan) release nothing from the SSNs.

    All of the above could be accomplished now, at low cost or zero cost.

    On the other hand, systems to remove debris are hugely expensive. No one is going to foot the bill of, ~$100 million to remove one piece of debris. All such missions also pose the serious risk of creating more debris (when they go wrong), and are essentially indistinguishable from anti-satellite weapons. Lastly, ADR-10 (active debris removal, 10 pieces per year) will only affect the long term (~200 years) debris evolution *probabilities*. A single collision, or anti-satellite weapons test, can undo decades worth of the "pick up trash slowly one piece at a time" approach by making a huge spike in the debris population, as we saw with the Chinese ASAT test and the Iridium / Cosmos collision.

    If one wants to spend some money on decreasing the risk of collisions between active satellites and space debris, the best way to spend it is on high-accuracy, open, space surveillance systems: A constellation of small-sats with telescopes tasked to observe and refine the trajectories of potentially dangerous debris objects. When the radar fence (low accuracy, tracks all objects) warns of a potential close approach, the satellites (high accuracy) can refine the trajectory and determine much tighter bounds on the trajectory, essentially eliminating the 99% of "false positives". It is these false positives that are at the root of the collision-avoidance problem.

    To avoid collisions between debris and debris (or between any non-manueverable objects) we could use something something like LightForce, but that's another story.