Thursday, 20 November 2014

Ballistic pancakes for orbital hygeine

(Credit to Rob Planke for helping in fleshing this idea out.)

Kessler syndrome is a problem that we don't have a solution for yet. It refers to case where the flotsam that accumulates in low Earth orbit accumulates to the point where pieces of it collide with each other, thus breaking apart into more pieces, which make more collisions and so on. If this starts to happen on a large scale, there will be too much garbage in that orbit to make anything else up there safe for long, even with how large space is.

As a younger, I had imagined a curved, ruggedized  'sweeper' satellite to knock trash out of orbit by deflecting it downwards and slowing it enough to destabilize the orbit of the trash. But if the satellite were traditionally made or rigid materials, it could make matters worse. Only three satellites have been destroyed in orbit (many more have been deorbited safely), and their remains are now clouds of thousands of pieces of junk. One sweeper would have to pick up thousands of pieces just to break even if collisions were knocking pieces off of it.

...but what if it were made of something more fluid, cohesive and/or deformable? What if most of the parts that could come off of the sweeper would break down in the vacuum and solar wind?  A substance based on ballistics gel comes out as a strong candidate. Its density is comparable to water, it's effective at stopping bullets, and it's cheap. Have a look at some of the pictures here:

Now imagine a pancake shaped object composed of that stuff, perhaps with a membrane to reduce premature breakdown and spillage from impacts. It's pancake shaped to maximize collisions for a fixed amount of payload that has to be brought up.

I'm still a bit concerned about the foam leaving the block of gel in these pictures at the entry and exit points of bullets. Even a paint fleck at orbital speeds is dangerous, but foam is likely less dangerous than a stray bolt or wingnut.

Also, to move the pancake toward the orbit of the centre of a cloud of junk, and to and correct for collisions that arise, there is a 'spine' which holds thrusters along the outer edge. The 'spine' also aids in maintaining the pancake shape when a ball is more natural.

With regards to cost and scaling: A disc of ballistic gel two metres across and 10cm thick, and the density of water has a mass of about... 300kg? Plus skin and thrusters. That's running in the region of $500k for just one disc. How many would be needed? Is there a stickier or lighter material that could be used instead? Is space junk clustered enough for a few discs can catch most of it? What are the limits on mass deflected and time in orbit for such a pancake?

This is going to be an ongoing problem for the rest of the space age because of entropy and accidents, so we should practice good orbital hygiene now before things get ugly.