Wayne Hale: “Everything I Know About Orbital Debris – Almost”

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Wayne Hale was Flight Director for 40 Space Shuttle missions and Shuttle Program Manager or Deputy for 5 years. He is now retired after 32 years at NASA. In this blog post, “Everything I Know About Orbital Debris – Almost”, he sums up his knowledge and experience with the growing problem of space junk in low Earth orbit.

Traveling at 5 miles per second, collisions between objects in orbit generate tremendous energy. A rule of thumb states that the energy released by an orbital impact is equivalent to exploding 25 times the weight of the impactor of TNT. A very small item can carry a huge wallop.

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The oft-cited tracking limit is 8 cm/4.5 inches – softball size. The exact capability is, as you might guess, classified. The lower size limit of objects that can be tracked depends on their altitude and reflectivity. Many debris objects are smaller than the trackable limit: metallic residue from solid rocket motor firings, paint flakes and the like. Worse are the slightly bigger objects, still untrackable, like bolts and other metallic object shed when a rocket releases its payload in orbit. As modern CubeSats reduce in size, difficulty to track them increases; the smallest CubeSats in LEO are trackable only if their radio transmitter is active. Only the big stuff in LEO is easily tracked: large satellites either active or derelict, upper stage rocket bodies expended but still in orbit. The catalog of trackable objects in LEO numbers nearly 10,000. It has been estimated that there are over one million debris objects greater than 2 mm in LEO. A metal part 2 mm in size is a very dangerous object traveling at 5 miles per second.

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Trickier than orbital mechanics is space law. Seems that dead satellites still belong to the nation that launched them. Unlike maritime law, salvage of derelict craft in orbit is not allowed. Grabbing somebody else’s satellite even to clear it out as trash is illegal. In the worst case, it could be considered an act of war. Think about that.

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Just a minor quibble – the discarded debris bolt is travelling at 5 miles per second, but surely so is the working satellite with which it interacts? This suggests that many collisions involving debris would have significantly lower velocity differences between the two objects, and consequently be much less energetic and less damaging.

I hate to sound optimistic, but perhaps the debris issue can be lived with? Of course, every reasonable effort should still be taken to minimize future additions to orbiting debris.

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This is only the case if the two objects that collide are in the same orbital plane and have the same eccentricity. This is rarely the case. If debris in a polar orbit collides with an object in an equatorial orbit (a common case), then the velocity difference is equal to full orbital velocity. The rule of thumb figure of five miles per second (8 km/sec) is an estimation of the average velocity of real-world collisions in low Earth orbit, recognising that there may be a wide dispersion in actual velocities.

An anti-satellite interceptor will almost always have a velocity in this range, as it usually pops up and collides with an orbiting target while not, itself, achieving orbital velocity in the same direction.

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