Time to Closest Approach

Being at a conference on Orbital Debris has turned my thoughts back to being a Flight Director and experiences I would rather forget.

There is a lot of junk in earth orbit, and some of it endangers our astronauts every day.  Paint flecks and particles of solid rocket exhaust are big enough to damage the shuttle windows.  We now replace the shuttle windows every flight because of the damage that these microscopic particles cause. 

At 5 miles per second, there is a lot of energy in “collisions” between orbiting objects.  Every bit of space junk packs the equivalent of 25 times its weight in TNT because of the extreme speeds of orbital encounters.  We cannot track the small stuff.  Even pieces as big as loose bolts are untrackable and potentially fatal.  A one inch bolt in orbit could punch a hole right through the shuttle or the station causing huge damage and explosive decompression.  You don’t even want to think about what it would do to a spacewalker in their fabric suit.

Larger items are tracked by NORAD (they have a new name but I never remember it).  We know where the big pieces and can avoid them.  Or so you might think.  There are limits to how accurately the trajectories of space junk can be determined.  Trajectories are affected by the solar wind, transient and unmonitorable variations in the upper atmosphere, and some objects even have propulsive vents so their trajectories are constantly and irregularly changing. 

The shuttle, of course, is always maneuvering and changing attitude.  The shuttle thrusters are not completely symmetric so there are small changes to the shuttle’s trajectory every time they fire. 

Orbital trajectories are predicted into the future assuming none of these variations.  Even so, the very small uncertainties in a trajectory  gets multiplied over hours of prediction and this leads to a grey zone surrounding its predicted future position where the space object may or may not be.

Nowadays this is a sophisticated science with much better tools.  Better radars and lots of mathematics and probabilities give a much more complete notion of where and when encounters may take place. 

In the early days of shuttle we just knew that anything predicted to come within a few miles could be a hazard.  Missing by inches is OK; missing by a mile is good; but it was all like Russian roulette in those days.

Knowing how to maneuver to avoid a predicted “conjunction” is critical.  If you guess wrong and maneuver to the part of the uncertainty box where the space junk actually is: POW.  Sometimes doing nothing is the best option.

Operationally there are other impacts.  Since shuttle maneuvers are initiated by the crew, obviously the crew must be awake to maneuver the ship.  If the crew sleep is interrupted, their performance the next day may be affected.  Think about being awakened in the middle of the night to do a precise task and then trying to go back to sleep, wake up the next morning at the regular time, and have a big work event that day.  Not really good.

So the early shuttle it was thought that we should not wake the crew up for debris avoidance maneuvers.  Even though space junk was predicted to be coming close by and could hit us, the odds were in our favor for a miss.  In the cold calculation of the risks involved it was thought better to let the crew sleep rather than wake them for something that might not happen.  We codified this in the Flight Rules.

On exactly three occasions I was the Flight Director on the crew sleep shift when we got the word a “conjunction” was imminent.  I remember each event like it was yesterday with crystal clarity.  Some things do not leave you.  I made all the appropriate notifications; phone calls to the management confirmed that we should follow the rules, let the crew sleep, and bet on the odds in our favor.

So GC would set a clock on the big board counting down to “TCA”.  Meanwhile we all tried to do the mundane work of monitoring the shuttle systems and planning the crew’s activities for the next day.   On the assumption that there would be a next day. 

 But as the clock counted down close to zero, Mission Control would get very quiet.  We all knew what might happen.  It’s tough to sit on your hands when your friends are in danger and you can’t do anything about it. 

In my imagination, the worst case scenario played out:  instantaneous cessation of telemetry transmission from the shuttle followed some time later by NORAD tracking confirming a multiple pieces in an orbit where only the shuttle had been before.  Then the notifications, the investigations, the whole drawn out parade of mourning and recrimination.  I could see it all. 

So as we waited for the clock to count to zero, there was plenty of time to contemplate metaphysical topics:  life, death, courage, risk, achievement, probability, dishonor.  They are all fellow travelers, intimately bound together.  No great accomplishment comes without difficulty or risk.  Miscalculation or failure results in death and dishonor.  But it is what it is; you do the best you can, make the best rational choice you can given what you know, and then wait for the result. 

Going to Las Vegas holds no enticement for me.