Trembling fear and anxiety.
My buddies decided we should each jump off the high diving board at the community pool the summer I turned eleven. That diving board was not over 8 feet – possibly less – but at the time it seemed to be a mile high. Steeling my nerves, I climbed up the metal ladder to the platform, my way out to the end of the board, took a look down . . . and retreated back down the steps. The sting of humiliation, not courage, made me climb up again. This time I jumped. Newton’s Laws took effect as my body described a ballistic path.
Free fall seemed like forever before WHACK! The surface of the pool.
The sting of the impact is still fresh in my memory. No permanent damage was done. And best of all, my buddies cheered!
Public risk was more acceptable in those days. Now it is nearly impossible to find a high diving board at a community pool, even that old high board and its pool are long vanished.
A few years later in Physics class we learned to calculate free fall and final velocity using g = 32.2 feet per second per second acceleration (9.8 meters/s/s). Gravity pulling inexorably toward the center of the earth. My impact occurred with a velocity of no more than 8 feet per second – barely running speed – and free fall could not have lasted more than a half second, no matter what it seemed.
Free fall can be experienced briefly on a roller coaster, bungee jumping, parachuting, or in other high anxiety activities, but it is always brief. Flying a really great parabola on the Vomit Comet yields almost 30 seconds of free fall before the pilot has to pull the nose up. But Newton rules, gravity is always in control, accelerating everything at 32.2 ft/s/s directly toward the center of the earth.
With no action to prevent the inevitable, free fall always ends with a big WHACK when the ground meets you.
Technically it is cringe worthy to discuss the orbital experience as zero-gravity (or micro-gravity). Newton’s infallible laws dictate that gravity has its full effect on everything in orbit. The experience is not the absence of gravity, it is just free fall. Why, then, is there no big WHACK at the end? When time has passed to cover the distance down from orbit to the surface of the earth? To be in orbit means to travel forward fast enough so that when falling the distance to the surface of the earth, the earth is not there anymore. The curvature of the globe has made the earth fallaway from you.
The extraordinary speed necessary to do that is almost 5 miles per second (more than 7.5 kilometers per second). Compared to our everyday experience – where walking is about 3 miles per hour; driving on a clear highway can exceed 1 mile per minute – orbital velocity is incredible, unfathomable.
Strange and unusual things happen at such speeds. Orbital mechanics is counter-intuitive, unlike our common experiences in almost every respect.
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. WHACK indeed.
‘Low’ earth orbit – LEO – arbitrarily up to 1000 km (600 statute miles) altitude high. Some definitions include higher altitudes but compared with the vastness of space LEO is close. LEO is where the ISS, the Hubble Space Telescope, and many other important satellites travel. On the low side, the lowest altitude at which an object can remain in orbit is roughly 75 miles/120 km. Below that, atmospheric drag inevitably causes a spontaneous re-entry. The atmosphere there is thick enough to dissipate forward speed into heat energy, soon the earth no longer curves away below you fast enough to prevent that WHACK at the end.
Detection of orbital debris is difficult, impossible for the smallest stuff. Many nations have sensors scattered around the world used for debris tracking. These are primarily powerful radars but also include optical scanners and some other devices. This loose network is not continuous but has large geographic gaps. Many of the sensors are primarily designed and operated for other functions – such as the detection of ballistic missile launches. Commercial and academic organizations have started developing sensor networks to detect and track orbital debris. All these sensors have different capabilities: they vary in sensitivity, accuracy, continuity of operation. Some of them provide only classified data which is not available the public. For all sensors the accuracy of the measurements is degraded if the object orbits tracks nearer the horizon or at a longer distance.
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.
All the tracking sensors combined constitute a porous net. A new object can arrive in such an orbit that takes hours to pass over an active sensor for detection. After the initial detection, it takes additional time to get a reasonable track on a debris object. To get a really good position and velocity – a state vector – requires multiple passes over multiple sensors. Depending on the circumstances, getting a good state vector on a new orbital target can take hours to days to weeks.
Of course, the point is to be able to predict when a collision – a conjunction – will occur. The process of orbital propagation and results in an ephemeris or series of predictions where the object will be at future times. Hopefully the prediction can be made far enough in the future to do something about it.
Any uncertainty or error in the state vector will mean that a prediction of where the object will be in the future is inaccurate. If the velocity has an uncertainty of 1 foot per second (.3 meter per second) – an error of one part in 25,000 – the position prediction an hour hence will be off by half a mile or almost a kilometer. That error can mean the difference between a hit and a miss. Close only counts in horseshoes. The more precisely the current velocity of an object can be measured the better a future prediction of location will be. All state vectors have some uncertainty.
The process of orbital propagation is tricky. The Space Shuttle orbit was a hard subject to propagate because the shuttle had so active thrusting activities; water vents, flash evaporator exhaust, uncoupled attitude thruster firings – all these acted to change the state vector – the position and velocity of the Space Shuttle over time. Any vehicle with reaction control thrusters or a vent of any kind will be hard to propagate because the errors will grow fast and unpredictably.
Don’t forget that there are gravity differences as an orbit passes over parts of the earth because it is not a perfectly uniform sphere. It is fatter at the equator than over the poles and the substructure of the earth has regions of denser material and others of lighter. Those differences factor into the ephemeris calculation.
An orbiting object is further affected by the solar wind, fluctuations in the earth’s magnetic field, and even the pressure of the sunlight. The largest effect is the variation in the density of the upper atmosphere. It is sound strange to talk about density of the atmosphere where the vacuum is better than anything that can be created in a laboratory on the earth. Those few widely separated atmospheric molecules that are struck by an orbiting object determines the rate at which the orbit decays. Atmospheric density is hard to predict, the solar sunspot cycle alternately heats and cools the atmosphere at high altitudes making its density vary with time and location. Atmospheric density at LEO cannot be measured directly and only roughly predicted.
Orbital decay due to atmospheric drag depends on an object’s ballistic coefficient; a function of its surface area and density. It stands to reason that a larger lightweight object – say a fabric thermal blanket – will be slowed down by atmospheric drag faster than a smaller denser object like a steel bolt. Interestingly an object’s ballistic coefficient can change if the attitude is controlled – that blanket edge on to the direction of travel has a different ballistic coefficient than if it presents a full face to the ‘wind’.
Orbital decay works on everything in LEO, it just takes on time. At 350 miles high the Hubble Space Telescope has maybe three decades of orbital life after its last reboost. At 600 miles high, a high ballistic coefficient object may hold out for almost a century. Lower, at 100 miles high orbital lifetime may be only days. If the orbit is not circular but elliptical, the most drag comes at the lowest point. The perigee matters.
Given all of this, it is not hard to see why predicting upcoming collisions between objects in LEO is messy and sometimes imprecise. Given the uncertainties, it has become standard practice to analyze any predicted collision – or close pass – to mathematically determine the probability of actual contact. Usually, the number is really small. But not always.
The Two-Line Elements (TLE) that are made public to describe an object’s orbital state have little of that uncertainty information. In the official world, there is much more information to go into the calculation.
In the case of the ISS, if a conjunction is predicted and it falls into a highly likely probability of collision, the team springs into action. Well, springs may not be truly descriptive. Preparations proceed slowly. The ISS uses thrusters on the Russian segment to provide translation – to move away from a predicted collision. It doesn’t take much thrust to move the propagated position out of harm’s way given the given several hours to a few days between the DAM (debris avoidance maneuver) and the time of closest approach. But the Russian segment is old fashioned and requires a specific uplink command to fire the engines. Doing the math, building the command, and verifying it is correct can take several hours. Nothing very fast in this process. So, if a conjunction is predicted in a very short time, well, we all hope it’s a big sky and the uncertainties fall in your favor.
In such a case, the ISS crew is directed to the ‘Safe Haven’ procedure. Doing things like closing hatches between modules, putting the crew in their pressure suits, and having them hang out in their lifeboat return capsules is the best that can be done. As Capt. Young used to term some of our Shuttle abort procedures ‘it’s something to do while you’re waiting to die.’ Safe Haven is the best that can be done, it might even be of some use. But don’t kid yourself.
Many trackable objects are derelict and have no translation capability, no way to command movement out of them. Some trackable items do have maneuver capability and can be commanded to move out of the way of a collision. A couple of derelict inert satellites collided a few years ago and put a cloud of debris at the higher reaches of LEO where it will slowly descent for decades posing a problem for everybody.
For the very smallest debris – paint flakes and the like – resulting damage from an impact is acceptably small. There were pits found post flight in the shuttle windows on a regular basis. Damage of this kind is considered acceptable.
For slightly larger items, but still untrackable, the ISS has debris shields – called Whipple shields after the astronomer that proposed them before the start of the space age. These multi-layer structures protect all the critical pressurized volume of the ISS, but not its appendages like solar arrays and thermal radiators. None of that on an EVA suit, there is very little protection for space walkers working outside the station.
There is a larger size category of still untrackable debris that can defeat the Whipple shields and cause a hole – maybe a big hole – in the ISS. Or any other satellite in LEO. And by analysis and some indirect measurements there is a lot of that stuff in orbit. So, what is the defense?
It’s a big sky.
A debris collision is part of the ‘accepted risk’ of spaceflight in LEO. Hope you don’t get hit. So far, we’ve been pretty lucky. Will that be the case in the future? Quien sabe? Somebody has calculated the probability of that. But I wouldn’t use that number to place a bet in Las Vegas.
Which brings us to the popular idea to clean up the mess up there. Certainly, it would be technically possible, with significant expenditure, to launch a fleet of garbage gatherers and deorbit a number of the derelict big stages or defunct satellites that we can track. But the price tag will be bigger than you might think. There are a large number of those items in LEO, and they are really spread out. It is unlikely that a debris removal spacecraft could actually rendezvous and get rid of more than two or three targets. But their removal may be key to avoiding the Kessler Syndrome.
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.
As for the small stuff that can’t be tracked but could still cause catastrophic damage – it is simply too widely spread out to easily clean up. Space is big, even at LEO. The very simplistic idea of a fleet of spacecraft going around the earth sweeping up all of that stuff is, well, impractical. But somebody may try, bless them for the attempt. I hope I’m wrong, but it is a lot simpler and cheaper to clean up the Great Pacific Garbage Patch than Low Earth Orbit. Nobody is seriously attempting either.
The best way now to control space debris is to insist that upper stages, when they have finished their job, retain enough propellant to immediately deorbit themselves. And likewise, provision for satellites at the end of their life should be made to lower the orbit so that they naturally decay in relatively short times. At the very least, propellant tanks should be vented, and batteries discharged because sometimes those items deteriorate over time and cause the object to explode spontaneously.
On the hopefully rare occasions when a rocket stage or satellite explodes because its batteries degraded or its propellant tanks leaked, or if there is a collision, or if – hopefully never again – somebody tries to shoot down a satellite; the pieces will be flung all about the original orbit. They may go somewhat higher or lower, left or right, farther ahead or slightly behind. This cloud of debris will slowly expand over time as the residual velocity differences of each piece spreads them apart. None of the collision products will ‘fall to earth’ right away. It gets to be a complex problem.
And one of the controls should be, of course, making sure there are no future ASAT (anti-satellite missile) tests in low earth orbit. Good luck with that.
But if you ever heard the talk, like I did, by a fellow named Don Kessler, you would worry about the consequences of increasing orbital debris. Don held out the hope, still true, that if at least some of the bigger derelict objects could be eliminated, the space lanes could be kept open. Because if collisions start coming more frequently and the debris multiplies; well, they named it after him: The Kessler Syndrome. It would be bad. The loss of access to space is in the balance.
Last thing we need in orbit is another WHACK.
I have a lot of fear and trembling over that.
sAnd after nearly forty years in the space business, that is really about all I know about orbital debris. Mostly.