Party Pooper: Gravity Part 2

So we touched on the EVA things that drove me nuts on the last post, but that wasn't all that made me a little bit crazy with Gravity. And I'll be describing them, so they'll be spoilers, so STOP HERE if you don't want details on what happened in Gravity.

The premise behind the catastrophe at the center of Gravity is that the Russians shot and destroyed one of their own satellites for some reason. This is not outside the realm of possibility. Both Soviet Union/Russia shot down several of their satellites as did we in the 80's. As we now understand the debris concern better and hopefully got less paranoid, we stopped doing those sorts of experiments until the Chinese tried for the first time in 2007, to quite a bit of furor from everyone else. The furor caused, not only because no one likes the notion that someone else could shot down one's own satellites, but because it was at 500 miles altitude, a wonderful spot to rain down debris on a sizeable portion of our most expensive Low Earth Orbit (LEO) assets (and other countries' assets) such as HST, ISS, etc. The U.S. shot down one of our own soon after, ostensibly to prevent a tank of hydrazine from reentering, but it was at a much lower orbit so the impact on the orbital debris was pretty minimal.

(Lower orbits get cleaned up naturally by the friction of the atmosphere which slows them down and encourages them to reenter. Higher orbits have less friction so it takes much longer, sometimes decades, to fall out of the sky.)

Is orbital debris a big deal, a real and significant hazard for our space assets? Oh, absolutely. No doubt about it. Not just because of the junk we send up there and stunts like deliberately blowing up some of it (or inadvertently doing so, as has happened with leftover fuel in various discarded stages), but also because each time debris is left or created from an impact, it can go on to hit more hardware which creates more debris and so on until, even if we never send any more up there, the debris environment will get uglier on its own (something called the Kessler Effect).

So, if orbital debris is a legitimate problem and shooting down a satellite is within the realm of possibility, what's my beef? I don't have a problem with the notion of orbital debris causing cataclysm - that's a legitimate concern - but how this cataclysm is portrayed doesn't work for me, for several reasons.

• One of the reasons orbital debris is so very dangerous are the extremely high speeds with which it moves, and, if it's going to opposite direction in orbit, that relative speed is doubled  so that, instead of objects moving at 8 km/s (which is what orbital velocity of everything in low earth orbit is, more or less), the impact speed could be as high as 16 km/s for a head-on collision. Why is that important? Well, for one thing, one needs perspective. The muzzle velocity of an M16 rifle is ~1 km/s, so orbital velocity is 8-16X faster. That also means that, for the same size impactor (say 4 g like a bullet that impacts with ~1800 J of energy), the amount of energy imparted by impact is 64-256x greater (115,200-460,800 J=1/2 stick of dynamite for a 4g impactor) since impact energy is 1/2mv^2. That high energy/speed has two major implications.

1. The big chunks of debris depicted in movies, like Gravity and many other films, aren't necessary to do catastrophic damage. Tiny objects far too small to track, like a nut or a screw, can punch through heat shields or pressure vessels. Although we track the big stuff because we can track dead satellites and expended stages, and large chunks (say the size of a wrench or larger), in many ways they are less of a concern because we can track them and move out of the way. But the smaller items, which are 10s, 100s if not 1000s of times more prevalent, we can't track and can't avoid. That's one reason orbital debris is such a big threat - because the stuff we can't see can still kill us. It seems unimportant - after all the big chunk thing is way cooler in the movie, and that's quite true - but by reinforcing the stereotypical concern, there's a very real risk to expend efforts to battle a section of the orbital debris problem that is a tiny fraction of the whole. Sadly, this perception is not limited to laymen.
2. The other aspect is that, when stuff is coming toward you at a relative speed large enough to do real damage, that can easily mean several times faster than a speeding bullet. Which means dodging, even seeing the debris, is virtually impossible. Ironically, if you can see it approach, the relative velocity is a tiny fraction of what it could be and it is therefore far less destructive. It's important to note that, though I mentioned relative velocities of 16 km/s (head-on collision) and 8 km/s (effectively hit dead on from the side), one can also be hit from a similar close by orbit at velocities from close to 8 km/s to a fraction of a m/s. Its because of this relative velocity thing that ships can match velocities and dock in space without damaging each other.

• As part of the film, all our communications, notably with the Shuttle, are knocked out. Um, no, not possible while still being an immediate threat to the Space Shuttle etc. The primary mode for communication with the Shuttle is the TDRSS Satellite system, which whirls around the equator (along with almost all communication satellites of the world) in geosynchronous orbit, some 35,800 km above the surface of the earth as opposed to the altitude of HST (559 km), ISS (~417 km) and Tiangong (~370 km). Although it's not impossible that an impact would could send a few chunks high enough to intersect (not match) geosynchronous, they would be unlikely to be a big threat since they'd either be tossed out of orbit (if they obtain escape velocity) or are in a highly elliptical orbit where the bulk of its time is spent at other altitudes. To match geosynchronous orbit, it would need another influx of force to circularize the orbit. The volume of space in  geosynchronous is also much larger than LOE. True, most of the satellites are clustered around the equator, but any debris originating from LOE are likely to be at another higher inclination such that they would only intersect the equator twice an orbit, reducing the potential for impact in geosynchronous orbit.
• And all this discussion on why communication would not be wiped out brings up something else very important about debris: there is a great deal of it, but the volume of space is really vast, which is why debris tends to be a threat over time. Not that one can't be hit by debris in a limited window of time, but that the smaller the window is, the less likely there is to be a significant impact. That's one reason why the risk to ISS is so great over its entire lifespan, but the risk is small on a per year or per month basis.
• Now there are several possibilities for Russian satellites to shoot down, but most of them are of high inclination since their launch sites tend to be further north than the U.S. and China, but the different orbits, at different altitudes and inclinations argues that there would be minimum interaction with all three space assets (HST/Shuttle, ISS and Tiangong) which are all at different inclinations and altitudes. Now, eventually, over time, the debris cloud will expand and affect all orbits below it to some extent, but this would be a process that occurs over months and years, not hours. And would pretty much negate the 45 minute window of time that they described as the time to expect debris.  Remember (or discover), two orbits that have different inclinations will only intersect twice an orbit (assuming they are comparable altitudes) but there's no saying that the object of that inclination will be at exactly the same spot unless they have identical periods (which is unlikely for debris which is likely to be in a more elliptical orbit, especially if it threatens objects at different altitudes). Please note that discussing the intricacies and complexities of orbital mechanics with words is bound to be confusing.
• One other key thing to note about debris is that the most important risks for human spaceflight are pressure vessel damage (as they showed for Shuttle) and damage to the heat shields of reentry vehicles where even small damage can cause catastrophic damage (as evidenced by Columbia, unfortunately). Yet, most of the damage shown was more in keeping with damage inflicted by large (relatively) slow moving impactors.

 So, why is this important? True, we're taking liberties and amplifying the threat and time table of orbital debris, but the threat of orbital debris is real, if not as represented. Well, because the perception of the threat and the cause of the threat is important and determines what we do to address that threat. And even science minded folks can be misled. Space Safety Magazine, for instance, a magazine I admire greatly (I am a charter member of one of it's publishing organizations: the International Association for Advancement of Space Safety [IAASS]) recently published an editorial that talks about the plausibility of Gravity, saying "The film, however, has a stronger basis in fact than you might imagine," and then noting the real dangers of orbital debris. But by not clearly pointing out the impossibilities in the movie (as opposed to the unlikeliness of the various scenarios), the position, in my opinion, is misleading. 

And, yes, that bothers me. Given the length of this post, I'll have to hold off until next time why traipsing from HST to ISS to Tiangong (among many other impossibilities) is just not going to happen. And why it's important to understand that.