Last week, I was driving home to Los Angeles from Santa Barbara along the 101, when a random piece of debris flew off a truck in the right lane and struck the hood of my shiny red Prius. It was just a little piece of rock, but at highway speeds, it carried enough kinetic energy to gouge the paint when it struck. It was most distressing, even though I realize that cars begin to depreciate the second you drive them off the lot, so you really can't get too worked up over the inevitable wear and tear. Still, it was just so random and unnecessary. Amplify my reaction about a million-fold, and you might get some sense of how NASA astronauts and engineers feel when one of their prized spacecraft or satellites gets pinged with bits of orbital debris: otherwise known as "space junk." Sure, the damage is mostly minor -- a scratch here, a barely perceptible dent there, a pinged window on the Space Shuttle -- but as bits of discarded space junk accumulate to greater densities, the risk of collisions with the potential for serious damage is increasing at an alarming rate. And that debris is traveling a heckuva lot faster than 65 MPH. (I might be down-playing the truth there, rather than admit to driving above the speed limit.)
We should have seen this coming. Many years ago, while chatting with a few space scientists about various missions, I innocently asked, "So, how do we get all that stuff back down again once we're done with it?" The answer: we don't. It just sits up there, year after year, until various forces conspire to pull it out of orbit and back down to earth. It's not something many of us think about as we breathlessly watch each Space Shuttle launch, and marvel at the glorious images collected by the Hubble Space Telescope, Mars Rover, the Cassini mission (just renewed for another two years, so congrats!), and so forth. We certainly don't think about the potential consequences of littering the Earth's orbit with all our communication satellites: we need our GPS, our satellite radios, and our satellite TV. Those are very real, much appreciated technological benefits. But we gain them at a cost. The sad truth is, the "human footprint" in space is starting to get a bit too big to ignore. We make a mess wherever we go, it seems, and haven't been any better about cleaning up after ourselves in space as we have here on earth. It's just not "cost effective."
Well, it looks like we're now starting to pay the price for that short-sighted behavior, according to David Wright of the Union of Concerned Scientists (which maintains a satellite database), who addressed the problem of space debris at the APS April Meeting. It's not exactly a new concern: back in the mid-1990s, the United Nations deemed it a significant enough risk to implement mitigation measures aimed at reining in the proliferation of space debris, measures it re-introduced in June 2007. Mitigation worked, for awhile (throughout the 1990s), but as we continue not just to explore space, but also exploit it for commercial (and/or military) gain, the issue is fast approaching a crisis point. The big worry is supercriticality: what happens when the destruction of an orbiting object into lots of smaller fragments of debris initiates a chain reaction of satellites in nearby orbits.
"With their high speed in orbit, even relatively small pieces of debris can damage or destroy satellites in a collision,'" Wright wrote in the October 2007 issue of Physics Today. "Since debris at high altitudes can stay in orbit for decades or longer, it accumulates as more is produced. As the amount grows, the risk of collisions with satellites also grows. If the amount of debris at some altitudes becomes sufficiently large, it could become difficult to use these regions for satellites." It may already be too late: Wright cites a 2006 study by NASA's Orbital Debris Program that found certain parts of space (the 900 to 1000 km band in particular, part of the Lower Earth Orbit, or LEO) have already reached supercritical debris densities; in fact, it estimates that an active satellite in LEO will collide with a piece of debris larger than 1 centimeter every five to six years.
A one-centimeter piece of debris doesn't sound like much, but at the high orbital velocities in space, it can pack a wallop. Wright noted that orbital speeds in LEO are typically greater than 7 kilometers per second (30 times faster than a jet aircraft), and "the relative seed of a piece of debris approaching a satellite in an intersecting orbit may be 10 kilometers per second or higher." That's a lot of kinetic energy. According to the folks at Space Watch, "a tiny speck of paint from a satellite once dug a pit in a space shuttle window nearly a quarter-inch wide" (see picture). Returned solar arrays from the Hubble telescope had multiple impact craters from collisions with debris, each one counted and classified by the European Space Agency (ESA) for future reference.
No wonder the Space Shuttle chose to perform a collision avoidance maneuver with a 7-second reaction control burn to avoid colliding with debris from an old Cosmos satellite in September 1991. France wasn't so fortunate. In 1996, a collision with space debris tore off a boom from the French satellite Cerise.
Where does all that junk come from? It's a mix of things, ranging from spent rocket stages, defunct satellites, fragments from explosions of various space gear, paint flakes, dust, and so forth, all dating back to the launch of Sputnik in the 1950s, ushering in the dawn of the space age. Haven't you ever lost a glove? Astronaut Ed White did, on the very first US space-walk, except his just floated around for a weeks until it re-entered the atmosphere. Lost pliers, cameras, even jettisoned garbage bags, routinely drift around a bit, but they're not major contributors to the space debris problem because they don't stay up very long.
Usually, this re-entry isn't catastrophic. Sure, in 1996, an Oklahoma woman got hit on the shoulder by a small piece of metallic material that fell from the sky. It turned out to be part of a rocket fuel tank used to launch a satellite earlier that year. But sometimes it can wreak devastation. in July 1979, Skylab -- the 78-ton space station the US had since abandoned -- came crashing down a bit earlier than planned, raining debris all across the Australian outback. Fortunately, it's not a highly populated area, but imagine if the same thing happened along the Northeast coast, or the heart of Europe. And in 2006, wreckage from a Russian spy satellite came awfully close to colliding with an Airbus carrying 270 passengers over the Pacific Ocean. That could have been truly disastrous. (I'm told the odds are unlikely, since most debris burns up as it re-enters the atmosphere, or re-enters over water -- nearly 3/4 of the planet is wet, and there are still vast swatches of uninhabited dry land.)
It's the explosions that seem to cause most of the trouble when it comes to accumulated debris. Rockets or spacecraft with unspent fuel have been known to collide with other objects and explode, thereby producing even more bits of debris. And sometimes the explosions are deliberate: China made global headlines in January 2007 when it used an anti-satellite (ASAT) missile to destroy one of its old weather satellites orbiting about 537 miles above Earth, thereby creating the largest amount of space debris in history: more than 2300 pieces bigger than a golf ball, and over 35,000 pieces larger than 1 centimeter.
Perhaps you're thinking, hey, no biggie. What goes up, must come down, right? Well, yes -- eventually. Aye, there's the rub. The higher the altitude, the longer the stuff stays in orbit. The debris from the Chinese test was high enough that scientists estimate that stuff won't "deorbit" for 35 to 100 years. In the meantime, any other spacecraft and satellites out there better be prepared to duck. The ESA offers the image below as a visual representation of the distribution of "space junk" in the lower earth orbit (h/t to James H. of Island of Doubt, from whom I shamelessly snagged the image).
Thanks to these kinds of events, the amount of space junk keeps growing. In 2006, NASA scientists published a report in Science concluding that even if the global community stopped launching things into space today, it still wouldn't solve the problem. This is due to something rather grandiosely dubbed the Kessler Syndrome (a staple of science fiction by now): there's already so much aging crap in orbit, many of which may collide with existing debris and break into even more pieces, thereby compounding the clutter problem. And before you know it, we're at supercriticality. The man behind the moniker, Donald Kessler (former head of NASA's orbital debris program) told the New York Times in February that the worst-case scenario is an over-hyped exaggeration, but doesn't deny that space debris presents a very real, very worrisome problem.
So what can we do about all that junk up there? Um... not very much. There's nothing wrong with the UN's mitigation measures in principle, but such measures merely maintain the status quo, and frankly, they're more like guidelines, with no real regulatory teeth. Plus, any "gains" can be easily wiped out by one bust-up event. Wright noted that while the UN's measures caused the accumulation of debris to level off in the 1990s, last year's Chinese test of an anti-satellite mission to destroy an old weather satellite alone undid all the gains of the previous decade. Uh, thanks for nothing, China. Your little "test" is the reason NASA had to move its Terra environmental spacecraft to avoid colliding with all that debris.
Not that the US can really point fingers, mind you. Nor can Russia. The oldest piece of space junk still in orbit is a the US satellite Vanguard 1, first launched in 1958. It's like the Energizer bunny, except it does nothing except drift about in orbit with no sign of falling back to earth any time soon. At least it hasn't exploded into thousands of tiny pieces. A US Pegasus rocket exploded in 1996, generating a cloud of about 300,000 fragments. That alone doubled the collision risk for the plucky Hubble Space Telescope. We did it again (oops!) just this past February, using an SM-3 missile to destroy a defective satellite carrying toxic hydrazine fuel; sure, we did it a pretty low altitude, but still -- let he who has not sinned cast the first stone. And in February 2007, a Russian booster stage exploded in orbit over Australia (what does space debris have against that continent?), producing just as much debris as the Chinese ASAT test, albeit at a lower altitude, so the debris won't be stuck up there as long.
The NASA scientists who authored the Science article advocated removal of existing large objects from orbit to "prevent future problems for research in and commercialization of space." However, they concluded, "As of now, there is no viable solution, technically and economically, to remove objects from space." Lots of things have been proposed to "sweep" space debris back into the atmosphere: laser "brooms" that "vaporize or nudge particles into rapidly-decaying orbits, or huge aerogel blobs to absorb impacting junk and eventually fall out of orbit with them trapped inside," per Wikipedia. We could also design our satellites and spacecraft with engines to direct them back to Earth, but this is really expensive (it adds considerable weight, for starters), for what is deemed to be very little benefit. People have also toyed with the notion of using ground-based lasers to disturb the orbits of defunct satellites, but the darn things are so big, it would take a huge amount of laser energy to make any kind of difference.
My personal favorite is a proposed "terminator tether" for any future launched spacecraft or satellites, which would use electromagnetic effects to slow down a spacecraft sufficiently that it can no longer stay in orbit. Apparently France did this successfully in 2003 with one of its satellites, which is expected to re-enter the atmosphere in about 15 years. Yeah, that's considered a short-term solution. On cosmic scales, it is. On human scales? Not so much. Then there are those who think we shouldn't even bother trying to get all that stuff back down: why clutter up terrestrial landfills even more, when we have the vast expanse of space at our disposal? (Jen-Luc would like to note, for our more literal-minded readers, that this a rhetorical question, asked satirically.) Really, they think it wold be a terrific idea to gather discarded spacecraft and the larger bits of debris into a central orbital "junkyard" in some part of space we're not really using. At least not right now. Hey, we could even mine it for old parts in a pinch! That should inspire tons of confidence in future astronauts who want to come home, knowing their re-entry vehicle has undergone patched repairs with debris fragments from a few decades ago.
Personally, I hope global space agencies and their respective governments take a cold hard look at what's at stake, and ask themselves: can we really afford to keep on this path? Is mitigation really enough? And isn't a sustainable long-term solution preferable to short-term temporary "fixes" even if it means initial high capital outlays? I would like to think they would answer with a resounding "YES!" But considering the human race's historical record on mitigating global warming, extinction of species, over-fishing the oceans, over-farming land, and so forth, I'm not optimistic about that.