Space isn't empty, and near-Earth orbit is downright crowded. Every month, some junk burns up during re-entry as ever more is introduced into orbit. Poking around NASA's Orbital Debris Quarterly archives reveals the story of space junk from deliberate to accidental, and all of it hazardous.

Catalogued space debris over time. Image credit: NASA, annotated by Mika McKinnon

Prior to June 1961, the entire population of artificial objects in near-Earth orbit was just over 50 objects, all spacecraft and rocket bodies. Then the Ablestar launch vehicle deployed its payload, the Transit 4A satellite, and exploded just over an hour later. The explosion created nearly 300 debris fragments, over two-thirds of which were still in orbit in 2002. After that, space just got messier.


Anti-satellite testing caused a whole lot of mess as the Soviet Union and the United States took turns proving they could blow up their own satellites. Between 1968 and 1982, the former Soviet Union conducted 20 tests, creating somewhere over 700 catalogue debris fragments, 301 of which are still in orbit. In 1985, the United States tested its own system, producing a whiff of debris, none of which remains in orbit. Realizing that all these explosions were producing a terrific mess, by collective international agreement, no one conducted any more tests of anti-satellite systems that produced debris. This agreement held for 20 years.

In the late '80s and early '90s, a whole lot of people did a whole lot of talking, eventually agreeing to voluntarily reduce the amount of junk they were producing. It worked for a while, reducing the growth rate of new debris cluttering up near-Earth space from a fairly steep climb in 1968 through 1988 to a far flatter climb from 1992 to 2006.


Tracked space debris in 1963 and fifty years later in 2013. Image credit: NASA

That isn't saying the new techniques were perfect at minimizing the creation of space junk. The first collision between catalogued satellites was in 1991, although it took until 2005 to realize what had happened. It was low speed and pretty sedate: a retired Russian navigation satellite, Cosmos 1934, was smacked by a small bit of debris from its sister-satellite Cosmos 926. The collision broke two chunks off of Cosmos 1934, while the fragment of Cosmos 926 broke up into pieces smaller than 1 centimeter and could no longer be tracked.

A few years later was a far more spectacular explosion. In June 1996, an abandoned upper stage rocket "broke up," the orbital debris euphemism for "suddenly exploded." The abrupt fragmentation of the rocket stage produced 700 pieces of distinct debris. The stage was the Pegasus Hydrazine Auxiliary Propulsion System (HAPS) from the STEP II mission that had launched 2 years previously. The event produced an order of magnitude more debris than models suggested it should have, forcing NASA to rethink what they did with abandoned craft. Eventually they figured out that the explosion was enhanced by excess fuel, leading to a procedure change where spent stages perform a propellant depletion maneuver to both reduce fuel and to place objects in a decaying orbit where they will (hopefully) burn up on re-entering the Earth's atmosphere within five years.

Most tracked events are nowhere near that exuberant. The next month, the French CERISE spacecraft was pinged by a fragment of an Ariane 1 launch vehicle that had exploded a decade earlier. No new debris was created, the spacecraft recovered, and everything kept on whizzing about the planet.

In 1998, NASA started integrating data from the Goldstone radar antenna into their debris tracking program. Although uncertainty in radar cross sections keeps it from being used to determining inclination and eccentricity of space junk orbits, the data is helpful for determining size, radial speed, and altitude, all of which gets incorporated into debris models.

All was relatively quiet until the last quarter of 1999, when four breakups of larger space-junk into fragments produced a surge in debris by the January 2000 tracking report. In the same period, the International Space Station performed its first collision-avoidance maneuver, dodging a Pegasus HAPS from the mission that placed Orbcomm into orbit.

The next major surge in debris was 20 months later, with the breakup of Cosmos 2367 in November and an Indian PSLV fourth stage in December 2001. Each breakup produced over 300 fragments, with the Cosmos debris cloud triggering a risk evaluation for a then-upcoming Space Shuttle mission to the International Space Station. The breakup of the Indian PSLV was somewhat puzzling, as it was a newer design and should have been fine if they had successfully completed a propellant depletion maneuver.

After that, things calmed back down for a while, which is nice because the Orbital Debris Quarterly went on hiatus. About the most exciting thing that happened was in November 2003. The Chandra Space Telescope was pinged by a suspected-meteorite small enough to cause minimal damage, but big enough to jostle its stability. It was between observations and the telescope quickly corrected itself to make observations three hours later.

The STAR-48B solid rocket motor was worse for wear after re-entry, shedding 150 of 200 kilograms when the engine nozzle and part of the casing burned off. Image credit: NASA

The resumption of the Orbital Debris Quarterly in January 2004 started with another reminder as to threat space junk poses not just to spacecraft, but to unwary Earth-bound residents. In October 1993, a Delta II launch vehicle placed Navstar 35, a Global Positional Satellite, into a transfer orbit. On January 20, 2004, the third stage of that launch vehicle, PAM-D, re-entered the Earth's atmosphere, and landed in the extreme northeast of Argentina, much to the surprise of nearby humans. A week later, the second stage of another Delta II launch vehicle that had been used to place the Mars Exploration Rover Opportunity into orbit fell back to Earth with far less fanfare.

The next year, a chunk of the Chinese CZ-4 launch vehicle that had exploded in 2000 collided with a 31-year-old NASA Thor Burner 2A final stage rocket body. The collision knocked both objects into slightly perturbed orbits, and produced three new bits of space junk for the catalogue. Although not in itself catastrophic, adding only three new bits of trash to track, it's a sign of the positive feedback loop of the more junk is in orbit, the more likely they are to collide, producing yet more junk.

But it wasn't until 2007 that things that seriously ugly. The year started off with the Chinese testing out their fancy new anti-satellite system. The test was simple: fire a ballistic interceptor missile to blow up their retired meteorological spacecraft, Fengyun-1C. It worked incredibly well.

More than 1,600 debris fragments were officially cataloged and tracked in the month following the Fengyun-1C anti-satellite test. Image credit: NASA

The debris cloud from Fengyun-1C is the single worst source of contamination of low Earth orbit, extending from 200 to 4,000 kilometers altitude. Even worse, the explosion threw most of the debris into long-duration orbits where they will remain as a navigation hazard for decades to centuries. The weapon was highly effective, disintegrating the satellite into thousands of fragments at least a centimeter in diameter. The amount of space debris around the Earth increased by 75% due entirely to this deliberate event.

Even just trying to count the fragments was no easy task: the 1,600 fragments identified in the first month after the test kept climbing as more and more suspected debris were tracked and confirmed. The total fragments positively associated with Fengyun increased to over 1,900 within six months, 2,300 within a year, 2,800 by 2010, and now stands at over 3,000 fragments. By the end of 2007, just 22 Fengyun fragments, less than 1% of the debris cloud, had decayed into a re-entry orbit and burned up. In 2014, more than 90% of the debris cloud is still in orbit, messing up everything for everyone.

The test violated a 20-year moratorium on any anti-satellite testing that produced debris, and directly conflicted with the the Space Debris Mitigation Guidelines that the China National Space Administration agreed to back in October 2001. A month too late, the United Nations formally adopted guidelines to reduce the proliferation of space junk that also explicitly banned the test:

  1. Limit debris released during normal operations. If everything is working properly, no debris should be released.
  2. Minimize the potential for break-ups during operational phases. If something goes wrong, the craft still shouldn't break up. If a failure is detected, take steps to reduce the potential for a break-up and dispose of the craft safely.
  3. Limit the probability of accidental collision in orbit. If things are going to hit, move so they don't.
  4. Avoid intentional destruction and other harmful activities. Thou shalt not litter, the space-version.
  5. Minimize potential for post-mission break-ups resulting from stored energy. After the mission is complete, deplete all stored energy (fuel, batteries) so less can go boom.
  6. Limit the long-term presence of spacecraft and launch vehicle orbital stages in the low Earth orbit(LEO) region after the end of their mission. It's a busy zone, so don't linger. (And standing directly in front of subway exists is rude.)
  7. Limit the long-term interference of spacecraft and launch vehicle orbital stages with geosynchronous (GEO) region after the end of their mission. It's a busy orbit, so if you don't need it, get out of the way.

The same month, the breakup of the fourth stage of a Russian Proton launch vehicle (Briz-M or Breeze-M) demonstrated the utility of guideline 2. In February 2006, the Briz-M launched to carry the Arabsat 4A spacecraft into a parking orbit. Upon reaching orbit, the Briz-M malfunctioned during the second of four burns, shut down early, and refused to restart. The spacecraft was separated and brought back to Earth under a controlled re-entry, but the Briz-M stayed in orbit. Nine days short of a year later, it exploded into roughly a thousand fragments, probably due to the large amount of propellent left on-board after the malfunctions. The explosion could have been prevented by a backup command that vented unused propellent after irrecoverable failure. Although estimates are that Briz-M produced somewhere on the order of 1,000 fragments, the highly elliptical orbit makes it difficult to detect, identify, and track the debris. Three years later, only 85 fragments had been officially catalogued as coming from the explosion.

Fengyun-1C debris originally formed a narrow ring of chaos in the moth following the explosive test, but quickly dissipated to form a sphere of interference. Image credit: NASA/NASA/NASA

Within six months, the Fengyun debris had spread enough to be posing a serious irritation to any other craft in low-Earth orbit. In June, the NASA Terra satellite had to dodge out of the way of a piece of junk passing within just 19 meters. Two weeks later, the NASA Cloudsat satellite stepped aside to avoid a 100 meter pass-distance with the small Iranian Sinah 1.

2008 was a relatively quiet year in the aftermath of the Fengyun anti-satellite test, with a few minor breakups contributing new debris. In August, the International Space Station performed its first evasive maneuvers in five years, dodging out of the debris cloud from Cosmos 2421.

But the calm couldn't last. In February 2009, the operational American communications satellite Iridium 33 collided with the decommissioned Russian communications satellite Cosmos 2251.

Perpendicular ribbons of debris from Iridium 33 and Cosmos 2251. Image credit: NASA

Both satellites were in nearly-circular, high polar orbits, and slammed into each other at at nearly right angles. The crossed orbits meant a high relative velocity of 11 kilometers per second, making this the fourth accidental hypervelocity collision and the first hypervelocity collision between two spacecraft.

Debris from Fengyun-1 and Cosmos 2251 had spread into spheres by July 2012, although a polar orbit was hindering the spread of Iridium 33 debris. Image credit: NASA

Due to its larger mass, Cosmos 2251 produced roughly twice as much debris as Iridium 33. Considered individually or together, the Cosmos 2251-Iridium 33 collision was one of the largest contributors of space junk into orbit. Within a year, the identified debris from the collision totalled more than 1,740 fragments. In a period of high solar activity, up to 50% the debris might have fallen to Earth within 5 years, but as the sun was in a period of low solar activity, less than 4% had fallen out of orbit a year later. The debris cloud threatened Space Shuttle mission STS-125 to service the Hubble Space Telescope.

The biggest producers of space junk. Table credit: NASA

Considered separately or together, the Iridium-Cosmos collision was one of the largest producers of space junk, and continues to be the most recent event in the Top Ten list of ways we've filled our orbit with garbage.

After the massive collision, the year-end report on the manoeuvres that avoided collisions is a bit anti-climatic:

During 2009 conjunction assessments led to eight collision avoidance maneuvers by NASA spacecraft, in addition to a collision avoidance maneuver of a French satellite operating in concert with NASA Earth observation satellites. Only two of the maneuvers involved close approaches by intact vehicles (one a spacecraft and one a rocket body). The other maneuvers were needed to avoid collisions with smaller debris, including twice with debris from the Chinese anti-satellite test of 2007 and once with debris from the collision of the Iridium 33 and the Cosmos 2251 satellites in February of 2009.

Most debris is between 600 and 1,000 kilometers altitude. Image credit: NASA

But the report was just a sign of the times. With ever more spacecraft and space junk in orbit, what was once roomy was getting downright crowded. By 2010, NASA reported they instructed seven of their satellites to dodge collisions, France reported thirteen dances with catastrophe, and the European Space Agency claimed nine maneuvers to avoid collisions. The vulnerable International Space Station developed the habit of pulling out of the way of debris about once a year, any time the odds of collision rise greater than 1 in 10,000, before needing to increase the frequency of the dance to three steps per year in 2011 and 2012.

Malfunctions and explosions kept on happening. In October 2012, yet another malfunctioning fully-fuelled Briz-M exploded produced a few hundred more fragments to the crowd. Collisions keep happening, too. In January 2013, a tiny 8-kilogram 17-centimeter diameter spherical satellite was pinged with something big enough to perturb its orbit yet small enough to not cause any real damage, while two heftier satellites received similar treatment in May. That July, a Delta 1 rocket scattered debris across Zimbabwe. The International Space Station got a brief respite from constantly dodging junk before needing to resume avoidance manoeuvring in March and April of this year.

Objects in high-Earth orbit as of 2009, with a distinct ring of geostationary satellites. Image credit: NASA/Earth Observatory

Space is seriously crowded, with every year adding more active spacecraft, more dying missions, more debris fragments, and more chances for collisions. After immersing myself in the archives tracking junk past and present, I can only wonder why catastrophes don't happen more often.