The steady accumulation of orbital debris — commonly called “space junk” — over the past 50 + years of space exploration is reaching a “tipping point” according to a recent report by the National Research Council (NRC). The report calls on NASA to find more effective ways to monitor and reduce the growing bands of dangerous, fast-moving debris that surround the Earth.
Litter is not just a problem here on Earth; ever since the launch of Sputnik in 1957 by the former Soviet Union, and the commencement of the space race between U.S.S.R. and the U.S., a growing, orbiting field of debris has been slowly filling up the space surrounding our blue planet.
Where does all this ‘space junk’ come from? There are four primary sources: post-mission disposals (single purpose craft or sections), launch vehicles (mostly rocket stages), fuel tank/reactor core parts (due to explosions from un-vented gas/fuel), and collisions amongst these larger, castaway objects.
Currently, there are an estimated 22,000 pieces of ‘space junk’ measuring 10 centimeters (4 inches) in diameter or larger orbiting the Earth, according to the U.S. Strategic Command, a military arm charged with tracking the larger objects. This is up from a 2007 estimation of 17,000 objects (source: N. L. Johnson, NASA, 2007). Most of these are fragments of old engines and rocket stages, and have aggregated primarily in the 600 km to 11oo km orbital band. This is problematic because this is the orbital band where many communication satellites orbit as well.
However, according to NASA’s Office of Orbital Debris, the number of pieces of said orbital debris measuring less than 10 centimeters (which cannot currently be tracked) may number in the hundreds of thousands. And, debris (like paint chips from old rockets) measuring less than a centimeter most likely numbers in the millions. Smaller pieces such as these are forming a “debris cloud” around our planet; given enough time, this cloud will no longer be restricted to specific altitude bands.
The newest NRC report, ‘Limiting Future Collision Risk to Spacecraft: An Assessment of NASA’s Meteoroid and Orbital Debris Programs’, calls on NASA to improve both its debris tracking and remediation capabilities. The report is decidedly more urgent in its tone and recommendations than previous reports, stating that a threshold, or “tipping point”, has already been reach, based upon new estimations of the total population and mass of the orbital debris. NASA scientists estimate that the total mass of all space junk increases between 2 and 5% annually.
Previous reports typically down-played the media hype about the dangers of space junk.
For example, a 2 January, 2006 report in Science (‘Risk in Space from Orbiting Debris’) by J.C. Liou and N. L. Johnson, stated the risk in less alarmist terms:
“Post mission disposal will slow down the growth of future debris populations. However, this mitigation measure will be insufficient to constrain the Earth satellite population. Only remediation of the near-Earth environment—the removal of existing large objects from orbit—can prevent future problems for research in and commercialization of space.”
Note: the term satellite is frequently used to describe any orbiting object, whether an active satellite mission or actual “junk”
The same 2007 report stated that (at that time) the orbital debris field was “stable”, as the number of new pieces of junk entering orbit was matched by the number of pieces of junk decaying out of orbit (and which usually either burn up in the atmosphere, or land harmlessly in the oceans). The authors of the older report also noted that collision simulations predicted the total amount of space junk would start to increase after 2055.
However, the NRC report notes that since 2007, just two space junk collisions have nearly doubled the amount of trackable (larger than 10 cm) objects in orbit. This situation could produce what’s known as a “collision cascade” and, according to the older NASA report, such collisions are the main cause of new debris (of all sizes). Predicting the number of particles resulting from said collisions, or the number of secondary collisions resulting from these, is next to impossible.
The NRC report suggest that a feedback effect is growing increasingly likely, as collisions create more junk which in turn increase the likelihood of more collisions, more junk, etc.
This new report seems to describe a present condition that is much closer to the so-called Kessler Effect, which is the name given to a scenario predicted by Don Kessler (former NASA scientist turned science fiction writer) who proposed that the amount of space debris surrounding the planet would increase to such an extent that it would endanger any space mission, thus making space travel and missions too dangerous.
A corollary to the Kessler Effect goes like this: to protect missions from space junk collisions, rockets will require more shielding to make it safely into space; more shielding means more weight; more weight means more fuel; more fuel means greater cost. Thus, a growing space junk “population” will add such a financial burden as to make space travel prohibitive — for either government or the private sector.
Dr. Kessler, as it turns out, is also the chairman of the committee that issued the report. Quoting Kessler in a recent Washington Post article: “We’re going to have a lot more [debris] collisions, and at an increasingly frequent rate.”
A large percentage of the space debris is found in LEO (low Earth orbit), which is defined, roughly, as between 200 and 2000 kilometers (between 124 miles to 1260 miles). But a fair percentage of older debris (e.g., from lunar missions) occupies altitude bands much further out into space, with some coming within reach of GEO (geosynchronous/stationary orbiting) satellites. This higher altitude junk is far less affected by atmospheric drag and thus takes much longer to decay from its orbit. It is also traveling at slower average velocities (around 1.5 km per second) than LEO debris which means the debris field from any collision will not be as severe as would occur in LEO. But these micro-satellites can still take out a human-built satellite (like one of NASA’s Earth monitoring observatories) in short order. Most of this high-altitude junk will be in orbit for at least the next two centuries.
The International Space Station has dodged a large tracked object at least eight times since it has been in orbit (as of this publication).
The oldest debris still on orbit is the second US satellite, the Vanguard I, launched on 1958, March, the 17th, which worked only for 6 years.
In 1965, during the first American space walk, the Gemini 4 astronaut Edward White, lost a glove. For a month, the glove stayed on orbit with a speed of 28,000 km / h, becoming the most dangerous garment in history.
More than 200 objects, most of them rubbish bags, were released by the Mir space station during its first 10 years of operation
As for human-occupied missions, most of the tracked debris does not orbit in the band occupied by the International Space Station (ISS), which is about 250 km above the Earth (although the ISS had a recent “close call”). Also, long-term missions like the ISS (and the now deorbited Mir space station), and most observation and communication satellites, are able to maneuver out of the way of the junk — which speed through space, on average, between 8 and 11 km per second (nearly 23, 000 mph).
The ISS also possesses ‘Whipple shielding’ to protect against smaller, mircometeorites (the term most used for the tiniest bits of natural space debris) and small, human-made debris particles. A Whipple shield generally consists of “a relatively thin outer bumper placed a certain distance off of the wall of the spacecraft. This improves the shielding to mass ratio, critical for spaceflight components, but also increases the thickness of the spacecraft walls”. This bumper wall design can “shock” a speeding piece of debris and cause it to disintegrate. “This spreads out the impulse particle [impact] over a larger area of the inner wall of the spacecraft.” [source quoted: wikipedia.org]
However, these smaller pieces still pose a very real risk of damaging solar array panels that provide energy to these missions. This is what happened to the Hubble Space Telescope in 2002, when a tiny piece of debris measuring 2.5mm penetrated one of its solar array panels, necessitating its replacement on a subsequent space shuttle mission.
And larger objects? It has been estimated that a collision with a golf-ball sized object would have the same impact on a space craft as a 400 pound safe traveling at 60 mph.
The NRC report does not lay out a time line or schedule for NASA to improve and implement changes to its (Micro) Meteoroid Orbital Debris programs (MMOD) and lauds NASA for its past accomplishments and successes in meeting various challenges and, so far, avoiding the worst hazards posed by space junk and micrometeorites.
The report conveys a sense of a kindly boss telling its employee to stop slacking and do what it does best: anticipate and meet a pressing technical challenge.
Quoting Kessler again (from the above cited article):
“The earlier we [deal with the problem], the cheaper it’s going to be in the long run.”
So, what can be done, or is being done, to remedy the orbital debris, aka ‘space junk’, problem?
Both government and private companies are actively investigating solutions to the problem.
Among potential remediation strategies, some private sector engineers have examined using tethers to remove debris from geostationary orbits (GEO). According to Dr. William Ailor, chief engineer at Aerospace Corporation: “This concept would involve launching a satellite to an orbit near GEO, deploying a long tether above and below the satellite, and attaching debris objects to the ends of the tether. The tether would then be extended or contracted to move the object to a disposal orbit. Once the debris is released, the tether vehicle would repeat the process with other objects.”*
Other possibilities for dedicated space craft involve proposals for “space tugs” which wold grab hold of dead satellites and relocate them to “disposal orbits”. Indeed, some of the most practical proposals for keeping LEO debris-free suggest deorbiting old/dead satellites (before they break up or collide with others) via out-fitting each new satellite with a solar electric propulsion system. Advances in this technology make this approach feasible (though costly). However, there remains the possibility that a collision can occur during the lengthy, deorbiting process.
Still other scientists working on the problem assert that only reorbiting aging satellites well above LEO can solve the problem and minimize risks of collisions. Such an approach would require only minimal thrusting power and an on-board, collision avoidance system. Still, such a system has yet to be fully design and prototyped
But no single approach to removing space junk will solve the problem.
A helpful approach is to consider the problem of debris in the build stage, that is, to develop launch vehicles and satellites that generate as little debris as possible during all stages of the mission: launch, deployment, operations, and post-disposal. As an example of what has already been achieved here, private sector satellites and launch vehicles have been designed and deployed that enable venting of fuel and gas propellants at end-of-life stage — thereby reducing the possibility of on-orbit explosions — a major source of space debris.
Lastly, international cooperation between governments and private corporations, and the adoption of “best practices” standards for minimizing risk associated with space exploration, will go a long way towards not making the problem worse (though debris will still increase from existing risks).
Progress here has already been made: In 2007, the major space agencies (NASA, JAXA, ESA, the umbrella IADC, and the U.S. DOD and DOT), under the auspices of the UN, agreed to a ’25 year rule’ regarding “post-mission disposal”. This rule stipulates that any future mission’s debris must have an orbital lifetime equal to or less than 25 years.
Still, given all that’s already up there, without a many-pronged approach, some innovative space engineering, and public support, the total amount of debris will continue to grow. It would be a truly tragic state of affairs if all of humanity’s hopes for a future in space were cut down by too much space junk.
Then again, it should not surprise us that space garbage would come back to bite us in the arse — just look at the garbage problem we created back here on Earth.
* Quote, and additional material, comes from a 2008 interview with Dr. William Ailor, chief engineer at the Aerospace Corporation, conducted by this author for Space Lifestyle Magazine, now defunct, for an article entitled: ‘Space Junk – The Forgotten Legacy of the Space Race’ )
(Special Note: this author presented a power point mini-lecture, featuring designs from my Space Junk Remediation Project’, at the Yuri’s Night Bay Area 2008 celebration at NASA Ames Research Center. The original design project (or thought experiment) includes both practical and highly speculative solutions to the orbital debris problem)