Space Junk / February 01, 2007

Saturn has rings. So does the Earth.

The Center for Space Standards & Innovation, a Colorado group that provides space-tracking information for aerospace firms, reports that the debris from the pulverized [Chinese] satellite is now spreading out in a ring around the Earth.

The ring is not dense — there are some 500 pieces large enough to be tracked on radar — but it’s vast, and the path is an orbit that goes over the north and south poles.

Through their parent company, Analytical Graphics, Inc., they’ve put together some animation, which will give you a good idea of what’s happening. (Note: it’s a large file, which will open Windows Media Player if your computer is configured as mine is.) The image on this page, also theirs, gives you an idea of what’s going on up there. The International Space Station, though its orbit is in a different plane, will pass through that red zone twice every orbit. NASA says there’s no risk; CSSI says that can’t be said.

To people in the space business, this is part of a growing problem. Even a fleck of paint can be fatal at 17,200 miles an hour. Space shuttles have had to evade known pieces of space junk on several flights in the past, and at least three satellites have been disabled by orbiting debris since the early 1990s. Three? That’s it? Just remember that each represents millions–or hundreds of millions–of dollars, much of which came from us taxpayers.


What are Debris Clouds?

Any concentration of debris particles or fragments in a well-defined region of space is referred to as a debris cloud. Debris clouds are formed whenever debris is being created by a single source. For example, discarded upper stages generally are surrounded by a cloud of particulates that are released over time by degradation of various materials such as paint and multilayer insulation.

Whenever an orbital breakup occurs, a debris cloud is instantly formed. Such debris clouds first take on the form of an expanding 3D ellipsoid. The center of the debris cloud moves along a well-defined orbit, which for explosions is identical to the orbit of the original object. The debris cloud gradually spreads around this orbit in a spiral pattern. As time passes, the debris cloud eventually envelopes the entire orbit and any other satellites in the nearby vicinity.

Due to the laws of orbital motion and to physical processes involved in an explosion or collision, fragments are not spread uniformly throughout a debris cloud. At some locations, spatial density of fragments is much greater than at others. When spatial fragment density is high, the collision risk posed to satellites that fly through the cloud is greatly increased.

Certain regions of the debris cloud are constricted to nearly one or two dimensions. There are three types of debris cloud constrictions: pinch points, pinch lines, and pinch sheets. Spatial fragment density is very high at these constrictions.

Pinch points and pinch lines are particularly important to satellite constellations. Neither of these constrictions moves with the debris cloud around its orbit. They remain fixed in inertial space while the debris cloud repeatedly circulates through them. In many satellite constellations there are multiple satellites in each orbital ring. If one of these satellites breaks up, the remaining satellites in the ring will all repeatedly fly through the pinch point and pinch line. If many fragments are produced by the breakup, the risk of damaging another satellite in the ring may be significant.

If satellites from two orbital rings collide, two debris clouds will be formed, one in each ring. The constrictions of each cloud then pose a hazard to the remaining satellites in both rings.

“The master list of satellite orbital launches and launch attempts. At the time of writing, this is the most complete list available with accurate launch times: I’ve managed to track down a lot of launch times (to the minute) that were previously unavailable. There may be some errors in this list, if you spot any please let me know.”



“The Kessler Syndrome is a scenario, proposed by NASA consultant Donald J. Kessler, in which the volume of space debris in Low Earth Orbit is so high that objects in orbit are frequently struck by debris, creating even more debris and a greater risk of further impacts. The implication of this scenario is that the escalating amount of debris in orbit could eventually render space exploration, and even the use of satellites, too prone to loss to be feasible for many generations.

The Kessler Syndrome is especially insidious because of the “domino effect and Feedback runaway.” Any impact between two objects of sizable mass spalls off shrapnel debris from the force of collision. Each piece of shrapnel now has the potential to cause further damage, creating even more space debris. With a large enough collision (such as one between a space station and a defunct satellite), the amount of cascading debris could be enough to render Low Earth Orbit essentially impassable.

To minimize the chances of damage to other vehicles, designers of a new vehicle or satellite are frequently required to demonstrate that it can be safely disposed of at the end of its life, for example by use of a controlled atmospheric reentry system or a boost into a graveyard orbit.”


Collisional cascading: The limits of population growth in low earth orbit
BY Donald J. Kessler / NASA/Johnson Space Center, Houston, TX 77058 / online 25 Nov. 2002

“Predictions have been made by several authors that random collisions between made-made objects in Earth orbit will lead to a significant source of new orbital debris, possibly within the next century. The authors have also concluded that there are a number of uncertainties in these models, and additional analysis and data are required to fully characterize the future environment. However, the nature of these uncertainties are such that while the future environment is uncertain, the fact that collisions will control the future environment is less uncertain. The data that already exist is sufficient to show that cascading collisions will control the future debris environment with no, or very minor increases in the current low Earth orbit population. Two populations control this process: Explosion fragments and expended rocket bodies and payloads. Practices are already changing to limit explosions in low Earth orbit; it is now necessary to begin limiting the number of expended rocket bodies and payloads in orbit.”


and so:

“A graveyard orbit, also called a supersynchronous orbit, junk orbit or disposal orbit, is an orbit significantly above synchronous orbit where spacecraft are intentionally placed at the end of their operational life. It is a measure performed in order to lower the probability of collisions with operational spacecraft and of the generation of additional space debris. It is used when the delta-v required to perform a de-orbit maneuver would be too high. De-orbiting a geostationary satellite would require a delta-v of about 1,500 m/s while re-orbiting it to a graveyard orbit would require about 11 m/s.

For satellites in a geostationary orbit and geosynchronous orbits, the graveyard orbit would be few hundred kilometers above the operational orbit. The transfer to graveyard orbit above geostationary orbit however requires the same amount of fuel that a satellite needs for approximately three months of stationkeeping. It also requires a reliable attitude control during the transfer maneuver. While most satellite operators try to perform such a maneuver at the end of the operational life, only one-third succeed in doing so.[citation needed]

FCC Enters Orbital Debris Debate
by Peter de Selding

The U.S. Federal Communications Commission (FCC) stepped into a years-long debate on orbital debris by ordering tough new measures governing how satellites are disposed of by their owners. Over the objections of several of the world’s largest commercial satellite-fleet operators, the FCC ruled that all U.S.-licensed satellites launched after March 18, 2002, will have to be placed into so-called graveyard orbits between 200 and 300 kilometers above the geostationary arc, where most commercial satellites operate. The ruling, published June 21, will set a regulatory standard that will be difficult for other nations to avoid. “Up to now, orbital debris has been handled in gentlemen’s-agreement fashion, with soft recommendations that were ignored,” said Christophe Bonnal, a member of an international group that has been pushing the United Nations to adopt debris-mitigation standards. “It is time that people started pounding the table, and it was absolutely necessary that the FCC do it.”

The FCC selected March 18, 2002, as the cutoff date because that was when it notified satellite operators that it was considering rules on the subject. An FCC license is an indispensable ticket for any satellite operator proposing services in the United States. Commercial as well as government satellite operators have been saying for years that they share the concerns that increasing debris both in low Earth orbit and along the geostationary arc 36,000 kilometers above the equator is a problem that ultimately could shut down the space industry. Space industry pioneers including Arthur C. Clarke, who first discovered the properties of the geostationary orbital position, have warned that orbital debris is the single largest long-term threat to the continued use of space for satellites, and especially for manned missions.

The perils of fast-flying particles, even as small as a coin, left over from spent rocket stages and satellites is the main reason the U.S. space shuttle flies upside down and backwards once in orbit. In that position, shuttle astronauts are best protected from orbital junk. The FCC based its new rules on recommendations made by the Inter-Agency Space Debris Coordinating Committee (IADC), which includes members from 11 of the world’s biggest spacefaring nations. The recommendations complement measures taken by most launch-vehicle operators to assure that their rockets are as benign as possible once in orbit. A large portion of the thousands of pieces of orbital junk tracked by the U.S.-Canadian North American Aerospace Defense Command (NORAD) is the result of exploding rocket upper stages.

The most striking of the FCC measures deals with satellites operating in geostationary orbit. Even some of the largest, most profitable satellite-fleet operators have a checkered performance in moving their spent spacecraft well out of the operating lane. According to figures prepared for the IADC based on NORAD data, of 13 geostationary satellites retired in 2002, only five were moved to safe graveyard orbits. The other eight were placed into orbits that sooner or later will threaten to disrupt operations in the geostationary arc. The situation did not improve much in 2003, when just six of 15 geostationary satellites taken out of service were placed in safe orbits. One of the 15, Loral Space and Communications’ Telstar 4, abruptly failed in orbit and could not be moved.

Eight others — Telesat Canada’s Anik C1, the Intelsat 5A, the Eutelsat 2 F1, the PanAmSat Galaxy 6, the Hispasat 1A, the German DFS Kopernikus 3, Russia’s Gals 2 and India’s Insat 2C — were relocated to orbital positions that are insufficiently out of the way, according to a summary submitted to the IADC’s April meeting in Albano Terme, Italy, by Europe’s Esoc space operations center in Darmstadt, Germany. Under the new FCC rules, operators of geostationary satellites will have to commit to raising their satellites to between 200 and 300 kilometers above geostationary orbit as a condition of receiving a license to provide services in the United States. The exact minimum post-retirement altitude will depend on the satellite’s size and other factors that determine how likely it is to drift back into the geostationary arc. The FCC estimates that for a standard Boeing Satellite Systems 601-class satellite weighing 2,477 kilograms without its fuel, the owner will need to raise the orbit by 266 kilometers. In its ruling, the FCC says it considered making the new rule retroactive to all FCC-licensed satellites in orbit. But several satellite operators, including PanAmSat Corp. of Wilton, Conn., EchoStar Communications Corp. of Littleton, Colo., SES Americom of Princeton, N.J., and Inmarsat Ltd. of London said such a move would cost them huge amounts of money. That led to the compromise date of March 18, 2002, which the FCC says will reduce “the potentially significant financial impact of this new requirement.”

For a commercial satellite operator, a satellite in stable position nearing the end of its life is in most cases generating revenues that go straight to the bottom line. Some of this revenue will now be conceded as operators reserve several kilograms of on-board fuel to assure their ability to raise the satellite into the FCC-specified graveyard orbit. Current monitoring technology makes it difficult for operators to assess how much fuel is left in a satellite nearing retirement. In at least one case in 2003, a satellite operator intending to follow the IADC guidelines misjudged the fuel remaining and had to leave the spacecraft in a less-favorable orbit. “Operators are going to have to err on the side of caution,” said Bonnal, who is the outgoing chairman of the IADC debris-mitigation working group. “If they need 4 kilograms to raise the orbit, they are going to have to save maybe 6 kilograms to be sure.” Six kilograms of fuel is enough for between two and three months of satellite operations.



“In an attempt to lay a radio-reflective ring around the world, small metal dipole needles were allowed to sublimate out of a matrix. The experiment was greatly criticized by astronomers who feared optical and radio pollution. However the needles apparently didn’t work as a radio reflector and the feared effects did not come to pass.”
Earth’s Artificial Ring: Project West Ford
BY Anthony Kendall / May 2nd, 2006

At the height of the Cold War in the late 1950s, all international communications were either sent through undersea cables or bounced off of the natural ionosphere. The United States military was concerned that the Soviets (or other “Hostile Actors”) might cut those cables, forcing the unpredictable ionosphere to be the only means of communication with overseas forces. The Space Age had just begun, and the communications satellites we rely on today existed only in the sketches of futurists.

Nevertheless, the US Military looked to space to help solve their communications weakness. Their solution was to create an artificial ionosphere. In May 1963, the US Air Force launched 480 million tiny copper needles that briefly created a ring encircling the entire globe. They called it Project West Ford. The engineers behind the project hoped that it would serve as a prototype for two more permanent rings that would forever guarantee their ability to communicate across the globe.

The project itself was a virtually unqualified success. Though the first launch ended in failure, the second launch went without a hitch on May 10th, 1963. Inside the West Ford spacecraft, the needles were packed densely together in blocks made of a napthalene gel that would rapidly evaporate in space. This entire package of needles weighed only 20 kg. After being released, the hundreds of millions of copper needles gradually spread throughout their entire orbit over a period of two months. The final donut-shaped cloud was 15 km wide and 30 km thick and encircled the globe at an altitude of 3700 km.

Copper Dipoles from Project West FordCopper Dipoles from Project West FordThe West Ford copper needles were each 1.8 cm long and 0.0018 cm in diameter and weighed only 40 micrograms. They were designed to be exactly half of the wavelength of 8000 MHz microwaves. This length would create strong reflections when the microwaves struck the copper needles, in effect making them tiny dipole anttennae each repeating in all directions the exact same signal they received.

The first attempt at remote communications using the West Ford belt was made on May 14th, 4 days after the launch. At this point, the dipoles had not completely spread out to fill their entire orbit so they were much more densely spaced than in their final configuration. Using two 18.5 meter microwave dish antennae, Project West Ford engineers managed to send voice transmissions between Camp Parks, California and Millstone Hill, Massachusetts. The voice connection was described as “intelligible” and was transmitted at a data rate of approximately 20,000 bits per second– about the speed of a 1992-era telephone modem. But as the needles continued to disperse to their final cloud, the data rate dropped off significantly, so much so that by June 18th only 400 bits per second could be transmitted. On July 2nd, the experiment was terminated. At this time, the tiny needles were spaced about 400 meters from each other.

Camp Parks Station constructed for Project West Ford in Pleasanton, CA Despite its technical success, the ultimate goal behind Project West Ford was never attained. Serious scientific opposition to the project sprung up almost immediately after it was first proposed in the late 1950s. Though West Ford’s cloud of dipoles was carefully designed to return to Earth within a few years of its launch, a fully-functional cloud dense enough for robust communications might be a permanent fixture of Earth’s orbit.

Because of the great distance between the tiny needles, the West Ford belt was visible only in the first few days after launch when the spacing was much smaller. A denser belt intended for permanent communications would probably not have been visible except by very powerful optical telescopes. But, at radio and microwave frequencies, the final dipole clouds may have become scars on the night sky, forever obscuring the universe beyond.

However, it may not have been the opposition from prominent scientists that finally killed Project West Ford’s dream. By 1963, communications satellite technology had become more and more capable. Compared to those sleek products of Space Age technology, the relatively low-tech West Ford dipole cloud was an unsightly dinosaur. However, the West Ford engineers remained convinced of the feasibility of their endeavour, and largely blamed the end of the program on the opposing scientists rather than flaws in their own technology.

Most of the West Ford dipoles re-entered Earth’s atmosphere sometime around 1970, according to theoretical and observational evidence. The needles slowly drifted down to the Earth’s surface, unscathed by re-entry because of their size. Some consideration was given to recovering one or more of the dipoles in order to learn more about the space environment. Calculations showed that as many as five dipoles would have landed per square kilometer in the high Arctic. But the exceptional cost of recovering these tiny needles from the haystack of billions of tons of Arctic snow killed off any practical attempts at recovery. Back in space, the failed 1971 1961 spacecraft and some larger clumps of the 1973 1963 dipoles remain in orbit like so many other pieces of space junk, silently carrying the long-dead hopes of this nearly forgotten experiment.




Homework Assignment #6: Orbital Debris

In this assignment you will model the orbital debris collision hazard for spacecraft in Earth orbit. This will require you to program formulas representing the flux (impacts/ unit time/ unit area) of debris, numerically integrate these formulas forward in time, and perform an analysis where different inputs will affect the results (a parametric study).

The flux of orbital debris on a spacecraft can be represented by a fairly simple formula that depends on two sets of information: Information about the spacecraft such as altitude, inclination, and orientation; and, information about the environment such as particle sizes, growth rates, and the effects of the solar cycle (through changing atmospheric drag).

For a little perspective realize that while a .1-mm particle may cause serious surface erosion, a 1-mm particle can be very damaging. A 3-mm particle traveling at 10 km/s has the kinetic energy of a bowling ball thrown at 100 km/hr and a 1-cm particle would have the energy of a 180-kg safe thrown at the same speed. The U.S. space shuttles have already changed their orbits several times to avoid large debris. There has been pitting of tiles, and the loss of several panes of its multi-paned windshield due to impact with a small fleck of paint.

Quoting from Kessler, Reynolds, and Phillip, “NASA Technical Memorandum 100 471: Orbital Debris Environment for Spacecraft Designed to Operate in Low Earth Orbit”:

“The natural meteoroid environment has historically been a design consideration for spacecraft. Meteoroids are part of the interplanetary environment and sweep through Earth Orbital space at an average speed of 20 km/s. At any one instant, a total of 200 kg of meteoroid mass is within 2000 km of the Earth’s surface. Most of this mass is concentrated in 0.1-mm meteoroids.”

Within this same 2000 km above the Earth’s surface, however, are an estimated 3,000,000 kg of man-made orbiting objects. These objects are mostly in high inclination orbits and sweep past one another at an average speed of 10 km/s. Most of the mass is concentrated in approximately 3000 spent rocket stages, inactive payloads and a few active payloads. A smaller amount of mass, approximately 40,000 kg, is in the remaining 4000 objects currently being tracked by U.S. Space Command radars. Most of the objects are the result of more than 90 on-orbit satellite fragmentation. Recent ground telescope measurements of orbital debris combined with an analysis of hypervelocity impact pits on the returned surfaces of the Solar Maximum Mission (SMM) satellite indicate a total mass of approximately 2000 kg for orbital debris of 1 cm or smaller and approximately 300 kg for orbital debris smaller than 1 mm. This distribution of mass and relative velocity is sufficient to cause the orbital debris environment to be more hazardous than the meteoroid environment to most spacecraft orbiting below 2000 km altitude.

While low-altitude debris will fall to the Earth due to atmospheric drag, it is quickly replenished by particles higher up and from collisions. It has been proposed that if too much debris gets into orbit, collisions could cause an increasing number of breakups, leading to an exponential growth in the number of particles. Such a catastrophic chain-reaction has been referred to as the “Kessler Syndrome”. Even with the envisioned growth in launch rate, the growth of orbital debris can be greatly reduced by de-orbiting spent rocket stages and satellite at the end of their useful lifetimes. Better design and management can also reduce the likelihood of explosions (often related to propulsion systems) and reduce the amount of debris likely to be generated in a collision. Orbiting robots have also been proposed to scavenge for old satellites so they could be recycled before they disintegrate, perhaps to be used as small source of materials in future space development.

The Assignment
1. Prepare a multi-curve plot that illustrates the average number of impacts on a spacecraft (y-axis) vs. altitude in km (x-axis) for particle sizes of 0.1, 0.2, and 0.3 cm. The range of heights should be 100 km to 2,000 km, and assume the 10-year time interval: t1=1985, t2=1995. Use the default values listed with the equations, i.e., k=1; random tumbling surface; I = 28.5 degrees inclination, etc.

2. Make at least two other graphs of your choice, in each one varying the effect of altering one variable – change k, S, t2, i () or p. For comparison keep d = {0.1, 0.2, 0.3 cm} or d = 0.1 cm depending on your graph. Explain your choices.

* k – represents type / attitude of spacecraft
* S – measure of solar cycle effects
* t2 – represents mission duration effects
* , i – represents orbital inclination effects
* p – represents growth rate effects



That cylindrical object you see pictured above is a roughly school-bus sized structure which was deployed into space in 1984. It orbited the Earth for five and a half years with nothing expected of it other than to float there, getting battered about by whatever the great black yonder saw fit to throw at it. You see, every inch of its outside surface was covered with Science. 57 separate experiments, mounted in 86 trays, involving the participation of “more than 200 principal investigators from 33 private companies, 21 universities, seven NASA centers, nine Department of Defense laboratories and eight foreign countries.” Its purpose was to study the effects of space on a multitude of materials. Its name is the Long Duration Exposure Facility (LDEF) and I am deeply in love with it.

As longtime readers are no doubt painfully aware, when it comes to certain items I have the tendency to objectify, to glaze over purpose and function and context and just splash about in the shallows of aesthetics. The recent post on microscope slides is a good example. Well, I should warn you straight off that I’m about to indulge that tendency yet again, because every bit of practical and thoughtful background information I plan to relay was contained in that first paragraph. Believe it or not LDEF, somewhat humble punching-bag though it may be, is gorgeous. More specifically the neatly arranged trays which cover every inch of its surface are, as a group and individually, some damned handsome items. Visually they represent the confluence of so many things I’m partial to that the LDEF might in some ways represent the perfect object for my ogling pleasure.

For me it brings to mind the abstraction of certain painters, a gridded and measured minimalism in graphic design, failed utopian architecture, and the shapes and surface textures of every science fictional interior ever put on film. Add to that the subtle color palette peppered here and there with super saturated counterpoints, the unintentional, almost accidental, nature of its beauty, and (having been well battered during its 32,422 Earth orbits) the indelible stamp of decay… and, well, what can I say? The images [see gallery] are each of individual sections of the LDEF’s exterior. I have cropped them where I thought necessary but they are otherwise exactly as they were photographed in 1990.



-As of april, 2005 at least 13 nuclear reactor fuel cores, 8 thermoelectric generators, and 32 nuclear reactors are known to be in Earth orbits below 1700 km.
-Low Earth orbit ~ 250-600 km from earth – International space station
-Geosynchronous stationary orbit ~ 35,785 km from earth – communications satellites
-~ 20,000 km from earth – Global Positioning System (GPS) satellites

Before 1961, the entire Earth satellite population was just over 50 objects. Now earth orbit is cluttered with: ~11,000 objects bigger than 10 cm, of which ~9,000 are catalogued and tracked (~8000 are of US or USSR origin) – including around 600 functional spacecraft; ~100,000 objects from 1-10 cm — too small to track, dangerous to spacecraft; and several million objects smaller than 1cm.

What are they? Jettisoned mission junk, rocket fuel, space station garbage, abandoned rocket parts, used nuclear reactors, leaked radioactive coolant and exploded bits (~150 unplanned explosions of rockets and satellites have occured to date). Collisions between orbiting debris make even more debris. Hundreds of close passes (less than 1 km apart) occur daily between catalogued objects. Each year around 100 objects fall out of orbit and survive re-entry, crash landing somewhere on earth. Dozens of earth orbit satellites launched by the USSR and the USA between 1965-1988 used nuclear power. Several have fallen out of orbit and crash landed. Nuclear power systems are being considered for projects in the next decade. About 75 – 100 new satellites are launched each year.


Orbiting Junk, Once a Nuisance, Is Now a Threat
by WILLIAM J. BROAD / February 6, 2007

For decades, space experts have worried that a speeding bit of orbital debris might one day smash a large spacecraft into hundreds of pieces and start a chain reaction, a slow cascade of collisions that would expand for centuries, spreading chaos through the heavens. In the last decade or so, as scientists came to agree that the number of objects in orbit had surpassed a critical mass — or, in their terms, the critical spatial density, the point at which a chain reaction becomes inevitable — they grew more anxious. Early this year, after a half-century of growth, the federal list of detectable objects (four inches wide or larger) reached 10,000, including dead satellites, spent rocket stages, a camera, a hand tool and junkyards of whirling debris left over from chance explosions and destructive tests.

Now, experts say, China’s test on Jan. 11 of an antisatellite rocket that shattered an old satellite into hundreds of large fragments means the chain reaction will most likely start sooner. If their predictions are right, the cascade could put billions of dollars’ worth of advanced satellites at risk and eventually threaten to limit humanity’s reach for the stars. Federal and private experts say that early estimates of 800 pieces of detectable debris from the shattering of the satellite will grow to nearly 1,000 as observations continue by tracking radars and space cameras. At either number, it is the worst such episode in space history.

Today, next year or next decade, some piece of whirling debris will start the cascade, experts say. “It’s inevitable,” said Nicholas L. Johnson, chief scientist for orbital debris at the National Aeronautics and Space Administration. “A significant piece of debris will run into an old rocket body, and that will create more debris. It’s a bad situation.” Geoffrey E. Forden, an arms expert at the Massachusetts Institute of Technology who is analyzing the Chinese satellite debris, said China perhaps failed to realize the magnitude of the test’s indirect hazards.

Dr. Forden suggested that Chinese engineers might have understood the risks but failed to communicate them. In China, he said, “the decision process is still so opaque that maybe they didn’t know who to talk to. Maybe you have a disconnect between the engineers and the people who think about policy.” China, experts note, has 39 satellites of its own — many of them now facing a heightened risk of destruction. Politically, the situation is delicate. In recent years China has played a growing international role in fighting the proliferation of space junk. In 2002, for instance, it joined with other spacefaring nations to suggest voluntary guidelines for debris control.

In April, Beijing is to play host to the annual meeting of the advocacy group, known as the Inter-Agency Space Debris Coordination Committee. Donald J. Kessler, a former head of the orbital debris program at NASA and a pioneer analyst of the space threat, said Chinese officials at the forum would probably feel “some embarrassment.” Mr. Kessler said Western analysts agreed that China’s new satellite fragments would speed the chain reaction’s onset. “If the Chinese didn’t do the test, it would still happen,” he said. “It just wouldn’t happen as quickly.”

Last week in Beijing, a foreign ministry spokeswoman failed to respond directly to a debris question. Asked if the satellite’s remains would threaten other spacecraft, she asserted that China’s policy was to keep space free of weapons. “We are ready to strengthen international cooperation in this regard,” the spokeswoman, Jiang Yu, told reporters. Cascade warnings began as early as 1978. Mr. Kessler and his NASA colleague, Burton G. Cour-Palais, wrote in The Journal of Geophysical Research that speeding junk that formed more junk would produce “an exponential increase in the number of objects with time, creating a belt of debris around the Earth.”

During the cold war, Moscow and Washington generally ignored the danger and, from 1968 to 1986, conducted more than 20 tests of antisatellite arms that created clouds of jagged scraps. Often, they did so at low altitudes from which the resulting debris soon plunged earthward. Still, the number of objects grew as more nations launched rockets and satellites into orbit. In 1995, as the count passed 8,000, the National Academy of Sciences warned in a thick report that some crowded orbits appeared to have already reached the “critical density” needed to sustain a chain reaction.

A year later, apprehension rose as the fuel tank of an abandoned American rocket engine exploded, breaking the craft into 713 detectable fragments — until now, the record. Amid such developments, space experts identified the first collisions that threatened to start a chain reaction, putting analysts increasingly on edge. On Jan. 17, 2005, for instance, a piece of speeding debris from an exploded Chinese rocket collided with a derelict American rocket body that had been shot into space 31 years earlier. Warily, investigators searched though orbital neighborhoods but found to their relief that the crackup had produced only four pieces of detectable debris.

A year later, Mr. Johnson, the chief scientist for NASA’s orbital debris program, and his colleague J. -C. Liou, published an article in the journal Science that detailed the growing threat. They said orbits were now so cluttered that the chain reaction was sure to start even if spacefaring nations refrained from launching any more spacecraft. “The environment is unstable,” they wrote, “and collisions will become the most dominant debris-generating mechanism.” It was in this atmosphere of rising tension that China last month fired a rocket into space that shattered an old weather satellite — its first successful test of an antisatellite weapon.

David C. Wright, a senior scientist at the Union of Concerned Scientists, a private group in Cambridge, Mass., calculated that the old satellite had broken into 1,000 fragments four inches wide or larger, and millions of smaller ones. Federal sky-watchers who catalogue objects in the Earth orbit work slowly and deliberately. As of yesterday, they publicly listed 647 detectable pieces of the satellite but were said to be tracking hundreds more. The breakup was dangerous because the satellite’s orbit was relatively high, some 530 miles up. That means the debris will remain in space for tens, thousands or even millions of years.

Mr. Kessler, the former NASA official, now a private consultant in Asheville, N.C., said China might have chosen a relatively high target to avoid directly threatening the International Space Station and its astronaut crew, which orbit at a height of about 220 miles. “Maybe the choice was to endanger the station in the short term or to cause a long-term problem,” he said. “Maybe that forced them to raise the orbit.” Even so, the paths of the speeding Chinese debris, following the laws of physics and of celestial mechanics, expanded in many directions, including upward and downward. As of last week, outliers from the central cloud stretched from roughly 100 miles to more than 2,000 miles above the Earth.

A solution to the cascade threat exists but is costly. In his Science paper and in recent interviews, Mr. Johnson of NASA argued that the only sure answer was environmental remediation, including the removal of existing large objects from orbit. Robots might install rocket engines to send dead spacecraft careering back into the atmosphere, or ground-based lasers might be used to zap debris. The bad news, Mr. Johnson said in his paper, is that “for the near term, no single remediation technique appears to be both technically feasible and economically viable.”

If nothing is done, a kind of orbital crisis might ensue that is known as the Kessler Syndrome, after Mr. Kessler. A staple of science fiction, it holds that the space around Earth becomes so riddled with junk that launchings are almost impossible. Vehicles that entered space would quickly be destroyed. In an interview, Mr. Kessler called the worst-case scenario an exaggeration. “It’s been overdone,” he said of the syndrome. Still, he warned of an economic barrier to space exploration that could arise. To fight debris, he said, designers will have to give spacecraft more and more shielding, struggling to protect the craft from destruction and making them heavier and more costly in the process. At some point, he said, perhaps centuries from now, the costs will outweigh the benefits. “It gets more and more expensive,” he said. “Sooner or later it gets too expensive to do business in space.”




Rings around the Earth: A clue to climate change? / September 11, 2002
Large-object collisions with Earth — comets, asteroids, or large meteors — could interact with the Earth’s surface and atmosphere to eject materials, including melted and reformed materials, called tektities to create a debris ring.

While most of us know about rings around Saturn and Jupiter, some scientists believe there once were rings of rock debris around our own planet. Two scientists — Peter J. Fawcett, of the University of New Mexico, and Mark B.E. Boslough, of the U.S. Department of Energy’s Sandia National Laboratories — have suggested that a geologically “recent” collision (about 35 million years ago) may have caused such a temporary debris ring. The two also suggest that such temporary rings — lasting from 100,000 to a few millions of years — may explain some patterns of climate change observed in the earth’s geological record. These conclusions are spelled out in an article in the Journal of Geophysical Research, Atmospheres, August 16 edition.

Lore of the Rings
“One way to get a ring,” says Sandia’s Boslough, “is with an impact.” There is a growing body of evidence showing that the earth has been subjected to numerous impacts by comets and asteroids throughout its history. Among these impacts are the Meteor Crater, in Arizona, the buried Chixulub crater, in the Yucatan Peninsula of Mexico, and a chain of at least five craters spread across several continents. Several studies, both theoretical and with laboratory data, suggest that some large impacts are capable of ejecting material into space in the form of debris rings, if the mechanics of the impact meet certain requirements. The authors conclude that the mostly likely scenario for ring creation is a low-angle impact by a large asteroid. Some earth materials and melted meteoric debris, called “tektites” would form the ring materials. Boslough describes an impact where the collision object ricochets back into the atmosphere. The ricochet becomes part of an expanding vapor cloud, setting up an interaction that allows some of the debris to attain orbit velocity. The orbiting debris will collapse into a single plane by the same mechanics that led to the rings of Saturn and other planets, Boslough explains. Such a ring would most likely form near the equator, because of the dynamics involved with the moon and the earth’s equatorial bulge.

Speculation on climates past
The effects of the larger impact events on earth’s environment and climate have been the subjects of much speculation and research over the past two decades. “Clearly, large impacts have affected the evolution of the earth, life on it and its atmospheric environment,” says Fawcett. Much of the work has focused on the Cretaceous-Tertiary (K-T) boundary event, which marked a mass extinction and the end of the age of the dinosaurs about 65 million years ago. A number of these studies suggest an impact resulting in the suspension of a layer of dust in the upper atmosphere blocking sunlight and cooling the earth. The two researchers asked could other impacts result in different atmosphere-altering phenomena?

An equatorial ring would cast a shadow primarily in the tropics, as it does for Saturn. Depending on location, surface area, and darkness of the ring shadow, the amount of incoming solar warmth, or insolation, could be significantly altered, the two authors conclude. To test their theory, the two assumed an opaque ring, like Saturn’s B-ring, scaled to earth-size and tested global climate affects using a climate model. The model selected and modified for the simulation was developed by the National Center for Atmospheric Research (NCAR.) The Center’s “Genesis” climate model includes atmospheric circulation information and layers of vegetation, soil, snow, sea temperature and land ice data. The goals of the internally funded project were for Sandia to adapt a popular climate code to run on distributed-memory parallel computers and to establish relationships with the climate change research community, Boslough explained. The Labs made use of its Sandia University Research Program to fund Fawcett’s efforts to analyze the data from the adapted code.

A Ring World
“The equatorial debris ring has a profound effect on climate, because it reflects a significant fraction of tropical insolation back to space before it can interact with the atmosphere,” the authors conclude. Surface and atmospheric temperatures, changes in temperature ranges from equator to poles, circulation patterns and the rain and snow cycles were all impacted by the ring, the model shows. The two scientists looked at changes shown in the model to predict changes that might be found in the earth’s geologic record as a way to test their work. In addition to the K-T boundary event, they looked at a more recent impacts and a much older one.

The most recent event — about 35 million years ago — is identified by an iridium layer (often associated with meteors) and two pronounced mico-tektite fields, where these melted meteor-related materials have been found and dated. Climatic records from sedimentary materials just above the iridium/micro-tektite interval indicate a 100,000-year cooling interval. Orbiting debris in a ring, casting its shadow in the subtropics could have sustained such a cooling trend, the authors suggest. The K-T boundary impact — about 65 million years ago — was much larger than the more recent impact and had a much larger immediate effect on the environment as measured by extinctions and atmospheric changes. But there were no long-term effects on the climate, leading the authors to conclude the event probably did not generate a debris ring.

Snowball Earth
Another interesting aspect of the modeling work is its implications for the so-called “Snowball Earth” theory. This theory holds that the earth was completely frozen over at the surface as many as four times in the neoproterozoic period — 750 to 580 million years ago. While much remains to be learned about the geologic evidence for this theory, “an opaque ring could have acted as the trigger to at least one episode of global glaciation,” the two researchers say. This would address one difficult question for the theorists: how did earth come to be frozen?

Mark Boslough / email: mbboslo [at] sandia [dot] gov
Peter Fawcett / email : fawcett [at] unm [dot] edu