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First look at nike and off-white air force 1 in ghost grey.
January 9, 2023 . Leave a Comment
Images have emerged of Nike and Off-White ‘s ongoing series of Air Force 1 silhouettes, this time in an attractive ghost grey colorway. Images were debuted by Instagram account @zsneakerheadz, which noted the silhouette may launch as a Paris exclusive in the spring.
As with prior Nike and Off-White Air Force 1 shoes, the silhouette features a tone-on-tone Swoosh adorned with external stitching and a small red tag along with Abloh’s signature zip tie attached to the shoelace. Colorwise, the entire shoe is grey, the Swoosh is silver and the “AIR” graphic on the heel and the interior is a pale green, a shade long employed by Off-White for its shopping and dust bags.
Nike and Off-White recently released a bright green version of the Air Force 1, a shoe made in conjunction with Abloh’s “Figure of Speech” exhibition at the Brooklyn Museum . The shoe was gifted to the exhibit’s security staff, who previewed the shoe when the exhibit opened in July 2022. It was subsequently launched on SNKRS in September, priced at $160.
Additional new colorways of Nike and Off-White Air Force 1 sneakers were also previewed at Abloh’s “The Code c/o Architecture” exhibition held at Art Basel in Miami from December 1-4.
As of now, there are no official release details for Nike and Off-White’s latest Air Force 1 sneaker.
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The Off-White x Nike Air Force 1 “Ghost Grey” May Finally Be On Its Way
First surfacing at the end of 2022, fans of the AF1 can rejoice as the concrete-like “Ghost Grey” Off-White x Nike Air Force 1s may be dropping very soon.
The Air Force 1 has become a serious staple in recent memory. Many, such as Billie Eilish, Drake’s NOCTA , Louis Vuitton, and Stüssy have collaborated with the classic kicks and offered their own spin on the AF1.
The constant collaborations and the behaviour of secondary markets that’s ensured the popular “Triple White” AF1s low price continues to rise, have all contributed to the humble AF1 now arguably being Nike ‘s most globally cherished silhouette of all time.
The late Virgil Abloh was undoubtedly a big fan of the AF1 . Both Louis Vuitton and Off-White have previously offered iterations of the classic silhouette with the latter getting ready to launch the monochromatic “Ghost Grey” AF1 trainers
The Off-White x Nike Air Force 1 “Ghost Grey” features the Off-White metallic Swoosh that decorates the smooth greyscale leather aesthetic upper.
After over six months of speculation, a variety of reports tells us that the latest Off-White AF1 may finally be hitting the market as early as this Spring.
However, reports also tells us that its rumoured that the new “Grey Ghost” silhouette may be exclusive to Paris. A city-exclusive release would mark a new strategy for the Off-White x Nike collaborations. Given Virgil’s close ties to the city, a Paris exclusive drop does make sense – which is unfortunate for us in the UK.
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Who Was in Paris? The Nike x Off-White Air Force 1 Looms in Grey
Expected to drop soon as a Paris/France exclusive.
[Update 4 Jan 2023] Off-White site Canary Yellow has seemingly given us confirmation of the imminent release of the Off-White x Nike Air Force 1 Low “Ghost Grey” via a retweet of a leak .
As of now, the Off-White x Nike Air Force 1 Low “Ghost Grey” is expected to drop in Spring 2023 at US$160.
[Update 8 Aug 2022] We’ve finally got closer looks at a proper visual representative of the Off-White x Nike Air Force 1 Low “Ghost Grey”! Based on what we see, the sneaker is to retain the same silhouette and silver Swoosh of its predecessor, “Light Green Spark”, with the exception of colour – instead of bright green, a dark “Ghost Grey” covers the entire upper. White laces and inner lining provide another colour contrast to the design, and the black Abloh zip-tie matches the “AIR” quotations on the midsole. Based on this rep, the only break from the monochrome colour palette is a red tag attached to the bottom of the Swoosh. Besides colourway design, sources are also confirming that “Ghost Grey” is expected to release in Paris by late 2022 at earliest, up to early 2023 at latest. (Photo courtesy of @jfgrails )
What’s the point? Simply that excruciatingly little is known about this new colourway. “Ghost Grey” (if it even is to be named that) might feature the same silver foil stitched Swoosh, Abloh’s iconic zip-ties, and Off-White’s satirical quotations labelling obvious parts of the shoe, all in monochrome greys and whites. On the other hand, the luxury creative could opt to completely swap up the colourway by inverting colours and exchanging materials. All we currently have is speculation based on the known colourway.
When few facts are available, one must ask: what should we expect? Most sources state that contrary to the wider release of “Light Green Spark”, this grey colourway is expected to (initially) be a Paris-exclusive drop. The green AF1s are highly likely to undergo a lottery release very soon, so the younger grey sibling could drop as early as within the next couple of months, but if not, early 2023 is a good guess. Stay tuned for more updates on both Off-White x Nike Air Force 1 Low “Light Green Spark” and “(Ghost) Grey”!
In other sneaker-related news, check out all the latest news about Nike SNKRS Day 2022 .
*Reminder: the images shown are mock-ups depicting how the colourway is currently expected to look.
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A paris-exclusive off-white x nike air force 1 low is releasing in grey.
As we await for the eventual release of the Off-White Nike Air Force 1 Low, rumblings of yet another colorway of the model are beginning to heat up. According to sources, a grey colorway of the Off-White Air Force 1 Low is next in line, set to drop exclusively in Paris. Currently, the Figures Of Speech is viewable at the Brooklyn Museum through January 2023, so it’s not clear if this next grey colorway is due this year or next.
The current green pair of the Off-White x Air Force 1 Low has yet to release to the public, although they are expected to drop very soon through a lottery-type release. More details on the green pair are to come, so stay tuned for relevant updates.
In other footwear news updates, check out the fill release list for Yeezy Day 2022 .
*The above photo is just a mock-up representation of what is expected.
UPDATE 8/9/2022: According to @zsneakerheadz , the Paris-exclusive Off-White x Nike Air Force 1 could release at the end of 2022 or early 2023.
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The Off-White™ x Nike Air Force 1 Low Potentially Releasing in "Grey"
Reportedly paris-exclusive pairs..
As Louis Vuitton ‘s exhibit for the Nike Air Force 1 by Virgil Abloh continues in Brooklyn, New York, Off-White™ revealed a “Light Green Spark” colorway earlier this month. A few weeks later, reports now indicate that another Paris-exclusive “Grey” colorway from Off-White™ could be arriving in the coming months.
Found in gray and white hues, the upcoming pairs feature contrast-stitched silver foil Swooshes and circular cutouts on the tongue tabs. Abloh’s trademarked zip-ties are looped into the laces and “AIR” prints are found on the midsoles. Additional branding text is pasted on the medial side, “SHOELACES” text marks the laces, and mismatched Off-White™ motifs appear on the insoles.
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It Looks Like Russia’s ‘Massive’ Military Exercise Wasn’t So Massive After All
On August 20, 2018, Chinese fighters of the People’s Liberation Army Air Force (PLAAF) landed at Shagol Air Base in Chelyabinsk,...
By Nicholas J. Myers, The National Interest | Published Oct 1, 2018 4:45 PM EDT
On August 20, 2018, Chinese fighters of the People’s Liberation Army Air Force (PLAAF) landed at Shagol Air Base in Chelyabinsk, Russia. They were coming to participate in the Shanghai Cooperation Organization (SCO) anti-terrorist exercise “Peace Mission-2018” that was supposed to be famous for being the first joint exercise between India and Pakistan. However, the SCO exercise faded into obscurity almost immediately as that same day Russia announced sixteen snap exercises across the Central and Eastern Military Districts (including Chelyabinsk) as a prologue to Vostok 2018.
The press release seemed a major diplomatic blow to China and its decision to send fighters because Vostok is an exercise traditionally intended to practice defense of the Russian Far East, presumptively from China . However, the very last sentence of Moscow’s announcement declared that the PLA, and old Soviet ally Mongolia, would be joining Russia. World news then (appropriately) pivoted to the story of vastly increased Chinese-Russian military cooperation as China was welcomed into Vostok 2018.
But the Russian military conquered the news cycle anew as the size of Vostok 2018 was revealed. For instance, it Moscow stated from the start that it would be the largest Russian exercise since the legendary Soviet Zapad-81. On August 24, Krasnaya Zvezda (the Russian equivalent to Stars and Stripes) claimed that 262,000 Russians were involved in the snap exercises of that week. The following Tuesday (28 August), Russian Defense Minister Sergei Shoigu officially announced that Vostok 2018 would involve about 300,000 servicemen. After the 2017 furor over the size of Zapad for which the Russians declared a conveniently low number that was just under Vienna Document limitations , Vostok appeared to be going to the impossibly high extreme: 300,000 servicemen would be one-third of the entire Russian Armed Forces.
Vostok 2018 was indeed large, but it seems as many tricks were used to inflate its numbers as were used to deflate those of Zapad 2017. For a start, the Russian Northern Fleet typically conducts its summer exercises at about the same time as its annual strategic-operational exercise (Zapad, Vostok, Tsentr, and Kavkaz), but this has always been officially treated as a distinct event. This year, however, the Northern Fleet sent its summer contingent east across the Northern Sea Route to formally participate in Vostok 2018 in the Bering Sea and the Sea of Okhotsk. This is a significant event in its own right, but it offers an unprecedented accounting boon for the Russian bean counters.
It further seems that three hundred thousand is the sum of this flotilla sailing from Murmansk and virtually the entire Central and Eastern Military Districts’ paper strength despite virtually no evidence of participation by the Eastern Military District’s 5th Army. Furthermore, the total is incorrect because the East’s 68th Army Corps did not participate and ongoing exercises by contingents of the Central Military District at their home garrisons were not officially engaged in Vostok.
Reconstructing exactly how many Russians did participate in the Vostok 2018 maneuvers is challenging to do with exclusively open sources, but can be attempted. Using the Russian Ministry of Defense (MoD) press releases of the various components of the events within Vostok , about fifty distinct activities were outlined. These hardly represent all the activities of the exercise, but highlight the aspects the Russian MoD wanted the public to know. Taking the Ground Forces and Airborne Forces numbers at face value and assuming that all listed participating ships had fully-manned crews and that each aircraft had a total flight crew of twenty-five (based on this recent exercise figure), the MoD figures suggest an overall participation of:
- Ground Forces: 46,250
- Airborne Troops: 6,000
- Navy: 3,695
- Aerospace Force: 5,800
This gives a total figure of only 61,745 soldiers. The Navy figures are almost certainly a bit low considering the shore duty sailors and officers coordinating their activities. The other service figures likely exclude command staffs overseeing the exercise as well. Still, a generous inflation would boost the number only to 75,000.
Using this methodology for the very opaque official figures from last year’s Zapad-2017 exercise (during which no official Ground Forces numbers were divulged whatsoever) only yields 3,027 servicemen, conveniently just under the official claim of 5,500 Russian soldiers involved. This would officially suggest that the MoD figures are only about half of the actual story, which might theoretically boost the Vostok numbers up to 150,000. However, the number 3,027 suggests that only 2,473 servicemen comprise the entire 1st Guards Tank Army contingent deployed within both Russia and Belarus in 2017, itself an unlikely figure considering the large number of activities the army carried out over the course of that exercise.
An exercise 75,000-strong is nothing to sneeze at—even the 61,745 figure still makes this the biggest event of the past year by a significant margin for the Russian military. However, this is an extremely far cry from 300,000, raising questions about why the Russians would make such a claim. Most likely, Russia is merely trying to claim strength in the Far East to ensure stability on that front as unrest continues on its western and southern borders. This would also explain Russia's decision to welcome China into its major eastern defense exercise.
Nevertheless, grounding its strategic stability in the Far East and Asia-Pacific on such inflated figures, even if the exercise somehow did number 150,000, raises questions of just how much Russian strategy relies upon misdirection.
This article originally appeared on The National Interest
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Remembering the Chelyabinsk Impact 10 Years Ago, and Looking to the Future
On Feb. 15, 2013, the people of Chelyabinsk, Russia, experienced a shocking event, and yet it was a small fraction of the devastation an asteroid on a collision course with Earth could yield. As NASA’s Planetary Defense experts reflect on the Chelyabinsk impact 10 years ago, they also look forward to the future and all that the agency has since accomplished in the field of Planetary Defense.
Harmless meteoroids, and sometimes small asteroids, impact our planet’s atmosphere daily. When they do, they disintegrate and create meteors or “shooting stars” and sometimes bright fireballs or bolides. Such was the case on Feb. 12 when a very small asteroid impacted Earth’s atmosphere over Northern France soon after discovery, resulting in a spectacular light show for local onlookers. Much more rarely, a larger asteroid that is still too small to reach the ground intact, yet large enough to release considerable energy when it disintegrates, can do significant damage to the ground. On Feb. 15, 2013, one such bolide event garnered international attention when a house-sized asteroid impacted Earth’s atmosphere over Chelyabinsk, Russia, at a speed of eleven miles per second and exploded 14 miles above the ground. The explosion was equivalent to 440,000 tons of TNT, and the resulting air blast blew out windows over 200 square miles, damaged buildings, and injured over 1,600 people – mostly due to broken glass. Due to the asteroid’s approach from the daytime sky, it was not detected prior to impact, serving as a reminder that while there are no known asteroid threats to Earth for the next century, an Earth impact by an unknown asteroid could occur at any time.
Coincidentally, negotiations sponsored by the United Nations were finalizing formal recommendations for the establishment of Planetary Defense-related international collaborations – the International Asteroid Warning Network (IAWN) and the Space Missions Planning Advisory Group (SMPAG) – when the Chelyabinsk impact occurred. Since then, NASA established the agency’s Planetary Defense Coordination Office (PDCO) in 2016 to oversee and coordinate the agency’s ongoing mission of Planetary Defense. This includes acting as a national representative at international Planetary Defense-related caucuses and forums, such as IAWN and SMPAG, and playing a leading role in coordinating U.S. government planning for response to an actual asteroid impact threat if one were ever discovered. The PDCO also funds observatories around the world through NASA’s Near-Earth Object (NEO) Observations Program to find and characterize NEOs – asteroids and comets that come within 30 million miles of Earth – with a particular focus on finding asteroids 460 feet (140 meters) and larger that represent the most severe impact risks to Earth. To help accelerate its ability to find potentially hazardous NEOs, NASA is also actively developing the agency’s NEO Surveyor mission , which is designed to finish discovery of 90 percent of asteroids 140 meters in size or larger that can come near Earth within a decade of being launched.
In 2022, working together with the Italian Space Agency, NASA’s Double Asteroid Redirection Test (DART) mission successfully demonstrated the world’s first-ever test for deflecting an asteroid’s orbit. Launched in 2021 , DART successfully collided with a known asteroid – which posed no impact threat to Earth – demonstrating one method of asteroid deflection technology using a kinetic impactor spacecraft. Since DART’s impact, Planetary Defense experts have been continuing to analyze data returned from the mission to better understand its demonstrated effects on the asteroid, which contributes to the understanding of how a kinetic impactor spacecraft could be used to address an asteroid impact threat in the future if the need ever arose.
The Chelyabinsk impact was a spark that ignited global conversation in Planetary Defense, and much progress in the field has occurred since then. However, there is still more work to be done, and NASA is actively at the forefront. In addition to building NASA’s NEO Surveyor to find the rest of the population of asteroids that could pose a hazard to Earth, the agency is considering a “rapid response reconnaissance” capability to be able to quickly obtain a more detailed characterization of a hazardous asteroid once it is discovered. NASA is also considering sending out a reconnaissance spacecraft to study an asteroid making a close approach to Earth in 2029.
“A collision of a NEO with Earth is the only natural disaster we now know how humanity could completely prevent” said NASA Planetary Defense Officer Lindley Johnson. “We must keep searching for what we know is still out there, and we must continue to research and test Planetary Defense technologies and capabilities that could one day protect our planet’s inhabitants from a devastating event.”
Learn more about NASA’s Planetary Defense Coordination Office
Keep up to date on NASA’s Planetary Defense efforts by following Asteroid Watch on twitter
- Five Years after the Chelyabinsk Meteor: NASA Leads Efforts in Planetary Defense
- Around the World in Four Days: NASA Tracks Chelyabinsk Meteor Plume
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Chelyabinsk: Portrait of an asteroid airburst
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David A. Kring , Mark Boslough; Chelyabinsk: Portrait of an asteroid airburst. Physics Today 1 September 2014; 67 (9): 32–37. https://doi.org/10.1063/PT.3.2515
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On 15 February 2013, 100 km above the snow-covered Russia–Kazakhstan border, a 4.5-billion-year-old relic of the solar system pierced Earth’s atmosphere and began a fiery descent toward the surface. Moving in excess of 19 km/s (faster than Mach 60), it crossed over the glaciated and kettle-rich plains of Kurgan and Chelyabinsk Oblasts, trailed a pair of smoky, iridescent plumes, and repeatedly shed debris before abruptly decelerating and exploding in a final half-megaton detonation a little more than 15 seconds later.
The asteroid passed about 40 km south of the Chelyabinsk city center. It blasted residents with a shock wave from the explosion above the countryside and hammered them with repeated sonic booms from trailing fragments. A few rocky remnants continued to move westward, the smallest on paths that were altered by the wind as they fell; the largest landed 30 km farther in Lake Chebarkul at the foot of the Ural Mountains. The event was the most dramatic near-Earth asteroid airburst since the 1908 Tunguska impact blast in Siberia.
The speed of whatever collides with Earth’s atmosphere depends on its orbit, which in turn depends on its source. The impactor’s entry at 19 km/s means that it came from the asteroid belt between Mars and Jupiter, not from a ballistically launched missile, whose speed is less than 11.2 km/s; a short-period comet, with an average speed of 35 km/s; or a long-period comet with an average speed of 55 km/s. As investigators began retracing the path of the meteor that blazed across the sky, their reconstructed orbit bore out that provenance.
The altitude of the blast suggested the object was relatively small and weak. In the past, showers of meteorites have been produced by so-called brecciated, or fragmented, asteroids, and it was likely that Chelyabinsk’s was made of such material. A quick assessment of the energy of the airburst, when factored together with the observed velocity and assumed density of the impacting material, suggested an object about 20 m in diameter. Although all of those parameters have been refined with additional analyses, 1 , 2 , 3 20 m remains a good round-number estimate for its size.
The meteorite shown in figure 1 and many others were collected by local residents within days of the airburst. The debris is generally light colored, intensely fragmented, and crosscut with veins of jet-black material. The black material experienced high-pressure shock and was partly melted—evidence of a preexisting collisional history on a small planetary body, which is often called a planetesimal or the parent body of the meteorite. The juxtaposition of highly shocked and less shocked (light-colored) material is a hallmark of impact-cratering processes on asteroids, planetesimals, and any other solid-surface planetary body.
Figure 1. The Chelyabinsk asteroid was a fragmented, metal-poor rock heavily damaged during its history wandering the solar system. (a) One of its meteorites has been broken open to show its fragmented interior on the left side and the dark-colored crust on the right side. The crust was produced when the surface was melted as it passed through the atmosphere. (Image courtesy of Svend Buhl/Meteorite Recon.) (b) A microscopic view of the interior of the meteorite. A few chondrules—molten droplets that crystallized in the solar nebula—are circled among various mineral grains. Black shock veins (the two near-vertical lines) penetrate the center of the sample and are also evidence of prior melting. (Courtesy of Amy L. Fagan.)
The Chelyabinsk meteorites range in size from dust particles to a 1.5-m boulder. Chemical and petrologic analyses indicate the meteorites are in a class called ordinary chondrites, the most common types of meteoritic material to hit Earth. Ordinary chondrites are composed mostly of stone (mixtures of silicate and oxide minerals), but they also contain small amounts of iron, nickel metal, and sulfides. There are three groups of ordinary chondrites, each with slightly different chemistry: H chondrites, which are high in iron; L chondrites, which are low in iron; and LL chondrites, which contain low total iron and low metal. Ordinary chondrites represent different planetesimals that once existed between the orbits of Mars and Jupiter.
Those planetesimals have a remarkable history. They are called chondritic bodies because they contain chondrules, millimeter-size spherules that formed in the solar nebula—the disk of dust and gas that surrounded the proto-Sun 4.56 billion years ago. The spherules have the same textures as igneous rocks, indicating they were once molten droplets of silicate, metal, and sulfide material produced by intense, high-temperature nebular storms before they cooled and accreted to form small planetary bodies.
The compositions of minerals in the meteorites—the silicates olivine and pyroxene and the metal alloy kamacite—can be used to determine the specific planetary source of the Chelyabinsk samples. Those analyses indicate the meteorites are related to the LL chondrites. The group is not uncommon: Some 40 000 ordinary chondrites populate meteoriticists’ collections and nearly 6000 of them have LL-chondrite affinities. In addition, in 2010 the Japan Aerospace Exploration Agency returned samples from the Itokawa near-Earth asteroid and showed that it, too, has LL-chondrite affinities. Pictured in figure 2 , the Itokawa asteroid is 540 m long and contains many boulders similar in size to the Chelyabinsk asteroid. Both objects, however, are small compared with the original LL-chondrite parent body, which is estimated to have been at least 100 km in diameter. (It is also generally assumed that all LL-chondritic materials come from the same parent body, though the possibility of multiple parent bodies cannot yet be dismissed.)
Figure 2. The Itokawa near-Earth asteroid is 540 m long and compositionally related to the Chelyabinsk asteroid. Both are assumed to come from the same LL-chondrite parent body that was originally 100 km or more in diameter. Boulders on Itokawa, such as the one circled, are similar in size to the 20-m-diameter Chelyabinsk. Many other objects like Chelyabinsk and Itokawa are thought to occupy near-Earth orbits. (Photograph courtesy of the Japan Aerospace Exploration Agency.)
The interior of that parent did not get hot enough to melt, but the deeper material was nonetheless altered by the heat given off from radioactively decaying isotopes like short-lived aluminum-26. The degree of thermal metamorphism that affected the chondrules in the Chelyabinsk samples suggests what became the asteroid was initially buried several kilometers beneath the surface of the LL-chondrite parent.
Numerous studies have shown that the parent had an extensive collisional history over the past 4.5 billion years. 4 The collisions heated and, in some cases, melted portions of the parent, which reset the radiometric ages in those areas. Several meteoritic samples were affected by one or more impact events about 4.20–4.35 billion years ago that reset their ages. Two additional meteorites have impact ages of 3.9 billion years, which corresponds to a period of collisional history first recognized among samples collected by Apollo astronauts on the Moon. That period of bombardment was initially called the lunar cataclysm, but we now understand that the bombardment affected the entire inner solar system.
There is a dearth of impact ages among LL chondrites over the next 2 billion years, and only a few ages are indicative of impact events during the past billion years. In general, the collision rate has decreased as the solar system has aged and material has been swept up by the larger planets. Small but abrupt increases in the impact flux may occur, however, following the catastrophic breakup of a minor planet or large asteroid. That type of event appears to have occurred as recently as 500 million years ago when an L-chondrite parent body broke up and produced a large number of meteorites that found their way to Earth shortly thereafter.
The Chelyabinsk samples fill in other details about the collisional evolution of the LL-chondrite parent body. According to uranium–lead and lead–lead isotopic analyses of the black shock-melted material, the sample was affected by an impact event about 30 million years after the solar nebula formed, 5 after 115 million–125 million years, 3 , 6 or both. Thus Chelyabinsk provides evidence of collisions affecting the LL chondrite at the same time the proto-Earth was still accreting and before the Earth–Moon system had formed (see the article by Robin Canup in Physics Today , April 2004, page 56 ). The argon–argon isotopic system was also reset, but by a much younger event that occurred only 29 million years ago. 7 Analyses of other cosmogenic radionuclides indicate that the surface of the Chelyabinsk asteroid was exposed to cosmic radiation about 1.2 million years ago, 8 which suggests yet another collision or fragmentation event. Based on the dynamical lifetimes of debris that size, Chelyabinsk probably entered a gravitational resonance in the main asteroid belt between the last two events. That increased its orbital eccentricity and put it into the Earth-crossing orbit that led to its final fate.
The strength of near-Earth asteroids appears to be limited by structural flaws, 9 , 10 such as the fractures and material contrasts produced by the repeated collisions that Chelyabinsk underwent. Observations of other falling meteorites indicate that fragmental asteroids tend to produce meteorite showers. Not surprisingly, the surviving samples of the Chelyabinsk asteroid are intensely fragmented and crosscut with impact melt veins generated by older collisional events. The Chelyabinsk near-Earth asteroid came with built-in weaknesses.
The Chelyabinsk asteroid first felt the presence of Earth’s atmosphere when it was thousands of kilometers above the Pacific Ocean. For the next dozen minutes, the 10 000-ton rock fell swiftly, silently, and unseen, passing at a shallow angle through the rarefied exosphere where the molecular mean free path is much greater than the 20-m diameter of the rock. Collisions with molecules did nothing to slow the gravitational acceleration as it descended over China and Kazakhstan. When it crossed over the border into Russia at 3:20:20 UT and was 100 km above the ground, 99.99997% of the atmosphere was still beneath it.
Because the asteroid was moving much faster than air molecules could get out of its way, the molecules began to pile up into a compressed layer of high-temperature plasma pushing a shock wave forward. Atmospheric density increases exponentially with depth, so as the asteroid plunged, the plasma layer thickened and its optical opacity rapidly increased. About one second later, at 95 km above the surface, it became bright enough to be seen from the ground. That was the first warning that something big was about to happen.
For the better part of 10 seconds, the asteroid pushed through the air as a rigid body, moving at a shallow angle—about 17° from the horizon—and descending 1 km for every 3 km of horizontal flight. At that altitude, the air density is so low that the dynamic pressure, even at 19 km/s, is too small to deform or break a rock—even one as weak and damaged as Chelyabinsk. Like the fastest supersonic jet (but 20 times as fast), the asteroid generated a bow shock that wrapped around it into a conical shape. The cone was very slender, blunt at the front end, and more like a cylinder surrounding the wake. The bow shock was invisible from the ground, but the meteor got steadily brighter as the plasma layer continued to thicken. The ionized air also radiated energy backward toward the asteroid, which absorbed it in a thin layer that vaporized and was swept away by the flow into the trailing wake.
At about 45 km above Earth’s surface, the nature of the entry began to change. The dynamic pressure built up to 0.7 MPa—not enough to slow the asteroid but enough to exceed its strength. Within a couple more seconds, below 40 km, pressure on the now-fracturing asteroid increased past 1 MPa, breaking it into a number of large fragments. As the pressure grew exponentially, the process cascaded and formed ever-smaller fragments that rapidly increased the total surface-to-volume ratio. As fragments in the dense collection ablated, the hot gas between them began to build up.
The pressure buildup, in turn, caused an outward expansion that further increased the surface area on which the rising aerodynamic drag could act. The only possible result of such a chain reaction is a massive explosion from the abrupt conversion of the asteroid’s kinetic energy into heat and pressure. Even as that massive explosion was under way, with half the original kinetic energy lost, one relatively unbroken main fragment, trailed by about 20 boulders, continued downrange, with a barely measurable loss of speed as it descended below 29 km.
The cascading breakup continued as the mass and energy of the remaining fragments were spent in the deeper, denser parts of the atmosphere until only one prominently visible piece remained to pop out, like a spark from the front of the explosion. That piece was visible until it slowed too much to generate a plasma, stopped ablating, and blinked out. It continued to fall as a ballistic missile in “dark flight” at terminal velocity until it punched a hole in the ice of the frozen Lake Chebarkul. The 1.5-m-wide boulder dredged up from the muddy lake bottom eight months later was the largest Chelyabinsk meteorite found.
Much of the evidence for the fragmentation cascade and the determination of altitude comes from the dramatic audio portions of recordings of the event. At some locations, the main blast is heard, followed by a long sequence of weaker booms, each generated by its own fragment or fragmentation event.
Two processes can generate strong air shocks: the passage of a supersonic body and the detonation of an explosive charge. The first process is normally associated with a high-speed jet that generates a conical bow shock along the flight path. When the wave intersects the ground, a sonic boom moves underneath the aircraft. Under textbook conditions, the intensity of the boom is constant along the ground track and decays laterally, which gives the appearance of cylindrical symmetry.
An explosion, on the other hand, is a point source. A purely explosive airburst, like the detonation of a bomb, radiates energy in all directions; it generates a spherical air shock with a peak pressure on the ground directly below and decays radially.
An asteroid airburst does not fit neatly into either category. At the beginning of its flight, the asteroid acts more like the supersonic aircraft, and at the end it acts more like an explosion. In Chelyabinsk, the energy deposition that led to the explosion took place in stages and was spread out over a long distance because of the shallow entry angle. Energy was deposited at linear densities greater than 1 kiloton per kilometer and rose to a peak value of 80 kt/km; most of the energy deposition occurred at altitudes from about 38 km down to about 23 km. It took about 4 seconds for that to happen, during which the asteroid left a 50-km-long wake of hot, expanding air and ablation products.
That slug of energetic material carried much of the original momentum of the asteroid and continued to push its way downrange as it exploded—still moving much faster than a fighter jet. Because of the long distance over which energy was deposited, the geographic pattern of the shock intensity, inferred from observed damage, looked more like an inclined cylindrical bow shock than a spherical explosion.
One might assume that a rare event like this can be generalized. But not all asteroids enter at such a shallow angle, and not all asteroids break up in stages over a long distance. There is no reason to think that a steeply entering asteroid could not break up and deposit most of its energy over a much smaller distance and yield a shock pattern on the ground that looks more like a point-source explosion. In fact, the appearance of the tree-fall pattern at Tunguska in 1908 indicates a more abrupt explosion.
Much of our understanding of airburst damage, including the concept of an “optimum height of burst” for which damage from a point-source explosion is maximized, comes from the literature on nuclear weapons effects. The damage is estimated by the area on the ground that experiences an overpressure above some threshold value. A lower-altitude source concentrates its energy into a smaller area but falls off more rapidly at a distance. The blast from a higher-altitude source diverges more before it reaches the ground. It may affect a larger area but is weaker.
The Chelyabinsk airburst caused damage out to 120 km from the supersonic path. 3 In the city of Chelyabinsk, glass was shattered and broken in more than 3600 apartment and commercial buildings (see figure 3 ), people were blown off their feet, and well over 1000 injuries were reported. Based on pre-Chelyabinsk scaling, blast damage over an area from 100 km 2 to 1000 km 2 is expected for a half-megaton explosive event.
Figure 3. The airburst damaged buildings throughout Chelyabinsk, shattering windows and, in one case, causing the wall of a zinc factory to collapse. This photograph was taken inside the glass-strewn lobby of the local drama theater. (Photo by Nikita Plekhanov.)
The damage could have been far worse for two main reasons: The explosion occurred well above its optimum height of burst, and it was not a point-source but a “line-source” explosion. For some combination of entry angle and strength, an asteroid can effectively deliver its energy at its optimum height of burst. Fortunately, that did not happen in Chelyabinsk.
The 1994 impact of comet Shoemaker–Levy 9 on Jupiter yielded enormous insight on hypervelocity-airburst physics and has informed much of what we understand about the threat from space. For example, the phenomenon of ballistic plumes, predicted just prior to impact, 11 was observed for the first time. Large parcels of air and comet debris were ejected above the cloud tops to an altitude of 3000 km before smashing back into Jupiter’s atmosphere. When the comet entered it broke up and was ablated by much the same process observed in the Chelyabinsk event, but it approached at the much steeper angle of 45°. Its wake was more vertical and better aligned with the atmospheric density gradient, so the expanding linear explosion was boosted backward along the same path into a suborbital trajectory. Numerical modeling suggests that smaller airbursts on Earth, such as the one at Tunguska, should launch similar ballistic plumes to altitudes greater than a few hundred kilometers. 12 Such airbursts would thus pose a risk to satellites.
Because the Chelyabinsk asteroid arrived on a shallow-angle trajectory, such a plume did not form. However, one of the most stunning and unexpected sights was the splitting of the wake into bilateral, contrarotating vortices containing asteroid vapor that condensed and made them visible, separated by a thin band of clear air with spots of blue sky. Preliminary models suggest that the splitting was driven by buoyancy. Immediately after entry, the wake approximated a very hot cylinder of expanding gas. The center of the cylinder rose faster than the edges, which caused both sides of the rising parcel to rotate outward in a way analogous to the toroidal vortex surrounding the buoyant fireball in a nuclear explosion. The photograph on page 32 illustrates the phenomenon seconds after the asteroid passed overhead, and figure 4 outlines the trails’ time evolution.
Figure 4. This computational simulation of the evolution of the asteroid’s atmospheric wake was adapted from code developed at Sandia National Laboratories for modeling nuclear explosions. The simplified two- dimensional model assumes that energy is deposited in a horizontal cylinder of air, which expands, rises buoyantly, and separates into two contrarotating vortices by a mechanism similar to that which forms a toroidal vortex in the mushroom cloud from a nuclear airburst. Each panel is a cross-sectional slice at the same location along the meteor’s path, for different times since the energy was deposited. Temperatures range from several hundred kelvin (red) to a few thousand kelvin (yellow). (For a movie of the simulation , see the online version of this article.)
The Chelyabinsk airburst was roughly two orders of magnitude more energetic than the approximately 10-kt Sikhote-Alin asteroid event of 1947 and roughly an order of magnitude less energetic than the 3- to 15-MT Tunguska blast of 1908. The considerable uncertainty in estimates of the energy of those and other relatively small impact events makes it difficult to quantify future hazards. For that reason, the Chelyabinsk impact event is incredibly important: It produced the first high-precision values for the energy of a blast—see the box on this page—and the ground damage it caused.
Fortunately, Earth’s atmosphere screens most objects from reaching the surface with cosmic velocities. Some objects are large enough and strong enough, however, to penetrate deeply into the atmosphere before exploding in an airburst or reaching Earth’s surface to produce a crater.
Although stony near-Earth asteroids dominate the impact flux, nearly all of the smallest craters on Earth were produced by iron asteroids. Small stony asteroids, like Chelyabinsk, preferentially produce airbursts before reaching the ground. Small iron asteroids, however, survive atmospheric passage and cause explosions at the surface that excavate impact craters. The most famous is the 49 000-year-old Barringer meteorite crater in northern Arizona. That breathtaking impact site was produced by a roughly 30-m-diameter iron asteroid, remnants of which are known as the Canyon Diablo meteorites.
Although the Barringer crater’s 1.2-km diameter is relatively small compared with some of Earth’s largest, such as the dinosaur-killing Chicxulub impact crater, 180 km in diameter, the kinetic energy of Barringer’s impact is sufficient to destroy a modern city. 13 The pressure pulse and airblast can be devastating. The blast was immediately lethal for human-sized animals within 6 km of it. Those within 10–12 km suffered severe lung damage from the pressure pulse alone. Winds in excess of 1500 km/h were produced within the inner 6-km-diameter zone and still exceeded 100 km/h at radial distances of 20 km.
With Chelyabinsk, scientists can, for the first time, link the damage from an impact event to a well- determined impact energy in order to assess the future hazards of asteroids to lives and property. Using methods of quantitative risk assessment, one can estimate the range of probabilities of various events and the consequences of those events. Asteroid impacts represent a classic low-probability, high-consequence risk: very unlikely but potentially catastrophic. 14 Moreover, the greatest contributor to long-term risk is from the most improbable but largest impacts, which can lead to civilization collapse or even human extinction. Fortunately, the largest are also the easiest to discover, and about 90% of nearby objects greater than 1 km in diameter have been cataloged. And because none are on a collision course, the assessed risk from large asteroids has dropped since the survey began by more than an order of magnitude.
The remaining risk comes from smaller, crater-forming impacts and airbursts. Most of the objects in that size range remain undiscovered, and the impact community’s assessment of the airburst risk has increased for two reasons. First, the vast majority of asteroids enter the atmosphere at steeper angles than Chelyabinsk and do more damage on the ground than nuclear explosions of the same energy; 15 our understanding of the risk assessment has progressively increased since the original assessments of the early 1990s. Second, the occurrence of the Chelyabinsk event and other, remotely detected airbursts over the last few decades has caused some scientists to question astronomically based estimates of the frequency of large airbursts. According to some studies, that frequency may be underestimated by as much as an order of magnitude. 2
The impact community’s asteroid risk mitigation strategy has consisted of three parts: Understand the impact process through experiments, field studies of craters, laboratory analyses of meteorites, and computer modeling; survey, track, and characterize asteroids in Earth-crossing orbits; and develop the means to deflect an asteroid, if one is found with sufficient warning on a collision course.
As a small object that approached Earth from the Sun, Chelyabinsk struck our planet with no prior warning. The desire for some warning about future events, particularly if they are more energetic, implies a fourth component to the strategy—that scientists develop the tools required to evaluate the consequences of an unavoidable impact and develop rapid communication protocols with the appropriate civil authorities. They would be the ones having to make some difficult decisions.
Most of the kinetic energy released during the collision of the Chelyabinsk asteroid with Earth was produced during its midair explosion. To estimate the size of the explosion and thus the kinetic energy, researchers used four sources: 2 the energy of infrasonic airwaves from the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty (see the article by Matthias Auer and Mark Prior on page 39 in this issue of Physics Today ); the energy of Rayleigh “ground roll” seismic waves generated when the airburst shock wave hit Earth’s surface; the energy of radiated light derived from more than 400 video camera and smartphone images; and radiated energy measured by US government satellites.
The total energy yield determined by those sources ranged from the explosive equivalent of 200–990 kilotons of TNT, where 1 kt = 4.184 × 10 12 joules. The best impact-energy estimate in that range is 500 ± 100 kt, which is about 25 times the yield of Trinity, the first atomic explosion.
David Kring is a senior staff scientist at the Lunar and Planetary Institute and principal investigator of the LPI’s Center for Lunar Science and Exploration in Houston, Texas. Mark Boslough is a physicist at Sandia National Laboratories in Albuquerque, New Mexico. To read his interview with Physics Today, see the online Singularities column “A passion for asteroids,” August 2014.
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