Calibrating Asteroid Impact Clark R. Chapman Science 342, 1051 (2013); DOI: 10.1126/science.1246250
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of December 4, 2013 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/342/6162/1051.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/342/6162/1051.full.html#related This article cites 5 articles, 2 of which can be accessed free: http://www.sciencemag.org/content/342/6162/1051.full.html#ref-list-1
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2013 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on December 4, 2013
This copy is for your personal, non-commercial use only.
PERSPECTIVES ASTRONOMY
Calibrating Asteroid Impact Clark R. Chapman
The airburst over the Russian city of Chelyabinsk earlier this year provides a calibration point to assess the possible damage due to asteroid strikes.
CREDIT: KYODO VIA AP IMAGES
A
n asteroid impact on Earth about 65 million years ago caused a mass extinction, opening an opportunity for mammals and, eventually, human beings to evolve. We could suffer the dinosaurs’ fate this century, but chances are extremely tiny. More realistic, though less catastrophic, threats come from much more numerous but much smaller near-Earth asteroids (NEAs). NEA impacts with the potential to kill millions of people, like the very largest floods, earthquakes, and hurricanes, occur far less than 1% as often as such natural terrestrial calamities. Indeed, truly dangerous NEA impacts, discounting mere meteorites that puncture roofs, occur so rarely that none have been reliably observed until this year. On page 1069, Popova et al. (1) describe the impact and atmospheric explosion of a 20-m-wide NEA over the Russian city of Chelyabinsk (population 1.2 million) on 15 February 2013, the first NEA impact disaster in modern history. An order of magnitude more energetic airburst in 1908 over the remote Tunguska region of Siberia was witnessed by few people, but two decades elapsed before the first scientific expedition reached the site. Chelyabinsk, with an explosive energy of half-amegaton of TNT, is the largest known impact since Tunguska (although one or two larger remote impacts might have been missed or marginally detected many decades ago). Despite scientific studies of the impact hazard (2), the impact frequencies of small NEAs (1 to 100 m in diameter) remain poorly known, and the potential lethality and damage to infrastructure and the environment remain speculative, relying on theory based on only partially relevant data from 1960s nuclear tests and the 1994 impacts of fragments of Comet Shoemaker-Levy 9 into Jupiter. Thus, Chelyabinsk presents the first opportunity to calibrate the impact hazard by direct observations of the consequences. Instead of waiting two decades to get to the site, researchers (including many among the 59 in Popova et al.’s “Chelyabinsk Airburst Consortium”) arrived within hours or days, and returned during subsequent weeks and Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, CO 80302, USA. E-mail: [email protected]
Making an impact. A photo of the point of impact in the frozen lake from which a half-ton fragment of the meteorite that exploded in the skies of Chelyabinsk was later recovered.
months, to collect data while meteorites were still in the snow and eyewitness memories were still sharp. Chelyabinsk alone cannot greatly improve our knowledge of impact frequencies, but it does augment suspicions that Earth is being struck more frequently than we thought. After all, Tunguska, estimated to be a oncein-1000-year event, occurred just over a century ago and now a 20-m NEA just struck, thought to be a once-in-100-year event. There are other indications of higher impact frequencies, but they are counterbalanced by extrapolations from the recent Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) survey of NEAs (3) suggesting fewer small NEAs. The prime contribution of Popova et al.’s study of the Chelyabinsk airburst is direct observation of the damage. A NASA study (4) had concluded that airbursts by NEAs up to 10 times more massive than Chelyabinsk would be essentially harmless. It was argued (5) that those predictions, based on stationary nuclear tests, ignored the downward momentum of an impacting NEA, which causes its airburst to descend dangerously toward the ground.
Actually, Chelyabinsk struck at an unusually shallow angle, just 18°, so the airburst at ~30 km altitude had little downward momentum and damage was widely distributed along and to the sides of a path roughly 100 km long. Impact at a more typical, higher angle, would have concentrated more serious damage near ground zero. Therefore, emergency management officials should note that any predicted impacts by NEAs even several times less massive than Chelyabinsk should be taken very seriously, perhaps meriting evacuation. Although Chelyabinsk-sized impacts now happen without warning (only 0.01% of such small NEAs have been discovered), at least one soon-to-be-operational search program, Asteroid Terrestrial-Impact Last Alert System (ATLAS) (6), can potentially discover up to half of small NEAs as they approach Earth, providing perhaps days to weeks of advance warning. The Chelyabinsk airburst killed nobody, despite occurring over an unusually densely populated region. Nevertheless, over 1600 people requested medical assistance at hospitals (~0.1% of the population). Several times as many suffered less serious injuries (sunburn
www.sciencemag.org SCIENCE VOL 342 29 NOVEMBER 2013 Published by AAAS
1051
PERSPECTIVES and temporary blindness or deafness), according to in-person interviews in ~50 towns and Internet-questionnaire surveys (1754 respondents, mostly from the city). The worst injuries were cuts and bruises from flying glass. Some people noticed the brilliant flash (many times brighter than the Sun), rushed to windows, perhaps remaining to gaze at the contrail, and were unprepared for the shock wave that arrived a couple of minutes later. Over 7300 buildings were damaged, mainly with broken windows. Popova et al. collected abundant, valuable reports of observations (sounds and smells) and damage for social scientists, engineers, and emergency managers to study. The Chelyabinsk event was surely a disaster, comparable in casualties and damage to some events that in the United States are presidentially declared disasters. However, it was modest in scale compared with other headline-grabbing disasters like the November 2013 typhoon in the Philippines. Small NEA impacts remain a tiny natural hazard. However, because time and location of a future impact may be predicted with great accuracy, the local emergency manager can now know from Chelyabinsk the potential dangers and can take prudent measures.
Popova et al. also describe the physical and compositional characteristics of the recovered meteorites. Mineralogical, compositional, and isotopic analyses demonstrate that Chelyabinsk is an unusually strongly shocked LL chondrite. Although just 8% of ordinary chondrites (the most common meteorite in museums) are of the LL (ironpoor) type, they are abundant among NEAs [e.g., Itokawa, from which dust samples were returned (7)] and in the inner asteroid belt. Popova et al. determine a trajectory for the Chelyabinsk asteroid consistent with its being derived from the large Flora family of asteroids in the inner belt. They suggest that Chelyabinsk may have been unusually weak and more heterogeneous than most meteorites, but all research groups must compare notes and converge on a definitive description of this meteorite. The largest fragment proceeded westward toward a lake where a 7-m-wide hole appeared in the ice (see the figure). Popova et al. show a video record of it striking the lake’s surface. On 16 October 2013, the half-ton fragment was removed from a depth of 20 m. Although just a tiny fraction of the asteroid’s estimated original mass of 13,000 metric tons
has been recovered, much will be gleaned from this best-ever observed meteorite fall, thanks to security and dashboard cameras. Meanwhile, efforts proceed to find other dangerous NEAs before they find us. The privately funded Sentinel mission (8) may launch in 2018 to provide a census of 90% of NEAs larger than 140 m (and many smaller ones), and the ATLAS project may soon provide advance warning of future Chelyabinsklike impacts, which may be more dangerous, and perhaps more probable, than had previously been estimated. References 1. O. P. Popova et al., Science 342, 1069 (2013); 10.1126/ science.1242642. 2. National Research Council, Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies (National Academies Press, Washington, DC, 2010). 3. A. Mainzer et al., Astrophys. J. 743, 156 (2011). 4. Near-Earth Object Science Definition Team, Study to Determine the Feasibility of Extending the Search for Near-Earth Objects to Smaller Limiting Diameters (NASA, Washington, DC, 2003). 5. M. Boslough, D. Crawford, Int. J. Impact Eng. 35, 1441 (2008). 6. J. L. Tonry, Publ. Astron. Soc. Pac. 123, 58 (2011). 7. T. Nakamura et al., Science 333, 1113 (2011). 8. B612 Sentinel Mission; b612foundation.org/sentinelmission 10.1126/science.1246250
EVOLUTION
Protein Expression Under Pressure
Cellular protein concentrations are generally under stronger evolutionary pressure than mRNA concentrations.
Christine Vogel
W
hich is more conserved across species—the concentrations of proteins or the concentrations of the messenger RNAs (mRNAs) that encode them? When examining orthologous genes, it’s protein concentrations that are more similar to each other. This observation was first made in worm and fly (1), and later for eight organisms ranging from bacteria to yeast, plant, and human (2). However, because the measurement platforms, data sets, and cell samples were heterogeneous in these studies, it has been difficult to separate possible biological trends from technical artifacts. On page 1100 of this issue, Khan et al. (3) show that the biological trend is very real. The authors show that protein concentrations from identical cell types across three primate species are under stronger evolutionary constraints than the respective mRNA expression levels. New York University, Center for Genomics and Systems Biology, New York, NY 10003, USA. E-mail: [email protected]
1052
Khan et al. subjected lymphoblastoid cell lines from humans, chimpanzees, and rhesus macaques (five of each) to RNA sequencing and mass spectrometry–based proteomics experiments. Protein and mRNA concentrations were quantified for ~3400 proteins across at least three individuals from each species, controlling strictly for variation across replicates, ambiguous quantification, and artifacts introduced by extremely highor low-abundance genes. The authors found that the expression is more tightly controlled for orthologous proteins compared to corresponding mRNAs (see the figure). Evolutionarily, this observation seems obvious: Proteins are the cell’s workhorses, and for proper cellular function, one would expect their concentrations to be firmly set at desired levels. Khan et al. demonstrate that protein concentrations diverge at a slower rate than mRNA concentrations, suggesting higher evolutionary constraints. These constraints may be larger for some protein functions than for others. Due to mass spectrom-
etry’s bias toward high-abundance proteins, it remains to be seen how the observation holds true for less-abundant proteins, such as transcription factors. One hypothesis to explain the conservation of protein concentrations is inspired by work on the chaperone heat shock protein 90 (HSP90), which supports proper folding of protein substrates. HSP90 can act as an evolutionary capacitor and enable the accumulation of mutant proteins across a cell population to provide a repository of functional variants when needed (4). This explanation can now be extended to possible capacitor roles in gene expression regulation. On the basis of Khan et al.’s observation, protein concentrations may be assumed to vary less across a population of cells than the respective mRNA concentrations—i.e., protein concentrations are buffered—and the regulation of transcription and therefore mRNA abundances are allowed to evolve more freely. Indeed, this is what Khan et al. conclude from their data, and it is consistent with the previously
29 NOVEMBER 2013 VOL 342 SCIENCE www.sciencemag.org Published by AAAS
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of December 4, 2013 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/342/6162/1051.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/342/6162/1051.full.html#related This article cites 5 articles, 2 of which can be accessed free: http://www.sciencemag.org/content/342/6162/1051.full.html#ref-list-1
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2013 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on December 4, 2013
This copy is for your personal, non-commercial use only.
PERSPECTIVES ASTRONOMY
Calibrating Asteroid Impact Clark R. Chapman
The airburst over the Russian city of Chelyabinsk earlier this year provides a calibration point to assess the possible damage due to asteroid strikes.
CREDIT: KYODO VIA AP IMAGES
A
n asteroid impact on Earth about 65 million years ago caused a mass extinction, opening an opportunity for mammals and, eventually, human beings to evolve. We could suffer the dinosaurs’ fate this century, but chances are extremely tiny. More realistic, though less catastrophic, threats come from much more numerous but much smaller near-Earth asteroids (NEAs). NEA impacts with the potential to kill millions of people, like the very largest floods, earthquakes, and hurricanes, occur far less than 1% as often as such natural terrestrial calamities. Indeed, truly dangerous NEA impacts, discounting mere meteorites that puncture roofs, occur so rarely that none have been reliably observed until this year. On page 1069, Popova et al. (1) describe the impact and atmospheric explosion of a 20-m-wide NEA over the Russian city of Chelyabinsk (population 1.2 million) on 15 February 2013, the first NEA impact disaster in modern history. An order of magnitude more energetic airburst in 1908 over the remote Tunguska region of Siberia was witnessed by few people, but two decades elapsed before the first scientific expedition reached the site. Chelyabinsk, with an explosive energy of half-amegaton of TNT, is the largest known impact since Tunguska (although one or two larger remote impacts might have been missed or marginally detected many decades ago). Despite scientific studies of the impact hazard (2), the impact frequencies of small NEAs (1 to 100 m in diameter) remain poorly known, and the potential lethality and damage to infrastructure and the environment remain speculative, relying on theory based on only partially relevant data from 1960s nuclear tests and the 1994 impacts of fragments of Comet Shoemaker-Levy 9 into Jupiter. Thus, Chelyabinsk presents the first opportunity to calibrate the impact hazard by direct observations of the consequences. Instead of waiting two decades to get to the site, researchers (including many among the 59 in Popova et al.’s “Chelyabinsk Airburst Consortium”) arrived within hours or days, and returned during subsequent weeks and Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, CO 80302, USA. E-mail: [email protected]
Making an impact. A photo of the point of impact in the frozen lake from which a half-ton fragment of the meteorite that exploded in the skies of Chelyabinsk was later recovered.
months, to collect data while meteorites were still in the snow and eyewitness memories were still sharp. Chelyabinsk alone cannot greatly improve our knowledge of impact frequencies, but it does augment suspicions that Earth is being struck more frequently than we thought. After all, Tunguska, estimated to be a oncein-1000-year event, occurred just over a century ago and now a 20-m NEA just struck, thought to be a once-in-100-year event. There are other indications of higher impact frequencies, but they are counterbalanced by extrapolations from the recent Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) survey of NEAs (3) suggesting fewer small NEAs. The prime contribution of Popova et al.’s study of the Chelyabinsk airburst is direct observation of the damage. A NASA study (4) had concluded that airbursts by NEAs up to 10 times more massive than Chelyabinsk would be essentially harmless. It was argued (5) that those predictions, based on stationary nuclear tests, ignored the downward momentum of an impacting NEA, which causes its airburst to descend dangerously toward the ground.
Actually, Chelyabinsk struck at an unusually shallow angle, just 18°, so the airburst at ~30 km altitude had little downward momentum and damage was widely distributed along and to the sides of a path roughly 100 km long. Impact at a more typical, higher angle, would have concentrated more serious damage near ground zero. Therefore, emergency management officials should note that any predicted impacts by NEAs even several times less massive than Chelyabinsk should be taken very seriously, perhaps meriting evacuation. Although Chelyabinsk-sized impacts now happen without warning (only 0.01% of such small NEAs have been discovered), at least one soon-to-be-operational search program, Asteroid Terrestrial-Impact Last Alert System (ATLAS) (6), can potentially discover up to half of small NEAs as they approach Earth, providing perhaps days to weeks of advance warning. The Chelyabinsk airburst killed nobody, despite occurring over an unusually densely populated region. Nevertheless, over 1600 people requested medical assistance at hospitals (~0.1% of the population). Several times as many suffered less serious injuries (sunburn
www.sciencemag.org SCIENCE VOL 342 29 NOVEMBER 2013 Published by AAAS
1051
PERSPECTIVES and temporary blindness or deafness), according to in-person interviews in ~50 towns and Internet-questionnaire surveys (1754 respondents, mostly from the city). The worst injuries were cuts and bruises from flying glass. Some people noticed the brilliant flash (many times brighter than the Sun), rushed to windows, perhaps remaining to gaze at the contrail, and were unprepared for the shock wave that arrived a couple of minutes later. Over 7300 buildings were damaged, mainly with broken windows. Popova et al. collected abundant, valuable reports of observations (sounds and smells) and damage for social scientists, engineers, and emergency managers to study. The Chelyabinsk event was surely a disaster, comparable in casualties and damage to some events that in the United States are presidentially declared disasters. However, it was modest in scale compared with other headline-grabbing disasters like the November 2013 typhoon in the Philippines. Small NEA impacts remain a tiny natural hazard. However, because time and location of a future impact may be predicted with great accuracy, the local emergency manager can now know from Chelyabinsk the potential dangers and can take prudent measures.
Popova et al. also describe the physical and compositional characteristics of the recovered meteorites. Mineralogical, compositional, and isotopic analyses demonstrate that Chelyabinsk is an unusually strongly shocked LL chondrite. Although just 8% of ordinary chondrites (the most common meteorite in museums) are of the LL (ironpoor) type, they are abundant among NEAs [e.g., Itokawa, from which dust samples were returned (7)] and in the inner asteroid belt. Popova et al. determine a trajectory for the Chelyabinsk asteroid consistent with its being derived from the large Flora family of asteroids in the inner belt. They suggest that Chelyabinsk may have been unusually weak and more heterogeneous than most meteorites, but all research groups must compare notes and converge on a definitive description of this meteorite. The largest fragment proceeded westward toward a lake where a 7-m-wide hole appeared in the ice (see the figure). Popova et al. show a video record of it striking the lake’s surface. On 16 October 2013, the half-ton fragment was removed from a depth of 20 m. Although just a tiny fraction of the asteroid’s estimated original mass of 13,000 metric tons
has been recovered, much will be gleaned from this best-ever observed meteorite fall, thanks to security and dashboard cameras. Meanwhile, efforts proceed to find other dangerous NEAs before they find us. The privately funded Sentinel mission (8) may launch in 2018 to provide a census of 90% of NEAs larger than 140 m (and many smaller ones), and the ATLAS project may soon provide advance warning of future Chelyabinsklike impacts, which may be more dangerous, and perhaps more probable, than had previously been estimated. References 1. O. P. Popova et al., Science 342, 1069 (2013); 10.1126/ science.1242642. 2. National Research Council, Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies (National Academies Press, Washington, DC, 2010). 3. A. Mainzer et al., Astrophys. J. 743, 156 (2011). 4. Near-Earth Object Science Definition Team, Study to Determine the Feasibility of Extending the Search for Near-Earth Objects to Smaller Limiting Diameters (NASA, Washington, DC, 2003). 5. M. Boslough, D. Crawford, Int. J. Impact Eng. 35, 1441 (2008). 6. J. L. Tonry, Publ. Astron. Soc. Pac. 123, 58 (2011). 7. T. Nakamura et al., Science 333, 1113 (2011). 8. B612 Sentinel Mission; b612foundation.org/sentinelmission 10.1126/science.1246250
EVOLUTION
Protein Expression Under Pressure
Cellular protein concentrations are generally under stronger evolutionary pressure than mRNA concentrations.
Christine Vogel
W
hich is more conserved across species—the concentrations of proteins or the concentrations of the messenger RNAs (mRNAs) that encode them? When examining orthologous genes, it’s protein concentrations that are more similar to each other. This observation was first made in worm and fly (1), and later for eight organisms ranging from bacteria to yeast, plant, and human (2). However, because the measurement platforms, data sets, and cell samples were heterogeneous in these studies, it has been difficult to separate possible biological trends from technical artifacts. On page 1100 of this issue, Khan et al. (3) show that the biological trend is very real. The authors show that protein concentrations from identical cell types across three primate species are under stronger evolutionary constraints than the respective mRNA expression levels. New York University, Center for Genomics and Systems Biology, New York, NY 10003, USA. E-mail: [email protected]
1052
Khan et al. subjected lymphoblastoid cell lines from humans, chimpanzees, and rhesus macaques (five of each) to RNA sequencing and mass spectrometry–based proteomics experiments. Protein and mRNA concentrations were quantified for ~3400 proteins across at least three individuals from each species, controlling strictly for variation across replicates, ambiguous quantification, and artifacts introduced by extremely highor low-abundance genes. The authors found that the expression is more tightly controlled for orthologous proteins compared to corresponding mRNAs (see the figure). Evolutionarily, this observation seems obvious: Proteins are the cell’s workhorses, and for proper cellular function, one would expect their concentrations to be firmly set at desired levels. Khan et al. demonstrate that protein concentrations diverge at a slower rate than mRNA concentrations, suggesting higher evolutionary constraints. These constraints may be larger for some protein functions than for others. Due to mass spectrom-
etry’s bias toward high-abundance proteins, it remains to be seen how the observation holds true for less-abundant proteins, such as transcription factors. One hypothesis to explain the conservation of protein concentrations is inspired by work on the chaperone heat shock protein 90 (HSP90), which supports proper folding of protein substrates. HSP90 can act as an evolutionary capacitor and enable the accumulation of mutant proteins across a cell population to provide a repository of functional variants when needed (4). This explanation can now be extended to possible capacitor roles in gene expression regulation. On the basis of Khan et al.’s observation, protein concentrations may be assumed to vary less across a population of cells than the respective mRNA concentrations—i.e., protein concentrations are buffered—and the regulation of transcription and therefore mRNA abundances are allowed to evolve more freely. Indeed, this is what Khan et al. conclude from their data, and it is consistent with the previously
29 NOVEMBER 2013 VOL 342 SCIENCE www.sciencemag.org Published by AAAS
