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On 12 November 2014, during the descent of the Rosetta lander Philae to the surface of comet 67P/Churyumov–Gerasimenko the Dust Impact Monitor (DIM) on board Philae recorded an impact of a cometary dust impact of a cometary dust particle at 2.4 km from the comet surface (5 km from the nucleus’ barycentre). In this work, we report further experiments that support the identification of this particle. We use aerogel as a comet analog material to characterise the properties of this particle. Our experiments show that this particle has a radius of 0.9 mm, a low density of 0.25 g/cm 3 and a high porosity close to 90%. The particle likely moved at near 4 m/s with respect to the comet. [ABSTRACT FROM AUTHOR]

The article discusses natural hazards and disasters that may cause the extermination of the human species. Topics discussed include geomagnetic storms that can be triggered by the eruption of coronal mass ejection (CME) from the sun which could destroy of thousands of power transformers, the prevention of collision from large asteroids or comets, and the explosive eruption of a supervolcano.

The article discusses the potential consequences and likelihood of the appearance of centaurs, giant comets from the trans-Neptunian region that reach the inner solar system via short-term, in the Earth. Topics include assessments of the risk posed by near-Earth objects (NEO) and the enhancement of their population, the disintegration of centaurs, and stratigraphic studies of craters in Earth.

The article explores the development of a strategic nuclear weapons for planetary defense on small meteoroids and space debris. Topics discussed include an overview on the planetary significant threat to the planet, the scientific study on the asteroid and comet population to protect the plant from collision and the development of collision mitigation techniques. An overview of the U.S. federal government's study on nuclear weapons complex is also explained.

Asteroids and comets have collided with the Earth in the past and will do so again in the future. Throughout Earth׳s history these collisions have played a significant role in shaping Earth׳s biological and geological histories. The planetary defense community has been examining a variety of options for mitigating the impact threat of asteroids and comets that approach or cross Earth׳s orbit, known as near-Earth objects (NEOs). This paper discusses the preliminary study results of selecting small (100-m class) NEO targets and mission analysis and design trade-offs for validating the effectiveness of a Hypervelocity Asteroid Intercept Vehicle (HAIV) concept, currently being investigated for a NIAC (NASA Advanced Innovative Concepts) Phase 2 study. In particular this paper will focus on the mission analysis and design for single spacecraft direct impact trajectories, as well as several mission types that enable a secondary rendezvous spacecraft to observe the HAIV impact and evaluate it׳s effectiveness. [ABSTRACT FROM AUTHOR]

A recrystallized band of pale feldspathic impact melt in a gneissic impact breccia from the approximately 10 km Paasselkä impact structure in southeast Finland was dated via 40Ar/39Ar step-heating. The newly obtained plateau age of 228.7 ± 1.8 (2.2) Ma (2σ) ( MSWD = 0.32; p = 0.93) is equal to the previously published pseudoplateau age of 228.7 ± 3.0 (3.4) (2σ) for the impact event. According to the current international chronostratigraphic chart and using the most recent published suggestions for the K decay constants, a Carnian (Late Triassic) age for the Paasselkä impact structure of 231.0 ± 1.8 (2.2) Ma (2σ) is calculated and considered the most precise and accurate age for this impact structure. The new plateau age for Paasselkä confirms the previous dating result but is, based on its internal statistics, much more compelling. [ABSTRACT FROM AUTHOR]

The strong 14C increase in the year AD 774/5 detected in one German and two Japanese trees was recently suggested to have been caused by an impact of a comet onto Earth and a deposition of large amounts of 14C into the atmosphere (Liu et al. 2014). The authors supported their claim using a report of a historic Chinese observation of a comet ostensibly colliding with Earth's atmosphere in AD 773 January. We show here that the Chinese text presented by those authors is not an original historic text, but that it is comprised of several different sources. Moreover, the translation presented in Liu et al. is misleading and inaccurate. We give the exact Chinese wordings and our English translations. According to the original sources, the Chinese observed a comet in mid January 773, but they report neither a collision nor a large coma, just a long tail. Also, there is no report in any of the source texts about 'dust rain in the daytime' as claimed by Liu et al. (2014), but simply a normal dust storm. Ho (1962) reports sightings of this comet in China on AD 773 Jan 15 and/or 17 and in Japan on AD 773 Jan 20 (Ho 1962). At the relevant historic time, the Chinese held that comets were produced within the Earth's atmosphere, so that it would have been impossible for them to report a 'collision' of a comet with Earth's atmosphere. The translation and conclusions made by Liu et al. (2014) are not supported by the historical record. Therefore, postulating a sudden increase in 14C in corals off the Chinese coast precisely in mid January 773 (Liu et al. 2014) is not justified given just the 230Th dating for AD 783 ± 14. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

A kinetic impactor spacecraft is a viable method to deflect an asteroid which poses a threat to the Earth. The technology to perform such a deflection has been demonstrated by the Deep Impact (DI) mission, which successfully collided with comet Tempel 1 in July 2005 using an onboard autonomous navigation system, called AutoNav, for the terminal phase of the mission. In this paper, we evaluate the ability of AutoNav to impact a wider range of scenarios that a deflection mission could encounter, varying parameters such as the approach velocity, phase angle, size of the asteroid, and the attitude determination accuracy. In particular, we evaluated the capability of AutoNav to impact 100–300 m size asteroids at speeds between 7.5 and 20 km/s at various phase angles. Using realistic Monte Carlo simulations, we tabulated the probability of success of the deflection as a function of these parameters and find the highest sensitivity to be due to the spacecraft attitude determination error. In addition, we also specifically analyzed the impact probability for a proposed mission (called ISIS) which would send an impactor to the asteroid 1999RQ36. We conclude with some recommendations for future work. [ABSTRACT FROM AUTHOR]

A large and sudden increase in radiocarbon (14C) aroundAD773 are documented in coral skeletons from the South China Sea. The 14C increased by ~15%during winter, and remain elevated for more than 4 months, then increased and dropped down within two months, forming a spike of 45%high in late spring, followed by two smaller spikes. The 14C anomalies coincide with an historic comet collision with the Earth's atmosphere on 17 January AD 773. Comas are known to have percent-levels of nitrogen by weight, and are exposed to cosmic radiation in space. Hence they may be expected to contain highly elevated 14C/12C ratios, as compared to the Earth's atmosphere. The significant input of 14C by comets may have contributed to the fluctuation of 14C in the atmosphere throughout the Earth's history, which should be considered carefully to better constrain the cosmic ray fluctuation. [ABSTRACT FROM AUTHOR]

The paper analyzes the disintegration of small cosmic bodies in a planetary atmosphere and most important facts observed during the fall of the Chelyabinsk superbolide: its speed loss during passage through the upper atmosphere, its strength and the character of destruction, the altitude of its explosion, and the energy release. Detailed data are presented on the aerodynamic phenomena accompanying the supersonic atmospheric entry and destruction of the superbolide. The strength of the original meteorite is evaluated as a function of its initial disintegration altitude. Principal data obtained on the collision between comet Shoemaker-Levy 9 and Jupiter are reported. [ABSTRACT FROM AUTHOR]

The article discusses threats posed by the possible collision of asteroids and comets with Earth and strategies to detect and neutralize these threats. It comments on the asteroid Apophis (2004-MN4), which was discovered in 2004, and the possibility of it colliding with Earth. The authors consider the need for research on the physical and chemical properties of asteroids and comets. Other topics include the cost and feasibility of deterring threats from asteroids and comets, international scientific and governmental cooperation, and the possible use of resources and minerals found on asteroids.

Abstract: The Younger Dryas (YD) impact hypothesis is a recent theory that suggests that a cometary or meteoritic body or bodies hit and/or exploded over North America 12,900years ago, causing the YD climate episode, extinction of Pleistocene megafauna, demise of the Clovis archeological culture, and a range of other effects. Since gaining widespread attention in 2007, substantial research has focused on testing the 12 main signatures presented as evidence of a catastrophic extraterrestrial event 12,900years ago. Here we present a review of the impact hypothesis, including its evolution and current variants, and of efforts to test and corroborate the hypothesis. The physical evidence interpreted as signatures of an impact event can be separated into two groups. The first group consists of evidence that has been largely rejected by the scientific community and is no longer in widespread discussion, including: particle tracks in archeological chert; magnetic nodules in Pleistocene bones; impact origin of the Carolina Bays; and elevated concentrations of radioactivity, iridium, and fullerenes enriched in 3He. The second group consists of evidence that has been active in recent research and discussions: carbon spheres and elongates, magnetic grains and magnetic spherules, byproducts of catastrophic wildfire, and nanodiamonds. Over time, however, these signatures have also seen contrary evidence rather than support. Recent studies have shown that carbon spheres and elongates do not represent extraterrestrial carbon nor impact-induced megafires, but are indistinguishable from fungal sclerotia and arthropod fecal material that are a small but common component of many terrestrial deposits. Magnetic grains and spherules are heterogeneously distributed in sediments, but reported measurements of unique peaks in concentrations at the YD onset have yet to be reproduced. The magnetic grains are certainly just iron-rich detrital grains, whereas reported YD magnetic spherules are consistent with the diffuse, non-catastrophic input of micrometeorite ablation fallout, probably augmented by anthropogenic and other terrestrial spherular grains. Results here also show considerable subjectivity in the reported sampling methods that may explain the purported YD spherule concentration peaks. Fire is a pervasive earth-surface process, and reanalyses of the original YD sites and of coeval records show episodic fire on the landscape through the latest Pleistocene, with no unique fire event at the onset of the YD. Lastly, with YD impact proponents increasingly retreating to nanodiamonds (cubic, hexagonal [lonsdaleite], and the proposed n-diamond) as evidence of impact, those data have been called into question. The presence of lonsdaleite was reported as proof of impact-related shock processes, but the evidence presented was inconsistent with lonsdaleite and consistent instead with polycrystalline aggregates of graphene and graphane mixtures that are ubiquitous in carbon forms isolated from sediments ranging from modern to pre-YD age. Important questions remain regarding the origins and distribution of other diamond forms (e.g., cubic nanodiamonds). In summary, none of the original YD impact signatures have been subsequently corroborated by independent tests. Of the 12 original lines of evidence, seven have so far proven to be non-reproducible. The remaining signatures instead seem to represent either (1) non-catastrophic mechanisms, and/or (2) terrestrial rather than extraterrestrial or impact-related sources. In all of these cases, sparse but ubiquitous materials seem to have been misreported and misinterpreted as singular peaks at the onset of the YD. Throughout the arc of this hypothesis, recognized and expected impact markers were not found, leading to proposed YD impactors and impact processes that were novel, self-contradictory, rapidly changing, and sometimes defying the laws of physics. The YD impact hypothesis provides a cautionary tale for researchers, the scientific community, the press, and the broader public. [Copyright &y& Elsevier]

Delivery of prebiotic compounds to early Earth from an impacting comet is thought to be an unlikely mechanism for the origins of life because of unfavourable chemical conditions on the planet and the high heat from impact. In contrast, we find that impact-induced shock compression of cometary ices followed by expansion to ambient conditions can produce complexes that resemble the amino acid glycine. Our ab initio molecular dynamics simulations show that shock waves drive the synthesis of transient C-N bonded oligomers at extreme pressures and temperatures. On post impact quenching to lower pressures, the oligomers break apart to form a metastable glycine-containing complex. We show that impact from cometary ice could possibly yield amino acids by a synthetic route independent of the pre-existing atmospheric conditions and materials on the planet. [ABSTRACT FROM AUTHOR]

review of the current state of investigations into the complex global problem of asteroid-comet hazard (ACH) is presented. Main attention is drawn to a comprehensive analysis of prospects of developing the intra-Russia cooperation of studies on the ACH problem and initiating a working program of solving this problem on the national basis. Special space projects are briefly discussed, aimed to investigate and solve the ACH problem, in particular, the project of a mission of spacecraft with an onboard transponder to a hazardous asteroid (conventionally, asteroid Apophis is chosen as a target). [ABSTRACT FROM AUTHOR]

Abstract: In the current view of planet formation, the final assembly of the Earth involved giant collisions between proto-planets (>1000km radius), with the Moon formed as a result of one such impact. At this stage the colliding bodies had likely differentiated into a metallic core surrounded by a silicate mantle. During the Moon-forming impact, nearly all metal sank into the Earth''s core. We investigate to what extent large self-gravitating iron cores can mix with surrounding silicate and how this influences the short-lived chronometer, Hf–W, used to infer the age of the Moon. We present fluid dynamical models of turbulent mixing in fully liquid systems, attempting to place constraints on the degree of mixing. Erosion of sinking cores driven by Rayleigh–Taylor instability does lead to intimate mixing and equilibration, but large blobs (>10km diameter) do not emulsify entirely. Emulsification is enhanced if most of the accreting metal cores deform into thin structures during descent through the Earth''s mantle. Yet, only 1–20% of Earth''s core would equilibrate with silicate during Earth''s accretion. The initial speed of the impactor is of little importance. We proceed to evaluate the mixing potential for shear instabilities where silicate entrainment across vertical walls causes mixing. The turbulent structure indicates that vortices remain at the largest scale and do not mix to centimeter length scale, where diffusion operates and isotopes can equilibrate. Thus, incomplete emulsification and equilibration of accreting iron cores is likely to occur. The extent of metal–silicate equilibration provides key information for interpretation of siderophile budgets and the timing of core formation using the Hf–W chronometer. The time scale of core formation derived from the Hf–W chronometer is usually tied to the last major metal–silicate re-equilibration, believed to coincide with time of the Moon-forming impact. However, we show that large cores have limited ability to reset the Hf–W system in the silicate Earth. Excess 182W in bulk silicate Earth is more sensitive to early core formation processes than to radiogenic ingrowth after the last giant impact. [Copyright &y& Elsevier]

Abstract: The Younger Dryas event is by far the best studied of the millennial-scale cold snaps of glacial time. Yet its origin remains a subject of debate. The long-held scenario that the Younger Dryas was a one-time outlier triggered by a flood of water stored in proglacial Lake Agassiz has fallen from favor due to lack of a clear geomorphic signature at the correct time and place on the landscape. The recent suggestion that the Younger Dryas was triggered by the impact of a comet has not gained traction. Instead, evidence from Chinese stalagmites suggests that, rather than being a freak occurrence, the Younger Dryas is an integral part of the deglacial sequence of events that produced the last termination on a global scale. [Copyright &y& Elsevier]

The article discusses the study on predicting the collision of natural cosmic bodies with the Earth's atmosphere and surface basing on the solution to a meteor physics solution. The solution depends on two parameters of the drag altitude and the role of mass loss on the meteor body while moving in the atmosphere. The study shows that preliminary estimates of natural cosmic body's parameter in the near-earth orbits provides reliable prediction on the body's amplitude of collision with Earth.

Based on elevated concentrations of a set of "impact markers" at the onset of the Younger Dryas stadial from sedimentary contexts across North America, Firestone, Kennett, West, and others have argued that 12.9 ka the Earth experienced an impact by an extraterrestrial body, an event that had devastating ecological consequences for humans, plants, and animals in the New World [Firestone RB, et al. (2007) Proc. Natl. Acad. Sd. USA 104:16016- 16021]. Herein, we report the results of an independent analysis of magnetic minerals and microspherules from seven sites of similar age, including two examined by Firestone et al. We were unable to reproduce any results of the Firestone et al. study and find no support for Younger Dryas extraterrestrial impact. [ABSTRACT FROM AUTHOR]

Over the past decade the risk of asteroids and comets colliding with Earth has emerged as one of the most significant natural hazards faced by the terrestrial biosphere. Ongoing studies of comets and asteroids, their distribution and dynamics, the cratering record on Earth and elsewhere in the solar system, all point to a long-term risk whose magnitude exceeds many others that generate substantial international concern. The origins and composition of the solar system, and particularly potentially hazardous asteroids and comets are described with references to meteorites and impact structures. Additionally, the nature and extent of the NEO (Near-Earth Objects) impact hazard, and the measures that are required to reduce the risk are considered. [ABSTRACT FROM AUTHOR]

The threat of collision from asteroids and comets calls for a three-step approach to mitigating the risks: first, find and track objects that are potentially hazardous to the Earth; second, study their characteristics so as to understand which mitigation schemes are likely to be effective; and third, test various deflection techniques to ascertain the best way to adjust the orbits of asteroids and comets, and possibly field a planetary-defence system. Each of these steps would benefit from international cooperation or agreement. [ABSTRACT FROM AUTHOR]

The article provides a discussion on the study of near-earth objects (NEOs). NEOs include comets and asteroids which have the possibility of crashing into earth. NEOs that pass 7.5 million kilometers within the Earth's orbit must be detected immediately. On December 2004, the asteroid Apophis was estimated to collide with Earth on April 2029. Additional studies moved the date to April 13, 2036. The incident prompted the development of measures to track NEOs including the completion of the Panoramic Survey Telescope and Rapid Response System. An overview of efforts to track NEOs is provided.

Based on the model concepts of destruction of the substance of a celestial body in the shock wave initiated by a strong explosion of weak penetration, the radius of the destroyed region, the ejected mass, and the recoil momentum have been evaluated. The values of the charges necessary for destroying completely bodies of different size and composition or withdrawing bodies from the Earth for a required distance have been determined. The efficiencies of the explosion and sublimation methods of changing orbits for the case where the hazardous bodies are comets have been compared. Problems on improving the efficiency of the explosion method of action on a hazardous body due to the high relative velocity of impact of this body with the charge-carrying rocket have been discussed. [ABSTRACT FROM AUTHOR]

On 4 July 2005, many observatories around the world and in space observed the collision of Deep Impact with comet 9P/Tempel 1 or its aftermath. This was an unprecedented coordinated observational campaign. These data show that (i) there was new material after impact that was compositionally different from that seen before impact; (ii) the ratio of dust mass to gas mass in the ejecta was much larger than before impact; (iii) the new activity did not last more than a few days, and by 9 July the comet's behavior was indistinguishable from its pre-impact behavior; and (iv) there were interesting transient phenomena that may be correlated with cratering physics. [ABSTRACT FROM AUTHOR]

Using the cometary nucleus model developed by Espinasse et al., we calculate the thermodynamical evolution of Comet 9P/Tempel 1 over a period of 360 yr. Starting from an initially amorphous cometary nucleus which incorporates an icy mixture of H2O and CO, we show that, at the time of Deep Impact collision, the crater is expected to form at depths where ice is in its crystalline form. Hence the subsurface exposed to space should not be primordial. We also attempt an order-of-magnitude estimate of the heating and material ablation effects on the crater activity caused by the 370-kg projectile released by the Deep Impact spacecraft. We thus show that heating effects play no role in the evolution of crater activity. We calculate that the CO production rate from the impacted region should be about 300–400 times higher from the crater resulting from the impact with a 35-m ablation than over the unperturbed nucleus in the immediate post-impact period. We also show that the H2O production rate is decreased by several orders of magnitude at the crater base just after ablation. [ABSTRACT FROM AUTHOR]

A large extraterrestrial object striking Earth at cosmic velocity melts and vaporizes silicate materials, which can condense into highly spheroidal, sand-size particles that get deposited hundreds to thousands of kilometers from the point of impact. These particles, known as impact spherules, have been detected in great abundance in a relatively small number of thin, discrete layers ranging in age from less than a million years to 3.47 billion years. Unaltered impact spherules consist entirely of glass (microtektites) or a combination of glass and crystals grown in flight (microkrystites). Impact spherule layers form very rapidly and can be very extensive, even global in extent [e.g., the Cretaceous-Tertiary (K/T) boundary layer], so they form excellent time-stratigraphic markers. Because they are always found in a stratigraphic context, spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts. A record of impacts whose craters have since been obliterated, most notably those in pre-Mesozoic oceanic crust, could survive in the form of spherule layers. Secular changes in surface environments and/or the nature of the impactors striking Earth through its history could also be reflected in differences in spherules and spherule layers as a function of geologic age. In this paper, we briefly review what spherules and spherule layers are and the processes that create them, then speculate about what might be learned through wider identification of and more extensive study of impact spherule layers. [ABSTRACT FROM AUTHOR]

Predictions of future potential Earth impacts by near-Earth objects (NEOs) have become commonplace in recent years, and the rate of these detections is likely to accelerate as asteroid survey efforts continue to mature. In order to conveniently compare and categorize the numerous potential impact solutions being discovered we propose a new hazard scale that will describe the risk posed by a particular potential impact in both absolute and relative terms. To this end, we measure each event in two ways, first without any consideration of the event''s time proximity or its significance relative to the so-called background threat, and then in the context of the expected risk from other objects over the intervening years until the impact. This approach is designed principally to facilitate communication among astronomers, and it is not intended for public communication of impact risks. The scale characterizes impacts across all impact energies, probabilities and dates, and it is useful, in particular, when dealing with those cases which fall below the threshold of public interest. The scale also reflects the urgency of the situation in a natural way and thus can guide specialists in assessing the computational and observational effort appropriate for a given situation. In this paper we describe the metrics introduced, and we give numerous examples of their application. This enables us to establish in rough terms the levels at which events become interesting to various parties. [Copyright &y& Elsevier]

We consider a small sample of known near Earth objects (NEOs), both asteroids and comets, with low minimum orbital intersection distance (MOID). Through a simple numerical procedure we generate slightly different orbits from this sample in such a way that these bodies will collide with the Earth at a specific epoch. Then we study the required change in orbital velocity (along track Δv) in order to deflect these NEOs at different epochs before the impact event. The orbital evolution of these NEOs is performed through a full N-body numerical integrator. A comparison with analytical estimates is also performed in selected cases. Interesting features in the Δv/time before impact plots are found; as a prominent result, we find that close approaches to the Earth before the epoch of the impact can make the overall deflection easier. [Copyright &y& Elsevier]

Time variation in impact probability is studied by assuming that the periodic flux of the Oort Cloud comets within 15 au arises from the motion of the Sun with respect to the Galactic mid-plane. The periodic flux clearly shows up in the impact rate of the captured Oort Cloud cometary population, with a phase shift caused by the orbital evolution. Depending on the assumed flux of comets and the size distribution of comets, the impact rate of the Oort Cloud comets of 1 km in diameter or greater is from 5 to 700 impacts Myr[sup -1] on the Earth and from 0.5 to 70 impacts per 1000 yr on Jupiter. The relative fractions of impacts are 0.09, 0.11, 0.26 and 0.54 for long-period comets, Halley type comets, Jupiter family comets and near-Earth objects, respectively. For Jupiter, the corresponding fractions in the first three categories are 0.18, 0.31 and 0.51. If we consider physical fading of comet activity that is compatible with the observations, then the impact rates of active comets are two orders of magnitude smaller than the total impact rates by all kinds of comets and cometary asteroids of size 1 km or greater. [ABSTRACT FROM AUTHOR]

Presents a realistic assessment of the threat to Earth from Near-Earth Objects. Earth's long and violent history of collisions with extraterrestrial bodies such as asteroids and comet nuclei; Effects of such impacts on the environment, living things, weather and geography; Existence of technology to predict and prevent such events; Absence of co-ordinated international response to such event.