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The threat of asteroid impacts on the Earth can be mitigated through deflection missions. The deflection capability has previously been omitted in asteroid impact risk analysis work that quantifies impact damage on the Earth. Here we investigate the effect of pre-designed deflection missions on the risk profile of the fictitious impact scenario 2019 PDC. The impact risk analysis is based on a statistical approach. We sample orbital and physical property distributions that provide a complete representation of possible impact scenarios. Three pre-designed kinetic impactor deflection missions are considered in separate simulations. The deflection mission imposes a deflection Δ V on the asteroid and the impulse enhancement factor β is calculated as part of the analysis. The post-deflection impact risk is compared to the undeflected risk situation. Variations in physical and orbital properties make the outcome of deflection missions uncertain. It can therefore be inadequate to label deflection mission outcomes in binary terms of success and failure. Instead, defining a satisfactory risk reduction goal and scaling the deflection mission capability to deliver this risk reduction is more accurate. In addition, the results highlight dilemmas that decision-makers would face in an asteroid threat scenario. By implementing a deflection mission, the potential impact point of the asteroid is moved, threatening new populations and potentially increasing damage in case of an impact.

著者人数: 69名 (所属. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS): 吉川, 真), Number of authors: 69 (Affiliation. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS): Yoshikawa, Makoto), Accepted: 2019-02-14, 資料番号: SA1190045000

The June 2, 2018, impact of asteroid 2018 LA over Botswana is only the second asteroid detected in space prior to impacting over land. Here, we report on the successful recovery of meteorites. Additional astrometric data refine the approach orbit and define the spin period and shape of the asteroid. Video observations of the fireball constrain the asteroid's position in its orbit and were used to triangulate the location of the fireball's main flare over the Central Kalahari Game Reserve. 23 meteorites were recovered. A consortium study of eight of these classifies Motopi Pan as a HED polymict breccia derived from howardite, cumulate and basaltic eucrite, and diogenite lithologies. Before impact, 2018 LA was a solid rock of about 156 cm diameter with high bulk density about 2.85 g/cm3, a relatively low albedo pV about 0.25, no significant opposition effect on the asteroid brightness, and an impact kinetic energy of about 0.2 kt. The orbit of 2018 LA is consistent with an origin at Vesta (or its Vestoids) and delivery into an Earth-impacting orbit via the nu_6 resonance. The impact that ejected 2018 LA in an orbit towards Earth occurred 22.8 +/- 3.8 Ma ago. Zircons record a concordant U-Pb age of 4563 +/- 11 Ma and a consistent 207Pb/206Pb age of 4563 +/- 6 Ma. A much younger Pb-Pb phosphate resetting age of 4234 +/- 41 Ma was found. From this impact chronology, we discuss what is the possible source crater of Motopi Pan and the age of Vesta's Veneneia impact basin., Comment: Meteoritics & Planetary Science (2021)

‘Oumuamua (1I/2017 U1) is the first known object of interstellar origin to have entered the Solar System on an unbound and hyperbolic trajectory with respect to the Sun1. Various physical observations collected during its visit to the Solar System showed that it has an unusually elongated shape and a tumbling rotation state1–4 and that the physical properties of its surface resemble those of cometary nuclei5,6, even though it showed no evidence of cometary activity1,5,7. The motion of all celestial bodies is governed mostly by gravity, but the trajectories of comets can also be affected by non-gravitational forces due to cometary outgassing8. Because non-gravitational accelerations are at least three to four orders of magnitude weaker than gravitational acceleration, the detection of any deviation from a purely gravity-driven trajectory requires high-quality astrometry over a long arc. As a result, non-gravitational effects have been measured on only a limited subset of the small-body population9. Here we report the detection, at 30σ significance, of non-gravitational acceleration in the motion of ‘Oumuamua. We analyse imaging data from extensive observations by ground-based and orbiting facilities. This analysis rules out systematic biases and shows that all astrometric data can be described once a non-gravitational component representing a heliocentric radial acceleration proportional to r−2 or r−1 (where r is the heliocentric distance) is included in the model. After ruling out solar-radiation pressure, drag- and friction-like forces, interaction with solar wind for a highly magnetized object, and geometric effects originating from ‘Oumuamua potentially being composed of several spatially separated bodies or having a pronounced offset between its photocentre and centre of mass, we find comet-like outgassing to be a physically viable explanation, provided that ‘Oumuamua has thermal properties similar to comets.

The B-plane is a fundamental tool to analyze planetary encounters of small bodies and spacecraft flybys. In this paper, we review the B-plane formulation with a full derivation of its coordinates and their partial derivatives, which allow the mapping of orbital uncertainties onto the B-plane. We find that this mapping can be sensitive to variations in the inbound asymptote, especially for low-velocity encounters, and to non-Keplerian dynamics for distant encounters. Under linearity assumptions, we show how to derive close approach boundaries and impact probabilities from the orbital uncertainty mapped onto the B-plane.

Asteroids and comets often capture and sometimes transit near a planet by traveling through the $$\hbox {L}_1$$ and $$\hbox {L}_2$$ libration point gateways, and these regions are therefore key to understanding the mechanism by which captures, transits, and some potential impacts of these bodies occur. Isolating blocks have recently been used to provide a theoretically rigorous method for computing the invariant manifolds of libration point periodic orbits in the circular restricted three-body problem (CRTBP), and for an appropriate energy range, they can allow us to compute all possible transit trajectories at a particular Jacobi constant in the CRTBP. In this study, both $$\hbox {L}_1$$ and $$\hbox {L}_2$$ isolating blocks are found for the Sun–Earth and Sun–Jupiter CRTBP systems to rigorously compute trajectories transiting near the Earth and Jupiter in the low-energy regime common for asteroids and comets. The characteristics of these transit trajectories are explored, and individual trajectory solutions are analyzed in more detail. The transit trajectories are also characterized using their orbital elements and compared to known comets and asteroids.

Near-Earth asteroid 2014 AA entered the Earth’s atmosphere on 2014 January 2, only 21 h after being discovered by the Catalina Sky Survey. In this paper we compute the trajectory of 2014 AA by combining the available optical astrometry, seven ground-based observations over 69 min, and the International Monitoring System detection of the atmospheric impact infrasonic airwaves in a least-squares orbit estimation filter. The combination of these two sources of observations results in a tremendous improvement in the orbit uncertainties. The impact time is 3:05 UT with a 1σ uncertainty of 6 min, while the impact location corresponds to a west longitude of 44.2° and a latitude of 13.1° with a 1σ uncertainty of 140 km. The minimum impact energy estimated from the infrasound data and the impact velocity result in an estimated minimum mass of 22.6 t. By propagating the trajectory of 2014 AA backwards we find that the only window for finding precovery observations is for the three days before its discovery.

1I/`Oumuamua is the first confirmed interstellar body in our Solar System. Here we report on observations of `Oumuamua made with the Spitzer Space Telescope on 2017 November 21--22 (UT). We integrated for 30.2~hours at 4.5 micron (IRAC channel 2). We did not detect the object and place an upper limit on the flux of 0.3 uJy (3sigma). This implies an effective spherical diameter less than [98, 140, 440] meters and albedo greater than [0.2, 0.1, 0.01] under the assumption of low, middle, or high thermal beaming parameter eta, respectively. With an aspect ratio for `Oumuamua of 6:1, these results correspond to dimensions of [240:40, 341:57, 1080:180] meters, respectively. We place upper limits on the amount of dust, CO, and CO2 coming from this object that are lower than previous results; we are unable to constrain the production of other gas species. Both our size and outgassing limits are important because `Oumuamua's trajectory shows non-gravitational accelerations that are sensitive to size and mass and presumably caused by gas emission. We suggest that `Oumuamua may have experienced low-level post-perihelion volatile emission that produced a fresh, bright, icy mantle. This model is consistent with the expected eta value and implied high albedo value for this solution, but, given our strict limits on CO and CO2, requires another gas species --- probably H2O --- to explain the observed non-gravitational acceleration. Our results extend the mystery of `Oumuamua's origin and evolution.

This paper reports the first OH 18-cm line observation of the first detected interstellar object 1I/2017 U1 (`Oumuamua) using the Green Bank Telescope. We have observed the OH lines at 1665.402 MHz, 1667.359, and 1720.53 MHz frequencies with a spectral resolution of 357 Hz (approximately 0.06 km-s^{-1}). At the time of the observation, `Oumuamua was at topocentric distance and velocity of 1.07 au and 63.4 km-s^{-1}, respectively, or at heliocentric distance and velocity of 1.8 au and 39 km-s^{-1}, respectively. Based on a detailed data reduction and an analogy-based inversion, our final results confirm the asteroidal origin of `Oumuamua (as discussed in Meech et al., 2017) with an upper bound of OH production of Q[OH] < 0.17 x 10^{28} s^{-1}., accepted for publication in Astronomical Journal, 7 pages, 1 figure, 1 table

Near-infrared and mid-infrared observations of the site of the 2009 July 19 impact of an unknown object with Jupiter were obtained within days of the event. The observations were used to assess the properties of a particulate debris field, elevated temperatures, and the extent of ammonia gas redistributed from the troposphere into Jupiter's stratosphere. The impact strongly influenced the atmosphere in a central region, as well as having weaker effects in a separate field to its west, similar to the Comet Shoemaker-Levy 9 (SL9) impact sites in 1994. Temperatures were elevated by as much as 6K at pressures of about 50-70mbar in Jupiter's lower stratosphere near the center of the impact site, but no changes above the noise level (1K) were observed in the upper stratosphere at atmospheric pressures less than ∼1mbar. The impact transported at least ∼2×1015g of gas from the troposphere to the stratosphere, an amount less than derived for the SL9 C fragment impact. From thermal heating and mass-transport considerations, the diameter of the impactor was roughly in the range of 200-500m, assuming a mean density of 2.5g/cm3. Models with temperature perturbations and ammonia redistribution alone are unable to fit the observed thermal emission; non-gray emission from particulate emission is needed. Mid-infrared spectroscopy of material delivered by the impacting body implies that, in addition to a silicate component, it contains a strong signature that is consistent with silica, distinguishing it from SL9, which contained no evidence for silica. Because no comet has a significant abundance of silica, this result is more consistent with a " rocky" or " asteroidal" origin for the impactor than an " icy" or " cometary" one. This is surprising because the only objects generally considered likely to collide with Jupiter and its satellites are Jupiter-Family Comets, whose populations appear to be orders of magnitude larger than the Jupiter-encountering asteroids. Nonetheless, our conclusion that there is good evidence for at least a major asteroidal component of the impactor composition is also consistent both with constraints on the geometry of the impactor and with results of contemporaneous Hubble Space Telescope observations. If the impact was not simply a statistical fluke, then our conclusion that the impactor contained more rocky material than was the case for the desiccated Comet SL9 implies a larger population of Jupiter-crossing asteroidal bodies than previously estimated, an asteroidal component within the Jupiter-Family Comet population, or compositional differentiation within these bodies. © 2010 Elsevier Inc.

On October 7, 2008, asteroid 2008 TC{sub 3} impacted Earth and fragmented at 37 km altitude above the Nubian Desert in northern Sudan. The area surrounding the asteroid's approach path was searched, resulting in the first recovery of meteorites from an asteroid observed in space. This was also the first recovery of remains from a fragile 'cometary' PE = IIIa/b type fireball. In subsequent searches, over 600 mostly small 0.2-379 g meteorites (named 'Almahata Sitta') with a total mass 10.7 kg were recovered from a 30 x 7 km area. Meteorites fell along the track at 1.3 kg km{sup -1}, nearly independent of mass between 1 and 400 g, with a total fallen mass of 39 {+-} 6 kg. The strewn field was shifted nearly 1.8 km south from the calculated approach path. The influence of winds on the distribution of the meteorites, and on the motion of the dust train, is investigated. The majority of meteorites are ureilites with densities around 2.8 g cm{sup -3}, some of an anomalous (porous, high in carbon) polymict ureilite variety with densities as low as 1.5 g cm{sup -3}. In addition, an estimated 20-30% (in mass) of recovered meteorites were ordinary, enstatite, andmore » carbonaceous chondrites. Their fresh look and matching distribution of fragments in the strewn field imply that they were part of 2008 TC{sub 3}. For that reason, they are all referred to as 'Almahata Sitta.' No ureilite meteorites were found that still held foreign clasts, suggesting that the asteroid's clasts were only loosely bound.« less

Given the critical role of the bright sungrazer C/1843 D1, also called the Great March Comet of 1843, in hypotheses of the origin and evolution of the Kreutz system, a new orbital investigation of this comet was desirable. Closely inspecting Kreutz's classical work, we conclude that his result of 512 yr for the orbital period, which has been endlessly quoted in the literature and employed in studies of the sungrazer system's evolution, is not realistic. We derive improved sets of orbital elements based on the best astrometric observations available from 1843, using new comparison star positions from the Hipparcos and Tycho Catalogues. We find that the 1843 osculating value of the orbital period was most probably between 600 and 800 yr and that the observations are consistent with a forced value of 742 yr, in which case comet C/1843 D1 could be a major (and possibly the most massive) fragment of X/1106 C1, the celebrated sungrazer of 1106.

We examine the process of cascading fragmentation for the Kreutz sungrazer system to continue our exploration of its birth, orbital evolution, and temporal clumping. We modify and broaden the two-superfragment model from Paper I to include clusters of ~30 bright comets spanning four centuries and 1000 SOHO sungrazers from 1996 to 2006. The spectacular parent sungrazer X/1106 C1 is assumed to have tidally split shortly after perihelion into a train of major protofragments immersed in a cloud of particulate debris, which at larger heliocentric distances were breaking up nontidally over and over again. We describe potential evolutionary paths for the Kreutz system by linking X/1106 C1 in subgroup I-type orbit with the comet of February 423 in one scenario or with the comet of February 467 in another. The latter scenario accounts for sungrazer clusters in as early as the 16th century, suggests that the progenitor object may have been observed as the comet of 214 BCE, is quite consistent with the orbital distribution of the SOHO sungrazers that sample the central filament of the Kreutz system between the clusters of major sungrazers, and predicts future clusters until ~2120. Comet X/1106 C1 and the common parent of C/1882 R1 and C/1965 S1 were two first-generation fragments of the progenitor that split nontidally on the way to its 5th century perihelion, reminiscent of the superfragments in Paper I. We provide computational tools needed for solving the problem of the Kreutz system's orbital evolution, but no unique scenarios are presented for the individual comets. Another cluster of bright sungrazers is expected to arrive in the coming decades, its earliest member possibly just several years from now.

Of the three major groups of comets approaching the Sun to between 6 and 12 solar radii and discovered with the coronagraphs on board SOHO, we investigate the Marsden and Kracht groups.We call these comets ''sunskirters'' to distinguish them from the Kreutz system sungrazers. Our objective is to understand the origin, history, and orbital evolution of the two groups.

We analyze the orbital behavior of four new co-orbital NEOs and the Earth horseshoe object 2002 AA29. The new objects are 2001 CK32, a 3753 Cruithne-like co-orbital of Venus, 2001 GO2 and 2003 YN107, two objects with motion similar to 2002 AA29. 2001 CK32 is on a compound orbit. The asteroid reverses its path when the mean longitude difference is −50°. Its motion is chaotic. 2001 GO2 is an Earth HS orbiter with repeated transitions to the QS phase, the next occurring 200 years from now. The HS libration period is 190 years and the QS phases last 45 years. For 2002 AA29, our simulations permit us to find useful theoretical insights into the HS–QS transitions. Its orbit can be simulated with adequate accuracy for 4400 years into the future and 1483 years into the past. The new co-orbital 2003 YN107 is at present an Earth QS. It has entered this phase in 1997 and will leave it again in 2006, completing one QS cycle. Like 2002 AA29, it has frequent transitions between HS and QS. One HS cycle takes 133 years.

The newly discovered asteroid 2003 YN107 is currently a quasi-satellite of the Earth, making a satellite-like orbit of high inclination with apparent period of one year. The term quasisatellite is used since these large orbits are not completely closed, but rather perturbed portions of the asteroid's orbit around the Sun. Due to its extremely Earth-like orbit, this asteroid is influenced by Earth's gravity to remain within 0.1 AU of the Earth for approximately 10 years (1997 to 2006). Prior to this, it had been on a horseshoe orbit closely following Earth's orbit for several hundred years. It will re-enter such an orbit, and make one final libration of 123 years, after which it will have a close interaction with the Earth and transition to a circulating orbit. Chaotic effects limit our ability to determine the origin or fate of this object.

A back-and-forth orbit integration technique, developed for our previous investigation of the splitting of the parent of the sungrazers C/1882 R1 and C/1965 S1, is now applied in an effort to understand the history and orbital evolution of the Kreutz sungrazer system, starting with the birth of two subgroups, which show prominently among the bright members and whose inception dates back to the progenitor's breakup into two superfragments. The integration technique is used to reproduce the motion of comet C/1843 D1 - the second brightest sungrazer known and presumably the most massive surviving piece of superfragment I - from the motion of C/1882 R1 - the brightest sungrazer on record and arguably the most massive surviving piece of superfragment II. Running the orbit of C/1882 R1 back to A.D. 326, the progenitor comet is found to have split at a heliocentric distance of 50 AU and nearly 30 yr before perihelion. The superfragments acquired separation velocities of similar to8 m s(-1) in opposite directions. Using the same technique, we show next that (1) the motions of two additional sungrazers, C/1880 C1 and C/1887 B1, are matched extremely well if these objects shared a common parent with C/1843 D1, and (2) C/1963 R1 (Pereyra), the second brightest subgroup I member on record, is more closely related to subgroup II objects (such as C/1882 R1 and C/1965 S1) than to C/1843 D1. This finding raises serious doubts about the major role of the subgroups in the system's orbital history and offers an incentive for considering an alternative dynamical scenario. The fragmentation models for C/1963 R1 and two additional bright sungrazers, C/1945 X1 and C/1970 K1, suggest that (1) these comets may have been the most massive pieces of the fragment populations formed from their respective disintegrating parents, and (2) the course of evolution of the Kreutz system at the upper end of the mass spectrum may be better ascertained from the distribution of the sungrazers' arrival times than from the sources of subgroups. If so, the fragment hierarchy should be determined primarily by the cascading nature of the fragmentation process, which was recently shown by Sekanina to control the evolution of minor fragments as well. The sungrazer system's estimated age is in any case very short, less than 1700 yr.

Radar echoes from Earth co-orbital Asteroid 2002 AA29 yield a total-power radar cross section of 2.9×10 −5 km 2 ±25%, a circular polarization ratio of SC/OC=0.26±0.07, and an echo bandwidth of at least 1.5 Hz. Combining these results with the estimate of its visual absolute magnitude, H V =25.23±0.24, from reported Spacewatch photometry indicates an effective diameter of 25±5 m, a rotation period no longer than 33 min, and an average surface bulk density no larger than 2.0 g cm −3 ; the asteroid is radar dark and optically bright, and its statistically most likely spectral class is S. The H V estimate from LINEAR photometry (23.58±0.38) is not compatible with either Spacewatch's H V or our radar results. If a bias this large were generally present in LINEAR's estimates of H V for asteroids it has discovered or observed, then estimates of the current completeness of the Spaceguard Survey would have to be revised downward.

Distant comets C/2002 A1 and C/2002 A2 make up a peculiar pair that moves about the Sun in virtually identical, somewhat unstable orbits, extending currently between about 4.7 and 29 AU from the Sun. The two objects, observed since late 2001, are unquestionably of common origin. Analysis of their relative motion indicates that their parent body split nontidally most probably between mid-1977 and early 1979 at a heliocentric distance of 22.5 ± 0.1 AU and about 2.5 AU below the ecliptic, with a separation velocity of 2.7 ± 0.2 m s-1. The motion of C/2002 A2, the secondary component that trails behind the primary, is found to be affected by a nongravitational deceleration of (13.4 ± 1.5) × 10-5 units of solar gravitational acceleration relative to C/2002 A1, which is in a range of decelerations that companions of split comets are usually subjected to. C/2002 A2 was somewhat less condensed and, interestingly, brighter than C/2002 A1.

We describe a computational technique to assess the near-term Earth impact hazard posed by newly discovered asteroids. In these late warning cases the observational data sets will often include only an hour or so of tracking, leading to a severe degeneracy in the orbit estimation. The systematic ranging approach attacks this problem by exploring the poorly-constrained space of geocentric range and range rate, while the plane of sky position and motion is readily derived from the recorded observations. A raster scan in the two-dimensional range-range rate space allows us to identify regions corresponding to collision solutions, from which we derive rigorous impact probabilities, as well as potential impact times and locations. As an example, we shall consider the case of 2014 AA, a small asteroid that was discovered from Arizona by the Catalina Sky Survey early on January 1, 2014, and—as evidenced by infrasound monitoring—impacted the Atlantic Ocean less than a day later.

Five potential methods of preventing an asteroid from colliding with Earth—Kinetic Impactor (KI), Ion Beam Deflection (IBD), Gravity Tractor (GT), Enhanced Gravity Tractor (EGT), and Laser Ablation (LA)—are compared, with the objective of helping to inform a NASA decision regarding which technology or technologies could be demonstrated in space on the proposed 2019 Asteroid Robotic Redirect Mission (ARRM). Three design blocks are considered, which differ in the power available to the deflection technology and the mass at low-Earth orbit. We plot the required warning time (up to 30 years between discovery of the hazard and potential collision with Earth) vs. asteroid diameter for each of four technologies (gravity tractor without enhancement is considered only as a trim/verification candidate) under each design block, and illustrate sensitivities of results to important parameters.

This chapter addresses the current state of the art in assessing the impact risk from any of the known near-Earth objects over the next century or so. The assessment is made by determining the orbits of potentially hazardous asteroids and comets using the latest sets of tracking measurements and then projecting into the future the possible positions for these objects during close approaches to the Earth. The chapter discusses various computerized risk assessment systems that perform these assessments, along with the scales that are used to assess these risks. The second part of the chapter addresses two important issues in accurately predicting the risk of impact on Earth that have been identified and addressed in recent years. The first of these is the “Yarkovsky effect,” which is the small recoil acceleration acting on an asteroid due to thermal emissions from its surface and which can now be detected, modeled, and accounted for. The second topic to be discussed is the phenomenon of “keyholes,” which are gravitational gateways that can take an asteroid from a close approach on one passage by the S. Chesley (*) • P. Chodas Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA e-mail: steve.chesley@jpl.nasa.gov; paul.w.chodas@jpl.nasa.gov # Springer International Publishing Switzerland 2015 J.N. Pelton, F. Allahdadi (eds.), Handbook of Cosmic Hazards and Planetary Defense, DOI 10.1007/978-3-319-03952-7_87 651 Earth to a later impact with our planet. Mapping the keyholes for a potential impactor is an important step in assessing the asteroid’s impact hazard and in planning a possible deflection mission.

Following our recent successful modeling of the common origin of two of the brightest members of the Kreutz system of sungrazing comets, we now examine three other objects: C/1970 K1 (White-Ortiz-Bolelli), the most recent sungrazer discovered from Earth, C/1880 C1 (Great Southern Comet), and C/1843 D1 (Great March Comet). For White-Ortiz-Bolelli, five possible origin and orbit evolution scenarios are explored. We find that its parent was neither C/1965 S1 (Ikeya-Seki), nor C/1882 R1 (Great September Comet), nor the comet of 1106 (the presumed parent of Ikeya-Seki and the 1882 comet). The motion of C/1970 K1 is consistent with a scenario in which the parent was an unknown fragment that separated from the 1106 comet at the same time as, or shortly before, Ikeya-Seki and passed through perihelion in 1970 June-July, shortly after White-Ortiz-Bolelli. The separation of White-Ortiz-Bolelli from this fragment is found to have occurred around the mid-eighteenth century, at a heliocentric distance of about 150 AU, with a relative velocity of 3-5 m s-1 in the general direction of the Sun and to the north of the orbital plane. On the other hand, we conclude that the 1880 comet separated directly from C/1843 D1, the second brightest known sungrazer, some 100-150 days after the 1843 comet's previous perihelion passage in the eleventh century, at 2.5-3 AU from the Sun, with a relative velocity of slightly more than 7 m s-1 in the generally antisolar direction and to the south of the orbital plane. The pattern of fragmentation of the Kreutz system's members discovered from Earth begins to resemble the evolution of the system's minor fragments detected coronagraphically from aboard the SOHO spacecraft, and there is significant qualitative similarity with fragmentation of comet D/1993 F2 (Shoemaker-Levy 9).

Our extensive orbital calculations show that the motion of comet C/1965 S1 (Ikeya-Seki), a major member of the Kreutz sungrazing system, can be derived from the motion of its "sister" comet C/1882 R1 (Great September Comet) on the assumption that the two objects are fragments of a common parent that split in the year 1106, at the time when a very bright comet appeared near the Sun according to a number of historical records. Specifically, the orbit of Ikeya-Seki derived from astrometric positions in 1965-1966 is matched with remarkably high accuracy, well within the errors of observation, by (1) integrating the motion of comet C/1882 R1 back to 1106; (2) launching from the parent a fragment some 18 days after perihelion, 0.75 AU from the Sun, with a relative velocity of about 7 m s-1 nearly in the antisolar direction; and (3) integrating the fragment's motion forward to 1965. We find that having the break-up closer to the Sun or before perihelion yields grossly inferior solutions. We conclude that the fragmentation event itself was not tidal in nature, but appears to have been due to rotational, and possibly other, forces acting on the parent comet, afflicted with cracks and fissures caused by the Sun's tidal forces a few weeks earlier. We note that the derived separation velocity is in the range established for nontidal fragmentation of minor sungrazers at large heliocentric distances and that there are obvious similarities with the behavior of comet D/1993 F2 (Shoemaker-Levy 9) following its close encounter with Jupiter in 1992 July.

The newly discovered asteroid 2002 AA29 moves in a very Earth-like orbit that relative to Earth has a unique horseshoe shape and allows transitions to a quasi-satellite state. This is the first body known to be in a simple heliocentric horseshoe orbit, moving along its parent planet's orbit. It is similarly also the first true co-orbital object of Earth, since other asteroids in 1: 1 resonance with Earth have orbits very dissimilar from that of our planet. When a quasi-satellite, it remains within 0.2 AU of the Earth for several decades. 2002 AA29 is the first asteroid known to exhibit this behavior. 2002 AA29 introduces an important new class of objects offering potential targets for space missions and clues to asteroid orbit transfer evolution.

NASA is currently studying an Asteroid Redirect Mission (ARM) concept that could capture an entire 2 to 12 m asteroid or boulder from a larger asteroid and place it in orbit around the Moon. Once at the Moon, astronauts would be sent to visit the asteroid or boulder and use it as a test bed for developing technologies and procedures needed for deep space human missions. In the case of the redirection of an entire asteroid, the asteroid mass could be from 100 to 1000 metric tonnes, and the ARM spacecraft would only be able to apply relatively small ∆V to the asteroid. Therefore, ARM would rely on Lunar Gravity Assists (LGAs) to provide most of the ∆V needed for capture. This paper presents a method based upon the Tisserand Criterion for identifying asteroids that can be redirected with a small ∆V (< 200 m/s) to an LGA that would capture the asteroid.

This paper will describe the benefits of execution of the Asteroid Redirect Mission as an early mission in deep space, demonstrating solar electric propulsion, deep space robotics, ground and on-board navigation, docking, and EVA. The paper will also discuss how staging in trans-lunar space and the elements associated with this mission are excellent building blocks for subsequent deep space missions to Mars or other destinations.

Comet C/2013 A1 (siding Spring) will experience a high velocity encounter with Mars on October 19, 2014 at a distance of 135,000 km +- 5000 km from the planet center. We present a comprehensive analysis of the trajectory of both the comet nucleus and the dust tail. The nucleus of C/2013 A1 cannot impact on Mars even in the case of unexpectedly large nongravitational perturbations. Furthermore, we compute the required ejection velocities for the dust grains of the tail to reach Mars as a function of particle radius and density and heliocentric distance of the ejection. A comparison between our results and the most current modeling of the ejection velocities suggests that impacts are possible only for millimeter to centimeter size particles released more than 13 au from the Sun. However, this level of cometary activity that far from the Sun is considered extremely unlikely. The arrival time of these particles spans a 20-minute time interval centered at October 19, 2014 at 20:09 TDB, i.e., around the time that Mars crosses the orbital plane of C/2013 A1. Ejection velocities larger than currently estimated by a factor >2 would allow impacts for smaller particles ejected as close as 3 au from the Sun. These particles would reach Mars from 43 to 130 min after the nominal close approach epoch of the purely gravitational trajectory of the nucleus.

We report initial results of daily delay/Doppler observations of Toutatis with the Goldstone 8510-MHz (3.5-cm) radar during November 25 - December 3, 1996. Using the physical model of Toutatis derived form 1992 radar observations (Hudson and Ostro 1995, Science 270, 84-86) to analyze the new data, we obtain refined estimates of the asteroid's orbit, spin state, and surface properties.

Comprehensive analysis of discrete events of secondary fragmentation leads to a conceptually new understanding of the process of disintegration of comet Shoemaker-Levy 9. We submit that the jovian tidal forces inflicted extensive cracks throughout the interior of the original nucleus but did not split it apart. The initial disruption was apparently accomplished by stresses exerted on the cracked object by its fast rotation during the early post-perijove period of time. We argue that this disruption was in fact a rapid sequence of episodes during July 1992 that gave birth to the 12 on-train, or primary, fragments: A, C, D, E, G, H, K, L, Q (later Q(sub 1)), R, S, and W. The discrete events of secondary fragmentation, which gave birth to the off-train fragments, are understood in this scenario as stochastic manifestations of the continuing process of progressive disintegration. Of the 13 off-train fragments considered, nine were secondary--B, F, G(sub 2), M, N, P (later P(sub 2) or P(sub 2a)), Q(sub 2), U, and V--and four tertiary (J, P(sub 1), P(sub 2b), and T). The separation parameters of 11 off-train fragments were determined. The vectorial distribution of separation velocities of these fragments shows a strong concentration toward a great circle, unquestionably an effect of the approximately conserved angular momentum of the progenitor comet since the time of its initial disruption. Also apparent is their clumping (except for P(sub 1)) to a segment along the great circle, implying that the fragments were consistently released from one side of their parents, thus explaining for the first time why the off-train fragments preferentially appeared on one side of the nuclear train. In order to obtain a consistent solution, our model requires that the points of separation be on the antisolar side of the parent fragments, where thermal stresses are likely to enhance the effect of rotation. The episodes of secondary fragmentation are found to have nine months after the close encounter with Jupiter in early July 1992, and the separation velocities ranged between 0.36 and 1.7 m/s. The spin-axis position is determined to have been nearly in the jovicentric orbit plane, which rules out the Asphaug-Benz-Solem strengthless aggregate model as a plausible breakup hypothesis. Since the separation velocities are rotational in nature, they cannot substantially exceed the critical limit for centrifugal breakup and offer an estimate for the original nuclear dimensions. The comet's nucleus is found to have been approximately 10 km in diameter and spinning rapidly. With the exception of P(sub 1) and apparently also P(sub 2) and F, no nongravitational deceleration was detected in the motions of the off-train fragments. Serious doubts are cast on continuing appreciable activity of any of these fragments. Indeed, when it was necessary to introduce a deceleration into the equations of motion, the effect appears to have been due to the action of solar radiation pressure on the centroid of centimeter-sized particulates in the disintegrating condensations.

The target asteroid of the OSIRIS-REx asteroid sample return mission, (101955) Bennu (formerly 1999 RQ$_{36}$), is a half-kilometer near-Earth asteroid with an extraordinarily well constrained orbit. An extensive data set of optical astrometry from 1999--2013 and high-quality radar delay measurements to Bennu in 1999, 2005, and 2011 reveal the action of the Yarkovsky effect, with a mean semimajor axis drift rate $da/dt = (-19.0 \pm 0.1)\times 10^{-4}$ au/Myr or $284\pm 1.5\;\rm{m/yr}$. The accuracy of this result depends critically on the fidelity of the observational and dynamical model. As an example, neglecting the relativistic perturbations of the Earth during close approaches affects the orbit with $3\sigma$ significance in $da/dt$. The orbital deviations from purely gravitational dynamics allow us to deduce the acceleration of the Yarkovsky effect, while the known physical characterization of Bennu allows us to independently model the force due to thermal emissions. The combination of these two analyses yields a bulk density of $\rho = 1260\pm70\,\rm{kg/m^3}$, which indicates a macroporosity in the range $40\pm10$% for the bulk densities of likely analog meteorites, suggesting a rubble-pile internal structure. The associated mass estimate is $(7.8\pm0.9)\times 10^{10}\, \rm{kg}$ and $GM = 5.2\pm0.6\,\rm{m^3/s^2}$. Bennu's Earth close approaches are deterministic over the interval 1654--2135, beyond which the predictions are statistical in nature. In particular, the 2135 close approach is likely within the lunar distance and leads to strong scattering and therefore numerous potential impacts in subsequent years, from 2175--2196. The highest individual impact probability is $9.5\times 10^{-5}$ in 2196, and the cumulative impact probability is $3.7\times 10^{-4}$, leading to a cumulative Palermo Scale of -1.70.

Radar observations of comet Hyakutake (C/1996 B2) made at the Goldstone Deep Space Communications Complex in California have detected echoes from the nucleus and from large grains in the inner coma. The nucleus of this bright comet was estimated to be only 2 to 3 kilometers in diameter. Models of the coma echo indicate backscatter from porous, centimeter-size grains ejected anisotropically at velocities of tens of meters per second. The radar observations suggest that a comet's activity may be a poor indicator of its size and provide evidence that large grains constitute an important component of the mass loss from a typical active comet.

Two months after the discovery of comet Shoemaker-Levy 9 came the astonishing announcement that the comet would impact Jupiter in July 1994. Computing the orbital motion of this remarkable comet presented several unusual challenges. We review the pre-impact orbit computations and impact predictions for SL9, from the preliminary orbit solutions shortly after discovery to the final set of predictions before the impacts. The final set of predicted impact times were systematically early by an average of 7 minutes, probably due to systematic errors in the reference star catalogs used in the reduction of the fragments' astrometric positions. The actual impact times were inferred from the times of observed phenomena for 16 of the impacts. Orbit solutions for the fragments were refined by using the actual impact times as additional data, and by estimating and removing measurement biases from the astrometric observations. The final orbit solutions for 21 fragments are tabulated, along with final estimates of the impact times and locations. The pre-breakup orbital history of the comet was investigated statistically, via a Monte Carlo analysis. The progenitor nucleus of SL9 was most likely captured by Jupiter around 1929 ± 9 years. Prior to capture, the comet was in a low-eccentricity, low-inclination heliocentric orbit entirely inside Jupiter's orbit, or, less likely, entirely outside. The ensemble of possible pre-capture orbits is consistent with a group of Jupiter family comets known as the quasi-Hildas.

We describe the physical and orbital properties of C/2011 W3. After surviving perihelion, the comet underwent major changes (permanent loss of nuclear condensation, formation of spine tail). The process of disintegration culminated with an outburst on December 17.6 (T+1.6 d) and this delayed response is inconsistent with the rubble pile model. Probable cause was thermal stress from the heat pulse into the nucleus after perihelion, which could also produce fragmentation of sungrazers far from the Sun. The spine tail was a synchronic feature, made up of dust released at, Submitted to Astrophysical Journal; 35 pages, 18 figures, 8 tables

We have combined the most recent Pioneer Venus Orbiter (PVO) and Magellan (MGN) data with the earlier 1978-1982 PVO data set to obtain a new 60th degree and order spherical harmonic gravity model and a 120th degree and order spherical harmonic topography model. Free-air gravity maps are shown over regions where the most marked improvement has been obtained (Ishtar-Terra, Alpha, Bell and Artemis). Gravity versus topography relationships are presented as correlations per degree and axes orientation.

On 2009 July 19, we observed a single, large impact on Jupiter at a planetocentric latitude of 55^{\circ}S. This and the Shoemaker-Levy 9 (SL9) impacts on Jupiter in 1994 are the only planetary-scale impacts ever observed. The 2009 impact had an entry trajectory opposite and with a lower incidence angle than that of SL9. Comparison of the initial aerosol cloud debris properties, spanning 4,800 km east-west and 2,500 km north-south, with those produced by the SL9 fragments, and dynamical calculations of pre-impact orbit, indicate that the impactor was most probably an icy body with a size of 0.5-1 km. The collision rate of events of this magnitude may be five to ten times more frequent than previously thought. The search for unpredicted impacts, such as the current one, could be best performed in 890-nm and K (2.03-2.36 {\mu}m) filters in strong gaseous absorption, where the high-altitude aerosols are more reflective than Jupiter's primary cloud.

Observations of near-Earth asteroid 1998 KY26 shortly after its discovery reveal a slightly elongated spheroid with a diameter of about 30 meters, a composition analogous to carbonaceous chondritic meteorites, and a rotation period of 10.7 minutes, which is an order of magnitude shorter than that measured for any other solar system object. The rotation is too rapid for 1998 KY26 to consist of multiple components bound together just by their mutual gravitational attraction. This monolithic object probably is a fragment derived from cratering or collisional destruction of a much larger asteroid.

In July 1994, the fragments of Comet Shoemaker-Levy 9 plunged into Jupiter. We present preliminary results from analysis of the Hubble Space Telescope (HST) imaging observations of Jupiter obtained before, during, and after the impacts. We discuss observations of observed phenomena ranging from plumes to waves to atmospheric evolution. We report positions of all detected impact sites (we could not locate F, P2, T, U, and V). By comparing predicted and observed impact longitudes, we estimate the actual impact time. On average, impacts occurred about 8 minutes later than predicted, although this varied from on time to more than 15 minutes late. We also present an assessment of the relative size of the disturbance for each event. HST images showed the development of several of the erupting plumes (A, E, G, and W) over a period of about 20 minutes. In three cases (A, G, and W), we may have detected the incoming bolide, although alternative explanations for pre-plume flashes are possible. For fragments E, G, and W, we detected the 10 to 15 km/sec. A fascinating and unexpected feature of the impact plumes is that they all appear to reach the same height (about 3300 km) regardless of the size of the impactor that generated them. This point greatly confounded the impact modelers, who planned to use the plume heights to estimate the diameter of the impacting comet fragments.