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We describe results of a planetary defense exercise conducted during the close approach to Earth by the near-Earth asteroid (99942) Apophis during 2020 December–2021 March. The planetary defense community has been conducting observational campaigns since 2017 to test the operational readiness of the global planetary defense capabilities. These community-led global exercises were carried out with the support of NASA’s Planetary Defense Coordination Office and the International Asteroid Warning Network. The Apophis campaign is the third in our series of planetary defense exercises. The goal of this campaign was to recover, track, and characterize Apophis as a potential impactor to exercise the planetary defense system including observations, hypothetical risk assessment and risk prediction, and hazard communication. Based on the campaign results, we present lessons learned about our ability to observe and model a potential impactor. Data products derived from astrometric observations were available for inclusion in our risk assessment model almost immediately, allowing real-time updates to the impact probability calculation and possible impact locations. An early NEOWISE diameter measurement provided a significant improvement in the uncertainty on the range of hypothetical impact outcomes. The availability of different characterization methods such as photometry, spectroscopy, and radar provided robustness to our ability to assess the potential impact risk.

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

We have completed a low-inclination ecliptic survey for distant and slow-moving bright objects in the outer solar system. This survey used data taken over 34 months by the University of Arizona's Spacewatch Project based at Steward Observatory, Kitt Peak. Spacewatch revisits the same sky area every three to seven nights in order to track cohorts of main-belt asteroids. This survey used a multiple-night detection scheme to extend our rate sensitivity to as low as 0.012 arcsec hr-1. When combined with our plate scale and flux sensitivity (V ≈ 21), this survey was sensitive to Mars-sized objects out to 300 AU and Jupiter-sized planets out to 1200 AU. The survey covered approximately 8000 deg2 of raw sky, mostly within 10° of the ecliptic but away from the Galactic center. An automated motion-detection program was modified for this multinight search and processed approximately 2 terabytes of imagery into motion candidates. This survey discovered 2003 MW12, currently the tenth largest classical Kuiper Belt object. In addition, several known large Kuiper Belt objects and Centaurs were detected, and the detections were used with a model of our observational biases to make population estimates as a check on our survey efficiency. We found no large objects at low inclinations despite having sufficient sensitivity in both flux and rate to see them out as far as 1200 AU. For low inclinations, we can rule out more than one to two Pluto-sized objects out to 100 AU and one to two Mars-sized objects to 200 AU.

The Spacewatch Project uses four telescopes of apertures 0.9-m, 1.8-m, 2.3-m, and 4-m on Kitt Peak mountain in Arizona for followup astrometry of priority NEOs. Objects as faint as V=23 on the MPC's NEO Confirmation Page, targets of radar, potential impactors, targets of spacecraft observations or visits, and PHAs with future close approaches to Earth receive priority for astrometry.

Results from a series of long-term numerical integrations of orbits centred on that of C/1995 O1 (Hale-Bopp) are presented. Initially, 33 orbits taken from various sources were integrated in realistic models of the Solar system for various time-scales about the present in the range (-3, +2) Myr and analysed to assess the probability of different dynamical outcomes, such as Sun-grazing state or the number of orbits since the comet was captured from a long-period orbit in the Oort cloud. Further integrations were performed using an ensemble of 26 orbits more closely clustered about the present orbit of the comet, based on a more accurate method of orbit determination. We find that the ensemble half-life for the comet tp be captured or ejected is on the order of 0.5 Myr in the backward integrations and 1.2 Myr in the forward integrations, although a few members of our ensemble were captured during strong encounters with Jupiter within 10 revolutions. Comet Hale-Bopp has a probability pb.15 of evolving to a Sun-grazing end-state and, though not a 'new' comet (in the sense of being recently captured from the Oort cloud), it may be considered dynamically young.

Since beginning an automated full-time survey for Near-Earth objects in September 1990, the Spacewatch project has discovered 95 new Near-Earth asteroids (NEAs), 3 new comets, and 3 new Centaur asteroids. Spacewatch typically identifies about 2000 main-belt asteroids each lunation while covering about 150 square degrees to a limiting magnitude of Vlim ∼ 20.9. We report automatically measured astrometric asteroid detections to the Minor Planet Center where known and multiply detected objects are identified. NEAs and other interesting objects are identified by their angular rates of motion near opposition at the time of discovery and are scheduled for astrometric follow-up on subsequent nights. Objects with exceptionally high rates of motion, called very fast moving objects, have been detected in near real-time by the observer and followed for several hours to several days. These objects are the smallest yet detected outside the Earth's atmosphere. Careful analysis of their discovery rates and orbits have indicated an enhancement of their magnitude-frequency distribution over that anticipated before the Spacewatch survey began – of about a factor of 40 for objects near absolute magnitude H ∼ 29 (Rabinowitz, 1993; 1994). A subset of these small objects which have almost circular orbits and perihelia near the orbit of Earth have been recognized as having significantly different orbits from those of the previously known NEAs (Rabinowitz et al., 1993). Their origin is still under debate, with possible sources including Earth or Lunar impact ejecta, Earth-Sun Trojans, or more complicated secular resonance interactions of NEA orbits with the giant planets combined with stochastic perturbational encounters with the inner planets (Bottke, 1994; N.W. Harris, 1995, personal communication). The large volume of asteroid detections allows magnitude–frequency studies of the detected main-belt asteroids and Jupiter Trojans. New discoveries of Comets and Centaur asteroids (whose orbits cross those of the outer planets) may allow studies of their magnitude–frequency distributions as well.

The Hubble Space Telescope made systematic observations of the split comet P/Shoemaker-Levy 9 (SL9) (P designates a periodic comet) starting in July 1993 and continuing through mid-July 1994 when the fragments plunged into Jupiter's atmosphere. Deconvolutions of Wide Field Planetary Camera images indicate that the diameters of some fragments may have been as large as approximately 2 to 4 kilometers, assuming a geometric albedo of 4 percent, but significantly smaller values (that is,1 kilometer) cannot be ruled out. Most of the fragments (or nuclei) were embedded in circularly symmetric inner comae from July 1993 until late June 1994, implying that there was continuous, but weak, cometary activity. At least a few nuclei fragmented into separate, condensed objects well after the breakup of the SL9 parent body, which argues against the hypothesis that the SL9 fragments were swarms of debris with no dominant, central bodies. Spectroscopic observations taken on 14 July 1994 showed an outburst in magnesium ion emission that was followed closely by a threefold increase in continuum emission, which may have been caused by the electrostatic charging and subsequent explosion of dust as the comet passed from interplanetary space into the jovian magnetosphere. No OH emission was detected, but the derived upper limit on the H2O production rate of approximately 10(27) molecules per second does not necessarily imply that the object was water-poor.

The Hubble Space Telescope observed the fragmented comet P/Shoemaker-Levy 9 (1993e) (P indicates that it is a periodic comet) on 1 July 1993. Approximately 20 individual nuclei and their comae were observed in images taken with the Planetary Camera. After subtraction of the comae light, the 11 brightest nuclei have magnitudes between approximately 23.7 and 24.8. Assuming that the geometric albedo is 0.04, these magnitudes imply that the nuclear diameters are in the range approximately 2.5 to 4.3 kilometers. If the density of each nucleus is 1 gram per cubic centimeter, the total energy deposited by the impact of these 11 nuclei into Jupiter's atmosphere next July will be approximately 4 x 10(30) ergs ( approximately 10(8) megatons of TNT). This latter number should be regarded as an upper limit because the nuclear magnitudes probably contain a small residual coma contribution. The Faint Object Spectrograph was used to search for fluorescence from OH, which is usually an excellent indicator of cometary activity. No OH emission was detected, and this can be translated into an upper limit on the water production rate of approximately 2 x 10(27) molecules per second.

The Spacewatch program at the University of Arizona has pioneered automatic methods of detecting Near Earth Objects. Our software presently includes three modes of object detection: automatic motion identification; automatic streak identification; and visual streak identification. For automatic motion detection at sidereal drift rates, the 4σ detection threshold is near magnitude V = 20.9 for nearly stellar asteroid images. The automatic streak detection is able to locate streaks whose peak signal is above ~4σ and whose length is longer than about 10 pixels. Some visually detected streaks have had peak signals near ~1σ.Between 1990 September 25 and 1993 June 30, 45 new Near Earth asteroids, two comets and two Centaur's have been discovered with the system. An additional six comets, five Near Earth asteroids, and one Centaur were also “re-discovered”. The system has directly detected for the first time Near Earth Objects in the complete size range from about 5 kilometers to about 5 meters. Each month ~2,000 main belt asteroids are also detected.Future upgrades in both hardware, software, and telescope aperture may allow an order of magnitude increase in the rate of discovery of Near Earth Objects in the next several years. Several of the techniques proposed for the Spaceguard Survey have already been tested by Spacewatch, and others will need to be tested in the near future before such a survey can be implemented.

With the NEOWISE portion of the \emph{Wide-field Infrared Survey Explorer} (WISE) project, we have carried out a highly uniform survey of the near-Earth object (NEO) population at thermal infrared wavelengths ranging from 3 to 22 $��$m, allowing us to refine estimates of their numbers, sizes, and albedos. The NEOWISE survey detected NEOs the same way whether they were previously known or not, subject to the availability of ground-based follow-up observations, resulting in the discovery of more than 130 new NEOs. The survey's uniformity in sensitivity, observing cadence, and image quality have permitted extrapolation of the 428 near-Earth asteroids (NEAs) detected by NEOWISE during the fully cryogenic portion of the WISE mission to the larger population. We find that there are 981$\pm$19 NEAs larger than 1 km and 20,500$\pm$3000 NEAs larger than 100 m. We show that the Spaceguard goal of detecting 90% of all 1 km NEAs has been met, and that the cumulative size distribution is best represented by a broken power law with a slope of 1.32$\pm$0.14 below 1.5 km. This power law slope produces $\sim13,200\pm$1,900 NEAs with $D>$140 m. Although previous studies predict another break in the cumulative size distribution below $D\sim$50-100 m, resulting in an increase in the number of NEOs in this size range and smaller, we did not detect enough objects to comment on this increase. The overall number for the NEA population between 100-1000 m are lower than previous estimates. The numbers of near-Earth comets will be the subject of future work., Accepted to ApJ

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.

The study of asteroid families has provided tremendous insight into the forces that sculpted the main belt and continue to drive the collisional and dynamical evolution of asteroids. The identification of asteroid families within the NEO population could provide a similar boon to studies of their formation and interiors. In this study we examine the purported identification of NEO families by Drummond (2000) and conclude that it is unlikely that they are anything more than random fluctuations in the distribution of NEO osculating orbital elements. We arrive at this conclusion after examining the expected formation rate of NEO families, the identification of NEO groups in synthetic populations that contain no genetically related NEOs, the orbital evolution of the largest association identified by Drummond (2000), and the decoherence of synthetic NEO families intended to reproduce the observed members of the same association. These studies allowed us to identify a new criterion that can be used to select real NEO families for further study in future analyses, based on the ratio of the number of pairs and the size of strings to the number of objects in an identified association., Comment: Accepted for publication in Icarus. 19 pages including 11 figures

THE spectacular 'string-of-pearls' appearance of periodic comet Shoemaker-Levy 9 (1993e) and its impending encounter with Jupiter's atmosphere have caused a flurry of speculation on the likely effects of the impact1. The magnitudes of the predicted effects depend critically on the masses of the fragments composing the chain, although these are currently poorly constrained. However, some limits on the comet's total size, and hence mass, can be obtained by considering its dynamical history. On its previous swing past Jupiter, the comet apparently passed within the Roche limit of the planet2,3, and it therefore seems likely that the present chain of about twenty nuclei was created when a single progenitor was split by tidal forces during closest approach. Here we describe a simple model of tidal splitting which is able to reproduce closely both the length of the observed chain and its position angle on the sky as a function of time. The length of the fragment chain is directly proportional to the size of the parent object, which we estimate to have been only about 2 km in diameter.

In January 1991, the 0.9-m Spacewatch telescope made the first observation of an asteroid outside Earth's atmosphere but in the neighborhood of the Earth-moon system. Since then, more than 40 Earth-approaching asteroids have been discovered, including 13 smaller than 50 m. Using these data, one of us has shown that there is an excess of Earth-approaching asteroids with diameters less than 50 m, relative to the population inferred from the distribution of larger objects. Here we argue that these smaller objects - characterized by low eccentricities, widely ranging inclinations and unusual spectral properties - form a previously undetected asteroid belt concentrated near Earth. The recent discovery of additional small Earth-approaching asteroids supports this conclusion.

THE 0.91-m Spacewatch Telescope, on Kitt Peak in Arizona, is being used to search for new Earth-approaching asteroids. On the night of 18 January 1991 a streaked image extending over more than 161 arcseconds was recorded. Further observations over the next 4.6 hours showed that the images were of an object travelling in an independent orbit around the Sun; it was given the asteroidal designation 1991 BA. Orbital computations, presented here, indicate that 1991 BA was only 0.0053 AU from the Earth at the time of discovery, closing to 0.0033 AU at the last detection. Extrapolation of the calculated orbit shows that the object passed only 0.0011 AU (170,000km) from the Earth 12 hours after it was found. The observed brightness translates into an absolute visual magnitude 28.9, corresponding to a diameter of only 5–10 m. 1991 BA is the closest and smallest asteroid yet observed outside the Earth's atmosphere.