吉川 真

In 2002, the Institute of Space and Astronautical Science (ISAS) of Japan will send the MUSES-C spacecraft to an asteroid to get the surface material of the asteroid, and return them to the Earth in 2006. The target asteroid is (4660) Nereus, and the backup target is 1989 ML. In this paper, we studied the orbital evolution of these asteroids. We found that both of these asteroids show chaotic motions. In particular the motion of Nereus is quite chaotic, so we cannot determine its orbital evolution for more than 200 years. However, we also found that these asteroids have been moving around the Sun in asteroid-like orbits for at least 5,000 years. © 1999 COSPAR. Published by Elsevier Science Ltd.

Unlike the major planets and main belt asteroids, some near-Earth objects (NEOs) make their closest approaches to Earth at high northern declinations and therefore are visible simultaneously from North America, Europe and/or Asia, providing occasional opportunities for intercontinental radar experiments. The first celestial target of intercontinental radar was the Earth-crossing asteroid 6489 Golevka (1991 JX), which in June 1995 passed 0.034 AU from Earth at a declination of similar to 40 deg. High power (similar to 0.5 MW), continuous-wave signals at 3.5cm wavelength were transmitted toward the asteroid from the 70 m antenna at the JPL/NASA Goldstone Deep Space Communication Complex (DSCC) in California on 13, 14 and 15 June 1995 during several hours on each date. This illumination of the asteroid created an artificial radio source for astronomers anywhere on the asteroid facing side of Earth. Five astronomical groups tried to detect the radar echoes and two succeeded. Detections were obtained on each of the three days by the 70 m antenna at Evpatoria DSCC in Crimea and on June 15 by the 34 m antenna at Kashima Space Research Center in Japan. The Goldstone 34 m antenna monitored echoes throughout all the observations. From the results of the Goldstone-Evpatoria experiment it can be inferred that the asteroid is about 0.5 km across, is not very elongated, possesses considerable surface irregularity and is very reflective presumably due to a large near-surface bulk density. This first inter-continental radar astronomy experiment can be considered as an initial step toward a global radar network for routine NEO investigations. (C) 1997 Elsevier Science Ltd.

多数の衛星を配置して通信ネットワークを構成する非静止衛星通信システムや, 測位システム, 静止クラスタ衛星等, 複数の衛星を使用する衛星システムの軌道設計を行う際には, 個々の衛星の軌道運動のみならず, 全ての衛星の相対的な位置関係も考慮しながら軌道解析をしなければならない. しかしながら, 理論的な解析で得られた軌道パラメータから衛星の3次元的な運動を正確に理解することは容易ではない. そこで, バーチャルリアリティの技術を軌道運動の解析に応用して, 衛星の複雑な動きを分かりやすく表現するシステムの開発を行っている. 本システムを用いて軌道運動を直感的に理解し, 画面を見ながら軌道パラメータを設定して動きを即座に表示することにより, 通信等の目的に応じた軌道設計を効果的に行うことができる.

The detailed activity profile of the Sextandids - one of the day-time meteor showers - is poorly known and still unclear. Using the forward-scatter radio technique we have successfully been able to obtain further detailed overall activity profile of the Sextantids for seven consecutive years: 1991-1997. Analysis confirmed the Sextantid activity duration in solar longitude (J2000) of at least 184-193°and the maximum solar longitude at 188.35 ± 0.10° with a full width at half maximum (FWHM) of 2.0 ± 0.2°. Performing the numerical integrations, we also substantiated a possibility of the association between Apollo-type asteroid (3200) Phaethon and the Sextantids. Furthermore, we roughly estimated relative maximum flux rate of Sextantids: Geminids as 1:3 amplitude ratio. Depending upon the flux rates and the time lags of the orbital evolution with Phaethon, we conclude that the Sextantids are at a more progressive stage of orbital evolution than the Geminids if both meteor streams are really associated with Phaethon.

The changes of surface reflectance in response to changing illumination and viewing geometry, depends upon physical and optical properties of surface materials and overall shape of the body. We are trying to estimate the surface reflectance properties of Phobos using the computer graphic (CG) of detail model of Phobos, on which reflectance conditions can be freely changed. Here we report the system configuration and procedures of the simulation study.

The effects of the mutual gravitational attraction between asteroids were analyzed by two N-body calculations, in which N=4,516 (the Sun, the nine planets, and 4,506 asteroids). In one calculation the gravity of the asteroids was taken into account, and in the other it was ignored. These calculations were carried out for a time period of about 100 years. The largest difference in the positions of the asteroids between these two calculations is about 10 AU. For the orbital elements of the semimajor axis, the eccentricity, and the inclination, the largest differences were 9 × 10 AU, 4 × 10 , and 5 × 10 degrees, respectively. It was found that the distribution of the differences of the semimajor axis between the two calculations is quite similar to the Cauchy distribution. © 1996 Kluwer Academic Publishers. -3 -6 -6 -4

The collisional probability of short-period (SP) comets with Jupiter was estimated from numerical integrations of the orbits of real 165 SP comets over 4400 years. The cumulative close-encounter distance vs frequency plot was extrapolated to the radius of Jupiter taking account of the gravitational focusing effects in order to obtain the mean collisional probability. The obtained value gave a fairly good agreement with that calculated by D. Olsson-Steel (1987, Mon. Not. R. Astron. Soc. 227, 501-524) based on a form of öpik’s method. Incorporating the estimated flux of invisible SP comets near the Jovian orbit in the observed range of H -magnitude, the intrinsic collision rate with Jupiter was found to be once per 950 years, which is nearly 15 times more frequent than the collision rate for long-period comets calculated by Olsson-Steel (1987). The total impact numbers of comets with each jovian satellite over the Solar System age were calculated for the steady-state flux model of comets. It is shown that the surface densities of ray-craters (youngest) on Ganymede and Callisto (D gt; 30 km) are in agreement, within a factor of two, with our calculated intrinsic flux of SP comets. From the fact that the number of large craters on Amalthea (JV) is nearly equal to the total impact number predicted from the steady-state flux of comets, it is proposed that Amalthea may be the remnant core of a larger parent body whose original surface was spalled by many catastrophic impacts, or it may be a new object recently captured into the present orbit. We also suggest that the maximum size of craters on the retrograde outer satellites, which could be measured by the Galileo spacecraft, be a good tester for the knowledge of the cometary flux integrated over 4.6 × 10 years near the orbit of Jupiter. It is discussed that current survival of some inner small satellites might give a clue to the upper limit of the integrated cometary flux there. © 1995 Academic Press, Inc. 10 9

Exponential growth behavior of the round-trip orbital error in numerical integrations of the orbits for periodic comets and comet-like bodies was examined in relation to the time scales of their orbits becoming chaotic and unstable. For the majority of the orbits of 159 short-period (SP) comets that we integrated for 4400 yr, the round trip error for each of the Keplerian elements increased approximately exponentially from a very small value (say, 10 ) to the order of unity within 1000-2000 yr; this period of time was found to be well correlated with the time scale of substantial orbital changes leading to capture/ejection for the SP comets. Similarly as a result of 0.2-0.4 Myr integration of the orbits for giant comet-like objects such as Chiron and Pholus, we found that their time scale of the round-trip orbital error growth is consistent with the time scale for the divergence of the ensemble of orbits with slightly different orbital elements while the range of orbital error growth is less than the order of 1 AU. Assuming that this approximate parallelism between the time scale of the round-trip error growth and that for the orbital divergence is valid for the objects with much slower orbital evolution, we here estimated from the round-trip error curve the time scale for the orbit of 1992 QB1, a candidate of the Kuiper belt members, to become unstable by the order of 1 AU. The time scale is found to be 24-46 Myr. Some resonant cases are discussed in which the round-trip error growth behavior may not be a measure of the dynamical instability. © 1995. -29

We derived direct evidence of the impulse effects on the Quadrantid meteor swarm by analysing the photographic orbital data along with performing backward numerical integrations. The standard Quadrantids, being observed in average years, have scarcely changed in the orbital elements during the past several decades. However, the orbit of the 1987 Quadrantid swarm was significantly different from those of the standard Quadrantids. The right ascension and declination of the radiant were less by 2 degrees-3 degrees and greater by 0.degrees 5-1 degrees, respectively, and the argument of the perihelion was consequently greater by 4 degrees-1 degrees. Our numerical integrations indicate that the 1987 Quadrantids was strongly perturbed by a close encounter with Jupiter up to 0.28 AU in 1984 late August, when it occurred at about a half orbital period before the observation. We conclude that the peculiar orbit of the 1987 Quadrantids should be due to the impulse effect of the Jovian perturbation. The orbital evolution of the 1987 Quadrantids before the impulse effect is also discussed in this paper.

We calculated the orbital motion of 4506 numbered asteroids taking into account the effect of the gravitational attraction of asteroids as well as the perturbation of nine planets. The calculation was carried out for about 130 years into the future, and such close encounters that distance between two asteroids is less than 0.01 AU were monitored for this period. In addition, close encounters between asteroids and planets and between asteroids and hypothetical spacecraft were also analyzed. By this calculation, we obtained a near-miss database which contains about 13,000 data of close encounters. If we assume that the diameters of most asteroids calculated here are more than 20 km, the probability of collision between asteroids is estimated to be once per 10 years. © 1994 by Academic Press, Inc. 8