吉川 真

On 12 September 2005, Hayabusa (MUSES-C) spacecraft arrived at near-Earth asteroid (25143) Itokawa, and made various kinds of observations while hovering around the asteroid. At first Hayabusa hovered around "Gate position" at an altitude of about 20 km from the Itokawa's surface. After the initial observation of the asteroid, Hayabusa descended from Gate position to "Home Position" at an altitude of about 7 km from the Itokawa's surface. Since then Hayabusa made detailed observations of Itokawa and attempted to touch down on its surface. In this paper we mention about the motion of Hayabusa around Itokawa, especially in terms of the solar radiation pressure and the gravity of Itokawa. In order to evaluate these values, we have used radiometric and optical hybrid orbit determination method. As the results of analysis, we have estimated the solar radiation pressure with the error of 2% and the Itokawa's mass with the error of 4%.

The Japanese spacecraft Hayabusa is planed to reach the Asteroid Itokawa in September 2005, and to bring back some samples of its surface to Earth in 2007. We have studied the future possible evolution of this asteroid by integrating numerically over 100 Myr a set of 39 initially indistinguishable orbits (clones), obtained either by small variations of the nominal initial conditions, or by using different computers (introducing different round-off errors). The results indicate that an Earth impact of this 500-m-size asteroid is likely within a million years, which is only a factor of four larger than the average impact frequency of asteroids of this size. The mission Hayabusa may thus sample a good candidate for being among the next 500-m-size Earth impactors. © 2005 Elsevier Inc. All rights reserved.

The Bisei spaceguard Center started its space debris observations from February, 2000. At the beginning, we used two telescopes with apertures of 25cm and 50cn to find those detectability of different artificial and natural objects. The 50cm telescope showed its ability to detect even LEO objects additionally to GEO objects. From January, 2002, the 1m telescope started its operation. Although each telescope had to stop its operation time to time, because of initial problems of the telescope itself and large format CCD camera. Here, we will show some numbers of error evaluation during the last 2 years. The Japan Spaceguard Association has the main target to detect near-earth asteroids in order to detect them and to present a timely precaution of possible asteroid collision to the earth. To make this work efficient, the whole sky should be surveyed as quickly as possible, and therefore telescopes with wider field were built at the Bisei Spaceguard Center. Detail descriptions are given in different papers (Isobe et al, 2000) Three telescopes with apertures of 25cm, 50cm, and 1m have circular fields with diameters of 1.5, 2, and 3 degrees. These field sizes are also good for a survey work of space debris. At the BSGC, such works were started in 2000 under a collaboration between the JSGA and JAXA. Since our experiences of satellite and space debris observations in an optical wavelength range, we started to observe objects with well-determined orbital elements. During these test observing run, we detected some interesting objects (Isobe et al, 2001). Since our telescope fields are large, we could track some objects detected under a survey mode, where we fixed telescope position during 10 minutes. This long tracking observations can produce the first order orbital elements of the detected objects, which make those identification carried our several nights later possible (Figure 1, Bisei Team, 2004). Figures 2 and 3 are magnitude of each objects observed by 50cm and 1m telescopes, respectively. We can not see much limiting magnitude difference of both telescopes. This result is probably caused by a difference of stellar image sizes. Usually, we make 5 exposures at one telescope position with an exposure time of 6 seconds under an interval around 2 minutes, during which most of the geosynchronous orbit objects move about several ten seconds of arc and can be used single orbital elements. Using them, we evaluate standard deviations of right ascension, O-C (dAcosD) where A and D are right ascension and declination, respectively, for each set of 5 exposures (Figure 4). Figure 5 shows same as those for declination, O-C (dD). The standard deviations of both right ascension and declination were as large as 6 seconds of arc during time to the beginning of June, 2004. Since the telescopes were built to observe near-earth asteroid, it is not necessary to have a high accuracy of time and each observed time had been obtained in an accuracy of 1 second of time, which caused high value of the standard deviations. We changed our clock system with an accuracy down to 0.1 second of time on June 4, 2004, and then those standard deviation went down to an order of 1 second of arc. Considering our current stellar image size, these values are under a proper level, since those for asteroids are around 0.3 second of arc. If we could improve those stellar image sizes and also have clock accuracy of 0.1 second of time, much improvement of the standard deviation can be expected. After finding these results, we started to make a systematic survey work to detect unknown space debris, additionally to regular observations of some specific satellites.

Japanese first Mars explorer NOZOMI, which was launched in July 1998, suffered several problems during the operation period of more than five years. It could have reached near Mars at the end of 2003, but it was not put into the orbit around Mars. Although NOZOMI was not able to execute its main mission, it provided us a lot of good experiences from the point of the orbit determination of spacecraft. One of the most difficult works was the orbit determination for the period without the telemetry. In this period, for the most of the time the high gain antenna did not point to the earth because of a constraint of the attitude. Therefore, the quality of the tracking data was not good, and for some period it was impossible to get the tracking data at all. Under such critical condition, we managed to get the solution of the orbit, and in a near-miraculous way, we were able to control NOZOMI and execute two earth swingbys successfully. Other issues related to the orbit determination are the spin modulation, the solar radiation pressure, the small force related to the attitude change, and the solar conjunction. We tried to solve these issues by the conventional way using range and Doppler data. However, we also tried the new method, that is the orbit determination by using the Delta-VLBI method (VLBI: Very Long Baseline Interferometry). In addition to this, we tried optical observations of NOZOMI at the earth swingbys. © 2005 Elsevier Ltd. All rights reserved.

HAYABUSA, which was launched in May 2003, is the first asteroid sample return mission in the world. It has arrived at its destination, Asteroid (25143) Itokawa, at the beginning of September 2005 successfully. HAYABUSA has an ion engine system (IBS) as the main thrusting system and it makes difficult to carry out the orbit determination because of the continuous low acceleration. We have tried many attempts to estimate the acceleration by IES. However, the accuracy of the orbit determination is not good enough when IBS is working, so we make ballistic period of at least three passes once in three weeks or so. We carry out accurate orbit determination at the ballistic period, and we propagate the orbit by using the telemetry information of IBS with some correction by estimating the acceleration by IBS. By this way we have carried out the orbital operation of HAYABUSA. We summarize here the status of the estimation of IBS acceleration. There are several important phases for the orbit operation, such as the launch, the earth swingby, the solar conjunction, and the asteroid arrival. The earth swingby was done on 19 May 2004, and it was successful. We were able to put the spacecraft into the orbit that approach to Asteroid Itokawa. In July of 2005, the solar conjunction occurred and HAYABUSA was opposite side of the Sun. We were able to go through the solar conjunction without any problem, and after that we carried out the optical navigation by using the optical camera on board. In this paper, we also summarize the results of our orbit determination at the launch, the earth swingby, and the solar conjunction. The first result of the optical navigation is mentioned briefly.

We carried out a series of VLBI observations of Nozomi by using a dedicated narrow bandwidth VLBI system. The three carrier waves with frequency interval of 515 kHz were recorded in 3 channels of the system and correlated by a software method. As a result of the correlation, the residual fringe phases of the main carrier wave are obtained for every 1.3 seconds. We can also continuously track them for 100 minutes. The variation of the residual fringe phase is +/- 150 degrees. Moreover, we can derive succesively the group delay for every 100 seconds by using these three carrier waves. The RMS of the group delays is 13 nsec and its average is well accorded with the delay determined by the range and Doppler measurements within an error of 2 nsec. Consequently, we confirmed the validity of the narrow bandwidth VLBI system, and it could be expected that this system, in addition to range and Doppler measurements, can be applied to three-dimensional tracking of a spacecraft and the precise gravity measurement of the Moon and the planets. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences.

The wide-field 1 m telescope with ten 2k×4k CCD chips was set up at the Bisei Spaceguard Center in Okayama prefecture, Japan. The Japan Spaceguard Association started to make space debris observations using this telescope in addition to the 50 and 25 cm telescopes. The 1 m telescope has a field of view of 2.5° by 3.0° and has the ability to detect space debris with a size of 30-50 cm in geosynchronous earth orbit. When the telescope's potential performance is reached, it will be one of the best telescopes to make space debris survey work. © 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

The infrared astronomical satellite ASTRO-F, which has a 67 cm cooled telescope, will be launched in early 2004 and it will carry out all sky survey observation. The obtained data will be used to study various fields of astronomy, such as formation and evolution of galaxies, star formation, interstellar matter, brown dwarf, and dark matter. In addition to these, ASTRO-F also observes objects in the solar system, such as asteroids, comets, and interplanetary dust. In this paper, we investigated the possible observation for Near-Earth Objects (NEOs), especially Near-Earth Asteroids (NEAs), by ASTRO-F. The definition of NEA is the asteroids whose perihelion distance is less than 1.3 AU. At the beginning of 2002, there were about 1750 NEAs. Among these NEAs, about 250 are numbered asteroids and about 1500 are unnumbered ones. At first we studied the number of NEAs that ASTRO-F can observe during the first survey period of six months in 2004. The result is that 140 NEAs (48 numbered and 92 unnumbered asteroids) will be observed if we assume that the limiting magnitude of the observation is 20 in V band. Next we checked infrared observations in the past for these 140 NEAs and found that most of them do not have infrared data. ASTRO-F has a capability to observe at 1.8-5, 5-12, 10-26, and 50-200 μm. Using these data, we can obtain the albedo and the size of these NEAs. We think that these data will be useful not only in the scientific researches but also in the hazard analysis of asteroid collision. © 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

As one of the potential candidates of the "post-MUSES-C" minor body exploration to be launched around 2010, we have investigated scientific justifications and feasible mission scenarios. Two mission types are focused on among many other proposed mission concepts. One is the multiple rendezvous and sample return mission to asteroids whose spectral type is already known. The other is the multiple fly-bys and sample return mission to an asteroid family. This paper reports the preliminary design results of these two proposed missions. © 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

A series of VLBI observations for spacecraft NOZOMI was performed in the period between the end of 2002 and July 2003 with aim of supporting the orbit determination. We have made astrometric analysis of the VLBI data for celestial coordinates estimation of the spacecraft with phase delay observables. Phase delay has potential to give 3-4 orders of improvement of delay resolution than group delay measurement, whereas main difficulty in using phase delay is ambiguity problem. We observed the spacecraft NOZOMI continuously for a long time with multiple VLBI stations, and we avoided the problem of phase ambiguity by connecting the phase delay. For correlation processing and analysis of VLBI data, relativistic VLBI delay model for finite distance radio source was developed, and it was used for the analysis. Coordinates of spacecraft in celestial sphere were estimated by least-square parameter fitting technique. © 2004 IEEE.

Japanese first Mars explorer NOZOMI, which was launched in July 1998, suffered several problems during the operation period of more than five years. It could have reached near Mars at the end of 2003, but it was not put into the orbit around Mars. Although NOZOMI was not able to execute its main mission, it provided us a lot of good experiences from the point of the orbit determination of spacecraft. One of the most difficult works was the orbit determination for the period without the telemetry. In this period, for the most of the time the high gain antenna did not point to the earth because of a constraint of the attitude. Therefore the quality of the tracking data was not good, and for some period it was impossible to get the tracking data at all. Under such critical condition, we managed to get the solution of the orbit, and in a near-miraculous way, we were able to control NOZOMI and execute two earth swingbys successfully. Other issues related to the orbit determination are the spin modulation, the solar radiation pressure, the small force related to the attitude change, and the solar conjunction. We tried to solve these issues by the conventional way using range and Doppler data. However, we also tried the new method, that is the orbit determination by using the Delta-VLBI method (VLBI: Very Long Baseline Interferometry). In addition to this, we tried optical observations of NOZOMI at the earth swingbys.

We present photometric observations of the near-Earth asteroid (25143) 1998 SF36 from the 2001 apparition campaign, and we discuss the corresponding physical model. The asteroid's photometric behaviour is consistent with an S-type object, it has a retrograde pole at λ = 355°, β = -84° ± 5°, and its sidereal rotation period is P = 12.132 ± 0.0005 hours. 1998 SF36 is elongated, with rough global dimension ratios a/b = 2.0, b/c = 1.3, but the elongation is not due to a bifurcated shape. The surface is not likely to contain major concavities. No significant albedo variegation was detected.

The Japanese Mars explorer NOZOMI was launched in July 1998. It was planed to arrive at Mars in October 1999. But a problem occurred when it left from the earth to Mars and it will reach Mars at the beginning of 2004. NOZOMI has several issues in its orbit determination, such as the spin modulation, the solar radiation pressure, the small force related to the attitude change, and the solar conjunction. We tried to solve these issues by using range and Doppler data. Recently, however, much more difficult problem has occurred. That is that the range and/or Doppler data cannot be obtained for some periods because we cannot turn the high gain antenna toward the earth due to the attitude constraint. Therefore we considered the orbit determination by using the Delta-VLBI method. In this paper, we summarize the issues of the orbit of NOZOMI up to now and show our recent activities on Delta-VLBI observations for NOZOMI.

Three short-duration "outbursts", in which more than 20-40 meteors appeared in a few seconds, have been reported during a recent Leonid storm. The meteors in these events were extremely localized within a few hundred km, which should be caused by clusters of meteoroids. The existence of such clusters indicates the fragmentation of meteoroids during orbital motion in interplanetary space. Considering the extent of the spatial distribution, the fragmentation should have occurred at around the perihelion passage of the meteoroids just before encountering the Earth. This may cause a possible enhancement of smaller meteoroids, even in old dust trails. A possible example of similar clusters in the past meteor storm of Giacobinids is also noted.

The orbital energies (a ) of released meteoroids (or fragments) from the periodic Comet 96P/Machholz must be slightly different from that of 96P. This results in the differences in their evolutional rates; therefore, the time-lags of the orbital evolutions should apparently occur in the course of time, stably holding the same amplitude in their orbital variations. In the present study, integrating the motion of 96P, we found that the orbital elements of 96P at epoch 2319 correspond to those of both the Arietids and the Marsden comet group, currently observed, and those of 96P at epoch 2408 agree with those of the Kracht comet group, similarly observable now. We also estimated a difference of the perihelion times, ΔT, as: ∼ 320 yr for 96P - Marsden comet group; ∼ 360 yr for 96P - Arietids; and ∼ 410 yr for 96P - Kracht comet group, respectively. These may indicate such time-lags of orbital evolutions between 96P and released meteoroids (or fragments) from 96P. As a consequence, we conclude that 96P, Arietids, the Marsden group, and the Kracht group are further possibly associated with each other now. -1

The Japanese Mars explorer NOZOMI was launched in July 1998. It was planed to arrive at Mars in October 1999. But a problem occurred when it left from the earth to Mars and it will reach Mars at the beginning of 2004. NOZOMI has several issues in its orbit determination, such as the spin modulation, the solar radiation pressure, the small force related to the attitude change, and the solar conjunction. We tried to solve these issues by using range and Doppler data. Recently, however, much more difficult problem has occurred. That is that the range and/or Doppler data cannot be obtained for some periods because we cannot turn the high gain antenna toward the earth due to the attitude constraint. Therefore we considered the orbit determination by using the Delta-VLBI method. In this paper, we summarize the issues of the orbit of NOZOMI up to now and show our recent activities on Delta-VLBI observations for NOZOMI.

The MIR re-entry brought much concern to Japanese government and public, because its final orbit had been announced to pass over Japanese populated area. We have carried out optical observations of the MIR space station from January 15 to March 11, 2001, when the MIR was visible from our observatory site and obtained enough accurate data for each observation night to determine its orbit. However, our observational capability is limited. It is necessary to develop a more accurate system and an idea is proposed. Using our data and NASA's two-line elements we were able to determine the MIR final reentry pass with sufficient accuracy and gave these information to the public through mass-media.

Japan Spaceguard Association (JSGA) carries out observations of asteroids and space debris at Bisei Spaceguard Center (BSGC) in Japan. Since the observations produce a lot of image data, we are considering to use them for education. One of the attempts is that we distribute the image data to school children and let them find asteroids. In order to find asteroids we developed software called "Asteroid Catcher B-612". This software is easy enough for children to handle. We are now distributing this software and image data to anybody who is interested in searching asteroids. We will supply image data by our web site, and then children can access much more recent data and can try to find asteroids. We call this next project "Spaceguard Detective Agency".

During the 2001 Leonids storm over Japan, several short-duration "outbursts" in which more than 20-40 meteors appeared in a few seconds, have been reported. The meteors in these events were extremely localized within a few hundred km, which indicate the recent fragmentation of meteoroids during the orbital motion in the interplanetary space. Considering the extent of the spatial distribution, the fragmentation should have occurred at around the perihelion passage of the meteoroids just before encountering the Earth. This suggests the possible existence of the glue, which connects several refractory particles in fresh meteoroids.

The MUSES-C mission is the first sample return mission from asteroid. It will be launched in the end of 2002, arrive at one of the Near-Earth Asteroid (25143) 1998 SF36 in 2005, and come back to the Earth in 2007. In this paper, the orbital evolution of this target asteroid is studied. Since this asteroid is almost in the ecliptic plane and the orbit crosses the orbits of the Earth and Mars, many close encounters with the Earth and Mars occur. Therefore, the motion of this asteroid is chaotic. We studied the orbital evolution of this asteroid by calculating the motions for many similar initial conditions. The results show that it is most probable that this asteroid exist around the present orbit for about several thousand years to the past or maybe for much longer period.

We developed a new 1.0 m telescope with a 3 degree flat focal plane to which a mosaic CCD camera with 10 2k×4k chips is fixed. The system was set up in February 2002, and is now undergoing the final fine adjustments. Since the telescope has a focal length of 3 m, a field of 7.5 square degrees is covered in one image. In good seeing conditions, 1.5 arc seconds, at the site located in Bisei town, Okayama prefecture in Japan, we can expect to detect down to 20th magnitude stars with an exposure time of 60 seconds. Considering a read-out time, 46 seconds, of the CCD camera, one image is taken in every two minutes, and about 2,100 square degrees of field is expected to be covered in one clear night This system is very effective for survey work, especially for Near-Earth-Asteroid detection.

Radar observations of two near-Earth asteroids were performed using a bi-static radar technique with Kashima 34-m antenna and Usuda 64-m antenna as receiving stations and Goldstone 70-m antenna as a transmitting station. The asteroid 6489 Golevka was observed on June 15, 1995 when its distance from the Earth became 0.048 AU and the radar echo from the asteroid was detected from the data observed with Kashima 34-m antenna. The success of the trans-continental bistatic radar observations became the first detection of a radar echo from a solar system object beyond Moon in Japan. The asteroid 4197 (1982 TA) was observed on October 24, 1996 when its distance to the Earth became 0.086 AU. The radar echo signal from the asteroid was detected from both of the data observed at Kashima and at Usuda. The received signal was coherently sampled and recorded at both stations. By using these data, interferometric data analysis was examined.

Leonids meteoroids raised attention in space organizations in the world due to probable collision hazards to artificial satellites. This paper describes the plan of the Leonids measurement campaign in Japan and the results. The correlation between optical and radiowave observations was especially focused on. The countermeasures to lessen the Leonids hazards to satellites will also be presented.

The Spaceguard Project proposes to set up a network of five ground-based telescopes, each with an aperture of 2-3 m. With ground-based telescopes, there is a blind celestial area surrounding the Sun (say, within 30 degrees), in which an asteroid approaching the Earth cannot be detected optically because of bright sky background. Our study shows that the number of close approaches of asteroids coming from this area to within a minimum distance of less than 0.01 AU of the Earth is more than 30% of the total.

We have estimated close asteroid encounters with the Earth by numerical integrations of a system with the Sun, 9 planets, and 188 near-earth-asteroids during the period 1994-4600. Asteroids approach the Earth from directions within 30° around the Sun in more than 20% of encounters with the closest distance less than 0.01 AU. Since groundbased observations cannot detect these objects, we should develop space-borne and/or lunar observatories in a short time to allow enough warning time before a catastrophic collision. © 1996 Kluwer Academic Publishers.

近年,将来の情報化社会にむけての世界的規模の情報通信基盤の構築が話題になっている.地上系の光ファイバーネットワークにおいてはGbitクラスの伝送速度が実現され実用化されているが衛星回線の伝送速度は100Mbps程度である.今後,衛星系を含めた地球規模の情報通信基盤ネットワークを構築するにあたってはこの衛星系の伝送速度を大幅にあげることが重要になる.郵政省通信総合研究所(CRL)では将来的にGbitクラスの衛星通信システムの構築を目指している.高速伝送を実現するためには中継器の広帯域化が必要であるが,この要求を満足するにはより高い周波数帯の利用が効果的である.CRLでは以前よりKa-bandを用いて衛星通信実験を行ってきており,この経験を生かして,このシステム実現のための第一歩として技術試験衛星VI型(ETS-VI)に搭載されたKa-band200MHzの帯域を持つ中継器を用いて伝送速度156MbpsでのHDTV伝送実験を計画していた.しかしながら,ETS-VIの静止化断念により,CRLではNTT所有の衛星であるN-Starを用いてNTTと共同で実験を行うことを決定した.本発表ではこの実験のシステムと予定されている実験項目について紹介する.