Asteroid Observations in Bisei Spaceguard Center

Transcription

1 Asteroid Observations in Bisei Spaceguard Center Makoto Yoshikawa, Atsuo Asami, David Asher, Tetsuharu Fuse, Nariyasu Hashimoto, Syuichi Nakano, Kota Nishiyama, Yoshiaki Ohshima, Noritsugu Takahashi, Jun-ya Terazono, Hiroaki Umehara, Takeshi Urata, Syuzo Isobe Japan Spaceguard Association Kyonan-cho, Musashino-shi, Tokyo , Japan / makoto@pub.isas.ac.jp ABSTRACT Bisei Spaceguard Center was constructed in 1999 for the purpose of asteroid and space debris observations, and observations were started since the beginning of Up to the end of 2001, observations were carried out by small telescopes (25 cm and 50 cm), and several important results were obtained, such as the discovery of a large near earth asteroid, the light curve observation of a mission target asteroid, and close encounter observations of artificial satellites, etc. In December 2001, the 1 m telescope was installed and adjustment operations and test observations have been carried out since then. In this paper, the current status of Bisei Spaceguard Center is summarized. 1. Introduction At present (June 2002), the number of the "numbered asteroids" has reached about 40,000 and including "unnumbered asteroids" about 180,000 asteroids have already been cataloged. The rate of discovery has greatly increased recently. The number of Near Earth Objects (NEOs) discovered is also increasing rapidly especially in the last five years, and we know more than 1,800 NEOs now. Since NEOs have the potential to collide with the earth in the future, it is very important to discover all the NEOs as soon as possible. The activity to search for NEOs is called "spaceguard", and there are several teams working intensively to detect NEOs in the world. In Japan, the spaceguard activity has been enhanced since Japan Spaceguard Association was established in 1996, after the International Spaceguard Foundation was founded. At that time, there were no specialized facilities to observe NEOs in Japan, so observations of NEOs were carried out by getting observation time at existing observatories. This was not sufficient because continuous observations are Fig 1. Bisei Spaceguard Center

2 necessary to find and follow up asteroids. Therefore, JSGA requested the Japanese government to make a special center to observe only NEOs (or asteroids). After long discussions, the Japanese government finally agreed to construct such a kind of center, although for space debris as well as asteroids. This is the Bisei Spaceguard Center (BSGC), which is shown in Fig.1. BSGC was built by the Japan Space Forum (JSF) using a grant from the former Science and Technology Agency (STA), now the Ministry of Education, Culture, Sport, Science and Technology (MEXT). The operation is funded by the National Space Development Agency of Japan (NASDA). The observations and research work are carried out by the Japan Spaceguard Association (JSGA). The project for asteroids and space debris is called the BATTeRS (Bisei Asteroid Tracking Telescopes for Rapid Survey) project (Fig.2). Fig 2. The mark of BATTeRS 2. Systems in BSGC Bisei Spaceguard Center was constructed in 1999 in Bisei town, Okayama prefecture in Japan. At first 25 cm and 50 cm telescopes were installed and observations started from the beginning of In December 2001, the 1 m telescope system was also installed (Fig.3). In Table 1, the specification of the 50 cm and 1 m telescopes is summarized. The unique features of these telescopes are that their field of view is very large and that the tracking speed is high. This is because the main purpose of these telescopes is to discover and to track asteroids and space debris. The CCD camera for the 1 m telescope has ten 2k x 4k CCD chips (Fig.4) and that for the 50 cm has two CCD chips of the same size. The CCD camera for 25 cm telescope has one CCD chip of 2k x 2k. The CCD cameras for 50 cm and 1 m telescopes are cooled by liquid nitrogen, and that for 25 cm is cooled by the Peltier device. Fig.3 The 50cm (left) and 1 m (right) telescopes

3 Table 1 Spec of telescopes 1 m telescope 50 cm telescope Focus mode Cassegrain-type focusing, F/3 Cassegrain-type focusing, F/2 Field of view 3 degrees 2 degrees Max. tracking speed More than 1 deg/sec. More than 5 deg./sec Structure Fork-type equatorial Fork-type equatorial CCD camera Approx. 160-mm imaging diameter Approx. 50-mm imaging diameter Ten 2k x 4k CCD chips Two 2k x 4k CCD chips CCD temperature Approx. 173 K during observation Approx.173K during observation In order to understand the field size of each CCD camera, we show here the images of M42, which were taken by each telescope system (Fig 5). We can imagine how efficiently we can get data of asteroids if we use the 1 m telescope system. These telescope systems will produce a huge amount of data and so the data reduction system (hardware and software) is also very important. In fact, it is estimated that about 50 GB data will be obtained in one night if the systems are operated fully. Fig.4 CCD chips in the camera for 1 m telescope Fig.5 Images of M42 taken by the three telescope systems in BSGC. The printed size of each image is arbitrary. Left: by the 25 cm telescope with one CCD chip of 2k x 2k Middle: by the 50 cm telescope with two CCD chips of 2k x 4k Right : by the 1 m telescope with ten CCD chips of 2k x 4k

4 3. Results of observations up to now The 1 m telescope was installed in December 2001; but it is still in the phase of test observations. Therefore, we have been carrying out our observations mainly using the 50 cm and 25 cm telescopes. However, since the beginning of 2000, we have observed a lot of asteroids and artificial satellites. We also discovered one new comet C/2001 W2, which is named as "BATTeRS", in November The statistics of the observations in the year 2000 and 2001 are summarized in Table 2. Table 2 Statistics of observations Year Asteroids number of observations number of objects 1733 (221) 1492 (78) NEOs number of observations number of objects 47 (1) 55 (0) Comets number of observations number of objects 31 (0) 46 (1) Space debris number of observations number of objects 680 (204) 676 (97) The numbers in the parentheses are the numbers of objects which are newly discovered in BSGC. Fig.6 shows the numbers of observations of asteroids in each month in June is the month in the rainy seasons in Japan, so the number of observation is small. The number of observation is also small in December 2001, because of the operations of installation of the 1 m telescope. Fig.7 shows the magnitude Fig.6 Numbers of observations in 2001 Fig.7 Magnitude distribution of observed asteroids

5 distribution of asteroids observed in Since the 25 cm telescope was mainly used in this year, the limiting magnitude is about Up to now, about 300 new asteroids have been discovered by the observations in BSGC. Most of them were main belt asteroids, but one new NEA was also found. This is 2000 UV13, an Apollo type asteroid (semimajor axis = AU, eccentricity = 0.62, inclination = 31.9 degs; Fig.8). Its orbit crosses the orbit of the earth (the perihelion distance is 0.91 AU). The main feature of this asteroid is that its absolute magnitude is This indicates that the diameter of this asteroid is from 5 km to 12 km. This is the second largest one among the asteroids that cross the orbit of the earth (Apollo and Aten type). It is widely believed that all Near Earth Asteroids (asteroids whose perihelion distance is less than 1.3AU) with a size of about 10 km or more have been discovered. However this discovery of 2000 UV13 implies that there are still such large asteroids that remain unknown. The orbit of 2000 UV13 was soon determined accurately because its image was found in past observations, and it was given the number (20826). Fig.8 Orbit of (20826) 2000 UV13 Another important observational result is the light curve observation of Asteroid (25143) 1998 SF36. This asteroid is the target of MUSES-C mission of Japan. In this mission, MUSES-C spacecraft will go to this asteroid, obtain a sample of the surface material, and come back to the earth. It will be launched at the end of It is very important to know physical characteristics of this asteroid before the spacecraft is launched. From our observations, it was found first that the rotational period is about 12 hours. As for the observations of space debris, several important results were also obtained. For example, we found a Russian Satellite approaches a Japanese meteorological satellite very closely. We also observed the Russian space station MIR before its reentry. 5. Data archive and educational project In addition to the observational activities, we are considering our data archive system and some educational projects using the image data taken in BSGC. Fig.9 shows the top page of the Web-based data archive system. This is still tentative one and at present it is planed to open to the public at the end of

6 2002. Our policy of data distribution is that the registered persons can access the image data which are taken more than two weeks ago. The inaccessible period of two weeks is extremely short in comparison with other astronomical observatories. We think astronomical researchers in various fields can utilize our data. As for the educational projects, we carried out a project called "International Asteroid Search Project" in the beginning of 2001 in collaboration with the British Council Fig.9 Top page of Web-based data of Japan and Yomiuri Shimbun (one of the largest newspaper archive system companies in Japan). In this project, we distributed the originally developed software "Asteroid Catcher B-612" (Fig.10) and image data files to the participants by two CD-ROMs. The participants tried to find asteroids and competed the number of asteroids that they found. This project was quite successful and many children and adults enjoyed finding asteroids. We plan to continue such projects in the future, and now we have finished making a elementary guide book to join our project. We named our educational project as "Spaceguard Detective Agency". Fig.10 The software to detect asteroids for education: "Asteroid Catcher B-612". (Left: Japanese version, Right: English version) 6. Summary At present (June 2002), the system of Bisei Spacegurad Center (BSGC) has almost been completed. BSGC has three telescope systems, which are 25 cm, 50 cm, and 1 m. But still we need some more works to start full observations. The test observations in BSGC were started in the beginning of 2000, and we have already obtained many important observational results both for asteroids and for space debris. The remarkable results for asteroid observations are the discovery of one large Near Earth Asteroid (2000 UV13), and the light curve observation of the mission target asteroid 1998 SF36. If the observations with the 1 m telescope start, it is expected that we can obtain much more interesting results.