CCD observations of Icarus (1566)

Chin.Astron.Astrophys.(1989) 13/l 69-72 a translation of Acta Astron.Sin. (1988) 29/3 260-263

0 Pergamon Press plc. Printed in Great Britain 0275-1062/89$10.00+.00

CCDOBSERVATIONS OF ICARUS(1566) ZHANG Jia-xiang YANG Ji-xing WU Yue-ehen Purple Mountain Observatory, Academia Sinica

Received 1987 December 31

Abstract We report here 18 precision positions of the asteroid Icarus (1566) obtained by us on 1987 July 30 and 31 using CCD equipment on the lm telescope of Yunnan Observatory.

1. INTRODUCTION The asteroid Icarus (No. 1566) has an orbital semi-amajor axis of 1.078 AU and an orbital eccentricity as high as 0.828, hence its perihelion distance is as small as 0.186 AU. With its orbit near the perihelion deep inside the orbit of Mercury, it is, apart from Mercury, the most suitable planet for testing the theory of general relativity from the precession of perihelion. For general relativity, the perihelion precession per century of a planet is given approximately by, [ll, X = 3.841 / (l-e2)aT

,

where e is the orbital eccentricity, a the semi-major axis in AU, Tthe period in years and X is in arcseconds. Because of the geometrical properties of the orbital parameters, it is not X itself, but eX that provides effective comparison with measured results. The relevant data on the four inner planets and Icarus are shown in TABLE 1. TABLE 1 PLANET Mercury Venus Earth Mars Icarus

x 43.03" 8.63 3.84 1.34 10.18

eX 8.82" 0.06 0.06 0.12 8.45

The importance of observations of Icarus can be seen. This planet was discovered in 1949, when it was very close to Earth and relatively bright. It approaches Earth about once every 19 years. At the close approach of 1968, a large amount of observation was done in the observatories worldwide (including some radar observations) with the aim of testing the theory of general relativity, and Shapiro et al. made special orbital studies [2]. In July 1987, Icarus again approached Earth to within 0.16 AU. The US observatory at Aricebo formulated a

70

Zhang Jia-xiang

et

al.

radar observing program for June 20-25, and the International Astronomical Telegraph Center urged worldwide support [31. Since an extension of the observing period can be of decisive importance in the study of orbital motion, we endeavoured to take part in this year’s observation with the hope of obtaining the best ever data since its discovery 38 years ago, thereby contributing to the study of the relativistic effect.

2.

INSTRUMENTATIONSUMMARY

Because Icarus is a faint object and its position is rather far south for us, we made our observations at Yunnan Observatory, using the lm telescope equipped with a CCD. The telescope has a focal length of 13.5 m and the basic data of the CCD are number of pixels: size of pixels: angular size of pixels: field of CCD:

320 x 512 3OP x 3o/.J 0.46” x 0.46” 2.5 ’ x 4.0’

The instrument has a large dynamic range, a high degree of linearity, high sensitivity and accuracy; apart from its use for astrophysical observations, it is also a new, powerful means for determining precise positions of specific objects.

Fig.

3.

1

A CCD picture of Icarus, taken on 1987 July between 14h 03m 37’ and 14h 13” 37’ UT

31

RESULTS OF OBSERVATION

In the last days of July, the brightness of Icarus was about magnitude 19 and its uosition was at declination -20” and lower. At the end of astronomical twilight in Kunming when we started observing, it was located in the southwestern sky at an altitude of under 30’ decreasing This, plus the to 17’, and the actual magnitude was as faint as 20. rapid motion of the asteroid and the extremely small field of the CCD,

a

Icarus

(1566)

71

Positions

On the two rare fine nights in meant a highly difficult observation. repeatedly refining the rainy season in Kunming, July 30 and 31, after the predicted position and velocity of Icarus at various times, and using handing guiding of star under visual monitoring during the observation, we eventually overcame the many difficulties and Figure 1 successfully obtained 18 frames giving 18 precise positions. shows one of the frames obtained. The final calibration stars for the measurement reduction were taken from the SAO Catalog. Because the observed field was very small, And at that usable calibration stars were rare or even non-existent. and we could no longer use the CCD for time, fine nights were rare, calibration observations; this meant great difficulty in the reduction of position. One of the authors (YANG Ji-xing) suggested and carried out a method of repeated calibration with star map recovery (this will and eventually reduced the position of be described in another paper), These results are the target object and obtained satisfactory results. given in TABLE 2

TABLE 2 U. T.

1987 7 30.55216

Observed

R. A. (1950)

Positions

of

Icarus

DECL. (1950)

(1566) o-c cfm 8 . ha A6

15h lZp 28!07

-240

09’ 07“3 .

30.57369

15

12 30.24

-24

09

30.59522

15 12 32.77

-24

09 37.3

30.61189

15

12 34.76

-24

09

54.4

2.2

-1.6

30.62891

15

12 36.54

-24

10 07.4

0.3

-1.6

20.6

2n1 -015 -1.1 0.5

2.9 2.7

30.64314

15 12 37.96

-24

10

31.56142 31.57080

15 14 23.97 15

14 24.88

-24 -24

22 03.2 22 10.5

-2.3

-0.7

31.57994

15

14 26.01

-24

22

-0.6

-1.5

14.2

18.0

-2.2 -0.6

2.4 -0.4

31.58932

15 14 27.05

-24

22 24.3

-0.5

-0.8

31.59661

15

14 27.76

-24

22 30.2

-1.8

-1.4 -1.3

31.60216

15

14 28.39

-24

22 34.2

-1.6

31.60876

15

14 29.06

-24

22 39.0

-2.4

31.61466

15 14 29.93

-24

22 44.6

0.6

-1.2 -2.5

31.61976

15

14 30.42

-24

22

48.3

-0.4

31.62647

15

14 30.97

-24

22 52.2

-3.1

-1.4

31.63503

15

14 32.03

-24

22 57.4

-1.6

-0.4

31.64314

15

14 32.91

-24

23 03.9

-1.9

-0.9

-2.4

The O-C values shown are the differences between the observed positions and the calculated positions based on the osculating elements. The latter were taken from the 1987 Ephemeris Volume [4] and are shown in TABLE 3 According to the data we have collected [5], during the second part of July 1987 when Icarus was relatively bright and its position higher in the sky, 27 positions were obtained with CCD on large telescopes The abroad, but since July, our observations seem to be the only ones. extended observing arc should be valuable in the numerical improvement

72

Zhang Jia-xiang et al. TABLE 3

Osculating Elements of Icarus (Equinox 1950.0) Epoch a e

1987 7 24.0 ET 1.0779614 0.8266639 31.17709° 87.49786" 22.88904" 64.38976"

z f

of the precise orbit of Icarus and in isolating effectively the relativity and other effects.

Acknowledgement Colleague WANG Qi participated in the measurement. Yunnan Observatory Stellar Division gave us great support, especially Colleague LI Zi-li rendered us great and prolonged assistance throughout our observing period and Colleague MAO Wei of the Astrometry Division discussed usefully with us; to these two Colleagues, our sincere thanks. We also acknowledge financial support from the Academy Astronomy Foundation.

REFERENCES [1 ]

Smart

[21

Shapiro

W. hf.,

Celestial

1. I., Smith

[31

Vol. 76(1971), 588. 1. A. U. Circular, No.

[ 4]

Ephemerides

[ 5]

The

MINOR

Mechanics

(1953),

245.

W. B., Ash hf. E. and Herrick

of Minor PLANET

S., The Ast~~w.xG~l

~ou~&,

4390(1987). Planets

(1987),

44.

CIRCULAR/MINOR

PLANETS

AND COMETS,

11887--12624(1987).