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Photometric determination of the rotation period of 1566 Icarus
XC~,RUS10, 436-440 (1969)
P h o t o m e t r i c D e t e r m i n a t i o n of t h e R o t a t i o n Period of 1566 Icarus 1 E. M I N E R A~D J . Y O U N G
Jet Propulsion Laboratory, Ualifornia Ins$it~e of Technology, Pasadena, Ualifornia 91103 Received December 26, 1968; revised February 25, 1969 Measurements made of Icarus on the nights of 1968 June 19 UT and June 20 UT indicate a rotation period of 2h16.~1 ± 0.~3 (estimated error). The amplitude of the brightness variation amounted to 0.~056. On a previous night (June 16 UT), UBV colors were found to be U - B = 0.~45 ± 0.~05 (RMS) and B - V = 0~.78 ± 0~.01 (RMS). All light-curve data were obtained with a pulse-counting photometer mounted at the Cassegrain focus of the 60-cm telescope at J P L ' s Table Mountain Observatory. T h e passage of I c a r u s close t o t h e E a r t h in J u n e o f 1968 w a s n o t e d w i t h considerable i n t e r e s t b y b o t h t h e scientific c o m m u n i t y a n d t h e public in general. P a r t o f this i n t e r e s t arises f r o m t h e o r b i t a l p a r a m e t e r s o f I c a r u s , which r e p r e s e n t several e x t r e m e s a m o n g k n o w n (and n u m b e r e d ) asteroids:
Photoelectric observations of Icarus were m a d e a t t h e Cassegrain focus o f t h e 60-cm reflector a t J e t P r o p u l s i o n L a b o r a t o r y ' s T a b l e M o u n t a i n O b s e r v a t o r y on 16, 19, a n d 20 J u n e U T . D u r i n g t h e first night, we o b t a i n e d no light curves. W e did, h o w e v e r , m a k e m e a s u r e m e n t s in t h e U B V s y s t e m . A (1) I c a r u s has t h e s h o r t e s t o r b i t a l period s t a n d a r d U B V p h o t o m e t e r was used w i t h a m o n g k n o w n asteroids (409 days). a n A S C O P 541A (S-4) p h o t o m u l t i p l i e r , (2) I c a r u s has t h e largest o r b i t a l eccen- cooled w i t h d r y ice. I n t e n s i t i e s were r e a d t r i c i t y a m o n g k n o w n asteroids (0.827). f r o m a strip chart. T h e c o m b i n e d R M S (3) I c a r u s comes closer t o t h e Sun a t error associated w i t h e x t i n c t i o n correction perihelion t h a n a n y o t h e r k n o w n asteroid a n d color t r a n s f o r m a t i o n was a b o u t 0.01 (0.186 a.u.). m a g n i t u d e . Twice d u r i n g t h e n i g h t we (4) I c a r u s comes closer to b o t h M e r c u r y o b t a i n e d color a n d m a g n i t u d e d a t a for a n d t h e E a r t h t h a n a n y o t h e r numbered I c a r u s ; t h e t w o o b s e r v a t i o n s were separasteroid (~0.10 a.u. a n d 0.04 a.u., respec- a t e d b y a b o u t h a l f a n hour. I n T a b l e I we tively). p r e s e n t t h e results. W e list t h e colors o f t h e T h e p u r p o s e of this n o t e is to p o i n t o u t Sun for comparison. T h e colors o f I c a r u s t h a t I c a r u s m a y h a v e y e t a n o t h e r e x t r e m e are quite t y p i c a l of t h o s e m e a s u r e d for o t h e r asteroids. characteristic, n a m e l y , A n I T T F W - 1 3 0 (S-20) p h o t o m u l t i p l i e r (5) I c a r u s r o t a t e s m o r e r a p i d l y t h a n a n y (also cooled w i t h d r y ice) was used t o o t h e r asteroid for which a r o t a t i o n period o b t a i n light curves on t h e 19th a n d 20th. has b e e n m e a s u r e d (2.268 hr). A C o m i n g 3384 filter served to eliminate u l t r a v i o l e t r a d i a t i o n a n d t h e r e b y reduce 1 This paper presents the results of one phase of research carried out at the Jet Propulsion e x t i n c t i o n effects. T h e p h o t o m u l t i p l i e r Laboratory, California Institute of Technology, o u t p u t was fed t h r o u g h a p u l s e - c o u n t i n g under Contract Number NAS 7-100, sponsored s y s t e m a n d r e c o r d e d on a digital printer. by the National Aeronautics and Space Ad- W e used this s y s t e m in f a v o r of t h e S-4 s y s t e m described a b o v e because o f its ministration. 436
ROTATION
PERIOD
OF
1566
ICARUS
437
TABLE I PHOTOI~IETRIC PARAMETERS FOR 1566 ICARUS V = 12759 ± 0?02 V = 1 2 7 6 4 4- 0 ? 0 2 B -- V = 0?78 ± 0701 (B - V)8~ ~ 0?63 U - B = 0?45 ± 0?05 (U -- B)~ = 0714 Rotation Period 2b.268 4- h.O01 = 2~1671 4- 071 E p o c h o f M a x i m u m L i g h t = 6h44 m o n 20 J u n e 1968 = J D 244 0 0 2 7 . 7 8 0 A m p l i t u d e o f L i g h t C u r v e f o r t h e a b o v e E p o c h "-- 0 ? 0 5 6 1968 J u n e 1 6 . 3 1 4 16.331
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FTG. 2. Light curve for 1566 Icart~ as measured on 20 June 1968 UT. greater sensitivity to small changes in the incident light flux. Our observational data consist of one asteroid reading and one sky background reading a minute during the approximately 4 hr of usable viewing time each night. Each reading represented a 16-sec integration. Those readings obviously influenced by the presence of stars in close angular proximity to Icarus were discarded. I t was originally our intention to construct the light curves by comparison with a nearby star, but difficulties in
relocating the asteroid rendered this procedure impractical. Thus comparison star measurements were made only prior to and following the entire series of Icarus readings each night. The stars used for this comparison and also to obtain extinction values were BS4550 (G8 V) and BS5659 (G5 V). BS4550 was closer in color (B -- V = 0m.75) to Icarus, and was consequently used to determine the extinction coefficient. Using this coefficient, we reduced all Icarus readings to the zenith. Figures 1 and 2 show the remaining scatter
R O T A T I O N P E R I O D OF 1 5 6 6 ICARUS
439
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F~G. 3. Mean light curve for 1566 Icarus. in the data points obtained. The ordinates represent the number of pulses (in units of 103) counted during the 16-sec integration time (less the corresponding counts of sky background). Universal time of the measurement is given along the abscissa. Small " P ' s " and "S's" are used to indicate the times of occurrence of primary and secondary maxima. We then corrected all measurements to unit distance and zero phase, using a phase coefficient of 0m03/degree of phase. The
resulting data were then analyzed to determine periodicities, amplitudes, and epochs of maximum light. Initially, we measured a period of 1.134 hr. I t was suspected, however, that the actual rotation period might be twice as long. Many asteroids with nonspherical shapes exhibit two reflection maxima within a single rotation. Closer analysis of the Icarus data seemed to show the same phenomenon. We therefore deduced that the true rotation period was 2.268 hr in length.
440
E. MINER AND J . YOUNG
This agrees with the radar results obtained by Goldstein (1968) from which he estimated a rotation period between 1.5 and 3.3 hr. Following determination of the period all data points were combined into the single light curve shown in Fig. 3. The plotted points represent the averages of 3 to 12 data points within a given 2.7-min segment of the period. The error bars represent the formal probable errors associated with each of the points. The average error is approximately ±0.~015. The solid line is a smoothed version of the data. We smoothed the data by successively fitting five consecutive points in Fig. 3 to a third order polynomial and iterating the process several times. A change of 0.m056 in the brightness of Icarus corresponds to a change in projected area of only 5 %, indicating near sphericity rather t h a n a highly irregular shape as noted for m a n y other asteroids. I t is difficult to understand how such a short rotation period could have originated in a nearly spherical body. Only one other asteroid (321 Florentina) has a measured
rotation period shorter than 4 hr (Florentina's period is 2h52.~2) and its brightness variation is about 0.~4, (Van HoutenGroeneveld and Van Houten, 1958), indicating a much larger deviation from sphericity t h a n for Icarus. Seen in context with the irregular (for an asteroid) orbital elements, the rapid rotation of Icarus possibly suggests a nonasteroidal origin for this unique object. On the other hand, the color of Icarus is by no means exceptional for an asteroid. Most asteroids tend to cluster around B - - V = 0 . ~ 8 2 and U - B = 0 ~ . 4 4 (Wood and Kuiper, 1963). When compared to the scatter in color about the mean, a deviation of 0.m04 in B - V for Icarus is insignificant. REFERENCES
GOLDSTEII~,R. M. (1968). Radar observations of
Icarus. Science 162, 903-904. VAN
HO U'rmN-GROENEVELD,
I.,
AND
VAN
HOUTEN, C. J. (1958). Photometric studies of asteroids. VII. Astrophys. J. 127, 253-273. WOOD, I-I. J . , AND KUIPER, G. P . (1963). Photometric studies of asteroids. X. Astrophys. J. 137, 1279-1285.
P h o t o m e t r i c D e t e r m i n a t i o n of t h e R o t a t i o n Period of 1566 Icarus 1 E. M I N E R A~D J . Y O U N G
Jet Propulsion Laboratory, Ualifornia Ins$it~e of Technology, Pasadena, Ualifornia 91103 Received December 26, 1968; revised February 25, 1969 Measurements made of Icarus on the nights of 1968 June 19 UT and June 20 UT indicate a rotation period of 2h16.~1 ± 0.~3 (estimated error). The amplitude of the brightness variation amounted to 0.~056. On a previous night (June 16 UT), UBV colors were found to be U - B = 0.~45 ± 0.~05 (RMS) and B - V = 0~.78 ± 0~.01 (RMS). All light-curve data were obtained with a pulse-counting photometer mounted at the Cassegrain focus of the 60-cm telescope at J P L ' s Table Mountain Observatory. T h e passage of I c a r u s close t o t h e E a r t h in J u n e o f 1968 w a s n o t e d w i t h considerable i n t e r e s t b y b o t h t h e scientific c o m m u n i t y a n d t h e public in general. P a r t o f this i n t e r e s t arises f r o m t h e o r b i t a l p a r a m e t e r s o f I c a r u s , which r e p r e s e n t several e x t r e m e s a m o n g k n o w n (and n u m b e r e d ) asteroids:
Photoelectric observations of Icarus were m a d e a t t h e Cassegrain focus o f t h e 60-cm reflector a t J e t P r o p u l s i o n L a b o r a t o r y ' s T a b l e M o u n t a i n O b s e r v a t o r y on 16, 19, a n d 20 J u n e U T . D u r i n g t h e first night, we o b t a i n e d no light curves. W e did, h o w e v e r , m a k e m e a s u r e m e n t s in t h e U B V s y s t e m . A (1) I c a r u s has t h e s h o r t e s t o r b i t a l period s t a n d a r d U B V p h o t o m e t e r was used w i t h a m o n g k n o w n asteroids (409 days). a n A S C O P 541A (S-4) p h o t o m u l t i p l i e r , (2) I c a r u s has t h e largest o r b i t a l eccen- cooled w i t h d r y ice. I n t e n s i t i e s were r e a d t r i c i t y a m o n g k n o w n asteroids (0.827). f r o m a strip chart. T h e c o m b i n e d R M S (3) I c a r u s comes closer t o t h e Sun a t error associated w i t h e x t i n c t i o n correction perihelion t h a n a n y o t h e r k n o w n asteroid a n d color t r a n s f o r m a t i o n was a b o u t 0.01 (0.186 a.u.). m a g n i t u d e . Twice d u r i n g t h e n i g h t we (4) I c a r u s comes closer to b o t h M e r c u r y o b t a i n e d color a n d m a g n i t u d e d a t a for a n d t h e E a r t h t h a n a n y o t h e r numbered I c a r u s ; t h e t w o o b s e r v a t i o n s were separasteroid (~0.10 a.u. a n d 0.04 a.u., respec- a t e d b y a b o u t h a l f a n hour. I n T a b l e I we tively). p r e s e n t t h e results. W e list t h e colors o f t h e T h e p u r p o s e of this n o t e is to p o i n t o u t Sun for comparison. T h e colors o f I c a r u s t h a t I c a r u s m a y h a v e y e t a n o t h e r e x t r e m e are quite t y p i c a l of t h o s e m e a s u r e d for o t h e r asteroids. characteristic, n a m e l y , A n I T T F W - 1 3 0 (S-20) p h o t o m u l t i p l i e r (5) I c a r u s r o t a t e s m o r e r a p i d l y t h a n a n y (also cooled w i t h d r y ice) was used t o o t h e r asteroid for which a r o t a t i o n period o b t a i n light curves on t h e 19th a n d 20th. has b e e n m e a s u r e d (2.268 hr). A C o m i n g 3384 filter served to eliminate u l t r a v i o l e t r a d i a t i o n a n d t h e r e b y reduce 1 This paper presents the results of one phase of research carried out at the Jet Propulsion e x t i n c t i o n effects. T h e p h o t o m u l t i p l i e r Laboratory, California Institute of Technology, o u t p u t was fed t h r o u g h a p u l s e - c o u n t i n g under Contract Number NAS 7-100, sponsored s y s t e m a n d r e c o r d e d on a digital printer. by the National Aeronautics and Space Ad- W e used this s y s t e m in f a v o r of t h e S-4 s y s t e m described a b o v e because o f its ministration. 436
ROTATION
PERIOD
OF
1566
ICARUS
437
TABLE I PHOTOI~IETRIC PARAMETERS FOR 1566 ICARUS V = 12759 ± 0?02 V = 1 2 7 6 4 4- 0 ? 0 2 B -- V = 0?78 ± 0701 (B - V)8~ ~ 0?63 U - B = 0?45 ± 0?05 (U -- B)~ = 0714 Rotation Period 2b.268 4- h.O01 = 2~1671 4- 071 E p o c h o f M a x i m u m L i g h t = 6h44 m o n 20 J u n e 1968 = J D 244 0 0 2 7 . 7 8 0 A m p l i t u d e o f L i g h t C u r v e f o r t h e a b o v e E p o c h "-- 0 ? 0 5 6 1968 J u n e 1 6 . 3 1 4 16.331
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F r o . 1. L i g h t c u r v e *'or 1566 T c ~ ' u s as m e a s u r e d o n 19 J u n e 1968 U T .
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FTG. 2. Light curve for 1566 Icart~ as measured on 20 June 1968 UT. greater sensitivity to small changes in the incident light flux. Our observational data consist of one asteroid reading and one sky background reading a minute during the approximately 4 hr of usable viewing time each night. Each reading represented a 16-sec integration. Those readings obviously influenced by the presence of stars in close angular proximity to Icarus were discarded. I t was originally our intention to construct the light curves by comparison with a nearby star, but difficulties in
relocating the asteroid rendered this procedure impractical. Thus comparison star measurements were made only prior to and following the entire series of Icarus readings each night. The stars used for this comparison and also to obtain extinction values were BS4550 (G8 V) and BS5659 (G5 V). BS4550 was closer in color (B -- V = 0m.75) to Icarus, and was consequently used to determine the extinction coefficient. Using this coefficient, we reduced all Icarus readings to the zenith. Figures 1 and 2 show the remaining scatter
R O T A T I O N P E R I O D OF 1 5 6 6 ICARUS
439
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F~G. 3. Mean light curve for 1566 Icarus. in the data points obtained. The ordinates represent the number of pulses (in units of 103) counted during the 16-sec integration time (less the corresponding counts of sky background). Universal time of the measurement is given along the abscissa. Small " P ' s " and "S's" are used to indicate the times of occurrence of primary and secondary maxima. We then corrected all measurements to unit distance and zero phase, using a phase coefficient of 0m03/degree of phase. The
resulting data were then analyzed to determine periodicities, amplitudes, and epochs of maximum light. Initially, we measured a period of 1.134 hr. I t was suspected, however, that the actual rotation period might be twice as long. Many asteroids with nonspherical shapes exhibit two reflection maxima within a single rotation. Closer analysis of the Icarus data seemed to show the same phenomenon. We therefore deduced that the true rotation period was 2.268 hr in length.
440
E. MINER AND J . YOUNG
This agrees with the radar results obtained by Goldstein (1968) from which he estimated a rotation period between 1.5 and 3.3 hr. Following determination of the period all data points were combined into the single light curve shown in Fig. 3. The plotted points represent the averages of 3 to 12 data points within a given 2.7-min segment of the period. The error bars represent the formal probable errors associated with each of the points. The average error is approximately ±0.~015. The solid line is a smoothed version of the data. We smoothed the data by successively fitting five consecutive points in Fig. 3 to a third order polynomial and iterating the process several times. A change of 0.m056 in the brightness of Icarus corresponds to a change in projected area of only 5 %, indicating near sphericity rather t h a n a highly irregular shape as noted for m a n y other asteroids. I t is difficult to understand how such a short rotation period could have originated in a nearly spherical body. Only one other asteroid (321 Florentina) has a measured
rotation period shorter than 4 hr (Florentina's period is 2h52.~2) and its brightness variation is about 0.~4, (Van HoutenGroeneveld and Van Houten, 1958), indicating a much larger deviation from sphericity t h a n for Icarus. Seen in context with the irregular (for an asteroid) orbital elements, the rapid rotation of Icarus possibly suggests a nonasteroidal origin for this unique object. On the other hand, the color of Icarus is by no means exceptional for an asteroid. Most asteroids tend to cluster around B - - V = 0 . ~ 8 2 and U - B = 0 ~ . 4 4 (Wood and Kuiper, 1963). When compared to the scatter in color about the mean, a deviation of 0.m04 in B - V for Icarus is insignificant. REFERENCES
GOLDSTEII~,R. M. (1968). Radar observations of
Icarus. Science 162, 903-904. VAN
HO U'rmN-GROENEVELD,
I.,
AND
VAN
HOUTEN, C. J. (1958). Photometric studies of asteroids. VII. Astrophys. J. 127, 253-273. WOOD, I-I. J . , AND KUIPER, G. P . (1963). Photometric studies of asteroids. X. Astrophys. J. 137, 1279-1285.