ACTA ASTRONOMICA Vol. 44 (1994) pp. 213±221
Orbit of the Enigmatic Object (2201) Oljato
by Krzysztof Z i o ø k o w s k i Space Research Center, Bartycka 18A, 00-716 Warsaw, Poland e-mail: [email protected]
Received April 13, 1994
ABSTRACT
The problem whether (2201) Oljato is an asteroid or an evolved comet is still open although many physical and dynamical aspects of this enigmatic object have been examined till now. The paper presents results of the new determination of its orbit made on the base of the complete observational material, covered the period from 1931 to 1993, and selected and weighted according to mathematical criteria. It turns out that the value of the root-mean-square (RMS) residual of all astrometric data for the nongravitational solution is signi®cant smaller than in the case of the pure gravitational one. It means that Oljato probably shows the residual cometary activity. Key words: Minor planets
1. Introduction
The title of this paper refers to the title of article by McFadden et al. (1993) in which the physical observations of the minor planet (2201) Oljato are reviewed and analyzedto answerthequestion: isit anasteroidoranevolved comet? The problem was opened when Russell et al. (1984) found a correlation between the variations in the interplanetary magnetic ®eld measured by the Pioneer Venus Orbiter in 1983 and thepassingofOljatonearVenus. Thehypotheticaltrailofmaterial along the orbit of Oljato, which can explain observed anomalies in the solar wind, found a support in the suggestions by Drummond (1982), by Babadzhanov and Obrubov (1983), and then by Olsson-Steel (1988), who proposed a link between this object and several meteor streams. Recent studies based on the spectrophotometric observations of Oljato made in 1979, 1983 and 1992, as well as the comparison of Oljato with comet P/Wilson-Harrington (1949 III) also known as asteroid (4015) 1979 VA, suggest that this object might be outgassing and thus be a remnant of a comet (McFadden et al. 1993, Schulz et al. 1993). Moreover, the orbital evolution of this near-Earth asteroid shows the properties, which might suggest a history related 214 A. A. to comets (Milani et al. 1989). A contribution to the problem resulted from the new determination of the orbit of Oljato derived on the base of all its astrometric observations available now is the main goal of this paper.
2. Historical Remarks
The minor planet (2201) Oljato was discovered in 1947 at the Lowell Obser- vatory by H.L. Giclas who made, however, only two its astrometric observations.
Basing on those two positions and two velocities, Rabe (1948) found provisional
= e = i = elements of its orbit ( q 0 12 AU, 0 98, 14 ) which show that the new asteroid 1949 XC is an unusual planet-crossing object. Unfortunately, the interest- ing minor planet was lost. Many years later J.G. Williams identi®cated this object with asteroid 1979 XA discovered by E.F. Helin in the Earth-Crossing Asteroid Survey (Helin and Shoemaker, 1979) and Marsden (1980) linked its 20 observa- tions, made during two oppositions in 1947 and 1979, by one system of orbital elements. Afterwards the asteroid was observed in 1980, 1982 and 1983, and it was named (according to a suggestion of the discoverer) after the place of Moon- light Water near Monument Valley, Utah, on the Navajo Indian Reservation (MPC 7782). Then the orbital elements of Oljato were computed by Landgraf (1985)
based on 53 astrometric observations that span the period only from 1979 to 1983 00
(RMS of residuals = 0. 8) and by Yeomans (1991) based on 49 astrometric optical
observations, and also four radar Doppler measurements, spanning the period from 00
1947 to 1983 (total RMS of residuals = 1. 06).
3. Former Results
Observational evidences that Oljato might be a still outgassing cometary rem- nant as well as dynamical properties of its orbit make reasonable a question if the orbital motion of this asteroid is affected by nongravitational effects that are common to the motions of comets. I have already tried to ®nd an answer to this problem some years ago (Zioøkowski 1991). To investigate whether the motion of
Oljato is perturbed by any additional forces I improved its orbit using a method in a which the secular change of semi-major axis ( _ ) of an object might be considered as a measure of hypothetical nongravitational anomalies (Sitarski 1981). The use of that model in searching for possible anomalies of nongravitational nature results not only from its simplicity but also from the fact that ± in contradiction to the Marsden's method commonly used in cometary researches (Marsden et al. 1973) ± it is independent of physical nature of phenomena responsible for anomalies being searched for and provides only the information about changes in orbital energy. Based on all 66 astrometric observations of Oljato, being made within the period from 1949 to 1983, I selected and weighted according to mathematical criteria (Bielicki and Sitarski 1991) 57 positions in right ascension and 61 positions Vol. 44 215 in declination, from which I found the following value of the secular change of semi-major axis of the orbit as a seventh unknown during improvement of six orbital elements:
10
a = ( ) _ 0 18 0 14 10 AU day
In the comparison with pure gravitational improvement the RMS of 118 residuals 00 has slightly decreased from the value of 0.00 95 to 0. 94. Thus it seems that it is impossible to ®nd even vestigial nongravitational effects in the motion of Oljato based on observational material that span the period from 1949to 1983. The useof
the Marsden's method to the determination of three nongravitational parameters A , has led to the same conclusion. It means that even though Oljato shows cometlike outgassing at present, it is at least not at a level suf®cient to detect the perceptible effects in the orbital motion. That con®rms the result of Yeomans (1991).
4. New Determination of the Orbit
Recently Bowell (1992) found an image of Oljato on the plate exposed by C.W. Tombaugh in 1931, and three new astrometric observations of the asteroid were made in 1992 and 1993. Moreover, the discoverer's observations from 1947 were corrected (MPC 20669). Thus the set of observations has extended and enriched on the very important positions. Therefore, to the new determination of the orbit of Oljato I could use 70 observations being made in 8 oppositions covered the period from 1931 to 1993. According to mathematical criteria (Bielicki and Sitarski 1991), I selectedand weighted 47 observations of the 1979/80opposition and ®nally I used 58 positions in right ascension and 63 positions in declination. Basing on those data, and taking into account the perturbations causedby the nine majorplanets and by the greatest minor planet Ceres, in the iterative process of the orbit improvement I obtained the following orbital elements of Oljato:
Epoch: 1993 Aug.1.0 ET Equinox: 2000.0
M
T = 1992Dec. 15.78023 ET = 70. 10438
11 4
q = 0.62872836 AU = 95. 93721
27 22
e = 0.71098420 = 76. 91007
13 22
i
a = 2.17541173 AU = 2. 51593
4 5
P
n = 0. 30717925 = 3.20858 ys 1 0
The improved orbit represents all the observations of Oljato with the mean residual of 1.00 06. 216 A. A.
5. Do Nongravitational Effects in the Motion of Oljato Exist?
Using the better observational material I renewed the investigation whether the orbital motion of Oljato is affected by nongravitational forces. An effort to detect, as former, the secular change of semi-major axis of the orbit has led now to the following value:
10
a = ( ) _ 0 10 0 04 10 AU day which seems to be more realistic then the previous one. It is interesting that in
this case the nongravitational orbit reduces the RMS residual of all observations to 00 1.00 03. More signi®cant reduction of the RMS residual ± to the value of 0. 98±I obtained then using the Marsden's model of nongravitational effects (Marsen et al. 1973). In the iterative process of correction of six orbital elements, together with
three nongravitational parameters A , I obtained the following solution:
Epoch: 1993 Aug.1.0 ET Equinox: 2000.0
M
T = 1992Dec. 15.78011 ET = 70. 10442
12 4
q = 0.62872887 AU = 95. 93864
40 51
e = 0.71098396 = 76. 90888
18 49
i
a = 2.17541167 AU = 2. 51587
5 5
P
n = 0. 30717926 = 3.20857 ys 1 0
9 2
= ( ) A1 0 13 0 07 10 AU day
12 2
= ( ) A2 0 22 0 08 10 AU day
10 2
= ( ) A3 0 72 0 27 10 AU day
As it was already mentioned, the above nongravitational orbit represents all the observations of Oljato with the mean residual of 0.00 98, that is signi®cantly less than in the case of pure gravitational solution (1.00 06). Thus it seems that the longer data interval made possible to detect the remnants of nongravitational effects in the motion of this object. It indicates that most likely Oljato shows the residual outgassing activity.
However, the negative sign of the determined value of A1 would imply that the outgassing is directed away from the Sun, which seems unlikely. It could only be explained if the lag angle between the object's noon and the point of maximum outgassing exceeded 90 , e.g., in the case of rapidly rotating body. Unfortunately, the uncertainties in estimates of the Oljato's spin period make impossible to solve now this question. Vol. 44 217
6. Conclusions
Determination of the orbit of Oljato on the base of the complete observational material available now shows that the motion of this asteroid seems to be perturbed by nongravitational forces that might indicate residual outgassing as in a comet. This effect could be detectedwhen the longerand richerdata sethasbeenused. The result should encourage to search for additional observations of Oljato obtained perhaps during the past oppositions. In order to make easier the inspection of archival plates the search ephemerides of Oljato for the apparitions of 1967, 1951, 1935,1931,1919,1916and1903arepresentedinTable1. Possible new astrometric observations would be very important for further orbit improvement of the object which appearsto be a most likely candidatefor an evolved comet among near-Earth asteroids.
Table1
Search ephemerides of (2201) Oljato
R
Date (ET) 1950 1950 mag 0 h m
1967 Mar. 31 3 13. 90 + 18 47. 4 1.234 0.826 15.2
Apr. 10 3 46.51 + 21 05.7 1.100 0.723
20 4 22.45 + 23 09.4 0.941 0.648 14.1
30 4 59.62 + 24 45.1 0.765 0.616
May 10 5 35.64 + 25 44.2 0.587 0.638 13.1
20 6 11.80 + 26 04.6 0.422 0.707
30 6 58.94 + 25 25.3 0.276 0.806 11.9
June 4 7 36.24 + 24 05.4 0.213 0.861
9 8 35.27 + 20 39.4 0.158 0.919 11.0
14 10 11.05 + 11 57.4 0.120 0.978
19 12 14.25 2 52.2 0.116 1.038 10.6
24 13 54.51 14 21.7 0.147 1.098
29 14 56.47 19 50.6 0.199 1.157 12.0
July 4 15 34.44 22 23.6 0.261 1.216
9 15 59.68 23 43.1 0.328 1.275 13.3
19 16 32.28 24 58.1 0.472 1.390
29 16 54.43 25 30.6 0.629 1.501 15.1
Aug. 8 17 12.43 25 46.5 0.797 1.608
18 17 28.72 25 53.8 0.976 1.711 16.3
28 17 44.28 25 55.1 1.163 1.810 218 A. A.
Table1
continued
R
Date (ET) 1950 1950 mag 0 h m
1951 Mar. 25 2 59. 60 + 17 42. 4 1.249 0.874 15.4
Apr. 4 3 30.00 + 20 03.1 1.128 0.767
14 4 03.91 + 22 15.3 0.982 0.681 14.3
24 4 39.80 + 24 06.2 0.815 0.631
May 4 5 15.08 + 25 25.5 0.640 0.631 13.2
14 5 48.93 + 26 10.3 0.472 0.682
24 6 26.99 + 26 16.8 0.321 0.768 12.2
29 6 53.29 + 25 53.3 0.253 0.820
June 3 7 32.17 + 24 40.3 0.191 0.875 11.3
8 8 37.20 + 21 04.2 0.137 0.933
13 10 27.46 + 10 53.2 0.103 0.991 10.3
18 12 41.79 5 31.8 0.107 1.050
23 14 17.64 16 05.9 0.145 1.109 11.2
28 15 12.47 20 40.0 0.200 1.168
July 3 15 45.41 22 46.4 0.263 1.227 12.7
13 16 23.45 24 32.8 0.401 1.342
23 16 46.87 25 15.4 0.551 1.454 14.7
Aug. 2 17 04.92 25 36.4 0.712 1.562
12 17 20.90 25 47.2 0.885 1.667 16.0
22 17 36.09 25 51.5 1.066 1.767
1935 Apr. 8 3 41.22 + 20 45.3 1.108 0.735 14.8
18 4 16.55 + 22 52.7 0.953 0.656
28 4 53.35 + 24 34.3 0.780 0.619 13.6
May 8 5 29.12 + 25 41.2 0.602 0.635
18 6 04.40 + 26 11.7 0.436 0.699 12.6
28 6 48.41 + 25 52.7 0.289 0.795
June 7 8 14.73 + 22 25.0 0.166 0.906 11.1
12 9 42.44 + 15 29.3 0.122 0.965
17 11 46.76 + 1 14.7 0.109 1.024 10.4
22 13 37.84 12 04.7 0.134 1.084
27 14 47.33 18 43.8 0.183 1.144 11.8
July 7 15 55.79 23 17.9 0.309 1.261
17 16 29.55 24 43.3 0.451 1.376 14.2
27 16 51.91 25 20.1 0.606 1.487
Aug. 6 17 09.88 25 38.7 0.773 1.595 15.7
16 17 26.05 25 47.9 0.949 1.698
26 17 41.48 25 50.8 1.134 1.798 16.7 Vol. 44 219
Table1
continued
R
Date (ET) 1950 1950 mag 0 h m
1931 Oct. 6 4 14. 73 + 18 42. 5 1.208 1.976 17.1
16 4 15.56 + 18 37.6 1.031 1.884
26 4 11.91 + 18 21.9 0.868 1.787 16.2
Nov. 5 4 02.32 + 17 51.6 0.722 1.687
15 3 45.08 + 17 00.0 0.597 1.583 15.1
25 3 18.94 + 15 38.4 0.494 1.475
Dec. 5 2 44.02 + 13 38.3 0.417 1.364 14.0
10 2 23.92 + 12 23.5 0.387 1.307
15 2 02.72 + 11 00.8 0.362 1.249 13.5
20 1 40.89 + 9 32.2 0.342 1.190
25 1 18.70 + 7 59.2 0.326 1.131 13.0
30 0 56.18 + 6 22.4 0.311 1.071
1932 Jan. 4 0 32.98 + 4 40.3 0.299 1.011 12.6
9 0 08.45 + 2 49.7 0.288 0.952
14 23 41.63 + 0 45.0 0.278 0.894 12.2
19 23 11.49 1 39.7 0.272 0.837
24 22 37.58 4 26.9 0.271 0.783 11.8
29 22 01.00 7 28.6 0.279 0.733
Mar. 4 20 14.77 17 46.2 0.663 0.663 13.4
14 20 31.58 17 34.7 0.798 0.746
24 20 51.54 16 55.2 0.910 0.851 14.6
Apr. 3 21 10.86 16 03.9 0.996 0.967
13 21 27.99 15 11.6 1.057 1.086 15.5
1919 Mar. 22 2 46.87 + 16 22.0 1.425 0.982 15.9
Apr. 1 3 15.83 + 18 42.6 1.324 0.867
11 3 49.12 + 21 00.5 1.198 0.761 15.0
21 4 26.79 + 23 04.8 1.049 0.677
May 1 5 07.96 + 24 41.3 0.880 0.630 13.9
11 5 51.23 + 25 36.6 0.703 0.633
21 6 37.58 + 25 39.3 0.535 0.687 13.0
31 7 34.99 + 24 16.7 0.388 0.775
June 10 9 01.90 + 19 16.2 0.273 0.882 12.1
15 10 03.22 + 13 48.0 0.235 0.940
20 11 14.35 + 5 53.4 0.218 0.998 11.9
25 12 25.60 2 50.2 0.227 1.058
30 13 26.98 10 04.4 0.258 1.117 12.5
July 5 14 15.16 15 04.6 0.306 1.176
10 14 51.94 18 19.7 0.364 1.234 13.5
15 15 20.35 20 27.1 0.430 1.292
20 15 42.93 21 52.9 0.501 1.349 14.3
30 16 17.32 23 36.1 0.656 1.461
Aug. 9 16 43.64 24 32.2 0.823 1.569 15.8
19 17 05.74 25 04.4 1.002 1.673
29 17 25.56 25 22.2 1.189 1.773 16.8 220 A. A.
Table1
concluded
R
Date (ET) 1950 1950 mag 0 h m
1915 Oct. 10 4 09 65 + 18 18. 7 1.148 1.963 17.0
20 4 07.67 + 18 05.5 0.979 1.870
30 4 00.51 + 17 38.8 0.826 1.773 16.0
Nov. 9 3 46.71 + 16 53.4 0.692 1.673
19 3 25.18 + 15 42.8 0.582 1.569 15.0
29 2 55.95 + 14 01.0 0.496 1.461
Dec. 9 2 21.06 + 11 50.0 0.435 1.349 14.0
19 1 44.37 + 9 24.2 0.394 1.235
29 1 08.77 + 6 59.5 0.365 1.117 13.3
1916 Jan. 8 0 33.73 + 4 36.7 0.340 0.999
18 23 54.05 + 1 48.0 0.314 0.883 12.4
23 23 29.64 0 03.7 0.303 0.827
28 23 00.79 2 23.0 0.295 0.775 12.0
Feb. 2 22 27.67 5 09.3 0.295 0.728
7 21 52.57 8 08.9 0.308 0.687 11.8
Mar. 8 20 28.24 16 52.6 0.634 0.676 13.4
18 20 41.92 16 49.5 0.761 0.761
28 20 59.45 16 16.9 0.865 0.866 14.6
Apr. 7 21 16.70 15 32.7 0.944 0.982
17 21 31.94 14 48.6 0.999 1.100 15.4
27 21 44.46 14 11.8 1.033 1.217
1903 Mar. 26 3 00.38 + 17 34.3 1.326 0.903 15.6
Apr. 5 3 31.33 + 19 55.5 1.209 0.793
15 4 06.42 + 22 08.9 1.068 0.700 14.6
25 4 44.92 + 24 01.8 0.905 0.640
May 5 5 25.13 + 25 21.2 0.730 0.627 13.5
15 6 06.37 + 25 58.8 0.558 0.665
25 6 53.71 + 25 41.5 0.402 0.744 12.6
30 7 24.25 + 24 55.2 0.332 0.794
June 4 8 04.06 + 23 14.9 0.271 0.847 12.0
9 8 58.46 + 19 47.9 0.219 0.903
14 10 11.58 + 13 12.1 0.184 0.961 11.4
19 11 37.75 + 3 16.0 0.173 1.020
24 12 58.68 6 41.6 0.191 1.079 11.8
29 14 01.38 13 40.4 0.232 1.138
July 4 14 46.34 17 52.4 0.286 1.197 12.9
9 15 18.76 20 23.2 0.349 1.255
14 15 43.09 21 57.6 0.417 1.313 13.9
19 16 02.20 22 60.0 0.490 1.370
24 16 17.93 23 43.2 0.567 1.426 14.7
Aug. 3 16 43.33 24 37.5 0.729 1.535
13 17 04.25 25 08.0 0.902 1.640 16.1
23 17 22.92 25 25.1 1.085 1.741 Vol. 44 221
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