Results
The spectra for 944 Hidalgo, and 2201 Oljato are essentially identical to the solar spectrum. No OH emission, or other emission lines were apparent.
Ground-based observations of these two objects were limited to several CCD images of the area in which 2201 Oljato was calculated to appear. Both 944 Hidalgo, and 2201 Oljato were near 19th magnitude. This placed them beyond the limit of most amateur telescopes.
None of the other three asteroids showed signs of the cometary emission that would be expected from a weakly active cometary nucleus. This should not be surprising, since the targets lie in stable main-belt orbits.
Ground-based observations of 182 Elsa, 224 Oceana, and 899 Jokaste were conducted visually, photographically, with VHS, and with CCD equipment. 224 Oceana was at 12th magnitude during the HST observations. 182 Elsa was also at 12th magnitude, and 899 Jokaste appeared at 15th magnitude.
All visual observations showed point images, with no sign of a dust coma. Photographic images gave the same result. A number of observers were able to accumulate CCD images in order to reach fainter magnitudes. Using this method, two observers reported signs of a possible short tail on the asteroid 899 Jokaste. This appeared around 17th magnitude. Studies of nearby stars on the original image suggest that this was due to a small amount of trailing during guiding. There was no further evidence of a coma around any of the three asteroids.
Observations of 944 Hidalgo and 2201 Oljato had to be conducted before the Hubble Space Telescope servicing mission. As a result, the two asteroids were not in convenient positions to detect OH. Hidalgo was imaged at 5 Astronomical Units from the sun, and moving away. This is approximately the same distance as SL-9 was before it impacted on Jupiter. The Hubble Space Telescope, using the same Faint Object Spectrograph and spectral element G270H, also failed to find any evidence of OH from SL-9.
2201 Oljato was near aphelion when it was observed. From the distance of the asteroid belt, any OH signature may have been hidden by noise in the data of the 19th magnitude asteroid. Ideally, both objects should have been studied shortly after perihelion passage.
The main-belt asteroids 224 Oceana, 182 Elsa, and 899 Jokaste were observed near opposition, but did not show any evidence of OH emission. The last two were observed when the Hubble Space Telescope was having guidance problems, and had to present special orientation to the sun due to the loss of one solar panel. The reports of previous coma may have been due to impacts.
Read more about this topic: Hubble Search For Transition Comets
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