Hubble Search For Transition Comets - Research and Study

Research and Study

The team used the Hubble Space Telescope to perform a spectroscopic search for OH emission from five asteroids. OH emission would indicate that the asteroids were once comets. 944 Hidalgo and 2201 Oljato move in elliptical, comet-like orbits. 182 Elsa, 224 Oceana, and 899 Jokaste are main-belt asteroids. The last three have been observed with coma (Kresak, 1977). Concurrently with the spectroscopic study, ground-based visual observations were carried out by 80 amateur astronomers in 22 countries.

Scientists suspect that some asteroids were once comets. A comet loses part of its mass with each passage around the Sun. It would appear that some would eventually use all of their volatiles, or perhaps cover these under a blanket of dust after repeated passages around the Sun. Such an object might then have an asteroid appearance.

The asteroid 944 Hidalgo is most frequently discussed as being in a comet-like orbit. In fact, Kresak (1977) identified it as an "extinct comet nucleus". In addition, comets tend to approach Jupiter closely while asteroids do not. Hidalgo approaches Jupiter at nearly the same distance as 3 comets: Comet Encke, Comet Arend-Rigaux, and Comet Neujmin I, all of which exhibit the low levels of activity expected of comets before they become extinct.

The Pioneer Venus Orbiter detected magnetic field disturbances that are correlated with 2201 Oljato. This could be caused by an outgassing of H at a rate of about 10-4 that for an active comet. Oljato's possible comet-like nature is supported by its unusual UV reflectance. It has been interpreted as Rayleigh scattering from a cloud of fine particles around it.

On December 13, 1923, the astronomer Josep Comas Solá observed the asteroid 224 Oceana with a coma. The asteroid was photographed with a faint halo 30 arc-seconds across. The asteroid's magnitude was determined to be 11.6, and at the asteroid's heliocentric distance of 167 million miles, this made the coma about 24,000 miles across.

The existence of volatiles on asteroids would be of great significance to future miners of the asteroid belt. Volatiles could supply water, fuel and oxygen for missions.

For the 1993 study involving amateur astronomers and the Hubble Space Telescope, 944 Hidalgo and 2201 Oljato were examined with the Hubble Space telescope's Faint Object Spectrograph for 3085 A emission of OH. These two asteroids were selected because of the nature of their orbits, their meteor-shower associations, and their other characteristics; see Weissman et al. (1989) for a full discussion of their comet-like features. The amber detector was used in the accumulation mode with spectral element G270H. This element covers wavelengths 2325-3225 A. The aperture was 1 arc-second.

The asteroids 182 Elsa, 224 Oceana, and 899 Jokaste were also observed with the Faint Object Spectrograph, and with the same spectral element G270H.

The team leaders placed advertisements in amateur astronomy publications for amateur observers who would be willing to observe these objects in the visible spectrum from the ground, while the HST was making studies in the UV. The ground-based observers were asked to check for evidence of a dust coma.