Important new information has become available today about 1) more molecules detected; 2) planned HST observations, and 3) ROSAT observations to be performed.
Richard M. West (ESO)
The head is now reported to be larger than 1 degree, when viewed from a dark site. The maximum size will probably be about 2 degrees (4 lunar diameters).
Ion tails up to 20 degrees long have been reported by naked-eye observers. The dust tail is still much fainter, although the recent increase in dust production, as testified by the new activity near the nucleus, indicates that this tail will from now on become more important. Predictions are extremely difficult, and estimates range from some 30 deg to perhaps twice this size for the straight ion tail and much less for the curved dust tail.
Yet another molecule, OCS, has now been detected by means of the NRAO 12-m telescope at Kitt Peak, Arizona, USA (IAUC 6344).
The additional detection with the same telescope of H2CO (formaldehyde) and CH3OH (methyl alcohol), and also again of HCN and CO, was reported on March 21 (IAUC 6345).
There is no doubt that the dust production by vents on the surface of the nucleus has now really begun. The onset of this phenomenon was expected for some time and it provides yet another confirmation of Comet Hyakutake's status as a major comet.
The ESO observations, carried out during a spell of very good seeing (down to 0.39 arcsec or 56 km projected), apparently do not show a signal from the nucleus itself. It is therefore likely that it is considerably smaller. It is expected that the upcoming Hubble Space Telescope observations (se below) will achieve significantly sharper images, although the comet's extremely high rate of motion must be exceedingly accurately compensated for in order to fully exploit this telescope's inherent very high angular resolution.
HST observations will be made on four occasions, from March 26 to April 2. They will involve both direct imaging in the UV and visual parts of the spectrum, as well as UV spectra. While the former will provide the sharpest images possible with existing telescopes, the UV spectra are particularly suited to study the atomic and molecular emissions from the coma and tail. The comet's unusual brightness and small distance will provide a unique opportunity for very detailed studies. More details are now available on a HST Press Advisory (March 21).
At NRAO's Very Large Array radio facility in New Mexico (USA), measurements will be made of the thermal (heat) emission from the nucleus. Moreover, a radar experiment will be carried out during which a radar beam from the Goldstone facility in California will be bounced off the cm-size dust surrounding the cometary nucleus (possibly - hopefully - also from the surface of the nucleus, although it is not yet sure whether the dust coma will permit the radar beam to penetrate to it) and the echo will be received by the VLA.
Infrared observations are planned with all major facilities on the Northern Hemisphere.
Observations with the ROSAT X-Ray satellite observatory will be performed on March 26 - 28. The total integration time will be nearly 20,000 seconds.
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