The images shown here are based on a unique series of fourty-four 2-sec exposures, obtained under excellent conditions (seeing 0.7 - 0.8 arcsec) in the morning of March 19, 1996 with the SUSI camera at the ESO NTT at La Silla, Chile. The observers, Olivier Hainaut (Institute of Astronomy, Honolulu, Hawaii) and Richard West (ESO), were assisted by Manuel Pizarro (ESO La Silla) and Vicente Reyes (ESO Garching).
These images were obtained during an observing run in remote control from the ESO Headquarters in Garching (Germany). In order to better show the interesting features, they are all reproduced in false colours.
The comet was 0.222 AU (33.2 million km) from the Earth and 1.165 AU (174 million km) from the Sun at the time of these observations. At this geocentric distance, 1 arcsec corresponds to 161 km. The nominal angular resolution, as indicated by the seeing, was about 110 kilometres projected at the distance of the comet.
The comet was moving at a speed of about 600 arcsec/hour in the sky and it was not possible to use the automatic guider. Instead, the NTT was set to perform blind tracking at the rate of the cometary motion. During each exposure, i.e. in 2 seconds, the comet moved 0.5 arcsec in the sky. The advantage of using a large telescope like the NTT for this type of observation is not only its intrinsically good imaging quality, but in particular that enough light to show even faint details may be collected in very short time, thereby avoiding additional image smearing because of the motion during the exposure.
In both frames, a straight ion tail is seen at p.a. 276 (towards West, i.e. right); it is noticeable at only 0.7 arcsec (115 km projected) from the peak of light (within which the nucleus is presumed to be located).
Strong dust jets are also visible. They trace the ejection of dust particles from the surface of the cometary nucleus. Of particular interest is the fact that there are substantial morphological changes on arcsec scale within the short time interval between these exposures. This is well brought out on a `differential' frame that has been obtained by subtracting the frame from the later epoch from that of the earlier one (ESO Press Photo 25d/96 [GIF,216k]).
This is the caption to ESO PR Photos 25a-d/96. They may be reproduced, if credit is given to the European Southern Observatory.
Go to the ESO Homepage for Comet Hyakutake.