Although there is uncertainty as to how bright comet Hale-Bopp will be at perihelion and there is no consensus as to whether or not it will be the "Comet of the Century", it is clear that this it will be a bright comet. Comet P/Halley may have been the most important comet scientifically this century because of the armada of spacecraft and intense ground-based observing effort, and certainly other comets have been better placed in the sky for viewing by the general public (including comet Hyakutake last spring), however, comets which are bright naked eye objects and are visible for several months from the northern hemisphere are very rare! This will make this an important comet both scientifically and for the public no matter what the final brightness. There are many scientific experiments which can only be done with very bright comets, either because we are spreading out the light into very small increments and therefore need a bright object (e.g. spectroscopy), or because the instrument technology is not as advanced as in other areas (e.g. in the infra-red, for detecting molecules).
Some examples of unique science which can only be done with a bright comet are discussed below for the case of comet C/1996 B2 (Hyakutake), and plans for coordinated observations of comet C/1995 O1 (Hale-Bopp) are described.
|Comet||Perihelion Date||Period||Perihelion||Eccentricity||Inclination||Arg. of Perihelion||Ascending Node|
|C/1995 O1||1997 04 01.146||2380 yr||0.914 AU||0.995||89.4||130.591||282.471|
|C/1996 B2||1996 05 01.396||29,270 yr||0.230 AU||0.999||124.9||130.2||187.3|
Jets and Rotation Rate - Observations during early March, 1996 from the European Southern Observatory (ESO; Chile), Pic-du-Midi in France, and other sites showed spectacular curved dust jets which were time-variable. Both the jets and observations made through narrow bandpass filters to isolate the light from either the dust or specific molecules showed that comet Hyakutake's brightness was changing on a timescale of 6.3 hours, probably due to nucleus rotation, and a single strong jet. The image at the right shows a false color image of the comet's inner coma which shows a prominent jet. The image was obtained with the ESO New Technology Telescope on March 19, 1996 (ESO Press Photo 25c/96). See larger version.
Fragmentation - During March, 1996, strong dust jets were reported in the inner coma of comet Hyakutake, and telescopes from sites with high resolution capabilities were reporting "knots" or "flakes" of material o moving away from the nucleus at velocities of 10-20 meters/sec. This type of behavior had not been seen before, but was interpreted as small pieces of the comet's surface flaking off. The image at the left from the Hubble Space Telescope taken on March 25, 1996 shows an image 3340 km across. Pieces which have broken off the comet and are forming their own tails are seen at upper left. Individual fragments could be traced from night to night in the images from the different observatories. See larger version.
Molecule Detection - Perhaps some of the most exciting discoveries with comet Hyakutake were the detections of many new molecules in the coma using radio telescopes and infra-red telescopes. Among the molecules discovered include a large suite of organic compounds such as methanol (CH3OH), methyl cyanide (CH3CN), hydrogen cyanide (HCN), formaldehyde (H2CO), methane (CH4), ethanol and ethane (C2H6). The ethane discovery was particularly exciting as this molecule had never before been seen on a comet. The relative abundance of ethane and methane were consistent with thermodynamic equilibrium, and suggested that they formed in a warm high-pressure region, which is inconsistent with our ideas of how comets formed. It is possible that maybe these comet constituents formed near the giant planets, say in a sub-nebula near Jupiter. However, if this interpretation is not possible, then a revision of the current astrochemical models may be needed since production of these molecules is believed to be inhibited in the ISM.
The Table below highlights some of the observed gases on comet Hyakutake and compares them to what we know about other comets and the ISM. The study of the abundance of many of these molecules, possible only with bright comets in many cases, is contributing greatly to our understanding of the origins of comets.
|CH3OH||99||4-5||1||1-5||1.5||0.023||0.049||Consistent with grains in ISM cloud cores|
|NH3||78||1||10||0.1-2||0.1-2||0.25||-||Consistent with molecular clouds|
|H2CO||64||0.2-1||-||0.5||0-5||0.002||-||Variable in P/Halley|
|CH4||31||0.2-7||0-5||0.02||-||0.7||-||First good detection in Hyakutake|
|CO||25||1000||13||1-7||7||5-5.8||6.4||Distributed source and nuclear source in Halley|
|S2||20||-||-||0.03||-||0.01||-||Seen in only 1 comet before|
|C2H6||-||-||-||-||-||0.4||0.13||Could be a lower limit|
At the Lowell Observatory astronomers D. Schleicher, B. Millis and T. Farnham are undertaking an extensive long-term observing campaign of comet Hale-Bopp. Schleicher and Millis are experts in the photometric observation of bright comets, and were key observers in the campaign to determine the rotation period of comet P/Halley when it was near perihelion in 1986.
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