Planetary Science Research Discoveries HOT IDEA HEADERposted February 14, 1997 (updated March 24, 1997) 1997 Apparition of Comet Hale-Bopp
What We Can Learn from Bright Comets
by Karen Meech

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 Hyakutake

Comet Hyakutake was discovered on January 31, 1996 at about 11th magnitude. Like comet Hale-Bopp, the comet is not fresh from the Oort Cloud, having last passed close to the sun about 8,000 years ago. In addition, the comet was not unusually large, nor was it instrinsically an especially active comet. What was notable about this comet was the fact that its orbit (see the table below for a comparison of the orbital parameters of comet Hale-Bopp) carried it very close to Earth, passing within 0.102 AU (15.3 million km) on March 25, 1996. This provided an opportunity for astronomers to observe a bright comet (because the reflected light intensity depends on the square of the distance from the object), and to get much better resolution since the comet was so close. The first tail observations were reported during mid-February, and near the time of the closest Earth approach, the tail was reported to be as long 100 degrees! Few historical comets have had tails this long. Unlike most comets which boast prominent dust tails, this comet was not very dusty, so it had a low surface-brightness gas tail.

Comparisons for C/1995 O1 Hale-Bopp and C/1996 B2 Hyakutake
CometPerihelion Date Period Perihelion Eccentricity Inclination Arg. of Perihelion Ascending Node
C/1995 O11997 04 01.1462380 yr 0.914 AU 0.995 89.4130.591282.471
C/1996 B21996 05 01.39629,270 yr 0.230 AU 0.999124.9130.2187.3


Nucleus Size - Because comet Hyakutake was passing so close to the Earth, astronomers had the unique opportunity to observe the nucleus directly by bouncing radar off the surface. S. Ostro (JPL) used the Goldstone Antenna to observe the comet on March 24 and 25, 1996. From the returned echo he was able to estimate a size for the nucleus of only 1-3 km, making the comet relatively small. Comparing the amount of dust and gas being produced to that of P/Halley, which had only 10% of the surface active, it was determined that a large fraction of this comet was outgassing (approximately 60%).

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.

Comet and ISM Composition Comparison
SpeciesT_s* ISM-GasISM-DustMost CometsP/HalleyHyakutakeHale-BoppImplications
H2O 152 100 100 100 100 100 100-
CH3OH 99 4-5 1 1-5 1.5 0.023 0.049Consistent with grains in ISM cloud cores
HCN 95 2 - 0.02 0.1 - --
NH378 1 10 0.1-2 0.1-2 0.25 -Consistent with molecular clouds
CO2 72 10 0-3 - 2-4 5 - -
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 6.4Distributed source and nuclear source in Halley
N2 22 10-1000 - 0.1 5 - --
S2 20 - - 0.03 - 0.01 -Seen in only 1 comet before
C2H6 - - - - - 0.4 0.13Could be a lower limit
CH4:CO - 0.001-0.0030.13-2.4- - 0.12 --
* T_s = Sublimation Temperature

Observing Plans for Comet Hale-Bopp

Because the comet is expected to be very bright, a large number of astronomers are planning observations of the comet. When a comet is bright, a large number of different observing techniques in different wavelength regimes may be used to investigate different aspects of cometary physics. Below are selected programs from some of the major teams which routinely observe bright comets.

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.

A team of comet scientists, lead by M. Mumma (Goddard) has been very active at looking at comets in the infra-red wavelengths to try to discover the composition of the parent molecules in the comet. The team (consisting of: N. Dello Russo, M. DiSanti, M. Fomenkova, K. Magee-Sauer, B. Novak, D. Reuter, and Y. Pendleton) plans to use the NASA Infrared Telescope on Mauna Kea in late January, late February and mid-April to do the following: Astronomer Harold Weaver, at the Johns Hopkins University has specialized in the observation of bright comets using the Hubble Space Telescope (HST) Facility. Since 1995 Weaver and his collaborators have been monitoring the comet with the HST which can achieve superior resolution compared to ground-based instruments, which allows astronomers to study jets in the inner coma. His specific observing plans include:

References

Mumma, M. J., M. A. DiSanti, M. Fomenkova, K. Magee-Sauer, N. R. Russo, D. X. Xie and C. D. Kaminski, 1996, "Discovery of Abundant Ethane and Methane in C/1996 B2 Hyakutake: Implications for Cometary and Interstellar Organic Chemistry", BAAS 28, 09.14. Mumma, M. et al., 1996, Science, in press.

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