The comet was discovered independently by two amateur comet observers: Alan Hale (New Mexico, and a professional astronomer by trade), and Thomas Bopp (Arizona), on July 23, 1995. At discovery, the comet was near magnitude 10.5 - which is not unusual for comets discovered typically within 1-2 AU from Earth. However, the slow motion of the comet against the background stars suggested that this comet was very far away, making the brightness very unusual. It was the brightest modern comet discovered by amateurs, appearing nearly 50,000 times brighter than comet P/Halley did as it approached the sun at the same distance.
Both observers were waiting for other objects to rise and were passing time by looking at some deep sky objects (globular clusters of stars) in the constellation of Sagittarius. It was immediately apparent that there was a fuzzy object in the field that was not on the usual star charts, and after confirming that the object was moving against the background stars, both observers reported the discovery to the Central Bureau for Astronomical Telegrams in Cambridge, MA. Once it was confirmed that this was a new comet, it was given the designation C/1995 O1 (Hale-Bopp).
|Event||UT Date||r [AU]|
|Pre Discovery Image||04/27/93||13.04|
|Earth Closest Approach||03/22/97||1.315|
|[Click on the figure to link to L. Koehn's website of astronomy-related animations, photographs, and graphics. The link will open in a new window.]|
|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|
|1811 I||1811 09 12.756||3065 yr||1.035 AU||0.995||106.9||314.502||95.631|
|Image (click on it to see a larger version)||Date||r [AU]||Telescope||Observer||Exposure Info.|
|8/25/95||6.9||La Palma, 1-m Kapteyn Telescope||A. Fitzsimmons||The jet was observed during 5 nights in August, with no significant change in the direction of the jet. The image is an R-band CCD image|
|9/26/95||6.6||Hubble Space Telescope||H. Weaver||7" field of view, clump from nucleus ejected 60 hrs earlier is visible. Material is expanding from nucleus at 30 km/sec.|
|3/20/96||5.3||European Southern Observatory 3.5m New Technology Telescope||O. Hainaut,|
|Nine 1-min R-filter images from the SUSI high resolution camera are combined in this image.|
|09/17/96||3.1||Canada France Hawaii 3.6m Telescope|| K.
|UH 8K CCD Mosaic camera (largest mosaic in the world) composite image, R filter, 2 min (tail) 15 s (core). Image processed by O. Hainaut.|
|11/12/96||2.4||UH 2.2m Telescope on Maunakea||K.
|R filter, 10 s. Jets were enhanced by subtracting off a coma varying inversely as the distance from the nucleus. Image processed by O. Hainaut.|
|02/17/97||1.18||UH 2.2m Telescope on Maunakea||K. Meech,|
|R filter, 0.5 s. Jets near the core enhanced by subtracting off a heavily smoothed version of the image (unsharp masking). Image FOV is 2.5 arcmin.|
|D. Bridges||4 min. exposure with Fujicolor SG+40 film, by amateur observer, shows well-developed plasma (blue) tail, and dust tail.|
|T. Puckett||AP-7 CCD camera composite of twenty-seven 30-second exposures.|
|3/12/97||0.98||180 mm lens|
|P. Stattmayer||12 min. exposure on Kodak Pro Gold 400 emulsion.|
What Do We Know About Hale-Bopp So Far?
Brightness - The brightness of comet
Hale-Bopp has been exceptional, and the predictions indicate that it
could rival the Great Comet of 1811. The Great Comet of 1811 was
observed for an unprecedented period of 17 months (naked eye), and at
the time of its discovery, it was around 5th magnitude at a distance of
2 AU from the sun. Comet Hale-Bopp reached this brightness at a
distance of 3 AU from the sun. The observations of Hale-Bopp are shown
in the figure below, with the white points showing the observed brightnesses (reduced to unit geocentric and heliocentric distances) and the blue points the predicted brightness based on its behavior so far. The brightness is steadily increasing, although more current predictions place the peak brightness between -0.5 and -1.0; not quite as bright as shown in the figure.
where the absolute mag is related to the size of the nucleus, delta is the geocentric distance, and r is the heliocentric distance. The value of n determines how fast the comet is brightening. The early observations suggested n=4, then the brightness leveled off, and now the comet is brightening with n=3, however, the comet should not be a disappointment.
Nucleus Size - Because of the great brightness, many believed that the comet may have an extremely large comet nucleus (for example the average radius of comet P/Halley was about 5 km). A team of astronomers lead by H. Weaver (Johns Hopkins University) has used the Hubble Space Telescope to get high resolution images of the inner region of dust (with resolutions of less than 500 km per pixel or detector element) surrounding the nucleus. They have used the high resolution images to model and subtract off the light contributed by the dust in the image core to infer that the nucleus is less than 40 km in diameter. This makes the nucleus large, but not exceptional in size. Ratio data taken in early March with the IRAM interferometer (Wink et al., 1997) also confirm this size since the signal they detected can by accounted for by a 380K sphere with a diameter of 45 km emitting radiation.
Activity Level - The comet was active (i.e. producing gas and dust) when discovered at 7.1 AU, and there was coma even in the pre-discovery image from 13.0 AU. Since the amount of solar radiation is too low at these distances to heat water-ice enough to initiate sublimation, it was known early on that other volatiles were playing a role in the activity. The first detection of CO was made at 6.6AU (Jewitt, et al., 1996), with the comet producing 1300 kg/sec of CO gas. In comparison, comet P/Halley attained a water production rate this high only near 2 AU! Between 4.7-4.1 AU the CO production flattened out, and the production of OH (a fragment of the water molecule) increased (Bockelee-Morvan, 1996). As the comet neared 2.4 AU, there was a rapid increase in water production. One of the first molecules usually detected in a comet is the CN molecule, which is typically released along with the sublimating water. In the case of comet P/Halley, this was first seen at the unusually large distance of 4.8 AU from the sun. In contrast, during August of 1995 the production of the CN molecule was measured at a distance of 6.8 AU. There is an indication from observations from Lowell Observatory in Arizona that in early March the gas progection began to level off. Additionally, dust production was increasing at a slower rate than the trends in mid-January and February.
Dust Level - Early measurements of the dust production, near 6.8 AU (A'Hearn et al., 1995), give the largest value ever observed. Because the comet is very dusty compared to other comets, the expectation is that it should develop a nice dust tail, unlike comet C/1996 B2 - Hyakutake, which displayed a prominent gas tail.
Dust Jets and Nucleus Rotation - Beginning around May 1996 observers began reporting strong jetting activity from the comet - consisting of several straight jets, which changed very little in appearance throughout the summer and fall. Jet Propulsion Laboratory scientist, Z. Sekanina (1997) has recently interpreted the jets as being boundaries of fan-shaped formations where dust is being ejected from 3-4 discrete active sources on the rotating nucleus. In the image at the right, a technique similar to that used by H. Weaver to estimate the nucleus size was used on this image of comet Hale-Bopp obtained using the UH 2.2m telescope on Maunakea (O. Hainaut). By removing the smooth coma variation, the jets stand out dramatically. Observations of the dust jets made in January and February, 1997 by astronomer J. Lecacheux at the Observatory of Paris (Meudon) showed that the comet nucleus is rotating with a period near 11.5 hours. Observations by Italian astronomers the following month confirmed this rotation period by looking in the infrared at shells (which are formed by periodic ejection of gas and dust as an active source rotates into and out of the sunlight). The Italian astronomers deduced that the dust was leaving the nucleus at a velocity of about 0.35-0.45 kilometers per second. Results, published on March 11, 1997 by Jorda and others in the International Astronomical Union Circulars, from observations by French and German astronomers using the Pic du Midi telescope in France suggest that the comet may exhibit complex rotation - that the rotation period is changing between 11.2 and 11.6 hours on a timescale of 22 days. This is like a top which has not only a simple rotation about its axis, but which also has a wobble, and may be caused by uneven outgassing from the jets.
Organic Molecules - Organic molecules are being detected in both the radio wavlengths and in the infra-red and are so far providing exciting information on how similar the comet nucleus material is to the material in the interstellar medium. Below is a table of the discoveries and the implications.
|CO||Carbon Monoxide||6.6||Jewitt et al., 1996||Producing 1300 tons/sec, compared to the production of water from P/Halley of 1000 tons/sec at 2AU|
|HNC/HCN||Hydrogen Cyanide||2.1||Matthews et al., 1996||The isomer HNC is unstable, and the ratio is consistent with interstellar cloud origin.|
|HCN/DCN||Hydrogen Cyanide||1.2||Matthews et al., 1997||D/H (deuterium/hydrogen) ratio is not enriched with respect to ISM as many comets have been.|
|HNC/HCN||Hydrogen Cyanide||1.196||Lis et al., 1997||Abuncance is 0.25.|
|HNC/HCN||Hydrogen Cyanide||1.003||Apponi et al., 1997||Abundance is 0.5.|
|H2CO||Formaldehyde||1.6||Womack et al., 1997||Ortho-Para spin state ratio - is similar to that observed in interstellar clouds. However, the interpretation of these observations is being contested.|
|1.5||Mumma et al., 1997||Abundance ratios similar to other comets for water:CO, and a little low for water:ethane from the only other comet (Hyakutake) where it has been detected.|
|14N/15N||Nitrogen isotopes||1.2||Matthews et al., 1997||Isotopic ratio is compatible with values seen on Earth.|
|HCO+||---||1.17||Veal, 1997||First detection in comets.|
|HCO+||---||1.165||Veal, 1997||First detection in comets.|
|SO||Sulfur monoxide||1.15||Lis et al., 1997||First secure detection in a comet.|
|1.124||Mumma et al., 1997||Molecules were detected in the infrared, and are most intense at the nucleus.|
|---||Organics||1.03||Mumma et al., 1997||Continuum is very strong - may be a signature of organic material.|
|SO2||Sulfur dioxide||0.949||Wink et al., 1997||First detection of molecule in comets. The molecule does not originate primarily from the nucleus.|
|HCOOH||Formic Acid||0.94||Wink et al., 1997||Abundance is 50 times lower than for the related molecule, methanol.|
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