Planetary Science Research Discoveries HOT IDEA HEADERposted February 14, 1997 (updated March 24, 1997) 1997 Apparition of Comet Hale-Bopp
Discovery of Comet Hale-Bopp
by Karen Meech

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).

Comet C/1995 O1 (Hale-Bopp)
EventUT Dater [AU]
Discovery 07/23/957.14
Pre Discovery Image 04/27/9313.04
Earth Closest Approach 03/22/971.315
Perihelion 04/01/970.914

The table above shows some of the important dates and distances for the comet. On August 2, 1995, shortly after its discovery pre-discovery images were found on photgraphic plates taken at the Anglo-Australian Observatory on 4/27/93, when the comet was 13.1 AU from the sun. The image showed that the comet was active (i.e. had a coma and tail) even at this distance. Because of the long time baseline of the observations, it was possible to quickly determine an orbit. It was found that the last perihelion passage of the comet was nearly 4200 years ago, and that in the absence of non- gravitational forces, it should next return in approximately 2380 years. This suggests that the comet is not making its first passage close to the sun from storage in the Oort cloud. It is interesting to note that the orbit of comet Hale-Bopp (see figure below) is very similar to that of the Great Comet of 1811.

position of comet
[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.]

Comparison of the Orbits of comet Hale-Bopp and the Great comet of 1811

CometPerihelion Date Period Perihelion Eccentricity Inclination Arg. of Perihelion Ascending Node
C/1995 O11997 04 01.1462380 yr 0.914 AU0.995 89.4130.591282.471
1811 I 1811 09 12.7563065 yr 1.035 AU0.995106.9314.50295.631

Chronology of Changes in comet Hale-Bopp

Image (click on it to see a larger version)Dater [AU]TelescopeObserverExposure Info.
8/25/95 6.9 La Palma, 1-m Kapteyn TelescopeA. 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
9/26/956.6Hubble 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.
3/20/965.3European Southern Observatory 3.5m New Technology TelescopeO. Hainaut,
R. West
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. Meech,
O. Hainaut,
J. Bauer
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. Meech,
O. Hainaut,
J. Bauer
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.
02/17/971.18UH 2.2m Telescope on MaunakeaK. Meech,
O. Hainaut,
J. Bauer
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.
3/6/971.0350-mm telescope,
f/2.0 lens
D. Bridges4 min. exposure with Fujicolor SG+40 film, by amateur observer, shows well-developed plasma (blue) tail, and dust tail.
3/9/971.00Meade 12-inch,
LX 200
f/6.0 lens
T. PuckettAP-7 CCD camera composite of twenty-seven 30-second exposures.
3/12/970.98180 mm lens
P. Stattmayer12 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.

How fast a comet will brighten as it approaches perihelion will depend on the type of orbit it has. Comets which have been close to the sun before tend to brighten much more quickly than those comets which are making their first passage, however, in both cases there can be unexpected brightness outbursts which can make predicting the increase difficult. Astronomers have also never observed a comet in this type of orbit so far before its perihelion, which makes predicting the brightness more difficult. Typically a comet's visual brightness may be represented by a power-law formula:

mag=absolute mag + 5 log(delta) + 2.5n log(r)

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.

Enhanced Jets 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.

Molecules Discovered in comet Hale-Bopp

FormulaMoleculer [AU]ReferenceSignificance
CO Carbon Monoxide 6.6Jewitt 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., 1996The isomer HNC is unstable, and the ratio is consistent with interstellar cloud origin.
HCN/DCNHydrogen Cyanide1.2Matthews et al., 1997 D/H (deuterium/hydrogen) ratio is not enriched with respect to ISM as many comets have been.
HNC/HCNHydrogen Cyanide1.196Lis et al., 1997Abuncance is 0.25.
HNC/HCNHydrogen Cyanide1.003Apponi et al., 1997Abundance is 0.5.
H2CO Formaldehyde 1.6Womack 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.
1.5Mumma et al., 1997Abundance 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/15NNitrogen isotopes1.2Matthews et al., 1997Isotopic ratio is compatible with values seen on Earth.
HCO+---1.17Veal, 1997First detection in comets.
HCO+---1.165Veal, 1997First detection in comets.
SOSulfur monoxide1.15Lis et al., 1997First secure detection in a comet.
1.124Mumma et al., 1997Molecules were detected in the infrared, and are most intense at the nucleus.
---Organics1.03Mumma et al., 1997Continuum is very strong - may be a signature of organic material.
SO2Sulfur dioxide0.949Wink et al., 1997First detection of molecule in comets. The molecule does not originate primarily from the nucleus.
HCOOHFormic Acid0.94Wink et al., 1997Abundance is 50 times lower than for the related molecule, methanol.

A'Hearn, M. F. et al. (1995). IAU Circular No. 6244. Apponi, et al. (1997). IAU Circular No. 6586. Bockelee-Morvan et al. (1996). IAU Circular No. 6511. Crovisier, Jacques, "Rapid Increase in Production of OH." Comet Hale-Bopp (1995 01) Observers' Bulletin Board and Archives June, 1996. Fitzsimmons, A. (1995). IAU Circular No. 6252. Jewitt, D., M. Senay and H. Matthews (1996) "Observations of Carbon Monoxide in Comet Hale-Bopp" Science 271 1110. Jorda, L., J. Lecacheux and F. Colas (1997). IAU Circular No. 6583. Lis, et al. (1997). IAU Circular No. 6566. Lis, et al. (1997). IAU Circular No. 6573. Matthews, H. E. et al. (1996). IAU Circular No. 6515. Matthews, H. E. et al. (1997). IAU Circular No. 6567. Mumma, et al. (1997). IAU Circular No. 6568. Mumma, et al. (1997). IAU Circular No. 6573. Sekanina, Z. (1997). IAU Circular No. 6542. Veal, (1997). IAU Circular No. 6575. Wink, et al. (1997). IAU Circular No. 6591. Wink, et al. (1997). IAU Circular No. 6587. Wink, et al. (1997). IAU Circular No. 6599. Womack, M. et al. (1997. IAU Circular No. 6542.

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