Produced at the Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, Massachusetts, U.S.A.
The discovery of Comet C/1995 O1 (Hale-Bopp) has generated a great number of inquiries from the news media and the general public. This information sheet addresses the most commonly asked questions. (Please see the Appendix at the end for definitions of astronomical terms in this discussion.)
A comet was discovered in July that may or may not become bright enough to be easily seen with the naked eye in early 1997. Comet C/1995 O1 (Hale-Bopp) is now far from the sun, and more months of observations are needed in order to get a better handle on what might or might not happen in the next 1-2 years. The comet is now intrinsically very bright for such a large distance from the sun (meaning that it is rare for a comet this far from the sun to be so bright, even though it is still only a telescopic object from Earth), which is why some excitement has been building.
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On July 23, two astronomers first each spotted the comet while looking at a cluster of stars known as Messier 70 (M70) in the constellation Sagittarius. Within minutes of each other, Alan Hale in New Mexico and Thomas Bopp in Arizona independently recognized a fuzzy object near M70 that was fainter than the star cluster itself, and both reported it to the worldwide clearinghouse for comet discoveries in Cambridge, Massachusetts, the Central Bureau for Astronomical Telegrams (CBAT). The Central Bureau, which is operated by the Smithsonian Astrophysical Observatory for the International Astronomical Union (IAU) then issued an IAU Circular announcing the discovery (IAUC 6187, 1995 July 23), as is the practice for new comet, nova, and supernova discoveries. [See question 11, below, for further information on IAU Circulars.]
Alan Hale has a Ph.D. in astronomy from New Mexico State University and resides in Cloudcroft, NM. He is one of the world's most active visual observers of comets and has seen nearly 200 different comets over many years. Thomas Bopp, who lives in a suburb of Phoenix, AZ, is an amateur astronomer who was observing at a "star party" with other amateur astronomers in the desert about 90 miles south of his home.
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The proper designation as used by the International Astronomical Union on its IAU Circulars is "Comet C/1995 O1 (Hale-Bopp)", though many people are referring to it simply as "Comet Hale-Bopp". Comets are normally named for their discoverers, and this is done by consultation between the CBAT and a special committee of nine astronomers within the IAU. The designation "C/1995 O1" means that this was the first comet found in the second halfmonth of July (letter O plus number 1) in the year 1995; halfmonths are given as letters, with "A" covering Jan. 1-15, "B" covering Jan. 16-31, "C" covering Feb. 1-15, etc. ("I" being omitted and "Z" not needed); the "C/" indicates that this is a long-period comet (that is, one with a solar-orbiting period of more than 200 years). Similar to the system used for designating asteroids, this system was brought into use for comets on 1995 Jan. 1.
The comet was 7.16 Astronomical Units (the equivalent of 1.07 billion kilometers, or 666 million miles) from the sun at discovery, and 6.20 AU (or 930 million km, or 577 million miles) from the earth. The comet will come no closer to us than about 1.315 AU (197 million km, or 122 million miles), and that will happen around 1997 March 22. This means that the comet will come no closer to the earth than 1.3 times the sun-earth distance. The comet will reach perihelion (closest approach to the sun) on 1997 March 31 at a distance of 0.91 AU (136 million km, or 85 million miles) from the sun.
Early observations suggested that the comet was quite far away from us because of very small parallax seen in near-simultaneous observations made by observers in Australia and Japan. As many positional (or astrometric) observations poured in to our offices from observers around the world, we were able to compute the comet's path, or orbit, about the sun. The orbit of C/1995 O1 (Hale-Bopp) is almost perpendicular to the earth's own orbital plane and takes the comet out quite far from the sun. A single, apparent image of the comet was found by astronomer Robert H. McNaught of the Anglo-Australian Observatory from a wide-field photographic plate taken in late April 1993 (when the comet was about 13 AU from both the sun and earth). This observation has strengthened the orbital calculations by greatly extending the arc of observation. It allows one to say that the comet has an orbital period of a few thousand years and extends out to some ten times the distance of Neptune at its furthest point.
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At discovery, and for a few months afterwards, the comet was moving slowly with respect to the background stars from night to night in the evening sky in Sagittarius. It has been visible in small amateur telescopes (with mirrors or lenses 4-6 inches across) at visual magnitude 10-11. It is now in the southwest sky just after evening dusk, and is moving closer to the sun's glare; comet C/1995 O1 (Hale-Bopp) will be lost in the sun's glare during December 1995-February 1996, after which time it will be in the morning sky.
Comet C/1995 O1 (Hale-Bopp) is quite faint and unimpressive at this time [10/95]. Only experienced astronomers are likely to be able to detect the comet now with any ease. Sagittarius is a very cluttered constellation: hosting the center of our Milky Way galaxy, it has a multitude of faint stars and nebulosities. The best way for inexperienced observers to see this comet now is to contact a local astronomy club, planetarium, or college observatory to find out about upcoming star parties or public observatory nights in which the comet will be shown to interested members of the public. For example, the Center for Astrophysics in Cambridge, MA, holds monthly observatory nights for the public on the third Thursday of the month throughout the year; call 617-495-7461 for additional information; the CfA's Whipple Observatory near Tucson, Arizona, holds quarterly star parties (call 520-670-5707 for information there).
Comets are very difficult to predict in terms of brightness, especially when first discovered. The question that needs to be answered first by astronomers is: "Has the comet been undergoing an extended outburst in brightness and coma activity that will gradually subside?" Comets at such large distances from the sun as this one are known to have extended, slow outbursts in brightness; they are rather far from the sun's intense radiation pressure, and things progress at a much slower rate than when a comet is inside the earth's orbit. If comet C/1995 O1 (Hale-Bopp) is in a significant temporary outburst, it is very difficult to say much of anything about the future (other than that this comet is sure to be well observed by astronomers for the next 2-3 years!). In fact, the comet has been fluctuating by as much as 2-4 times in total brightness during the first two months after discovery, so the increases and decreases in brightness appear to be an on-going event (suggesting that the comet will not fade excessively in the near future).
If, then, comet C/1995 O1 (Hale-Bopp) is not undergoing a significant, short-lived outburst and continues slowly to increase in brightness (as do most comets) between now and its perihelion passage in 1997, it should at the least become a very nice binocular object during late 1996 and early 1997 for northern-hemisphere observers -- and it has a good chance of becoming a naked-eye object. But as to whether or not it becomes the first "spectacular" naked-eye comet since comet C/1975 V1 (West), which in the morning skies of March 1976 was as bright as Sirius (the brightest star) or even the planet Jupiter, is difficult to predict. By looking at the brightness patterns of long-period comets during the last four decades, astronomers can project on that basis that comet C/1995 O1 (Hale-Bopp) should reach a peak brightness somewhere between magnitudes +4 (making it a faintly visible object to the unaided eye in a dark sky) and -4 (making it about as bright as the planet Venus). This refers to the brightness of the comet's coma (or head or atmosphere). Just as important as this brightness figure for the general public's ability to see (or not to see) this comet as an impressive naked-eye comet in early 1997 will be the development of a dust tail. [see question 7, below]
However, numerous comets have been observed completely to fall apart on approach to perihelion, as many comets are thought to have nuclei that are very loosely held together. It is too soon to tell if comet C/1995 O1 (Hale-Bopp) will warrant widespread public attention.
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Comet C/1973 E1 (Kohoutek) earned a bad reputation by "not living up to" the media hype, including stories about it being possibly the `Comet of the Century'. It turns out that C/1973 E1 (Kohoutek) had a very stable light curve; what we did not know then was that light curves for comets that are thought to be entering the inner solar system for the first time (as apparently did C/1973 E1) very often have much slower rises in brightness than do comets that have made many journeys through the inner solar system. By comparison, C/1995 O1 (Hale-Bopp) has evidently made numerous trips through the inner solar system, every few thousand years, and there is reason to think that it may rise more rapidly in brightness than did Kohoutek. But other comets have very discontinuous light curves that make them totally unpredictable, such as comet C/1989 X1 (Austin), which first rose steeply in brightness and then leveled off to finish several magnitudes fainter than expected.
Not all comets have tails. There are two types of tails --- gas (or ion) tails and dust tails. Gas tails tend to be more common in comets, but they are also usually fainter than dust tails to the naked eye; this is because gas tails emit light by fluorescence, in which gas atoms emitted from the comet's nucleus interact with solar-wind radiation, and they re-transmit energy received from solar radiation at different wavelengths. This fluoresced light in comet tails is very blue, which is difficult for the human eye to perceive. Dust tails tend to become prominent in comets that travel inside the earth's orbit (i.e., less than 1 AU from the sun), in regions where the warming solar radiation more strongly interacts with ice in the comet's nucleus, causing much overall coma and tail activity. Most of the so-called 'bright' comets of this past century displayed prominent naked-eye dust tails. One potential problem with C/1995 O1 (Hale-Bopp) is that its perihelion distance is much further out than most of the other "spectacular" comets of the 20th century (perihelion distance = 0.91 AU). The Great Comet of 1811 had an even greater perihelion distance (just outside the earth's orbit), but still displayed a tail at least 25 degrees long (or about a quarter of the way from the horizon to the zenith in the night sky). An intrinsically fainter comet, C/1983 H1 (IRAS-Araki-Alcock), became brighter than the stars of the Big Dipper in May 1983 but showed no dust tail, and thus appeared merely as a large fuzzy ball in the sky. Whether or not C/1995 O1 (Hale-Bopp) behaves as the Great Comet of 1811 or as C/1983 H1, in terms of tail development, remains to be seen.
No, not really. The coma size has translated into well over 1 million km (as of August 1995), though this size varies over time. This is very large for a comet this far from the sun, but we know that there is activity from sublimated carbon-monoxide (CO) ices (and probably from dust, as well). The activity may decrease or increase over the coming weeks and months. The source of this activity is actually a much tinier nucleus, or solid, dirty snowball. Most comets have nucleus sizes around 1-10 km; comet 1P/Halley had an oblong nucleus of size 8x15 km. Because of the dense shroud of coma material around the nucleus, we cannot tell the size of the nucleus itself while the coma is active (without a close rendezvous by an artificial spacecraft, as with 1P/Halley). Astronomers have been wondering if the large amount of coma activity (and corresponding total brightness) might mean a larger-than-usual comet nucleus for this comet. But recent calculations by comet scientist Zdenek Sekanina of the NASA's Jet Propulsion Laboratory suggest that the current activity need not require a nucleus larger than 10-15 km in size.
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During 1990-1994, an average of about 12 comets per year were discovered (plus about one rediscovery per year of a "long-lost" short-period comet), with roughly four discovered by amateur astronomers. However, at the end of 1994, two major professional search programs for comets ceased at Palomar Mountain in southern California, and these programs had discovered four or more comets per year over the past 10-12 years. So far, only three new discoveries have been made in 1995, plus one rediscovery of a long-lost comet that had not been seen for about 150 years (as of 1995 Sept. 29).
It depends on your definition of "spectacular", but the range is roughly every 20 years or so (or a couple of times in a lifetime), especially if one defines "spectacular" as being as bright as the brightest planets or brighter. The increase in light pollution will make comet C/1995 O1 (Hale-Bopp) harder to see for many people, regardless of its brightness. This, combined with a high standard for "spectacular" activities, could detract from public perception of this comet.
Be wary, then, that many members of the general public --- who are used to fireworks being spectacular (where fireworks are typically between the moon and sun in brightness) --- may not find anything fainter than a crescent moon (mag -8 or so) to be spectacular! Realize that there is a broad spectrum of listeners and readers out there! Light pollution is much bigger today than 20 or 30 years ago, and those stuck in a large city are perhaps unlikely to be impressed.
The Circulars are a publication of the Central Bureau for Astronomical Telegrams of the International Astronomical Union. The IAUCs are available both in paper form (by postal mail) and in electronic form via the CBAT Computer Service and via e-mail. IAUCs are the original source for discovery information regarding all new comets, novae, and supernovae. Newspapers and magazines, as well as libraries and professional and amateurs astronomers, subscribe to these useful astronomical news circulars. For subscription information, send e-mail to iausubs@cfa.harvard.edu or send postal mail to:
Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory
60 Garden St.
Cambridge, MA 02138; U.S.A.
And check out the Central Bureau's World Wide Web page with useful information at the following URL:
http://cfa-www.harvard.edu/cfa/ps/cbat.html
APPENDIX. Definitions of commonly-used terms.
Contact: Daniel W. E. Green (Associate Director, Central Bureau for Astronomical Telegrams). E-mail dgreen@cfa.harvard.edu. Telephone 617-495-7440.
Written with input from D. W. E. Green, B. G. Marsden, G. V. Williams, J. Hoskins, J. Corliss and J. Cornell.
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Map created by Alan Gould
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