IAU Central Bureau for Astronomical Telegrams

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PRESS INFORMATION SHEET: Comet C/1996 B2 (Hyakutake)

Produced at the Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, Massachusetts, U.S.A.

[Updated 1996 November 20.]

The discovery of Comet C/1996 B2 (Hyakutake) 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.)


1) What's this I hear about a comet that was recently discovered and was to become bright in March and April?

A comet was discovered in late January that became the brightest comet visible in a dark sky from the earth in 20 years, in the second half of March 1996. The comet is now visible only to southern-hemisphere observers, having passed perihelion on May 1. The comet has been followed by observers in the southern hemisphere, as it faded from fourth magnitude in May to sixth magnitude in late June (a binocular object), and the faint comet is now only visible in large telescopes. This comet did not appear spectacular to the general public, though astronomers have been pretty excited because few comets become as bright as the brightest stars, and many useful observations have already been made from visible wavelengths to ultraviolet to infrared to radio (as the comet emits radiation at all of these wavelengths). The comet was visible from all parts of the world until about March 23 or 24 (when southern-hemisphere observers will have more difficulty viewing it until May), but faded as it moved away from the earth and toward the sun's glare in April. It then re-emerged from the solar glare in mid-May after having moved far south in the sky.

The comet passed rather close to the earth (0.102 AU, or about 15 million kilometers or 9.3 million miles) on March 25 (at about 7 hours Greenwich Mean Time, or about 2 a.m. USA Eastern Standard Time). At that time, C/1996 B2 was moving some 10-20 degrees per day across the northern sky, and the comet will in fact be visible all night long from the U.S., Canada, Europe, and Japan (weather and light pollution conditions permitting) through the last week in March. Then it was visible in the northwest each evening in late twilight during the last week of April, though it was a difficult object due to low altitude and twilight. Comet C/1996 B2 (Hyakutake) temporarily "upstaged" the anticipated good showing of comet C/1995 O1 (Hale-Bopp) in early 1997, though C/1995 O1 may be substantially brighter than was C/1996 B2.

1a) What is a comet?

It is a small body in our solar system that orbits the sun much as do the earth and other planets. It has a "nucleus", or solid body, that is usually around 1-10 km across and is supposedly a "dirty snowball" consisting of ices and dust and rock. When far from the sun in the outer reaches of the solar system, there is very little activity coming off such a nucleus. However, when the comet nucleus gets closer to the sun, the sun's radiation warms the nucleus, causing the ices to sublimate (or "steam") outwards from the nucleus from various vents, carrying along various atoms and molecules that constitute different ices and dust and rock in the original nucleus. This venting outwards creates both the coma (or atmosphere) surrounding the nucleus --- out to thousands, hundreds of thousands, and sometimes millions of kilometers from the nucleus itself --- and also the tail of material that generally streams in the anti-sunward direction from the nucleus. Once this venting activity "turns on", the true nucleus is almost invariably invisible from Earth, as intense material in the inner coma then tends to mask the tiny nucleus.

Note that comets are NOT the same as meteors (or so-called "shooting stars"; see definition in Appendix below); while meteors typically streak through our atmosphere in a second or two and are sometimes much brighter than even a "bright" comet (when they are called "fireballs"), comets are much further away than the moon and move slowly with respect to the background stars from night to night, rising and setting each day just as do the sun, the moon, the planets, and the stars.

2) How was this comet discovered?

On January 30 Universal Time (Greenwich Mean Time), a Japanese amateur named Yuji Hyakutake from Hayato-machi, Aira-gun, Kagoshima-ken, Japan, made a visual discovery of a previously-unknown comet (his second) using 25x150 binoculars (these large binoculars have lenses that are 6 inches across). This comet was very close in the sky to the discovery location of another comet, C/1995 Y1, found by the same observer one month previously! The comet was rapidly confirmed by other experienced observers in Japan, and the results were reported 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 6299, 1996 January 31), as is the practice for new comet, nova, and supernova discoveries. The designation C/1996 B2 (Hyakutake) was assigned by the Central Bureau upon computation of preliminary orbital elements (IAUC 6303). [See question 11, below, for further information on IAU Circulars.]

3) What is the proper name of this comet, and how did it get that name?

The proper designation as used by the International Astronomical Union on its IAU Circulars is "Comet C/1996 B2 (Hyakutake)". 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/1996 B2" means that this was the second comet found in the second halfmonth of January (letter B plus number 2) in the year 1996; 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.

4) How far away is the comet now, and how close will it come to Earth?

The comet was 2.0 Astronomical Units (the equivalent of 300 million kilometers, or 186 million miles) from the sun at discovery, and 1.83 AU (or 275 million km, or 170 million miles) from the earth. The comet will come no closer to us than about 0.10 AU (15 million km, or 9.3 million miles), which is some 40 times the earth-moon distance, and that happened around 7 hours Greenwich Mean Time (about 2 a.m. USA Eastern Standard Time) on 1996 March 25; the comet's velocity when closest to the earth in late March was roughly 150,000 kilometers per hour (for comparison, the earth orbits the sun at about 108,000 km/hr). The comet will reach perihelion (closest approach to the sun) on 1996 May 1 at a distance of 0.23 AU (34 million km, or 21 million miles) from the sun; the comet's velocity at that time will be roughly 316,000 km/hr.

The approach of C/1996 B2 to the earth on March 25 (0.102 AU) was the closest for any comet since 1983 (when there were two comets coming to 0.06 AU and 0.03 AU within a month of each other), and it is the fifth closest approach of any comet during the past century. What is unique about this comet is that no other comet is known then to have gone on to pass anything like as close to the sun as this one does (0.23 AU on May 1). One of the 1983 comets had about twice this comet's perihelion distance, but the approach to the earth was well after perihelion. There was possibly a comet with a perihelion distance comparable to this one that came closer to the earth after perihelion in the year 400, but that is very uncertain. The time interval between passage near the earth and subsequent passage near the sun is longer for C/1996 B2 (37 days) than for any closer earth approach since that of the famous Lexell comet in 1770 (43 days), that comet holding the record confirmed approach to the earth (0.015 AU). C/1996 B2 is intrinsically the brightest earth-approacher since the early eighteenth century, and the 55 days between discovery and earth approach is a record for a preperihelic earth approach. A list of the closest cometary approaches to the earth is available.

4a) How do we know these distances and where the comet will be at any given time?

The distances of the comet now and in the future are known because of the existence of many accurate positional measurements of the comet, made mostly by amateur astronomers around the world using CCD cameras on numerous nights as the comet moves slowly with respect to the "fixed" background stars (whose positions are known from catalogues). As many positional (or astrometric) observations pour in to our offices from observers around the world, we are able to compute the comet's path, or orbit, about the sun. Two pre-discovery photographs containing faint images of C/1996 B2 were found by a Japanese amateur who had taken them on January 1, and these have permitted the comet's orbit about the sun to be more definitively determined. The Central Bureau and its associated Minor Planet Center maintain such positions and orbits of comets and minor planets. Comet C/1996 B2 (Hyakutake) apparently had an orbital period (i.e., the time it takes to make one complete revolution about the sun) of about 8000 years, prior to its current voyage to the inner solar system; the perturbations by the major planets will alter this figure, such that its period may increase to roughly 14,000 years (according to figures computed by Brian G. Marsden on 1996 March 28).

An orbital diagram for this comet is available. This plot shows a view from the north pole of the ecliptic, which is the plane of the earth's orbit about the sun (on our sky, the ecliptic is the apparent annual path of the sun, which is inclined with respect to the celestial equator by about 23 degrees); the dashed line for the comet shows its path when it is below this plane, and the solid line shows when it is north of the ecliptic (and thus best placed for northern-hemisphere observers). The small inset shows a side view in the plane of the earth's orbit, showing how the comet's orbit slices through the ecliptic at a high angle.

5) Can members of the general public see the comet now?

(see also question 5, above) Only if you are in the southern hemisphere, and then only with the aid of a large telescope. The best way for inexperienced observers to see any comet usually 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).

When comet C/1996 B2 (Hyakutake) was readily visible to observers in the northern hemisphere several months ago, it passed several degrees away from the bright star Arcturus (magnitude 0) during March 22-23, then a similar distance away from the stars in the handle of the Big Dipper on March 24-25. On March 26, comet C/1996 B2 passed by the Little Dipper, and on March 27 passed a couple of degrees away from Polaris, the North Star (at which time it was at a rather constant altitude above the horizon, looking due north, for northern-hemisphere observers all night long). The comet then moved toward the evening sky, from whence it was visible to the northwest in late evening twilight and during the evening hours during early- to mid-April. After mid-April, the comet neared the sun (and thus the sun's glare), becoming more of a twilight object that set around the time it got dark.

6) So how bright was this comet expected to become?

Comets are very difficult to predict in terms of brightness, especially when first discovered. The comet is now well past its expected peak brightness. It should now slowly fade for southern-hemisphere objects as it gradually recedes to more distant parts of the solar system. While C/1996 B2 (Hyakutake) is certainly the brightest comet visible in a dark sky since 1976 (and one of the most fascinating this century, due to its close approach to the earth), it will probably not be considered as 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. Such an accolade may be assigned to C/1995 O1 (Hale-Bopp) next year, and such accolades must partly be answered with respect to "the eye of the beholder" (see question 10 below); it would appear at this time that comet C/1995 O1 (Hale-Bopp) will become brighter in early 1997 than C/1996 B2 (Hyakutake) was in late March.

Comet C/1996 B2 (Hyakutake) peaked in brightness around magnitude 0 during the last week in March. It faded to magnitude 2 during April. This brightness refers to the brightness of the comet's coma (or head or atmosphere); note that the comet's coma is still now a half degree or more across (or equal to or larger than the apparent size of the moon), and it was as much as 2 or more degrees across around March 25-26. This means that C/1996 B2 is not likely to be as obvious as a star or planet of the same brightness, because the latter objects have the same amount of light concentrated in (essentially) a point. Thus, while experienced observers were measuring the "integrated" brightness of the entire coma of the comet to be near magnitude 0 or even brighter around March 25, in effect the comet appeared at a glance to be about as obvious as a first-magnitude star, because that 0-magnitude brightness was spread out over the better part of a degree on the sky (as opposed to such light appearing as a point for stars).

7) Did this comet have a nice tail?

Tail lengths when the comet was close to the earth in late March were reported to be well over 60 degrees, and the naked-eye lengths approached 80 to 100 degrees in April --- very rare for a comet. 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.

While experienced observers in dark skies reported tail lengths of more than 60 degrees in late March (and up to 100 degrees in mid-April), this refers to a very faint, tenuous gas tail that is all but invisible from more urban areas that suffer from heavy light pollution; such urban viewers should expect only to see a fuzzy star, or a large bright, fuzzy ball in binoculars that is progressively brighter toward the comet's center. However, the brighter part of the tail (especially any dust tail, which is more easily seen by the human eye) was expected to grow a bit during April, as the comet draws rapdily nearer to the sun.

The comet in March exhibited mainly a bluish gas tail, which makes it difficult to see with the human eye. There are some signs that more of a yellowish dust tail is forming now, but the coma is sure to be dominant in brightness over the tail during most of April.

8) Do we know how large this comet is?

Some recent radar bouncing has suggested that the nucleus may be on the order of 1-3 km in size, which is typical for such long-period comets (which normally are much more active than are short-period comets). The coma size is on the order of hundreds of thousands, or even millions, of kilometers (miles) across. 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).

9) How frequently are comets discovered?

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. In 1995 there were five discoveries of previously-unknown comets, plus one discovery of a comet that had been lost for 150 years (122P/de Vico). Comet C/1996 B2 was the third new comet discovery of this year.

10) How frequently do 'spectacular' comets become visible?

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. By that definition, comet C/1996 B2 is not likely to become "spectacular". The increase in light pollution made comet C/1996 B2 (Hyakutake) 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. We may be treated to two bright comets in the span of a single year, with C/1995 O1 (Hale-Bopp) expected to become a naked-eye object for early 1997.

11) What are the IAU Circulars?

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 either check out the on-line 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.

  • Arc minutes. There are 60 minutes of arc in 1 degree. In the sky, with an obstructed horizon (as on the ocean), one can see about 180 degrees of sky at once, and there are 90 degrees from the true horizon to the zenith. The full moon is about 30 arc minutes across, or half a degree.
  • Astrometry. The careful, precise measurement of astronomical objects, usually made with respect to standard catalogues of star positions.
  • Astronomical Unit (AU). Approximately equal to the mean earth-sun distance, which is about 150,000,000 km or 93,000,000 miles.
  • AU. see Astronomical Unit.
  • Coma. A comet's atmosphere surrounding its nucleus. The coma is rather tenuous (except very close to the nucleus), and stars can be occasionally easily seen through it, shining from behind.
  • km. kilometer = 0.6 mile.
  • Light curves. Literally, a plot of a comet's brightness as a function of time. A comet never follows a smooth light curve in a strict sense, because there are many night-to-night and week-to-week fluctuations (sometimes on scales of several magnitudes); however, in general, the average brightness of most comets can be fairly well represented over intervals of months.
  • Magnitude. The units used to describe brightness of astronomical objects. The smaller the numerical value, the brighter the object. The human eye can detect stars to 6th or 7th magnitude on a dark, clear night far from city lights; in suburbs or cities, stars may only be visible to mag 2 or 3 or 4, due to light pollution. The brightest star, Sirius, shines at visual magnitude -1.5. Jupiter can get about as bright as visual magnitude -3 and Venus as bright as -4. The full moon is near magnitude -13, and the sun near mag -26. The magnitude scale is logarithmic, with a difference of one magnitude corresponding to a change of about 2.5 times in brightness. In the case of comets, we speak of a magnitude that is "integrated" over an observed coma diameter of several arc minutes; thus, a 7th-magnitude comet is much harder to see than a 7th-magnitude star -- the latter having all its light in a pinpoint, and the former having the same amount of light spread out over a large area (imagine defocussing a 7th-magnitude star to the size of a diffuse comet). Typically, however, when comets become very bright, their apparent coma sizes shrink so that the majority of visible light is in a small, intense core of the comet's head (and the comet may appear starlike with a tail emanating from the comet's head).
  • Meteors. Small rocky and/or icy particles that are swept up by the earth in its orbit about the sun. Also called "shooting stars", they travel across the sky in a very short time, from less than a second to several seconds, and they do so because they are only a matter of tens of miles above the surface of the earth. Meteor showers are generally thought to be produced by the debris left by comets as the latter orbit the sun. (Comets, on the other hand, are not in our atmosphere but are much further away than is our own Moon; therefore, comets do not "streak" across the sky as do meteors -- a common misconception among the general public.)
  • Orbit. The path of one object about another (used here for an object orbiting the sun).
  • Parallax. the apparent displacement or the difference in apparent direction of an object as seen from two different points not on a straight line with the object (as from two different observing sites on earth).
  • Perihelion. The point where (and when) an object orbiting the sun is closest to the sun.
  • Sublimation. The change of a solid (such as ice) directly into a gaseous state (bypassing the liquid state). This happens in the vacuum of space with comets, as the heating effects of solar radiation cause ices in comets to "steam off" as gasses into space. The ice molecules present in the nucleus actually break up (or dissociate) into smaller atoms and molecules after leaving the nucleus in gas form.
  • Tail. see "Comet."
  • Zenith. The point directly overhead in the sky.

Contact: Daniel W. E. Green (Associate Director, Central Bureau for Astronomical Telegrams). E-mail dgreen@cfa.harvard.edu. Telephone 617-495-7440.

Written by D. W. E. Green, with input from B. G. Marsden, G. V. Williams, J. Hoskins, J. Corliss, and J. Cornell. [Updated 1996 November 20.]

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