For the first time in 20 years, it appears that we will have a truly great comet - C/1996 B2 (Hyakutake). With C/1995 O1 (Hale-Bopp) coming to perihelion in 1997, the drought of bright comets is finally over.
For those interested in general information on comets, consult the Other Sources of Comet Information Page, the C/1995 O1 (Hale-Bopp) Page or the Comet Definitions Page.
IAU Circular 6299 (January 31, 1996) reported the visual discovery of a comet by Yuji Hyakutake (Hayato-machi, Aira-gun, Kagoshima-ken, Japan) using 25x150 binoculars. At discovery, the comet was about 10th magnitude, 4' in diameter and moderately condensed.
The following is based on a review of Comets 1P/Halley, C/1990 K1 (Levy) = 1990c, and C/1983 H1 (IRAS-Araki-Alcock) as they approached 1 AU. Trying to forecast what a comet will do is always dangerous. Hopefully if I err, the comet will be better than expected.
The orbit for this comet indicates that it may become a very bright object in March, April, and May of this year. The ephemeris is given on the ephemeris page. The perihelion distance of this comet is only 0.23 AU. Perihelion will occur on May 1. However, the comet will also make a close approach to the Earth (0.10 AU) on March 25. The Northern Hemisphere is favored preperhelion, particularly in late March when the comet will be circumpolar reaching +84 degrees. The comet may be 1st magnitude at that time! However, the comet will be quite large. A coma size of more than half a degree in diameter is almost certain and a degree or more is possible (particularly if the comet has a faint outer coma). Thus, the comet will most likely look like a condensed, fuzzball to the naked eye (remember IRAS-Araki-Alcock in 1983? - similar, but probably much more condensed and smaller than I-A-A). During this time, the comet will move from the morning into the evening sky.
Prior to its close approach to the Earth, the tail of the comet will be significantly foreshortened (apparent length reduced by ~40% assuming the tail is pointing directly away from the Sun - an assumption that does not always hold for dust tails). However, as C/1996 B2 passes by the Earth, the foreshortening disappears and with the reduced distance, there is a real possibility of a very long tail (tens of degrees). The prime time for this is March 22-March 31. The question is: Will a tail be visible? The tail(s) will be poorly developed compared with the postperihelion time period. The tails, even if significant, will probably be faint. This is particularly true of the gas tail because the eye is much less sensitive to the gas emissions. It is possible that a long faint tail may be spotted with the naked eye or binoculars by experienced observers observing from dark sites (without the Moon). Using Comet Levy as an example, a tail as long as 20 - 40 degrees is possible. (For those of you who think that it is impossible to have such a long, faint tail, I suggest that you read the accounts of 1P/Halley at the end of April 1986. At that time Halley had a very faint 30 degree tail.) The likelihood of a bright tail seems remote at this point. For many observers, particularly those looking from a city, the comet will just be a fuzzball with little or no tail.
As the comet recedes from the Earth and approaches the Sun, foreshortening of the tail will not be a problem. However, the growth of the tail will be offset by the increasing Earth-comet distance so the tail will rapidly diminish in length in early April. As the comet approaches perihelion, it will become more condensed - that is, it will look more star-like. How spectacular the comet appears will depend on both the brightness of the coma and the amount of dust the comet generates. The human eye is most sensitive to the reflected light from the dust. For a bright tail to be obvious, the comet must produce a lot of dust. Remember P/Halley in 1986? Halley's dust tail was relatively faint and most general observers were disappointed. In late April, there is the potential that the tail will again become quite long (see the tail length prediction by Andreas Kammerer given below. Within a few days of perihelion, the tail quickly becomes foreshortened (by as much as 60%) as the comet swings around the Sun. The tail is likely to be quite a bit shorter at perihelion. The moon, which will be full at the time of perihelion (although in the opposite part of the sky), will not hinder observations as much as the twilight. By the last few days of April, the comet will only be visible in twilight.
Postperihelion is totally a Southern Hemisphere event. The tail development will likely reach a maximum a month or so after perihelion. The foreshortening of the tail will become less of a problem toward by the middle of May. These two effects should result in a rapid lengthening of the tail. At this point, I am not willing to speculate on how long the tail will become during this time period. However, the comet will be more than 1 AU from the Earth so a very long tail seems unlikely. The downside for the Southern Hemisphere observers is that the comet will rapidly fade. It is likely to be lost as a naked eye object in early June.
How bright the comet actually becomes will depend on its rate of brightening. Many observers will remember Comet Austin in 1990, which was suppose to reach 0 magnitude and only made it to 4.5. I believe that the peak brightness will be between magnitude 0 and +2 in late March and -1.5 to 2.0 at perihelion. We will know more by the end of February.
John Bortle, one of the foremost experts on comet observation and the appearance of comets, provides these comments on C/1996 B2:
"Yes, you and I are essentially in agreement as to how we expect the comet to perform. I had based my opinion on a look at the record of intrinsically bright comets with close pre-T Earth encounters observed since 1800. Among these I felt the closest analogy was Comet 1864 II, which passed Earth under almost identical circumstances. That object was well condensed, 1-2 magnitude, and had a tail up to 40 degrees in length according to the great Julius Schmidt.
I did not retain a copy of my own recent messages to S&T but the gist was that, near perigee, C/1996 B2 would appear as a rather strongly condensed, fairly high surface brightness object (unlike I-A-A) which would likely display distinct jet structure near the nucleus when viewed telescopically. With regard to the tail, I anticipated that a Type-I tail was the most likely and that, although faint, a length of 25 degrees would not in the least surprise me and that a 50 degree length was not out of the question - although any "long" tail would exist rather briefly because of the encounter circumstances. As to a dust tail, I felt such would probably be quite a bit shorter and but of equally low surface brightness.
I made a further prognostication that the comet might be marginally visible in the daytime telescopically some days either side of perihelion passage were it to attain an integrated magnitude of 0 to -1. This possibility, above all, is the most interesting to me personally.
Of course, all these predictions are made sight-unseen in my case! Cloudy weather has been very prevalent here this winter and I have not had a single opportunity to look for C/1996 B2. However, the visual descriptions by others do suggest an intrinsically bright, well developed comet and are quite different from either and "outburst" object or a Kohoutek. "
At the International Workshop on Cometary Astronomy in February 1994 I talked about my analysis of visual tail lengths. The analysis showed that there exists a correlation between the heliocentric magnitude and the tail length (see International Comet Quarterly, Oct. 1994, pp. 144-148).
Applying the formula to comet C/1996 B2 (Hyakutake) I make the following prediction about the development of the visual tail:
Because of severe foreshortening prior to its encounter with earth the tail length will not exceed 5 degrees prior to Mar. 20th. The following days, however, will see a rapid growing - due to diminishing foreshortening and distance - with a maximum on Mar. 27th. For that day I predict a visual tail length of 20 degress, but it should be of comparable length from Mar. 25th to 28th. The tail however will not be conspicuous. Thereafter, mainly because of the increasing distance to earth, the tail will shrink and reach a minimum of about 12 degrees around Apr. 5th. After that date it will grow again and gain brightness. Maximum length will be reached on Apr. 25th/26th. Thereafter foreshortening and increasing distance will cause a rapid shrinkage. The formula predicts the following lengths (0h UT): Apr. 15th: 17 degrees, Apr. 20th: 26 degrees, Apr. 25th: 35 degrees, Apr. 30th: 20 degrees, May 5th: 14 degrees , May 10th: 13 degrees, May 15th: 9 degrees, May 20th: 6 degrees.
The prediction has three important restrictions: First, the formula describes the average(!) tail development of comets. If comet Hyakutake will show high activity or will even break apart, the visible tail can get much longer. On the other side, in the case of low activity it will be shorter.
Second, the formula depends on the brightness development. The above values are calculated on the basis of the brightness parameters m0=5.5 and n=4 (neglecting a probably greater m0 and smaller n after perihelion, because assuming any different values for this period is not more than a guess). Again, if the comet will show a different brightness development, the tail length will differ from the above values.
Third, the formula is well tested only for distances down to about 0.35 AU. Because of its exponential nature, it has to be replaced by another kind of formula (I prefer a parabola) at a distance somewhere between about 0.3 AU and 0.2 AU. Comet Hyakutake will have sun distances of less than 0.3 AU from Apr. 26th to May 7th. For this dates, the given values are therefore not supported by any observations of past comets.
As a result, northern hemisphere observers at about 40-60 degrees latitude will have the best view (especially in the days of earth passage). To south hemisphere observers the comet will not be visible in a moderately dark sky prior to about May 7th. By then the tail length will have shortened to only about one third that displayed at the end of April. Only in the case of a splitting of the nucleus and a resulting increase of the absolute bright- ness of at least 3.5m would the tail achieve comparable lengths after perihelion.
Andreas Kammerer, Johann-Gregor-Breuer-Str. 28, 76275 Ettlingen, Germany, Internet address: firstname.lastname@example.org (alternatively: email@example.com)
The following ephemerides give daily positional information on C/1996 B2. Details on the ephemerides can be found on the Ephemerides Page.
Orbital elements are from MPEC C06 (Feb. 10, 1996) which states "Provided the prediscovery measurements can be relied on to arcsecond accuracy, the ephemeris uncertainty around the comet's approach to the earth is at most a few tenths of a degree, the main source of error being in the orbital eccentricity, which is clearly very close to unity."
1950 coordinates (Feb. - July 1996)
2000 coordinates (Feb. - July 1996)
The following ephemeris (2000 coordinates only) is from Donald Yeomans of JPL and is based on Reference Orbit #6 (Feb. 12, 1996). The errors are significantly reduced from previous orbits.
2000 coordinates (Feb. - July 1996)
For links to sites with finder charts and other information on C/1996 B2, Ron Baalke's list of links will point you in the right direction.
A general discussion of light curves can be found on the Light Curve Page.
Comments?Charles S. Morris / firstname.lastname@example.org