C/1996 B2 (Hyakutake)
Copyright�1996 by Herman Mikuz (Crni Vrh Observatory, Slovenia)
Wide-field mosaic image of comet Hyakutake, taken on 1996 Apr. 6, with the 90mm f/2.8 lens, a CCD camera, and a narrow-band H2O+ filter. Each frame was exposed for the 5 minutes, centered at 19:18 UT. The frame field of view is 2.9° tall and 13.0° wide.
Yuji Hyakutake (Hayato-machi, Aira-gun, Kagoshima-ken, Japan) discovered this comet with 25x150 binoculars on 1996 January 30.83 at a position of RA=14h 31.2m, DEC=-24° 55' (2000.0). This was his second comet discovery in five weeks and the position was less than 4 degrees from the spot where his last comet was discovered! The comet was estimated as magnitude 11.0, and was described as diffuse with a central condensation. The coma was 2.5 arc minutes across.
A prediscovery observation was announced on February 10. K. Takamizawa (Saku, Nagano, Japan) found images of this comet on photographs exposed through his 0.10-m f/4.0 patrol camera on 1996 January 1.84. The magnitude was 13.3 and the coma was 1 arc minute across.
On February 1 several observers had began to view this comet. It was described as very diffuse, with magnitude estimates ranging from 8.9 to 11.7. Experienced observers estimated the coma as 2.5 to 3 arc minutes across, with a bright, near-stellar central condensation. Moonlight interfered for several days thereafter, and few obervations were reported.
By mid-February the comet had become visible in small binoculars. Most magnitude estimates ranged from 8.3-8.6, while the comet's coma had already increased to 6-8 arc minutes. The comet steadily brightened during the latter half of the month and by the 26th, Terry Lovejoy (Australia) became the first person to view the comet with the naked eye. By the 29th, observers were reporting brightness estimates of 6.0 to 6.5, while the coma had increased to between 10 and 15 arc minutes. A tail was then reported as up to one degree long.
The moon was full on March 5. Observers were competing with its light during this period, but the comet's brightness continued to allow observations. The coma was typically reported as between 10 and 15 arc minutes across, indicating no increase since the end of February, but moonlight was the most likely deterent. Interestingly, Charles Morris and Terry Lovejoy both reported the magnitude had increased to 5.0 and 5.1, respectively, by the 6th, indicating the comet was very close to the brightness predictions. Gary Kronk saw the comet with 20x80 binoculars early on March 10 just 5.6 degrees from the moon. By the 13th, observers were reporting a coma diameter over 20 arc minutes, with an average naked-eye magnitude near 4.
Observers appear to have been out in force on the 18th, with a huge number of observations being reported to the ICQ, AOL, Compuserve, and numerous web sites (including this one). The most experienced observers were reporting the total magnitude as about 2.8 (naked eye), while estimates of the coma diameter were typically in the range of 40 to 55 arc minutes. The tail was also being observed without optical aid by some, with lengths estimated as between 2 and 4 degrees. On the 19th, observers estimated magnitudes which averaged about 2.5. The coma was definitely over one degree across, or twice the apparent size of the full moon! Meanwhile, the media had picked up on the comet and the general public was reporting the comet was visible to the naked eye!
Naked-eye observations by Kronk on the 22nd indicated the comet was magnitude 1.4, with a coma around 110 arc minutes across and a tail 15.2 degrees long.
This photograph was obtained by Kronk on March 23. He used Kodak 1000 ISO film and a 50 mm lens on his Minolta 35mm camera.
Observations on the 24th revealed the comet at about magnitude 0, with some estimates as high as -0.3. The coma was variously estimated as 1.5 to 2 degrees across (3 to 4 times the diameter of the full moon). Most observers reported a tail 15 to 20 degrees in length, but observers in clear skies could follow it to a distance of 35 to 45 degrees.
The comet passed closest to Earth (0.1018 AU) on March 25. There have only been 32 other comets in history that have passed closer (check the Closest Comets list). Magnitude estimates for the 25th were typically between +0.9 to -0.8, with the average near +0.1. Tail lengths were typically in the 40 to 50 degree range, while some observers, such as S. J. O'Meara (Hawaii), estimated it as long as 80 degrees.
On the 26th, the tail was estimated as 100 degrees long by O'Meara and J. V. Scotti (Arizona), while A. Baransky (Ukraine) found it 90 degrees long. Signs of the comet's fading as its distance from Earth increased became apparent within the next few days. The average magnitude was apparently near +0.2 (based on 18 estimates) on the 26th, while on the 27th it had apparently dropped to +0.4 (based on 13 estimates). Despite this slight fading, Vince Star (Illinois) was able to see the comet with the naked eye on the night of the 26th about one mile from Wrigley Field in Chicago. The light pollution prevented any observation of the tail, even with 7x35 binoculars and a 500mm Schmidt-style spotting scope. The comet reached its most northerly declination of +86.6 degrees on March 27. Observers on that date variously estimated the magnitude as -0.2 to +1.5, with an average around +0.5. The coma was still a bit over 1.5 degree across, and tail estimates from dark-sky observers were generally between 45 and 70 degrees. Magnitude estimates continued to be scattered through the end of the week but averaged 0.8 on the 28th, 1.1 on the 29th, and 1.5 on the 30th. On the last date, in bright moonlight, coma diameter estimates were generally near 50 arc minutes, while tail lengths ranged from 10 to 30 degrees.
By the end of March reports had been issued detailing observations of the nucleus. Lecacheux et al. (Pic-du-Midi) concluded the rotation period was in the range of 5-9 hours, with 6.6 hours seeming most likely, while D. Schleicher (Lowell Observatory) said single-peaked lightcurves with periods of 6.25 or 8.55 hours were viable. Meanwhile, the 230-foot antenna at the NASA/JPL Goldstone Deep Space Communication Complex observed the nucleus and concluded it was less than 2 miles across. Even more nucleus news came beginning with observations on March 23 which revealed knots of material had separated from the nucleus. The best photograph of these knots was obtained by the Hubble Space Telescope on March 26 (below). H. A. Weaver imaged the comet and revealed three separate objects. It has been suggested by the HST team that these objects are "made up of coarse-grained dust," because "Large fragments of the nucleus would not be accelerated in the tail." Meanwhile, on IAU Circular 6360, Zdenek Sekanina (Jet Propulsion Laboratory) commented that a solution using the available observations of the main "blob" of material indicated a probable separation from the main nucleus on March 21.0. He indicated the typical lifetime for such nuclei would be 10 to 20 days if the comet was situated one AU from the sun. The comet is obviously closer than that distance and decreasing.
Moonlight had become a major factor as April began, but the comet was still easily seen with the naked eye. Magnitude estimates were generally around 2, with coma diameters near 30 arc minutes. The tail was still 10 to 15 degrees long. There was a total lunar eclipse during April 3.98-4.04 and several observers in Europe and the United States saw the comet during that time. Magnitude estimates ranged from 2.1 to 2.3 and R. J. Bouma (the Netherlands) reported a tail length of 18 degrees.
After the eclipse, bright moonlight again affected observations, but it was no longer an issue by the 7th. Naked-eye magnitude estimates were then generally in the range of 2.1 to 2.4. Most tail lengths were still in the range of 7 to 12 degrees, although a few observers lucky enough to have very dark skies were still estimating lengths of 30 to 50 degrees. Brightness estimates changed little on the 8th and 9th, but according to O'Meara the gas tail was extending 60 degrees on the 8th and 90 degrees on the 9th. The comet passed nearest to Venus on April 11 (0.2311 AU).
On the 12th Kronk estimated the magnitude as 2.5. George Gliba, observing near Marshall, Virginia, said the comet seemed brighter than on preceeding days, and added that 20 degrees of tail was visible to the naked eye.
A change began on April 14. Gliba noted the condensation seemed brighter than two nights earlier and more yellowish than in the past. Late on the 15th European observers began reporting the total magnitude had brightened to between 1.7 and 2.0. This brightness held as observers in the United States began seeing the comet on the 16th. Unfortunately, the outburst seems to have been short-lived, and the comet was back to a brightness of 2.0 to 2.5 by the 19th. Kronk observed the nuclear region with magnifications of 200x and 500x with a 13.1-inch Dobsonian. There was a notable jet extending northwestward for a couple of arc minutes from the nuclear condensation on the 16th. By the 17th, the jet was gone, but was replaced by a cloud extending northward from the nuclear condensation, that gave the nuclear region an oval appearance. By the 18th, the cloud had become very faint and ill-defined and was northeast of the nuclear condensation. There was also a notable tail spike extending 5 arc minutes into the tail from the condensation. The ill-defined cloud was more or less centered near the base of this spike. Most magnitude estimates on the 20th ranged from 1.5 to 2.1.
The comet passed nearest to Mercury (0.2099 AU) on the 21st. Observers in extremely clear skies then estimated magnitudes of 1.5 to 3.1. As the week progressed, the number of reports declined as the comet dropped deeper into twilight, causing magnitude estimates to begin to vary considerably. By the 23rd, magnitude estimates ranged from 1.6 to 4.1, and by the 25th they ranged from 1.3 to 3.1. Following the 25th, observations became few and far between. The last observation prior to perihelion was apparently made by John E. Bortle (New York) who saw the comet in bright twilight on April 28.02. The comet was then at a 4 degree altitude. Bortle used 15x80 binoculars and estimated the magnitude as 2.5-3, but noted no tail. He said the comet was then 12 degrees from the sun.
The photograph above was obtained by the Solar and Heliospheric Observatory (SOHO) on May 1, the day the Comet passed closest to the sun (0.23 AU). The comet is at the top of the image, above the intense solar corona. The magnitude of the comet might have reached 0 at this time. Additional photographs were obtained by SOHO on May 2 and 3. The comet was at its closest angular distance (elongation) from the sun (5.4 degrees) on the 3rd.
Northern hemisphere observers were at a major disadvantage after the comet passed the sun, as it only rose above the horizon in the morning sky after strong twilight was present. On the other hand it climbed straight up into the sky for observers in the Southern Hemisphere. Terry Lovejoy (Australia) searched for the comet in very bright twilight on May 7 and 8, but could not find it. He estimated the comet's magnitude must have fainter than 1.0 on the first date and 2.5 on the second. The comet was finally picked up by Gordon Garradd (Australia) on May 9.82. He estimated the magnitude as 3 and said the comet was two arc minutes across. No tail was visible as the comet was in bright twilight and at a low altitude. The comet was situated 15 degrees from the sun on May 10, and by the 11th G. Ballan and L. Mansilla (Argentina) could see the comet with the naked eye. They estimated the magnitude as 1 and said two degrees of tail were visible.
Numerous observers began to detect the comet by the 13th. Magnitude estimates ranged from 3.1 to 3.6, and most observers found the tail extending about 3 degrees. Lovejoy found the coma about 5 arc minutes across in 10x50 binoculars. There was little change on the 14th, with magnitude estimates ranging from 3.1 to 3.8, although Lovejoy reported a 4 degree tail could be seen with the naked eye and binoculars. David Seargent saw the comet with the naked eye on May 15 and estimated the magnitude as 3.8.
Most magnitude estimates ranged from 3.9 to 4.5 during the period of May 19 to 23, while tail length estimates ranged from 1.5 to 7 degrees. With the comet completely out of twilight, it became an easy naked-eye object. Magnitude estimates continued to fade and for the period of May 24 to 31 it ranged from 4.2 to 5.0. Also, during these latter days of the month, the coma was very consistently estimated as 4 to 5 arc minutes, while tail lengths were 2 to 6 degrees.
The comet began this month at a declination of -22 degrees and finished at -57 degrees. As the comet's distances from the sun and Earth increased, the brightness continued to fade, with estimates at the beginning of the month near 5 and those at the end near 6.5. Lovejoy said the comet was still a naked-eye object on the 29th. Coma diameter estimates were consistently around 4 to 5 arc minutes for the entire month. The tail, although not seen by some observers, was described as faint and usually 1 or 2 degrees long.
Total magnitude estimates continued to drop during this month. Observers were estimating values of 6.5 to 7.1 as the month began, and 7.9 to 8.4 as the month ended. The only thing that stayed consistent was the coma diameter, which seemed to change little from the initial estimates of 4 to 6 arc minutes at the beginning of the month.
The comet faded very rapidly during August, beginning the month at about magnitude 8 and ending near magnitude 11. The coma also experienced a rapid decrease in size as observers estimated it as 3 to 5 arc minutes across during the first week of August, but only 1 arc minute across during the last week.
The comet faded from 11 to 12 during the first half of September and was still about 1 arc minute across. Only one attempt to see the comet was apparently made after the 16th, with A. Pearce (Australia) reporting that he failed to see the comet with his 0.20-m reflector on the 21st. He surmised the comet must then have been fainter than magnitude 12.0.
After a month of no observations, G. J. Garradd (New South Wales) finally detected the comet again on October 24. He used a 0.25-m reflector and a CCD camera and estimated the magnitude as 16.8. No coma was detected.
B. G. Marsden (Central Bureau for Astronomical Telegrams) computed the first orbit, which was published on IAU Circular 6303 (1996 February 3). This orbit was based on 33 positions obtained during the period of 1996 January 31 to February 3, and indicated the perihelion date was 1996 May 2.77, the perihelion distance was 0.22 AU, and the inclination was 122 degrees.
The most recent orbit was computed by Marsden and published on MPC 27287. It was based on 703 precise positions obtained during the period of January 31 to May 23, and resulted in the following elements:
|Epoch||1996 Apr. 27.0 TT|
|Perihelion Date (T)||1996 May 1.39500 TT|
|Perihelion Distance (q)||0.2302210 AU|
|Argument of Perihelion ()||130.17725°|
|Ascending Node ()||188.04538°|
On IAU Circular 6359 (March 28) Marsden noted that the comet's original orbital period before entering the solar system was about 8000 years, and that its future orbital period after it exits the solar system will be 14000 years.