THE NEXT BIG COMET is almost certain to be Comet Hale-Bopp, C/1995 O1, which will put on its peak display in late March and early April of next year. Brian Marsden (IAU Central Bureau for Astronomical Telegrams) estimates that Hale-Bopp could reach magnitude -1.8 around its time of perihelion at the end of March. By comparison Comet Hyakutake, C/1996 B2, reached only about magnitude 0.0 or -0.5 during its Earth flyby last March. Hale-Bopp will be beautifully placed for midnorthern observers, shining in the northwestern sky after evening twilight -- just about where Hyakutake was visible last April.
Despite their similar peak brightnesses in similar parts of the evening sky at the same time of year, Hale-Bopp and Hyakutake are in many ways opposites. Hale-Bopp is a giant, dust-rich comet with a solid nucleus probably at least 10 and maybe 40 or more kilometers in diameter. Hyakutake, by comparison, was dust-poor and had a 1- to 3-km nucleus as measured by radar. Hale-Bopp was discovered at the extraordinary distance of 7.2 astronomical units from the Sun more than a year and a half before its perihelion. Hyakutake came out of nowhere to fly past Earth just two months after its discovery. The bad news is that Hale-Bopp will be on the opposite side of the solar system from us when at its brightest; it will never get closer than 1.3 a.u. from Earth. Hyakutake passed by us a mere 0.1 a.u. away.
Accordingly, we can expect Hale-Bopp to look quite different when at its brightest. Its head might appear smaller and more concentrated, and its tail could be much brighter. The comet may look whitish yellow if its rate of dust expulsion is high, compared to the blue-green of Hyakutake's ionized gas. P> So far, Hale-Bopp seems to be right on track for a fine performance. Its output of water vapor increased earlier than expected. By April water production equaled the output of carbon monoxide, the gas that originally drove the comet's activity. This milestone was not expected until summer. Dust production continued strong too.
By mid-June Hale-Bopp was an easy sight in binoculars at magnitude 6-1/2, and observers with very dark skies were glimpsing it with the naked eye. Telescopic observers were describing a bright stellar condensation on the east end of the coma and a diffuse glow extending west. "The appearance was not of a fan tail, but rather an ellipse of material attached to the condensation," noted Charles S. Morris using a 20-inch telescope. "This comet continues to have a strange appearance."
(40K gif) Visible with binoculars in the evening sky this summer and fall, Comet Hale-Bopp works its way northward through Scutum, Serpens Cauda, and Ophiuchus. For several evenings around October 28th, when it should be about magnitude 4.5, the comet will be within 1/3° of the 7.6-magnitude globular cluster M14 and thus in the same telescopic field.
Hale-Bopp should be easy to find in binoculars for the rest of the summer and fall. Use the chart above to pinpoint it just west of the summer Milky Way. The purple constellation lines are the same as on our naked-eye chart in the center of each month's Sky & Telescope. (Also available is a chart [90K gif] showing Hale-Bopp's path throughout all of 1996.) According to Marsden the comet should glow at about magnitude 6 in July, 5-1/2 in August, 5 in September, and 4 in November. It will disappear into the sunset in early December, then reappear in the dawn sky in mid-January at about 2nd magnitude to begin its 1997 apparition.
The following ephemeris gives, for 0h Universal Time on each date, the comet's right ascension and declination in 2000 coordinates, its angular elongation from the Sun in the morning (Mo) or evening (Ev) sky, and Sky & Telescope's prediction of its total visual magnitude. The positions were calculated from the following orbital elements (epoch April 27, 1996) in Minor Planet Circular 27079: perihelion date T = 1997 April 1.14635 TT (ET), perihelion distance q = 0.9141261 a.u., eccentricity e = 0.9950884, inclination i = 89.42829°, argument of perihelion W = 130.59099°, longitude of ascending node w = 282.47081° (2000.0).
Comet Hale-Bopp, C/1995 O1Date R.A. (2000.0) Dec. Elong. Mag.
July 2 18h 53.4m -11° 48' 167° Mo 6.2 July 12 18h 39.1m -10° 48' 164° Ev 6.0 July 22 18h 24.6m -09° 49' 153° Ev 5.9 Aug. 1 18h 10.7m -08° 54' 141° Ev 5.7 Aug. 11 17h 58.2m -08° 03' 129° Ev 5.6 Aug. 21 17h 47.8m -07° 17' 117° Ev 5.5 Aug. 31 17h 39.6m -06° 38' 106° Ev 5.4 Sep. 10 17h 33.9m -06° 03' 95° Ev 5.2 Sep. 20 17h 30.6m -05° 32' 85° Ev 5.1 Sep. 30 17h 29.7m -05° 03' 76° Ev 5.0 Oct. 10 17h 30.8m -04° 34' 67° Ev 4.8 Oct. 20 17h 33.9m -04° 03' 58° Ev 4.7 Oct. 30 17h 38.7m -03° 27' 50° Ev 4.5 Nov. 9 17h 45.0m -02° 45' 43° Ev 4.2 Nov. 19 17h 52.9m -01° 54' 37° Ev 4.0 Nov. 29 18h 02.1m -00° 51' 32° Ev 3.7 Dec. 9 18h 12.7m +00° 27' 29° Ev 3.4 Dec. 19 18h 24.7m +02° 04' 27° Ev 3.0 Dec. 29 18h 38.1m +04° 04' 27° Ev 2.6
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