Comet Hyakutake - Update (August 3, 1996)

Dust Jets (ESO NTT Image)


This is most probably the final Update about this Great Comet of 1996 which is now rapidly receding into the remote regions of the solar system. The latest was published on the ESO Web on May 11, 1996. It is based on information received directly by email and also from IAU Circulars and on other Hyakutake WWW pages.

Richard M. West (ESO)
Munich, August 3, 1996; 14:00 UT

Current appearance

Following a glorious passage at only 15 million km from the Earth in late March 1996, Comet Hyakutake went on to its perihelion on May 1, 1996. The first observations thereafter confirmed the tendency, already noted in the course of April, that the magnitude was not quite as bright as hoped for. Indeed, it seems that this comet put on its finest display when its was closest to us!

It has since moved rapidly southwards and the declination is now so low (-70 deg) that it can only be observed from the southern hemisphere. The latest reports by observers date from late July and indicate a visual magnitude of about 8.5. A tail (of about 0.5 deg length and pointing directly south) was last seen on July 7.

Comet Hyakutake will continue to fade as it moves away from the Sun. At this moment, the heliocentric distance is about 310 million km; it will increase to nearly 600 million km at the end of 1996. Then the magnitude will be about 13-14, somewhat weak for visible observers, even with reasonably powerful amateur telescopes, but still an easy object for those equipped with CCD detectors.

Orbit and ephemeris

The most recent orbital calculation and ephemeris by Brian Marsden were published on May 23 (MPC 27287) and are based on over 700 observations obtained since the discovery date. It was necessary to include non-gravitational forces in the computation in order to fit well the astrometric observations which were made after the perihelion passage. They represent the repulsive action on the nucleus of the dust and gas jets emanating from vents on its surface.

The orbit is very nearly parabolic and this comet will not return to our neighbourhood during the next 100,000 years.


The passage of Comet Hyakutake proved a true bonanza to spectroscopic observers and many molecules were detected, first of all by sub-millimetric instruments, but also in the infrared and visible parts of the spectrum.

At a conference in Paris in early July, Jacques Crovisier of the Paris-Meudon Observatory presented the following impressive list of molecules observed in Comet Hyakutake: H2O, HDO, CO, CO2, CH4, C2H6, C2H2, CH3OH, H2CO, NH3, HCN, HNC, CH3CN, H2S, OCS and S2 . For most of these, the relative abundances were measured with reasonable accuracy.

Among these, ethane (C2H6) was detected for the first time in comets at the NASA Infrared Telescope Facility (Mauna Kea, Hawaii). There are several, most interesting implications of this discovery.

Optical observations in early April with a telescope at the Hoher List observatory (Germany) revealed that the CO+/H2O+ ratio > 1 in Comet Hyakutake (at a tailward distance of 64,000 km from the nucleus). In Comet Halley, this value was

X-ray observations of comets

It will be remembered that X-rays from Comet Hyakutake were detected from ROSAT observations on March 27; this was the first time in any comet. Meanwhile, the scientists working with this satellite observatory went back to have a look at earlier data, in order to see whether X-rays had possibly also been observed from other comets, when the telescope was scanning the sky.

On May 22, Konrad Dennerl (Max-Planck-Institute for Extraterrestrial Physics, Garching) sent me the following note:

We have searched archival X-ray data taken with the ROSAT satellite for cometary X-ray emission, and succeeded in finding comet Tsuchiya-Kiuchi (C/1990 N1) in the ROSAT all-sky survey data (cf. IAUC 6404). In order to detect the comet, it was necessary to correct the X-ray photons individually for its proper motion, since the comet was moving between the survey scans which were separated by 96 min.

This finding is exciting for several reasons:

While comet Hyakutake (C/1996 B2) was a spectacular case, comet Tsuchiya- Kiuchi (C/1990 N1) was too faint to be visible at all with the naked eye. Furthermore, the X-ray telescope was not pointing at the comet (as in the case of comet Hyakutake) but was scanning over the sky, so that the comet was in the field of view for not more than 30 sec per scan. In view of the optical faintness and the special observing conditions, the detection of X-rays from comet Tsuchiya-Kiuchi was a surprising event.

Besides the fact that X-rays from a second, much fainter comet have now been discovered, this detection has an additional scientific impact: it allows, for the first time, spectroscopic studies of the X-ray properties.

The observations of comet Hyakutake had been taken with X-ray/EUV detectors which had no sufficient intrinsic energy resolution, so that all the spectral information which was available so far had to be derived from the flux ratio between the X-ray and EUV bands. For the all-sky survey, however, a different detector with spectral capabilities, the Positional Sensitive Proportional Counter (PSPC), was in the focal plane of the X-ray telescope (it cannot be used regularly any more because it requires permanent gas supply, and there is not much gas left).

The spectral information now available definitely rules out the possibility of pure line emission due to oxygen and carbon fluorescence, and indicates a typical 'temperature' of kT = 0.4 keV for assumed thermal bremsstrahlung emission. Although the conditions for thermal bremsstrahlung are probably not all satisfied, we are using this model at the moment in some pragmatic approach to get a rough idea on the energetics involved; we intend to refine the spectral investigations as soon as a more adequate model will become available.

With this second detection of a comet in X-rays, it seems that a new branch of astrophysical research has opened.

But this is not the end of the story! Some days later, the ROSAT scientists published a list on IAUC 6413 with all the significant comet detections obtained so far (corresponding all-sky survey observing intervals indicated). With C/1996 B2 (Hyakutake) , this brings the total to four: C/1990 K1 (Levy) (1990 Sept. 6.50-7.10 UT, 1991 Jan. 10.82-11.96, 1991 Jan. 17.49-17.96); C/1990 N1 (Tsuchiya-Kiuchi) (1990 Nov. 18.76-20.36, 1991 Jan. 12.07-13.67) and 45P/Honda-Mrkos-Pajdusakova (1990 July 31.20-31.86). All comets were at heliocentric distances less than 2 AU, with visual magnitudes between 4.2 and 11.6. If anybody has obtained optical images of these comets taken at or near the specified times, please contact K. Dennerl (e-mail:

Comet Hyakutake was again observed after its perihelion passage with ROSAT on June 22-23, when the geocentric and heliocentric distances were 174 million km (1.16 AU) and 202 million km (1.35 AU), respectively (IAUC 6433). The visual magnitude was about 7.1. The new X-ray image was similar to the earlier one and the extent of the X-ray emitting region was at least 400,000 km in diameter. The peak intensity was a factor of 5 lower than during the previous ROSAT observations in March (IAUC 6373). This confirms the continuous behavior of X-ray emission in the comet.

ROSAT will continue to observe the comet until September 8. Obviously, simultaneous observations at other wavelengths (especially extreme ultraviolet and radio) will be extremely valuable .

Possible meteor showers

Alexandra K. Terentjeva (Institute of Astronomy of the Academy of Sciences of Russia, Moscow, Russia) has made calculations which indicate the possibility of meteors associated with Comet Hyakutake. Details about the predicted radiants (directions from where meteors from this comet might be seen) are available in a short note. If anybody observed meteors from the radiants mentioned in this note, would they please contact A. Terentjeva:

Publication of Hyakutake results

Wing Ip (Max-Planck-Institute for Aeronomy, Lindau, Germany) has published an announcement concerning a Special Thematic Issue on Comet Hyakutake now planned for the journal Planetary and Space Science.