Comet Hale-Bopp (June 13, 1997)

This is a summary of recent developments around this comet; the previous was published on the ESO Web on May 16, 1997. It is based on information received directly by email and also from IAU Circulars and on other Hale-Bopp WWW pages.

Richard M. West (ESO) 
Munich, June 13, 1997 

There has been very little activity during the past weeks; this is reason why the present Update appears almost one month after the previous one.

1. Current state

The comet is rapidly becoming fainter; the total magnitude is now estimated as about 2.5. Due to the comet's present position in the sky, only 21 o from the Sun, observations are very difficult and can only be made from the southern hemisphere.

The latest report filed by an experienced observer in the northern hemisphere dates from May 22.1 UT, when Gary Kronk (Troy. Illinois, USA) made the following notes: comet first detected at 9:07 p.m. CDT while sweeping with the 20x80 B; it was then about 3.1 deg above the horizon.... the comet then appeared as a rather bright nuclear region with a wedge-like emission with borders to the SW and S-SE of the nuclear region; the comet was last seen in the B at 9:10, as I was making a magnitude 9:17, the wedge of material was virtually gone and the comet's appearance was that of a diffuse star with a hint of some elongation southward; almost exactly as the time reached 9:23, the comet passed behind a bush nearly a mile away; the horizon was then within the same field as the comet and the altitude was 0.2 deg; the very last view of the comet showed it as nothing more than a very diffuse star-like object; there was no hint of any elongation; twilight was still strong throughout the session...

By June 10, very little of the dust tail was visible, at the most 1-2 o and there was less structure around the nucleus.

At this moment, Comet Hale-Bopp is moving close to the celestial equator, which it will cross about two weeks from now, on June 26. It is now about 2.45 AU (366 million km) from the Earth and is receding at 21 km/sec. The distance to the Sun is about 1.55 AU (232 million km), i.e. it is now just outside the orbit of planet Mars.

2. Comparison with other comets

A table with the apparently brightest comets has been added to Jacques Sauval's note on the Longest Visibility of Ancient Comets.

3. Molecular update

On June 9, 1997, Jacques Crovisier (Observatoire de Paris) informs that we have no other firmly detected molecules than those reported in the IAU Circulars. We have some marginal detections on which we are working and a lot of upper limits. We are still busy with all this and we have still observing time at IRAM (mainly for follow-up monitoring: there is little hope now to find new molecules) .

Moreover, Alan Stern (South-West Research Institute, Boulder, USA) has informed me that the earlier announced detection of Helium (He) is not valid. He writes: I have spoken with Mike Mumma about his IAUC stating He emission was observed by EUVE, and he can verify for you what I am telling you. I was suspicious of his claim because (i) He is so volatile it should not be retained in the ice, and (ii) we have a very strong upper limit on He in other comets showing it is more than 10 4 times depleted over cosmogonic He. When I asked Mike about the EUVE He detection, he revealed that what they are seeing is SOLAR WIND helium that is in charge exchange emission at the comet, NOT endogenic helium. I think that a lot of people believe He was actually seen in Hale-Bopp because of the... wording of Mumma's IAUC.

I have therefore removed this element from the list of molecules found. This is the latest version:

H 2 O, HDO, OH, H 2 O + , H 3 O + 
CO, CO 2 , CO + , HCO + 
H 2 S, SO, SO 2 , H 2 CS, OCS, CS 
HCN, DCN, CH 3 CN, HNC, HC 3 N, HNCO, CN, NH 3 , NH 2 , NH 2 CHO, NH 
CH 4 , C 2 H 2 , C 2 H 6 , C 3 , C 2 
Na, K 
O + 

and the following isotopes:

H 13 CN, HC 15 N 
C 34 S

4. Many small comets near the Earth?

Recently, information has become available about exciting observations with the POLAR satellite of possible, small `comet-like bodies' disintegrating near the Earth, see this WWW-site:

However, doubts have been expressed by several scientists about the interpretation of the light trails seen on these images. In particular, it seems to be the widespread opinion outside the circle of scientists directly connected to this project, that the statistics cannot be correct. If there were really so many, small comets near the Earth as indicated in the associated Press Release, then they would be seen with other techniques, for instance during the dedicated searches for Near-Earth Objects (NEO's) now carried out at various observatories. Moreover, there would be many more craters on the Moon and new ones would continue to appear. Also the amount of water that would be deposited in the Earth's atmosphere would have important consequences which would be observable.

Nevertheless, nobody doubts the reality of the observations shown on the WWW and if the trails do not originate by the disintegration of small comets, they must have another, still not understood cause. All of this is clearly a very exciting subject!

5. Another comet

One more comet ( C/1997 L1 - Xinglong ) has been found during the first half of June 1997. You will find full details at the corresponding webpages (the Headlines) at the Central Bureau for Astronomical Telegrams).

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