Soon after this problem was recognized, a debate arose about the reality of these estimates. The critics suggested that these estimates should be regarded as a result of physiological effects of the eye-brain-system (which tends to lengthen linear features at the threshold of detection) and of the influence the rapid publication of observations via the internet has on observers.
The defenders of the reality of these observations suggested that a thorough tail modeling should be made to support them. However, until now the author knows of no successful investigation.
In principle, there are three possible approaches to explain the problematic observations: an extraordinarily long tail or a tail deviating from the antisolar direction or a combination of both effects. A comet with a heliocentric magnitude around 5m normally displays a visual tail of around 20 million km. But even the assumption of a visual tail of 100 million km is not enough to explain all the problematic estimates! All of them can only be explained by the assumption of a 100 million km tail plus a deviation from the anti-solar direction of up to 40° toward the earth! What makes these assumptions even more unrealistic is the fact, that the tail would have to rotate in step with the changing position of the earth!!
The diagram demonstrates in which way the tail has to be deviating from the anti-solar direction (assuming a 100 million km visual tail). Regarding the average tail length of a comet with a heliocentric magnitude around 5 mag the depicted deviation angle must be regarded as the smallest required. Interestingly, the observers reporting problematic estimates noted position angles in accordance with a tail in anti-solar direction!
Conclusion: it is much more convincing to state that the problematic tail estimates were caused by physiological effects of the eye-brain-system, which should be taken in account in the future. The visual tail length of comet Hyakutake therefore did not exceed 75° (perhaps 80°) at maximum.
Ettlingen, June 6th, 2000.