Hale Bopp's Orbit
Fig. 5
Most comets have expected orbits, or that is predictable paths from where ever they come from to their journey around the sun. Many have expected arrivals and departures, documented well ahead of their returns. Some, however are not so easy to predict. As pointed out in other sections of this page, Hale Bopp has many peculiar aspects.
One has been the orbit and trajectory.
You can find two excellent 3D views of Hale Bopp's orbit by Eric Frappa
from the Saint Etienne Planetarium HERE and HERE.
Ephemeris' are numerical estimates of heavenly position, based upon a number of observations. The person who has probably calculated the most ephemeredes for Hale Bopp is Mr. Don Yeomans of JPL.
To the date of this writing, Mr. Yeomans has calculated 55 ephemeredes!
The obvious question is why so many?
Can't they repeat the process that has been done so many times before, and come up with one, or maybe only two ephemeredes, for the entire journey of the comet?
But to have 55, what's going on?
In the following quotes, may be one possible answer:
"The following orbits and ephemeredes include the rocket-like effects (non-gravitational accelerations) usually associated with active comets. Normally, these effects cannot be determined accurately with fewer than three apparitions so they are rarely used for one apparition long-period comets. While the formal errors of these non-gravitational parameters are quite small, realistic errors are probably several times larger.
Given the short observation interval and the relatively large radial non-gravitational acceleration noted, one cannot escape the conclusion that this comet is small (easy to push around...) - a conclusion consistent with the recent radar results reported by Steve Ostro."
Don Yeomans
-JPL May 13, 1996
This comment was made in reference to the recent comet Hyakutake. Two striking points made by this comment; first, this was a long roundabout way to say that Hyakutake had a "jet" that pushed it around. Kind of like a lateral rocket jet that helps change direction in outer space.
Second point, is that they imaged the nuclear size with radar.
Brian Marsden made the following comment on January 23, 1996, pertaining to the belief that an observation of a comet in 1993 was actually Hale Bopp:
"...The alternative of invoking the effects of non-gravitational forces on the comet seems very unlikely at the comet's great heliocentric distance. As noted on IAUC 6287, the comet's last perihelion passage occurred some 4200 years ago, while its return after 1997 will be some 3400 years hence [that has now been modified to a little over 2000 years!).
If the influence of non-gravitational forces as the comet approaches perihelion in 1997 remains negligible, the currently available ephemeris should have a maximum error no larger than 2 arc minutes."
Sorry Brian, There have been more ephemeredes calculated after your comments than before! Nineteen before the end of 1995 and that makes about 29 since. First, He is saying that Hale Bopp should not have had a "jet" that far out where, theoretically it is beyond the greater influence of the sun (i.e., the dirty snowball effect theory). But unlike Yeomans, Marsden does not come right out and say these "non-gravitational effects are "jets".
He calls them "forces", which I believe may be more accurate. Primarily the effect of gravimetrics and EM fields. It is my belief that these comets both have EM fields that interact with the sun and planets to shape and form their orbits. And in the case of Hale Bopp as opposed to Hyakutake, we have the concern of a field that is considerably larger.
One significant change came in the months from the end of February '96 to the beginning of April '96. This was the period of time that Hale Bopp was passing the giant Jupiter. Pictures prior to the passing, such as the Pinwheel Image, show that the comet was spinning on its axis more or less toward us. The retrograde action of the image appears to evidence that the comet was spinning clockwise with its northern pole facing us.
Following the passing of Jupiter, we see such images as the one showing the Great Northern Jet, as it is referred to. The comet passed a mere thirty million miles the other side of Jupiter, then passed through the Interplanetary Magnetic Field. The published ephemeris of December '95 was the last published, modified ephemeris before the changes really began on June 27th of this year.