2023-01-31, 07:07 PM
(This post was last modified: 2023-01-31, 07:49 PM by theskyhound.)
Phil, you mostly nailed it.
Phil, yes you could use HORIZONS to generate a set of elements with a custom epoch at a later date (months or years from now) and that will work with my plan, and it can be useful in some circumstances.
But... there is an important reason why you usually need to wait until a date closer to the pass to plan for these things, and its not perturbations.
Except for a few asteroids with very well-known orbits from previous close passes, what limits the accuracy of the plotted positions is how accurate the observational data is that the orbit is built on. The accuracy of the astrometry improves as the object gets closer. The accuracy required from the orbital elements also increases as it gets closer. The two work in lock step together. Usually, the most important factor in accurately plotting the position of an asteroid as it passes the earth is the accuracy of the astrometry, and that, in turn, is usually the most recent astrometry as it approaches. This is the main reason for releasing new orbital elements regularly for NEAs, rather than the perturbations from the earth. Phil--you can't tell from the epoch alone what observations are included in the orbit. A new orbit based on the same observations for an epoch ten days later isn't a useful improvement, because there is normally nothing to affect the osculating elements over such a short period of time. What's more useful is that new astrometry has been included that improves the orbit, not that the epoch has been updated.
So here is what I believe is needed:
1. Download of elements updated more often than the 200-day epoch (e.g. the NEA Today elements)
2. Patience, as the asteroid approaches and the astrometry improves
3. For very close passes only, wait until the last minute and import sets of HORIZONS elements every couple of hours during the relatively short period of the pass and afterward
4. A way for SkyTools to pick the proper set of HORIZONS elements, based on the time, during the pass when plotting the path on the chart
I am proposing making (3) easier, but mostly, I have a plan to implement (4) as a special case.
Note: I previously mistakenly said I wanted to make (1) easier.
Phil, yes you could use HORIZONS to generate a set of elements with a custom epoch at a later date (months or years from now) and that will work with my plan, and it can be useful in some circumstances.
But... there is an important reason why you usually need to wait until a date closer to the pass to plan for these things, and its not perturbations.
Except for a few asteroids with very well-known orbits from previous close passes, what limits the accuracy of the plotted positions is how accurate the observational data is that the orbit is built on. The accuracy of the astrometry improves as the object gets closer. The accuracy required from the orbital elements also increases as it gets closer. The two work in lock step together. Usually, the most important factor in accurately plotting the position of an asteroid as it passes the earth is the accuracy of the astrometry, and that, in turn, is usually the most recent astrometry as it approaches. This is the main reason for releasing new orbital elements regularly for NEAs, rather than the perturbations from the earth. Phil--you can't tell from the epoch alone what observations are included in the orbit. A new orbit based on the same observations for an epoch ten days later isn't a useful improvement, because there is normally nothing to affect the osculating elements over such a short period of time. What's more useful is that new astrometry has been included that improves the orbit, not that the epoch has been updated.
So here is what I believe is needed:
1. Download of elements updated more often than the 200-day epoch (e.g. the NEA Today elements)
2. Patience, as the asteroid approaches and the astrometry improves
3. For very close passes only, wait until the last minute and import sets of HORIZONS elements every couple of hours during the relatively short period of the pass and afterward
4. A way for SkyTools to pick the proper set of HORIZONS elements, based on the time, during the pass when plotting the path on the chart
I am proposing making (3) easier, but mostly, I have a plan to implement (4) as a special case.
Note: I previously mistakenly said I wanted to make (1) easier.
Clear skies,
Greg
Head Dude at Skyhound
Greg
Head Dude at Skyhound