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How to Download or Enter Orbital Elements for Quickly Changing Orbits
#1
Hello,

Here is a quick guide to obtaining the data needed in SkyTools to as accurately as possible plot the path of an asteroid as it passes close to the earth.

Here are a few things to know first:

1. SkyTools uses Osculating Orbital Elements to plot the position of an asteroid or minor planet. These elements are calculated as if only the asteroid and the sun exist. With time, other objects in the solar system will begin to tug (or perturb) the motion of the asteroid, and that will change the orbit. So you always need a fresh orbit, and this is why SkyTools has options available for downloading elements regularly. The moment in time when the orbit is valid is called the epoch of osculation (or just the epoch).

2. SkyTools accumulates orbital elements over time. The set of elements with an epoch that best matches the date of your calculation will be used. That way you can get new elements in the future, but SkyTools will still be able to accurately plot the position on the night you observed it, even if that was long ago.

3. The orbit of a Near Earth Asteroid is being perturbed by the earth and moon as it makes its close pass. This will cause a change in the orbital elements as it passes by the earth, so it is often critical to update the orbital elements just prior to when you are going to observe the asteroid.

4. Many Near Earth Asteroids don't actually have well defined orbits in the first place. This is because many of them are too faint to observe except when they are close to the earth. Many have only just been discovered as they approach the earth. As they approach, astrometry (carefully measured positions) is obtained every possible night, and these new positions are used to update and improve the orbital elements. As important as point No. 3 is, the improvements of the orbital elements as the asteroid approaches us can be even more important to accurately plotting the position of the asteroid as it passes by the earth. This is yet another reason to update your elements just prior to observing.

4. In order to make the database efficient, orbital elements are sorted into different "standard epochs" when they are stored. This is a period of time, 200 days long, that orbits are sorted into. The epoch dates in general that you see from the Minor Planet Center's MPCORB download are typically these same standard epochs.

SkyTools will only store one set of orbital elements for each standard epoch. This is where it gets a little tricky when it comes to Near Earth Objects (NEOs), because the MPC recently began publishing elements at a standard epoch date in the future when you download the MPCORB data. Aside: the alternative ASTORB database from Lowell Observatory is computed for an epoch close to the current date.

As a result, SkyTools needs some rules to follow to know when to replace an existing set of orbital elements with a new one. It can't simply keep the elements with the latest epoch, because you may want to update the elements near the time of a close pass, which may be prior to the date of the elements that are already in then database. To solve this problem in SkyTools, there are now cases when a new set of elements will always overwrite the ones already stored for that epoch.

When you download the MPC NEA Today or MPC Daily Update from within SkyTools, the downloaded elements will always overwrite the current orbital elements in the current standard epoch. That way, you can always update your elements when a close pass is coming up (and the elements improve). These elements are updated daily, to the current epoch, and hopefully with the latest data, so they will often be the most accurate. Prior to a close pass you should download these elements. This is all most observers of close approaching asteroids need to do.

For Advanced Users
For those who want to enter elements from JPL's HORIZONS system, this is done by pasting the elements into a SkyTools dialog. When manually entering data, such as that from HORIZONS: if you edit an existing set of elements, this will always overwrite what was there before. If you create a new set of elements, be sure it has an epoch more recent than the one in the database. Otherwise it will not overwrite the one already there. Do not kludge the epoch. It must be accurate.

How do you tell if everything is OK with your orbital elements?

1. Open The Minor Planets Database from the Data menu
2. Enter the designation of the minor planet under Designation and press the Enter key.
3. When it finds the minor planet, it will list all of the orbital elements by epoch. Double-click on any one to see the full data.

Prior to observing a close pass, you want to see the top set of elements have an epoch very close to the current date. If you are planning to manually add new elements, ensure that there is no set of orbital elements for a date in the future, beyond the date of closest approach. If there is one, delete it, or replace it manually, by editing the data. To delete it, close the Osculating Elements dialog, right-click on the elements you wish to delete, and select Delete Element Set.
Clear skies,
Greg

Technoking of Skyhound
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#2
Thanks Greg. What a succinct way to describe the needed actions. Been following your methods for a long time and they just work!
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