Imports comets and asteroids from the websites of NASA's Jet Propulsion Laboratory and the IAU's Minor Planet Center.
When you select this command, a dialog will be displayed showing the following different object types:
The data supplied by NASA's Jet Propulsion Laboratory (JPL) has more significant digits than the data supplied by the IAU's Minor Planet Center (MPC). However, more significant digits doesn't always mean more accurate data because the extra digits may be incorrect, so it is not always true that the JPL data is to be preferred to the MPC data. In addition, the epoch for an object's data is sometimes much more up-to-date in the MPC data, making the MPC data preferable in these cases. Also, if you're interested in a comet's magnitude, the MPC data is to be preferred because the JPL data lacks absolute magnitudes, forcing AstroGrav to make a guess that can be very inaccurate.
You can select one type of object, and then choose OK to continue or Cancel to abandon the import. After opting to continue, a table of appropriate objects that have been downloaded from the website will be displayed, together with a Fast import checkbox. After downloading from the website, the data is cached on your computer's disk, and it is normally this cached data that is displayed. The date and time at which the data was downloaded is displayed above the table, together with a Refresh button that allows you to download the latest version of the data if you desire. Refreshing the data can take quite a while, particularly if you have a slow internet connection, so it is a good idea to only refresh the data if you need to. On the first occasion that you choose a particular type of object, the data is automatically downloaded because there is no cached data to display.
The table of objects can be filtered by using the Category menu at the top-left. If you hover your mouse pointer over an item, a little window pops up to show you the exact definition that AstroGrav uses for that particular category. When you select a category, the number of objects are reduced to show just the ones in your chosen category. When you have a table of comets displayed, the following categories are available:
The table of objects can be sorted by clicking on the table headers, and this may be useful in helping you locate particular objects of interest. Having found the objects that you want to import, select them in the usual way, choose whether or not you want a Fast import, and then choose OK to continue or Cancel to abandon the import.
After opting to continue, what happens depends on whether or not you selected the Fast import checkbox. If you chose a fast import, AstroGrav will calculate the positions and velocities of the imported objects ignoring the gravitational influence of everything but the Sun. If you rejected a fast import, AstroGrav will calculate the positions and velocities of the imported objects taking into account the gravitational influence of all other objects that are more massive than the mass threshold specified in the evolver settings. To achieve this, the simulation will automatically evolve to the appropriate epochs to import the objects, and then finally evolve back to the original time (so that it looks as though no evolution has taken place). Screen updates are disabled while this takes place, so that all you will see is a progress bar that indicates how long the import will take to complete. As with any other type of evolution, you may stop it (using the Cancel button) if you desire. A fast import is almost instantaneous, but is not as accurate as a slow import, and in some cases the loss of accuracy can be severe. In most cases you will probably find that a slow import takes no more than a few seconds, so that there is no worthwhile advantage in choosing a fast import.
If you have used the Tools / Calculate Orbits... command to specify that orbits should be calculated relative to the barycenter of all inferior objects, you may notice a significant difference between the orbital elements in the import tables and the orbital elements in the simulation, particularly in the case of comets with near-parabolic orbits. This is because the orbital elements in the import tables are always relative to the Sun, whereas the orbital elements in the simulation are relative to the barycenter of all inferior objects. Switching back to calculating orbits relative to the Sun will greatly reduce this difference, but usually won't eliminate it entirely because orbital elements are always changing slowly with time.
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