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AstroGrav Help /Menus /Edit /Import Objects...Version 5.1)Imports comets and asteroids from the websites of NASA's Jet Propulsion Laboratory, the IAU's Minor Planet Center, and Lowell's Minor Planet Services, and also imports Earth satellites using two- or three-line elements.
When you select this command, a dialog will be displayed showing the following different options:
Below these options, there is an Append Sources to Names checkbox and an Append Epochs to Names checkbox. When the Append Sources to Names checkbox is selected, any object that is imported will automatically have its data source (JPL, MPC, or Lowell) appended to its name. This may be useful if you import the same object from more than one data source, because it allows you to easily distinguish between the different versions of the same object. When the Append Epochs to Names checkbox is selected, any object that is imported will automatically have its epoch appended to its name. This may be useful if you want to know how far the object's epoch is from the simulation's current time.
In the case of the JPL Search, MPC Search, and Lowell Search, there is a text field in which you can type the name, number, or designation of an object that you wish to import, and then choose OK to continue or Cancel to abandon the import. After opting to continue, AstroGrav will search for the object that you have specified, and import it into the simulation if it is found.
Alternatively, you can select one of the other options, 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 Sort Designations By Time and Fast import checkboxes. 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 large amounts of 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:
When you have a table of asteroids 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. The Sort Designations By Time checkbox allows you to specify that you'd prefer to have designations sorted by discovery time instead of lexicographically, which is the default. 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.
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.
The easiest way to study a co-orbital asteroid is to import it, use the evolver settings to set the time step to the orbital period of the associated planet, and then run the simulation either forward or backward. The associated planet will then appear stationary in a view window, making it easy to observe the relative motion of the asteroid.
When this is done, co-orbital asteroids fall into one of the following sub-categories, although it is possible for an individual asteroid to occasionally move from one sub-category to another.
If the associated planet is one of the giant planets, it's a good idea to first use the Tools / Calculate Orbits... command to ensure that orbits are being calculated relative to the barycenter of all inferior objects, and to use the Edit / Remove Objects... command to remove the inner solar system. Removing the inner solar system makes the simulation evolve much more quickly, which is important when evolving with time steps as long as the orbital periods of the giant planets.
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.
Importing Earth satellites works differently from importing comets and asteroids. Instead of the data being automatically downloaded from the appropriate website, you manually copy the two- or three-line elements of the desired satellites, and then paste them into the table. These two- or three-line elements maybe be obtained from any source, with the CelesTrak website being particularly useful in that respect.
)that undoes the latest change to the frontmost simulation.
)that redoes the most recently undone change to the frontmost simulation.
)that sets the evolver settings of the frontmost simulation.
)that provides a choice of two different ways of calculating orbital elements in the frontmost simulation.
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