Near Earth Asteroid Rendezvous

Planetarium Software

A number of programs are now available that you can run on your home computer to display the positions and motions of the planets in the solar system. Such software allows you to explore what appeared to be complex motions to the ancients, and to see the view not only from the Earth but from other positions in the solar system. Prices range from free to expensive, but are typically reasonable for the value.

Other lists of software

Software

Here is a list of some such programs:

More on Orbits
What Are Keplerian Elements? by AMSAT.

More Kepler by Michael Fowler. Also don't miss his Physics 109N.

Kepler's Laws of Planetary Motion by RCBrill

Orbital Elements

Quite a few planetarium programs allow you to add new objects, such as asteroids, and will then plot their positions as they move through the sky. To add a new asteroid you must describe it's orbit around the sun. This is done by giving values for the size, tilt, and orientation of the orbit, and where the asteroid is in the orbit at a certain time. These values are called the orbital elements. There are a standard set of orbital elements, but the programs expect them in slightly different formats. There are several sources of orbital elements in the expected formats:
Simplified web page to access the Minor Planet Ephemeris Service for Eros and the first four asteroids.

Full web page for the Minor Planet Ephemeris Service covering all asteroids.

Orbital Elements for Software Packages from the Minor Planet Center. Lists of elements organized into bright asteroids, unusual asteroids, distant asteroids, and comets.


A Step by Step Example

To show how to add objects to a planetarium program an example will be given here. The Starry Night program will be used.

First get the orbital elements for Eros. Use the Simplified web page, scroll down to its third section and click the button for Starry Night. Then below that click the button labeled Get orbital elements. There will be a pause while the request is transmitted back to the Minor Planet Center, then a save to disk dialog box will appear with the file name MPEph.COM. It's a good idea to change that name to something like eros_eph.txt before saving. For Starry Night that file looks like this.

You might also want to pick an application to view the file, it's a simple text file so any simple text editor should work. If the file was saved to disk open it in a text editor to view it. The orbital element values may be copied and pasted from the editor window into Starry Night.

To add Eros to Starry Night first make sure the Planets pallet is open, then select the Sun, and click on Add at the bottom of the pallet. This brings up Starry Night's orbit editor, in the Info mode. There are four items in this section, all should be changed. The first is the type of object, select Asteroid for this. Next is the object name, enter 433 Eros here. Change the orbit color if you wish, and finally enter the absolute magnitude which comes with the orbital elements. The absolute magnitude is used to compute the apparent magnitude of the asteroid as it moves around its orbit.

Now click on the pallet tab labeled Orbital Elements. This is the fun part. If you are new to orbits, or just new to Starry Night, take some time to play with this part of the orbit editor. The view that comes up in the editor is a working Starry Night window. That means you can turn off the stars and constellations (to simplify the view), turn orbits on and off, or anything else you can do for a main window. A default orbit appears in the window. You can flow time forward or backward and by dragging the mouse in the window change your viewpoint. An oblique viewpoint is best for many cases. Now use the mouse to drag each of the orbital element slider bars back and forth and watch what happens to the new orbit. If it's not clear what a slider bar does make sure you review the meaning of that orbital element. None of this is necessary to enter Eros, it just gives you a better feeling for the meaning of the elements. Don't forget to copy and paste each of the elements for Eros into the correct areas and click the save button before you leave the orbit editor.

Now that Eros has been entered you can view just like any other planet, trace its path through the sky, show its orbit, and so on. It is a very good idea to use the elements web page to get an ephemeris for Eros, a list of positions in the sky. Then make sure your program gives the same position at the same time or is very close. If it is far off then some of the orbital values may have been entered incorrectly.

Below are a few example images made using the orbital elements of Eros

Looking down on the Solar System from above the sun gives this view. From here Eros, orbit in blue, appears to endanger both Earth and Mars as it crosses their orbits.

But looking edge-on to both the orbits of Earth and Eros in the image below shows both planets are safe from Eros at this point in time.

Here is an example Quick-Time Movie (2.2MB) made by Starry Night. It is a first attempt and a bit rough. It shows the 1975 near flyby of Eros. It rotates the view in 3-D to show how the orbit of Eros is inclined out of the plane of the Solar System.

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