Keplerian Elements created 17 Apr 07
Describe the location and path of an orbiting object using the six Keplerian elements.Ephemeris
There is a lot of stuff in orbit around the Earth, especially after the Chinese ASAT test in Jan 2007. Orbital objects move so quickly that even miniscule ones are dangerous — a rogue paint chip pitted the window glass of an early Space Shuttle Mission. Consequently, space adventurers must be keenly aware of all the space junk to prevent disastrous collisions.
Currently, NORAD tracks anything in orbit of significant size (bigger than a softball) and disseminates the information using an orbit descriptor called a Two Line Element (TLE) set. This is composed of international designators, drag tems, reference times, and orbital element sets. These TLE sets will tell you exactly where an object is at a specific time. Given that satellites move in ellipses, you can then predict where they'll be at any time in the future… sort of.
There are several forces perturbing satellites while they are in orbit: gravitational fields aren't uniform because the Earth is neither uniform nor spherical, the Moon pulls on satellites, solar radiation (both direct and reflected) pushes satellites, and low fliers have aerodynamic drag terms. These forces will distort the standard orbital ellipse and will eventually cause things to fall out of the sky. Fortunately, there are accurate models of these disturbances called the SGP4 and SDP4 algorithms for low and high altitude satellites, respectively.
Simulation and Visualization
The applet below is an implementation of these algorithms. It will generate ephemeris, a table of precise positions for celestial bodies, and render the data over a 3D Earth. Upon loading, it will show you a few of the most recent launches. Press '1' to add half of the debris from the Chinese ASAT test (careful, you'll need a beefy computer for this visualization, or try the standalone — it runs faster). Press 'h' to see the other included sets. If you want to see more objects or need more recent TLE sets (they're accurate for ~30 days), they are readily available online. CelesTrak is an excellent source. Simply save them to your hard drive and press 'a' to import (this feature is not available on the online version for security reasons).
Figure 1. Satellite orbit visualizer. Click on applet for focus. Press h for a list of commands. Mouse Problems?
The attached jar files are compressed using the zip format. So, you can open and edit them in any zip application. If you go into the data folder, you'll find a jpeg called earth.jpg. This is the texture map used in the program. You can replace it with any other jpeg named earth.jpg. The options for lunacy are numerous, but I would recommend that you use a high resolution texture map of the Earth. NASA has some stunning textures available on their Blue Marble server. You will need to scale these down (less than ~2000 pixels per side) to work, or you can increase the memory allotted to the application.
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