• Finding a software gem!
• Odin - when will it decay?
• Another example - Freja

Finding a software gem!
Incredibly enough one can still find real gems for free on the Internet. The other day I bumped into a piece of software that NASA makes avialbale for free - the Debris Assessment Software (DAS) provided by NASA:s Orbital Debris program Office. It a marvelous piece of software (1.1 MB to download) that calculates the risk of being hit by space debris etc. It has some nifty features like being able to compute the lifetime of a satellite. Of course you have to know the initial orbit and the "ballistic coefficient", i.e. the ratio between projected surface area and mass. The projected area or cross section area in the direction of flight can be a bit tricky to estimate and it requires good knowledge of the spacecraft's geometry and orientation.

Odin - when will it decay?
As a pastime I tried to look at Sweden's Odin satellite. This satellite is in an unusual sun-synchronous orbit passing over the day-night line (called the terminator - no it is not Arnold!). The solar panels are normally aligned with the orbital plane, but often they are 10-20 degrees out of alignment with the orbital plane in order to cover observation targets in the sky and in the atmosphere. Below I have sketched what DAS calculates for a few different cases. If the satellite is kept operating as now, it will decay 12 years from launch, i.e. in 2013. The actual orbital elements six years after launch are also marked showing that my estimate of the crossection of the spacecraft is close to the truth. The graphs also show beautifully how the DAS package models the solar sycle. It is of course at peak solar activity that the risk of decay suddenly is very high. In between solar maxima the satellite does not decay in these examples. Indeed an interesting orbit!

 
Another example - Freja
So, this shows that DAS can be used for parametric studies of the sensitivity of orbital evolution to orbital parameters and spacecraft orientation. I then performed a similar analysis of the first satellite developed by SSC, i.e. the Freja satellite launched from China in 1992. The satellite used to be controlled so that the spin axis of the disk-shaped satellite was pointing at the sun, but now that we have no contact with it the spin axis points randomly. The graph below is very typical of decay from an elliptical orbit. Freja will last 363 years in orbit. The graph shows also that DAS models the sun's 11 and 22 year cycles.

So, get DAS and play with it! It has a space debris model that is updated by NASA at regular intervals...have fun!

Sven Grahn