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The University of Southern Indiana (USI) Undergraduate Nano Ionospheric Temperature Explorer (UNITE) CubeSat, funded by NASA’s Undergraduate Student Instrument Project – 2 (USIP-2), was deployed from the International Space Station on January 31, 2019 (Figure 1). UNITE was a 3U CubeSat without deployables that was passively stabilized magnetically and aerodynamically. The UNITE CubeSat reentered the atmosphere on October 21, 2021, its 995th day in orbit. One of the mission objectives of UNITE was to track its orbital decay with the intent of updating CubeSat drag models. The original predicted mission lifetime of 428 days was based on results from a spreadsheet available with Space Mission Engineering: The New SMAD, using standard CubeSat parameters and simply assuming “solar mean” for the solar cycle. To more accurately understand the actual orbital behavior known from the TLEs (two-line elements), a completely new algorithm was coded for orbital drag, but taking into account varying atmospheric densities consistent with the variability of the actual solar cycle experienced between January 2019 and October 2021. Using the algorithm and the updated atmospheric density model, the mission time was computed to be 976 days, only 19 days short of the actual mission time. Next, the drag coefficient was adjusted from 2.2 to 2.08, and with that change the algorithm correctly calculated the 995 days in orbit. Also of note, in the process o f the development of the algorithm, a slight error was discovered in the Bessel function table available through The New SMAD, and that was corrected, as well. The new algorithm, developed by the undergraduate student, was implemented in an Excel program and is readily available for use by other teams. With the implementation of a new algorithm that accounts for solar cycle activity and allows for drag coefficient adjustment, it is hoped that future CubeSat teams will be able to predict mission life and the orbital decay more accurately. |
