The NextSat spacecraft was designed and built by Ball Aerospace & Technologies Corp. as part of the DARPA-funded
Orbital Express mission. Orbital Express, launched in March of 2007, was a highly successful demonstration mission
proving the feasibility of autonomous on-orbit refueling and servicing of spacecraft. The Orbital Express mission
consisted of the Ball-built NextSat/CSC satellite and the
Boeing-built ASTRO satellite. Both satellites launched mated
into a 492km circular orbit on board a Lockheed-Martin Atlas V 401 launch vehicle from Cape Canaveral. The NextSat
satellite acted as both the next generation "serviceable" satellite and the commodities satellite. This paper discusses the
on-orbit mission experiences of the NextSat satellite. Key experiences include: launch and early orbit operations in
which the NextSat satellite was called on to perform critical attitude control functions for the mated stack, functionality
which was never tested or planned for; autonomous fluid transfers between ASTRO and NextSat; autonomous ORU
transfers between ASTRO and NextSat; autonomous separation, free-flying and rendezvous operations; and end-of-life
operations.
The Jupiter Magnetospheric Explorer (JMEX) is a UV observatory operating in an earth orbit proposed as part of NASA's Small Explorer (SMEX) class of missions. To meet mission requirements the residual jitter portion of the imaging error budget is set at 0.079 arcsec (3σ) over a 33.3 ms frame integration time and 0.01 arcsec (3σ) for all frequency content higher than 15 Hz. These requirements are challenging for a small, low cost mission and require some innovative system solutions to achieve these goals. The solution, discussed in the paper, was to combine several jitter rejection techniques fine-balanced reaction wheel mounted on an isolation assembly, post processing using science images and reaction wheel momentum control. This paper focuses primarily on meeting the high frequency portion of the requirements. To facilitate system performance verification, we leveraged an integrated model toolset, EOSyM (End-to-end Optical System Model), developed and used on various other advanced space-based missions over the last 9 years. Starting with individual subsystem models for the reaction wheel disturbances, the coupled payload/ spacecraft structural dynamics model, and the optical design, we were able to evaluate the end-to-end LOS performance under varying reaction wheel speeds. At the end we found that the requirements could be met by maintaining the reaction wheels operating range within a well-defined speed band. This paper describes the mission, the technical challenges, the integrated model, and system performance results.
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