Orbs: Femtosat Swarms for Space Science Applications

A femtosatellite, or femtosat, is a satellite with a mass of less than 100 grams. Compared to other orbit perturbations, Solar radiation pressure (SRP) has a relatively significant influence on the orbits of these very small satellites. Groups, or swarms, of orbiting radiation pressure barometer spheres (ORBS) could be released with identical inertial and magnetic properties, but with different optical coatings to isolate the effects of SRP on the relative motion of members of a given ORBS swarm. By cleverly assigning ORBS optical properties and implementing detailed solar radiation pressure models, ORBS tracking data could be used to estimate important atmospheric and solar quantities. First, detailed SRP modeling and orbit propagation tools are being developed to model the relative dynamics of the members of an ORBS swarm. Second, parameter estimation techniques are being developed to estimate atmospheric and solar parameters using ORBS tracking data. When complete, this set of modeling tools will enable the simulation of ORBS science missions. These simulations will enable us to assess the viability of ORBS swarms for various missions. The SRP modeling, orbit propagation, and parameter estimation tools have been developed individually and applied successfully to other problems as preliminary steps to modeling ORBS science missions. 1. ORBS Introduction Forces affecting satellite orbits such as residual air drag and solar radiation pressure (SRP) are experienced on very different relative scales by a femtosat than by conventional, larger satellites. This is illustrated in Figure 1, where the relative influence on spacecraft accelerations by solar radiation is shown to increase with smaller spacecraft. In Figure 1, ORBS would have characteristic length, λ, on the order of 10m. Note the relative significance of SRP (labeled SP here) with respect to air drag (AD), planetary albedo (PA), and others. This provides additional motivation for the small form factor of ORBS beyond low launch mass and cost. Additionally, the spherical geometry and uniform optical properties of each ORBS femtosat will decouple SRP accelerations from spacecraft pointing. Precise tracking of ORBS relative motion will provide input to parameter estimation tools. These parameter estimation tools will utilize an orbit propagation model which will include a highly detailed SRP force model. Since various solar and atmospheric quantities drive the detailed SRP model, these can be estimated using the parameter estimation tool and tracking data from the ORBS swarms. Figure 2 shows a conceptual rendering of an ORBS swarm, where the grouping of ORBS in orbit has occurred due to variations in the optical properties of members