# Novel 3U Stand-Alone CubeSat Architecture for Autonomous Near Earth Asteroid Fly-By
## 4 Subsystems Analysis
### Payload
Due to the various sizes, shapes, and possible fly-by altitudes of the targets, it is not straightforward to derive a unique requirement for the GSD, but a maximum value of 50 m/pixel at 300 km altitude can be defined; this requirement would allow to describe with at least 6 \(\times\) 6 pixels a 300 m diameter NEA, which is assumed as the minimum fair value for both the scientific output and relative navigation, as it should allow to reconstruct basic asteroid shapes and to perform centroiding in case of partial illumination of the surface. Clearly, the lower the GSD is, the better the scientific output and navigation accuracy will be. A proper characterization of the required minimum number of pixels describing the asteroid is left for future works, as it will depend on many factors such as the techniques to extract information from the images, and on the actual fly-by altitude, which can be triggered based on hardware constraints. Based on the product datasheet available on the market, it is possible to retrieve the GSD at a certain altitude, or, when the values are not available, the GSD at a certain altitude can be approximately computed from basic camera characteristics as:  
\[GSD=\frac{hS_{w}}{Fim_{w}}, \tag{2}\]  
where \(h\) is the altitude, \(S_{w}\) the sensor width, \(im_{w}\) the image size in pixels, and \(F\) the focal length. Based on Equation (2), Figure 2 reports the GSD as a function of the fly-by altitude for the most promising COTS cameras and for ASPECT [9], a Multi-spectral imager from VTT, that will be employed by ESA M-Argo. It is characterized by three acquisition channels (VIS, NearIR and ShortWave IR) and, even if it is not a COTS instrument, its small volume (1U), mass (950 g), and power consumption (7 W), make it a valid candidate alternative to the COTS camera and spectrometer, especially for larger applications. Other off-the-shelf components have not been included in the comparison, such as Crystal Space Micro Camera System [29] and XCAM C3D [30]. They represent very compact solutions in terms of mass, size, and power consumption, but their GSD is not sufficient to guarantee a significant scientific return. Hyperion Technologies IM200 [31] appears as the most promising solution in terms of mass and volume. Moreover, it can be used also as a navigation camera (NAVCam), as it has been discussed in the Navigation section. For larger CubeSats, where mass and volume requirements can be less strict, SCS Space Gecko Imager [32] represents an excellent alternative in terms of accuracy. If the requirements are even less strict, Simera Sense HyperScape100 and Simera Sense MultiScape100 would ensure an incomparable resolution together with the possibility of performing VIS and NIR measurements with the same instrument [33]. Due to its low mass and volume, Hyperion Technologies IM200 will be considered for this architecture. Argus 2000 represents the only currently available off-the-shelf IR spectrometer, and its IR range is smaller than ASPECT's range (1240-2000 nm compared to 900-2500 nm). Moreover, its narrow FOV (\(\sim\)0.15\({}^{\circ}\)) poses several challenges in a 300-500 km fly-by altitude. Nevertheless, its mass (300 g) and volume (\(\sim\)0.3 U) make it perfectly suitable for this application. The preliminary considerations and analysis reported in this section are useful to get an idea of the typical (and minimum) payload that a low-resources spacecraft, aimed to explore NEAs, should carry on-board. It is clear that the option for a custom payload cannot be excluded, as the obtainable scientific return is strongly influenced by actual hardware, whose design is therefore linked to the techniques to extract data. As an example, the technique to obtain the shape of the target from the image influences the requirement on the GSD. Similar argumentation can be derived for the rotational parameter or composition characterization.