# L2-CalSat: A Calibration Satellite for Ultra-Sensitive CMB Polarization Space Missions
## 4 L2-CalSat Description
On the other hand, a clear advantage of orbiting L2 instead of Earth comes from its lower risk in terms of micrometeorite bombing and, in particular, of space debris coming from the increasing number of satellites in that region of space. In this sense, the risk of impact can be calculated using models that estimate the background meteoroid flux (continuous and from certain directions of space) according to its mass. In addition to the influx of background meteoroids, there are quasiperiodic events caused by streams of materials from comets that pass close to Earth and therefore near L2. Other kind of threats as, for instance, solar wind could also affect to both, the navigation instrumentation and the payload stability. Previous missions in L2 as Gaia and Herschel have shown that the radiation environment can be considered similar to that of GEO and high inclination LEO orbits or even more benign in some aspects (trapped particles are avoided, for example). Although single events are more prone in this region, specially during periods of high solar activity, protections measures can be implemented in both software and hardware. Nevertheless, the technology of the navigation instrumentation and the payload has been used in previous missions (flying far from Earth like MarCO [44] and CMB missions like Planck, respectively) without detecting particular issues from that aspect, thanks to the shielding systems implemented on them. In order to study the general stability performance of L2-CalSat, in principle it is not considered the application of long-term vacuum chamber tests at space temperature because the instrumentation will be operating at a temperature close to the usual ambient one (typically between 0 and 50 Celsius, as will be seen later) thanks to the incident radiation coming from the Sun. On the other hand, previous experiences tell us that the operation of the instrumentation (both, navigation and payload) it is not affected by the fact of being in vacuum conditions. However, other laboratory tests are required to characterise the gain and, consequently, emitted power variations that can happen due to expected temperature changes in the previously referred range (0-50 Celsius). This kind of tests are performed using temperature chambers that are easily available and require only hours or days for a complete and accurate characterization. Due to the large technological development carried out during the last years and the low-cost of CubeSat platforms and Commercial off-the-shelf (COTS) components, we have studied their suitability for the L2-CalSat platform. In particular, a 6U CubeSat is expected to meet the volume needs of L2-CalSat. It would travel as a piggy-back of the CMB-Sat which, until the deployment in L2, must provide structural support and a minimum power supply to maintain the batteries of L2-CalSat. Once it is deployed, some hardware would remain attached to CMB-Sat for communication and navigation support tasks. These subsystems would operate as autonomously as possible, with their own solar panel and batteries. Nevertheless, it would be necessary a minimum capacity of communication with CMB-Sat for calibration synchronization and downloading data to ground. Following the CubeSat and COTS philosophy, there exist commercial separation systems, which fulfil the requirements. Other implementation option for L2-CalSat would be a micro-satellite capable of reaching L2 autonomously from the same launcher. As the thrust required in L2 is very small (the orbit presents low-force gradients), one of the main differences with the piggy-back option would be the required propellant, taking into account that only the transfer to L2 would require an impulse (\(\Delta v\)) of about 290 m/s, in comparison with the 17 m/s (see Section 5 ) of total \(\Delta\
u\) required in the base-line (piggyback) option. The micro-satellite option would be very flexible at system design level, but its cost would be significantly higher, limiting so the viability of the concept in terms of practical implementation.