Photometric monitoring is a widely used method to characterize variable brightness stars such as young active stars and stars around which exoplanets transit.
Space telescopes are seen as perfect tools for continuously observing this type of star because they eliminate the drawbacks of ground-based observations such as the day/night cycle and atmospheric disturbances (turbulence and uncertain weather).
In this context, we propose to carry out the CubeSat MARSU (Millimag Astrophotometry of Red SPIRou/SPIP stars as a University space mission), which consists of a nanosatellite for photometry in the near-infrared that will work in parallel with SPIRou and SPIP, two next-generation solar spectropolarimeters based respectively at the Canada France Hawaii Telescope (CFHT, Hawaii) and the Bernard Lyot Telescope (TBL, Pic du Midi, France). SPIRou is currently operational at CFHT (www.spirou.irap.omp.eu) while SPIP (its twin) will be installed at TBL starting in 2021.
The common observation program SPIRou/SPIP is a project initiated with SPIRou at TCFH and will be extended with SPIP at TBL. The main missions of this program are to detect and characterize Earth-like, habitable planets around red dwarf stars, as well as to study the birth of stars and planets.
With the MARSU CubeSat, the monitored stars and exoplanets detected by SPIRou/SPIP will benefit from simultaneous photometric tracking.
Like SPIRou and SPIP, MARSU will observe in the near infrared, a spectral range where the targeted stars (red dwarfs and young stars in formation) emit the most light. By combining spectroscopy and photometry, it will be possible to measure the masses and radii of exoplanets, thus inferring their densities and obtaining information about their internal structures in order to distinguish, for example, gas giants from terrestrial planets. Moreover, the Doppler effect observed by SPIRou and SPIP can be polluted by stellar magnetic activity, whose contribution may exceed the velocity signatures of the exoplanets. Through its photometric tracking, MARSU allows for recording the photometric fingerprint of stellar activity, leading to better filtering of this effect and thus improving the reliability of exoplanet detection and characterization.

The objective of MARSU will be to carry out continuous photometric monitoring (90% of the time in observation) in the YJH spectral bands (1-1.8µm) for stars up to magnitude H~11, with an accuracy better than 1 mmag for exposure times < 10 minutes, over maximum periods of 3 months and simultaneously with SPIRou and SPIP observations.
Technically, MARSU will have a camera consisting of an 8.5 cm lens (made up of 5 lenses) and a shortwave infrared (SWIR) detector operating in the spectral range of 0.9 – 1.8 µm. MARSU retains the possibility of accommodating one or two cameras (each camera occupying 1x1x3U) with enhanced performance in the latter case; this requires that the payload be integrated on a 6U or 12U platform depending on the chosen option.

In conclusion, the CubeSat MARSU not only proposes an ambitious scientific objective (in terms of the sample size covered), but also a technological innovation (with infrared observations, a first for the spatial study of photometric transits).