Scientific Observations of the Didymos Binary Asteroid System using the Milani NavCam

The Asteroid Impact and Deflection Assessment (AIDA) collaboration, consisting of NASA’s DART mission and ESA’s Hera mission, aims to test the capability of a kinetic impactor to deflect an asteroid. At the end of September 2022, DART successfully impacted the secondary of the binary asteroid system
Didymos, called Dimorphos. The resulting changes to the system are significant [1] and follow up observations by the Hera mission are of great importance. Hera will rendezvous with the binary system in early 2027 and part of its payload suite are two CubeSats which will orbit in close proximity of the asteroids. The two CubeSats, named Milani and Juventas, will be the first nanosatellites to orbit in the close proximity of a small celestial body and to perform scientific and technological operations around a binary asteroid. Milani’s main scientific objectives are to characterize the surface and dynamical environment of both bodies, investigate the dust environment around the system, and provide measurements for determining the gravity field. Besides the scientific objectives, the Milani mission also aims to achieve several technological objectives related to testing the effect of the environment on CubeSats grade hardware and validating novel autonomous navigation algorithms. One of the payloads of Milani is the NavCam, an optical imager with and RGB sensors which nominally provides information on the translational and rotational state of Milani but will also be used to perform scientific investigations and perform experiments related to autonomous navigation around asteroids.

The use of CubeSats allows for more riskier operations, including closer flybys of the system. This allows for high resolution imaging to be performed, which can be of great aid to the observations performed by Hera. Both the scientific and technological outputs of Milani will aid the main objectives of the Hera mission, and additionally help inform both the scientific payload selection and navigation system design for future CubeSat missions to asteroids. This work describes both the scientific investigations the NavCam will perform, and the setup of the autonomous navigation experiment.

Science Objectives

The main scientific objectives of the Milani NavCam are: to support the surface mapping and crater modelling in different color bands, obtain shape models of Didymos and Dimorphos, and support the gravity science investigation. For the surface mapping, a global mapping of the surfaces will be performed, with a higher resolution focus on the crater (if present) made by DART. The RGB filter of the NavCam, with bands centred on 470 nm (B), 550 nm (G), and 600 nm (R), allows for spectral information to also be included in these investigations. The NavCam will also provide its own shape models of both bodies, including their rotational state. These models can be combined with the shape models generated by the other spacecraft to improve the general global parameter estimation. Finally, the gravity science experiment can be aided by the NavCam observations in several ways. First, the tracking of optical observables like surface features will improve the accuracy of the state estimation of Milani and thus also improve the accuracy of the gravitational field. Second, the optical tracking of the landing maneuver of Juventas will also improve the accuracy due to the aid of another tracking instrument (besides the ISL). Finally, just as was done for OSIRIS-Rex [2], the tracking of natural particles around the system (if present) would allow for a significant improvement in the gravity field modelling.

Autonomous Navigation Experiment

Besides the scientific objectives, the NavCam will also be used for an opportunistic technology demonstration objective regarding the use of autonomous navigation algorithms on-board a CubeSat. Regarding the navigation experiment, the NavCam will provide several observables, e.g. the phase angle and centre of figure of both bodies, which are then used by a navigation filter to provide state estimates of Milani. These results are then compared with ground based orbit determination to estimate the achieved accuracy of the autonomously estimated state.

References

[1] Cheng AF, Agrusa HF, Barbee BW, et al (2023) Momentum transfer from the DART mission kinetic impact on asteroid Dimorphos. Nature 616:457–460. https://doi.org/10.1038/s41586-023-05878-z

[2] Chesley SR, French AS, Davis AB, et al (2020) Trajectory Estimation for Particles Observed in the Vicinity of (101955) Bennu. J Geophys Res Planets 125:e2019JE006363. https://doi.org/10.1029/2019JE006363