Janus: A NASA SIMPLEx mission to explore two NEO Binary Asteroids

Janus is a NASA SIMPLEx mission currently in Phase C/D. Janus was selected in 2019 to be co-manifested with the NASA Discovery mission Psyche, and is scheduled to be launched in August 2022.

Janus will send two spacecraft, each of which will fly by a Near Earth Binary Asteroids in early 2026. The targeted binary asteroid systems are (175706) 1996 FG3 and (35107) 1991 VH. These asteroids have been observed using Earth-based optical, IR and radar telescopes for over two decades. These observations have provided precise global shape models and orbit information for each system, which in turn have exposed additional questions that require higher resolution observations to address.

(175706) 1996 FG3 Binary 1996 FG3 is a primitive C-Type asteroid. It has been documented to lie in a singly-synchronous state and was the first binary system to be documented to lie in a Binary YORP — Tide equilibrium Observation of the thermal properties of the secondary will allow us to gain insight into tidal dissipation occurring in the primary body, which will be an unprecedented measurement for a small rubble pile asteroid.

(35107) 1991 VH Binary 1991 VH is a rocky S-Type asteroid. It has a secondary that is not settled into the usually observed, minimum energy singly synchronous state. Instead, the secondary has been seen to exchange angular momentum and energy with the system orbit, leading to an apparent chaotic dynamical evolution. We will use our visible and thermal observations of the secondary and the entire system to better understand why this system is not in a lower-energy state, as most binaries are.

The overall Janus science goals are to understand the formation and evolutionary mechanics of binary rubble pile asteroids, and to understand the key features of each of the binary asteroid systems outlined above. The Janus science goals will be achieved by combining flyby observations of the target binary asteroids with ground-based observations of the systems. This combination will enable the high resolution imaging and thermal data to be placed into a global context, leveraging all available data to construct an accurate topographical and morphological model of these bodies. In addition, the dynamics of the binary asteroid systems will be fit across the encounter, from approach to departure observations, in order to constrain the mass and inertias of the system components, where possible.

Understanding the formation and evolution of binary asteroids provides a key to understanding the physical evolution and lifecycles of rubble pile asteroids. Janus will provide insight into these larger-scale evolutionary scenarios.