The Janus mission was selected in 2019 as a NASA SIMPLEx mission to be launched in August of 2022 with the NASA Discovery mission Psyche. Janus will send two spacecraft, each of which will fly by different NEO binary asteroid systems in early 2026. The targeted binary asteroid systems are (175706) 1996 FG3 and (35107) 1991 VH, both of which have been observed repeatedly with photometry, spectrometry and radar. Janus is led by the University of Colorado, with the spacecraft being built and operated by Lockheed Martin. Each spacecraft carries a visible and IR imager, constructed by Malin Space Science Systems.
The 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. The Janus science goals will be achieved by combining flyby observations of the target binary asteroids with ground-based observations of the systems, leveraging decades of observations of these binary asteroids. 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 systems will be fit across the encounter, from approach to departure observations, to constrain the mass and inertias of the system components, where possible.
When unresolved, on approach and departure to the binaries, the asteroids will be observed to precisely determine the orbit and rotational phasing of the bodies, orbit periods and rotation periods. When resolved the spacecraft will track the binary asteroid systems through closest approach, allowing for a combination of absolute surface resolution, relative resolution across the target asteroids and phase angle coverage unparalleled in previous asteroid flyby missions.
Janus is currently in Phase C/D and preparing for launch. The NASA SIMPLEx program is designed around the idea of using secondary launch opportunities to explore interplanetary destinations. SIMPLEx missions are cost-capped at $55M USD, including all Phase E support.