Chimera is a NASA mission concept to orbit and explore 29P/Schwassmann-Wachmann 1 (SW1), an active, outbursting small icy body in the outer Solar System. The concept was developed in response to the 2019 NASA call for potential missions in the Discovery-class, and it would have been the first spacecraft encounter with a Centaur and the first orbital exploration of a small body in the outer Solar System. The Chimera proposal was ranked in the first tier (Category 1) of submissions, but was not selected for further development for the programmatic reason of maintaining scientific balance.
SW1 is a member of the Centaur group, a population of near-pristine objects that have been gravitationally perturbed from the Kuiper Belt into unstable orbits in the region between Jupiter and Neptune. Many Centaurs eventually migrate into the inner Solar System to become short period, 'Jupiter Family' comets (JFCs), and SW1 is believed to occupy the orbital ‘gateway’ through which they pass as they make this transition. SW1’s characteristics are a chimeric combination of icy small bodies at different points along their evolution from the fringes of the Solar System to active comets passing near the Sun. This provides a unique opportunity to study how these objects formed, are composed, and change over time. Over a more than two year orbital encounter, Chimera would sample the escaping gas coma of SW1, study its patterns of activity and outburst, map the composition and topography of its surface, probe its interior, and monitor for changes as it evolves.
Science
Icy small bodies are primordial echoes of the formation of the Solar System, with physical properties derived from the planet forming disk and an orbit distribution related to early migration of the giant planets. Investigation of their composition (ice and gas), shape, and interior structure all provide insight into the process of planetary development. The modern population of icy small bodies includes unmodified objects in stable orbits in the remote outer Solar System (e.g. The Kuiper belt and Oort cloud) and more evolved objects that have migrated inward toward the Sun to become long period comets (e.g. C/1995 O1 (Hale-Bopp), short period comets (e.g. 67P/Churyumov-Gerasimenko) and the Centaurs.
The Centaurs are the least altered icy bodies orbiting interior to Neptune, with physical characteristics that are intermediate between the small icy bodies explored by previous (e.g. Rosetta, New Horizons), initiated (e.g. Lucy) and planned (e.g. Comet Interceptor) spacecraft missions. Their orbits are unstable and, on timescales 1-10 Myr, they are either scattered back into their Trans-Neptunian source region or inward toward Sun where they become comets. Centaurs are too far from the Sun for large-scale, water-based cometary behavior to occur, but they are close enough that some experience a form of sporadic activity. This early stage processing provides an opportunity to explore the transition of icy planetesimals from their primordial origins to their heavily-weathered cometary end-state.
Since its discovery during an outburst in 1927, the characteristics of 29P/Schwassmann-Wachmann 1 have identified it as enigmatic compared to other known comets and a candidate for detailed study.
SW1 is the most active small body in the outer Solar System and the only Centaur known to be continuously active. SW1 has a century-long history of experiencing major (2-5 Visual Magnitude) outburst events that can expel ≥109 kg of dust, gas, and ice. Modern studies show an outburst rate of ~7 per year, making it the only known object where the opportunity for in situ study of these highly energetic events is guaranteed as part of a long duration spacecraft encounter. SW1 orbits inside a dynamical 'gateway' through which a majority of future JFCs pass. Forward modeling gives SW1 a 75% chance of becoming a JFC within the next 4000 years.
The physical characteristics of SW1 and its orbit simultaneously connect it to icy planetesimals in multiple evolutionary states. Its study provides insight into their distinctive histories.
SW1's orbital evolution is linked to both the KBOs (Kuiper Belt Objects) and JFCs. SW1's activity patterns mirror those of LPCs in the giant planet region. SW1 is experiencing physical processing common to embryonic JFCs. SW1's thermal environment is similar to that of the Jupiter Trojans during a possible early active period.