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Project Progress

“Small Body Missions”

There were two mission encounters during this reporting period: The EPOXI encounter with comet 103P/Hartley 2 on 4 November 2010 and the StardustNExT encounter with comet 9P/Tempel 1 on 14 February 2011. PI Meech, a CoI on both missions was in charge of coordinating the Earth-based observing effort for both missions. This included long-term characterization of the nuclei of both mission targets – both to help plan for the missions and as a baseline for activity over a longer time period than possible from the short flyby periods.

EPOXI was the extended Deep Impact mission utilizing the primary Deep Impact spacecraft to fly past a smaller, more active comet nucleus than had been visited before. Our team obtained observations of the comet far from the sun when it was in an inactive state in order to determine the rotation period. This was accomplished using a combination of the Gemini N and S 8-m telescopes and the Hubble Space Telescope, and our team found that the comet was rotating with a period of 16.4 hours. Once the comet became bright enough for smaller telescopes, after developing its tail, others started to monitor the rotation period by analyzing the jet features seen in the dust and gas coma that had originated on the nucleus. Our team coordinated the world-wide observing campaign in support of the mission.

Beginning in August 2010 and continuing throughout the fall, and including pre-encounter observations made from the spacecraft, it was clear that the rotation period was changing rapidly, likely from torques induced from the uneven outgassing. Prior to selection as a mission target, it was noted that this comet was unusually active – and a comparison of its surface area with the water production suggested that more than 100 percent of the surface was active. The encounter resolved this puzzling observation by discovering that the comet was surrounded by a swarm of large water-ice chunks that were also releasing gas, thus increasing the effective surface area of the comet. We developed a simple volatile sublimation model using the Earth-based data that showed that carbon dioxide played a very important role in the activity of the comet near perihelion – more than water. This was seen from the spacecraft at flyby and we are now developing a new paradigm for understanding the mechanism for comet activity that may have important implications for the chemistry of these early solar system planetesimals. A first paper has come out in Meech et. al. (2011), and observations and thermal models are being used to explore the evolution of this comet.

The goal of the StardustNExT mission was to execute a second in-situ visit a comet nucleus after it had made one orbit around the sun to look at (1) surface changes caused by the perihelion passage, (2) to image some parts of the nucleus not seen during Deep Impact, and (3) to image the crater made during the Deep Impact experiment. To accomplish this, we needed to have a precise knowledge of the spin state of the comet in spite of the fact that it was changing (slowing down) with time. To this end, a total of 500 whole or partial nights of data were obtained worldwide, in several coordinated observing campaigns, the last during fall 2010. A rotation model was developed which showed that we needed to use all of the spacecraft reserve fuel to adjust the time of arrival in order to accomplish the mission goals. The flyby on 14 February 2011 showed that the predictions had been exactly right, and we met all of the mission objectives. To our surprise the effects of the perihelion passage were more subtle than expected, but we were able to image the impact site from the Deep Impact event. Work continues on the analysis of the space- and ground-based data sets.

Comets provide one of the mechanisms for large-scale transport and delivery of water within our solar system, and asteroids provide another source of volatiles. However, neither comets nor asteroids can explain both Earth’s water and its noble gas inventory. A recently discovered new class of icy bodies in the outer asteroid belt, the Main Belt Comets (MBCs), are comets in near-circular orbits within the asteroid belt. Dynamics suggest they formed in-situ, beyond the primordial snow line, and as such represent a class of icy bodies that formed at a distance from the Sun that has not yet been studied in detail. During 2008-2010 our team developed a Discovery-class mission concept to rendezvous with a main belt comet using an ion propulsion system a state of the art mass spectrometer and an imager and dust analyzer. The mission goals are to (1) provide new knowledge of the history of water in our solar system, (2) contribute to the understanding of the proto-planetary disk structure and dynamics and the location of the snow line, (3) and explore a new class of objects in the main belt.