In September 2015, a team of astronomers from the National Astronomical Observatory of Japan, University of Michigan, Kyoto Sangyo University, Rikkyo University and the University of Tokyo successfully observed the entire hydrogen coma of the comet 67P/Churyumov-Gerasimenko, using the LAICA telescope onboard the PROCYON spacecraft. They also succeeded in obtaining the absolute rate of water discharge from the comet.
This comet was the target of ESA’s Rosetta mission in 2015. Because the Rosetta spacecraft was actually inside the cometary coma, it couldn’t observe the overall coma structure. There were bad observing conditions during the time the comet could be observed from Earth, so through our observations, we were able to test the coma models for the comet for the first time.
Comet observation by the PROCYON spacecraft had not been scheduled in the original mission plan. Thanks to the efforts of the spacecraft and telescope operation teams, observations were conducted shortly after we started discussing the possibility, producing results of great scientific importance.
This result is the first scientific achievement by a micro spacecraft for deep space exploration. Moreover, this provides an ideal example where observations by a low-cost mission (e.g., the PROCYON mission) support precise observations by a large mission (e.g., the Rosetta mission). We hope this will become a model case for micro spacecraft observations in support of large missions.
The Rosetta mission and its limits
The 2015 apparition (appearance) of the comet 67P/Churyumov-Gerasimenko was a target of ESA’s Rosetta mission. In the Rosetta mission, precise observations of the comet were carried out from close to the surface of the nucleus for more than two years including when the comet passed perihelion (closest approach to the Sun) on August 13, 2015. However, observation of the entire coma was difficult because the Rosetta spacecraft was located in the cometary coma.
To extrapolate from Rosetta’s observations of specific areas and estimate the total amount of water released by the comet per second (water production rate), we need a model for the coma. But the water production rate strongly depends on the coma model we use. To test the coma models, we have to compare the absolute water production rate derived from entire coma observations to predictions based on Rosetta’s results and the various coma models. Therefore, it was useful to observe the entire coma from farther away from the comet with another satellite.
Conventionally, the SWAN telescope onboard the SOHO spacecraft has often been used to observe such targets. Unfortunately, the comet moved to a region where there are many stars behind it, and because of the SWAN telescope’s low spatial-resolution it could not distinguish the comet from the background stars.