Discovery of an Extreme Asteroid Moon
Click on the image to watch this animation of the orbit of Daphne and its satellite. The fly-by begins with our vantage point from Earth on the night of the discovery, We then fly up to view the satellite’s orbit edge-on, being occasionally eclipsed by Daphne. Lastly, we fly directly above the orbit to see that the orbit is circular. The 3-dimensional shape of Daphne used here was determined from past light curves analysis (Kaasalainen, et al) and confirmed with the Keck AO images. Animation courtesy of Jack Drummond/PSI and Al Conrad/WMKO.
An image flashed up on the display. It was the second group of exposures of the night for a large main-belt asteroid called Daphne. W. M. Keck Observatory Support Astronomer Al Conrad had been working from a remote control room in Waimea while his research partner, Principal Scientist Bill Merline, looked at the same image from his computer at the Southwest Research Institute in Boulder, Colorado.
The scientists were working on two research programs, one to learn about the shapes and sizes of asteroids, and the other to look for moons. The two programs use different approaches to determine the basic properties of asteroids.
Conrad had been helping Merline use an image-correction technology called adaptive optics (AO) on the 10-meter Keck II telescope. This world-leading system uses high-speed computers and deformable optics to correct the blurring effects of the Earth’s atmosphere.
Tonight, Merline and Conrad were looking at four large main-belt asteroids: Sylvia, Camilla, Kalliope, and Daphne. The plan was to look at each asteroid in round-robin fashion. They would make a “group” of exposures of one asteroid, consisting of perhaps a dozen pictures, taking a total of about 10 minutes. Then they would move to the next asteroid, cycling through all four every 40 minutes.
Each asteroid would rotate on its axis once every four to six hours. By the end of the night, the researchers would have enough data to see the outline of each asteroid. From there, they could begin to narrow down important and elusive characteristics of these cosmic targets.
Conrad and Merline had just looked at three asteroids and were finishing the fourth. It was time for Daphne’s second session. They moved the telescope to Daphne, and set the exposure time. A minute or so passed. When the first image appeared on the screen, something unusual appeared on the display.
“Holy Cow,” Merline said. Conrad looked up. He saw a dot, just a few pixels wide, very close to the outer edge of the asteroid, near the two-o’clock position.
They had discovered an asteroid moon!