Because of the longer exposure times used by the Hyper Suprime-Cam survey, the asteroids look bean-shaped or elongated in the HSC search. This trailing is due to the asteroid’s relative motion to the Earth. We’re seeing it move and therefore its light is deposited in slightly different positions along the camera’s field-of-view during the exposure, creating a streak or trail. As far as we can find in the scientific literature, nearly all main-belt comets were detected in images will little trailing. We found one example, that we wanted to share with you. Below is a well-known image of Main Belt Comet 107P/Wilson-Harrington. This observation was actually taken back in 1949 (on glass plates!) by the 48-inch Oschin Schmidt telescope on Mt. Palomar for use in the Palomar Observatory Sky Survey (Fun Fact – I used the same telescope for my thesis survey for distant Kuiper belt objects nearly 60 years later).
Yanga Fernandez, Lucy McFadden, Carey Lisse, Eleanor Helin, and Alan Chamberlin went back to these observations in 1996/1997 and found that this asteroid was active as an MBC with a visible tail! You can find their paper here (unfortunately it’s behind a paywall, but you can read the paper abstract for free). The asteroid is very streaked, a bit more than what you typically see on the Comet Hunters HSC search, but it gives you an idea. The tail is faint and diffuse, but visible off to the left of the streak.
In case you need help spotting the tail, I’ve annotated this version below with magenta arrows pointing to the tail.
I wrote this sitting on a plane on the way to Grapevine, Texas. I’m on my way to the American Astronomical Society winter meeting. It is the largest meeting of astronomers in the United States. I’ll be presenting a talk on Comet Hunters about the Hyper Suprime-Cam search.
A question that pops up from time to time on Comet Hunters Talk is whether or not we can see the shape of an asteroid in the HSC and Suprime-Cam observations. The answer to that is no. The Subaru Telescope does not have the resolution to resolve the shape of the asteroid. The asteroid is viewed by the telescopes and cameras as point-like and smeared out to the turbulence in the atmosphere and the optics of the cameras just like the background stars, so we can’t infer anything about the shape of the asteroid in most case just from what we see in the Comet Hunters images displayed on the site. In the HSC workflow, many of the asteroids look streaked or ‘bean shaped’ compared to the stationary background stars. This is due to the asteroid’s motion. The HSC observations are close to 150 seconds, and in that amount of time some asteroid orbits have on-sky velocities that move a noticeable number of pixels elongating the asteroid’s appearance in the image.
But there are other ways to indirectly probe the shape of the asteroid. You can use the varying amount of light reflected by the asteroid over time to estimate the shape of the asteroid and how it rotates. Asteroids don’t produce their own light source. In the optical wavelengths (what our eyes can see), asteroids are reflecting a portion of the Sun’s light. How much surface area and the type of surface changes the amount of sunlight reflected back to the Earth. If the object is very round, you’ll see a nearly uniform amount of light from the object. If the asteroid is oblong, you’ll see the object brighter when the longer axis is facing Earth and a see it is fainter when the smaller axis as the body rotates. This picture can be complicated if there is compositional differences on the asteroid’s surface and how much light those different surface types absorb and reflect sunlight. I high recommend checking out Pedro Lacerda’s light curves of small solar system bodies website to see simulated light curves (brightness measurements over time) of small solar system bodies.