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The Shape of an Asteroid

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.


Very fast moving asteroid in two HSC observations

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.

Comets Lurking in the Asteroid Belt


Asteroids and comets are small bodies (several hundred miles across, or less) leftover after the construction of our Solar System’s planets. Asteroids are commonly assumed to be mostly rocky or metallic with the majority orbiting between Mars and Jupiter.  Comets on the other hand are a mixture of rock, dust, and ice with orbits more very elongated or hyperbolic orbits that typically originating from the outer Solar System and Oort Cloud (reservoir for long period comets)  As a result of their icy composition, when comets come close to the Sun and get hotter, they become “active” as their ice sublimates (changes from solid to gas), releasing gas and dust, creating the distinctive fuzzy halo (known as a coma) and tails that we associate with comets.

The assumed separation between comets and asteroids was seriously challenged in 2006 with the discovery of main-belt comets (MBCs), which have orbits in the main asteroid belt but have been observed showing cometary activity. Most of the MBCs known today were discovered by telescopes dedicated to surveying the night sky, which so far has been the most efficient way to discover these extremely rare objects. Unfortunately, these dedicated telescopes are rather small (less than 2 meters, or 6 feet, across). Much larger telescopes are available, but these are typically used by individual astronomers to conduct specific observations, and are not used for general surveys. However, the images taken by these telescopes is archived and later made publicly available after a certain amount of time (typically one year), and it turns out that we can re-purpose these observations to search for active asteroids.

Left: Images of the first three main-belt comets (MBCs) to be discovered, each showing the fuzzy appearance and tails characteristic of cometary activity - Image credit: H. Hsieh

Left: Images of the first three main-belt comets (MBCs) to be discovered, each showing the fuzzy appearance and tails characteristic of cometary activity – Image credit: H. Hsieh


Asteroids frequently appear by chance in astronomical observations of other targets (such as stars or galaxies). By compiling accidental, or “serendipitous”, observations of asteroids in archival observations of other targets, we can effectively conduct a “survey” of public data archives using much larger telescopes than are currently available for dedicated all-sky surveys. This is important because we believe that we should be able to discover many more active asteroids if we can detect fainter activity, and the way to detect fainter activity is to use larger telescopes. Detecting activity itself is not easy, however. Comets can have a wide variety of appearances, and while computer algorithms can be designed to detect some comets, it is difficult to design an automated system to detect all the different types of comets that might exist. In contrast, the human eye is much more flexible in terms of spotting different combinations and levels of “fuzziness” and tails that could indicate cometary activity.

For Comet Hunters, we have extracted images of known main-belt asteroids from the archives of the Subaru Telescope, one of the largest telescopes in the world (at 8.2 meters, or 27 feet, across), located on Mauna Kea in Hawaii, one of the best astronomical observing sites in the world. None of the objects we are targeting have been previously known to show activity, but most have so far only previously been studied using small telescopes that may have missed faint activity. By going through and classifying the images on this website, you will help us identify candidates for new comets that we will then analyze in detail and possibly re-observe with follow-up observations to confirm the activity. If we can confirm a comet candidate is real, you will have helped discover a new comet! You will also have helped us along the way to our goal of greatly increasing the number of known active asteroids, and in doing so, contributing to the progress of this new and exciting field of astronomy.

Fancy giving it a try?

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