Waiting for Shadows from the Distant Solar System

How can we hope to measure the hundreds of thousands of objects in our distant solar system? A team of astronomers is harnessing citizen science to begin to tackle this problem!

RECON light curve

A light curve from an occultation collected by a RECON site in Quincy, California. As the object’s shadow passes, the background star’s light dims. [RECON/Charley Arrowsmith (Feather River College)]

Occultation Information

Estimates currently place the number of Kuiper belt objects larger than 100 km across at over 100,000. Knowing the sizes and characteristics of these objects is important for understanding the composition of the outer solar system and constraining models of the solar system’s formation and evolution.

Unfortunately, measuring small, dim bodies at large distances is incredibly difficult! One of the best ways to obtain the sizes of these objects is to watch as they occult a distant star. Timing the object as it passes across the face of the star can give us a good measure of its size and shape, when observed from multiple stations in the path of the shadow.

An Extended Network

Occultations by nearby objects (like main-belt asteroids) can be predicted fairly accurately, but those by trans-Neptunian objects are much more poorly constrained. Only ~900 trans-Neptunian objects have approximately known paths, and occultation-shadow predictions for these objects are often only accurate to ~1000km on the Earth’s surface. So how can we ensure that there’s a telescope in the right location, ready to observe when an occultation occurs?

RECON site map

Map of the 56 RECON sites distributed over 2000 km in the western United States. [Buie et al. 2016]

The simplest answer is to set up a huge network of observing stations, and wait for the shadows to come to the network. With this approach, even if the predicted path isn’t precisely known, some of the stations will still observe the occultation.

Due to the number of stations needed, this project lends itself perfectly to citizen science. In a recently published paper by Marc Buie (Southwest Research Institute) and John Keller (California Polytechnic State University), the team describes the Research and Education Collaborative Occultation Network (RECON).

RECON of Distant Objects

RECON consists of 56 communities in the western United States that have each been armed with a telescope, camera, and timing device. The observing groups include teachers and their students, amateur astronomers, and other community members, and telescopes are primarily located at schools.

Because the shadows from occultations generally travel from east to west, the communities are based in a roughly north-south network spanning 2000 km. They’re spaced no more than 50 km apart, providing enough coverage to obtain sizes for 100-km objects crossing the baseline.

RECON is a great example of how citizen science can be used to advance astronomy. The project reached full operating status in April 2015, and it has already conducted two official observing campaigns of trans-Neptunian objects, as well as roughly 30 additional campaigns, including training runs and local projects. The team is now publishing some of its first results in an upcoming paper, so keep an eye out for future publications to find out what they’ve learned!


Check out this awesome video of an asteroid occulting a star, as observed by a RECON system. The grey field shows the actual video image collected by one of the RECON cameras, in which one of the two visible stars (the one on the right) is occulted. The asteroid itself is too dim for us to see. The inset at the top left shows the light curve collected during the occultation, and the upper right-hand corner shows an animation of the asteroid as it occults the star. [RECON]



Marc W. Buie and John M. Keller 2016 AJ 151 73. doi:10.3847/0004-6256/151/3/73