Citizen Scientists in Active Asteroids Program Find Rare Solar System Objects

Category: Press Release

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March 21, 2024, Tucson, Ariz. Citizen scientists from around the world, working with professional planetary scientists, have made an array of discoveries, including new members of a population of rare Solar System objects that have asteroid-like orbits but have comet-like appearances.

“We have successfully demonstrated the power of citizen science as an approach to finding active small bodies in our Solar System. Based on the early results published in this paper, we are confident that we should be able to discover many more active small bodies in this way. This should greatly advance our understanding of comet-like objects in our Solar System and learn more about related topics like the amount and distribution of ice in the present-day Solar System, and how other activity-producing processes like impacts and rotational disruption operate,” said Henry Hsieh, a Senior Scientist at the Planetary Science Institute and co-author of “The Active Asteroids Citizen Science Program: Overview and First Results” that appears in The Astronomical  Journal. Colin Chandler of the University of Washington is the lead author.

The Active Asteroids initiative is a NASA partner program that calls upon volunteers from around the world to assist in the search for active asteroids — a category of rare and elusive small Solar System objects that have asteroid-like orbits, usually between the orbits of Mars and Jupiter, but also display comet-like tails or hazy clouds around their central nuclei known as comae. Studying these objects is crucial for scientists to understand fundamental questions about the formation and evolution of the Solar System, including the origins of water here on Earth. Additionally, active asteroids may be valuable for future space exploration because the same ices that are responsible for comet-like tails can also be used for critical resources, such as rocket fuel, drinkable water, and breathable air.

“Our paper reports previously unknown activity discovered by the project in 15 asteroids and one Centaur, although more active objects are being discovered all the time. These objects include active asteroids, but also other types of active objects on more typical comet-like orbits,” Hsieh said. “While the project was mainly created to find more active asteroids, discovering new comets in general is also always exciting.”

Identifying and tracking active asteroids whose activity specifically appears to be due to the sublimation of ice – known as main-belt comets – is a particular interest of the project team, as it is an essential part of understanding the abundance and distribution of volatile material like ice in the Solar System.

“The more main-belt comets we discover, the better our ability gets to map out their locations in the Solar System as tracers of ice that would otherwise be invisible to us. This allows us to better understand how widespread icy objects are, and also determine what properties they have in common, which may in turn allow us to discover more,” Hsieh said.

“From a more practical standpoint, water ice is recognized as an extremely valuable resource from a space exploration perspective, and so learning more about what kinds of asteroids may contain water and how to identify them, as well as what specific asteroids are known to contain water, could be very valuable for future space missions,” Hsieh said.

The project, utilizing publicly available Dark Energy Camera (DECam) data from the Victor M. Blanco telescope in Chile, involved the examination of more than 430,000 images of known minor planets by 8,300 volunteers, where images identified by citizen scientists as being likely to contain active asteroids were then passed on to the science team for confirmation and additional analysis.

“This showcased the power of citizen engagement in advancing scientific knowledge. This is definitely a collaborative project, so no, I’m not worried that citizen scientists might put us out of work,” Hsieh said. “They are more of a force multiplier that enables us to go through much more data than our team could look through on our own to find the most interesting objects, allowing us to then focus our attention on those objects.  If anything, they are doing work typically done by computer algorithms which have been used to try to automatically detect comets, and showing that for all of the recent advances in technology, the human eye is still superior in many ways when it comes to pattern recognition and detection of the unexpected.”

The Active Asteroids project is ongoing. Anyone interested in participating can visit

Hsieh’s work on this project was funded by a NASA Solar System Observations program grant, which also provided direct support to the project via a subcontract to Northern Arizona University.


Alan Fischer
Public Information Officer
[email protected]

Henry Hsieh
Senior Scientist
[email protected]

Mark V. Sykes
[email protected]





The Planetary Science Institute is a private, nonprofit 501(c)(3) corporation dedicated to Solar System exploration. It is headquartered in Tucson, Arizona, where it was founded in 1972.

PSI scientists are involved in numerous NASA and international missions, the study of Mars and other planets, the Moon, asteroids, comets, interplanetary dust, impact physics, the origin of the Solar System, extra-solar planet formation, dynamics, the rise of life, and other areas of research. They conduct fieldwork on all continents around the world. They also are actively involved in science education and public outreach through school programs, children’s books, popular science books and art.

PSI scientists are based in 35 states and the District of Columbia.



Images of six of the active objects discovered so far by the Active Asteroids project. Credit: H. Hsieh/PSI.