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Hubble’s data reveal a large unseen population of small asteroids.


The image of the barred spiral galaxy UGC 12158, captured by the Hubble Space Telescope, features a distinctive appearance reminiscent of a white marking pen streak. However, this effect is the result of a foreground asteroid passing through Hubble’s field of view during multiple time exposures. The dashed pattern evident in the galaxy’s image indicates the presence of several exposures. The asteroid’s curved trail is due to parallax, a consequence of Hubble’s orbital motion around Earth, creating the illusion of its movement along a curved path. Positioned within the asteroid belt of our solar system, this uncharted asteroid is approximately 10 trillion times closer to Hubble than the distant galaxy it photobombed. Rather than a hindrance, such occurrences provide valuable data for astronomers conducting a census of the solar system’s asteroid population.

Scattered across the cosmic landscape like boulders, rocks, and pebbles, asteroids vary widely in size, making their cataloging a challenging task. Unlike stationary objects, these celestial wanderers dart along their orbits around the Sun, evading easy observation. However, astronomers recently undertook a treasure hunt using a trove of archived images captured by NASA’s Hubble Space Telescope. Spanning 19 years and consisting of 37,000 images, this archival goldmine revealed a previously unseen population of smaller asteroids. Their diligent search yielded 1,701 asteroid trails, with a staggering 1,031 of them previously unrecorded. Notably, around 400 of these newfound celestial objects measure below 1 kilometer in size, shedding light on the diverse composition of the asteroid belt.

Volunteers from across the globe, known as “citizen scientists,” played a crucial role in identifying this treasure trove of asteroids. Collaborating with professional scientists, their efforts were augmented by machine learning algorithms, marking a novel approach to uncovering asteroids in astronomical archives spanning decades. According to lead author Pablo García Martín of the Autonomous University of Madrid, Spain, this collective effort has revealed a surprising abundance of candidate objects, shedding light on the previously elusive population of main belt asteroids. This newfound understanding holds significance in refining evolutionary models of our solar system.

The extensive, random sample obtained offers fresh perspectives on the asteroid belt’s formation and evolution. The prevalence of small asteroids lends support to the notion that they are remnants of larger bodies shattered through collisions, akin to fragments of smashed pottery, a process spanning billions of years. An alternative theory suggests that these smaller fragments originated billions of years ago, yet questions linger regarding the mechanisms preventing their aggregation into larger sizes from accumulating dust in the planet-forming circumstellar disk around our Sun. Co-author Bruno Merín of the European Space Astronomy Centre in Madrid, Spain, highlights the potential of collision signatures to test the current main belt population hypothesis.

Can capture wandering asteroids through their telltale trails in the Hubble exposure.

Hubble’s rapid orbit around Earth enables it to capture wandering asteroids by detecting their distinctive trails in its exposures. These asteroids, known as “photobombers,” leave unmistakable curved streaks across Hubble’s photographs, easily distinguishable from other celestial objects. As Hubble observes an asteroid, its changing viewpoint, combined with the asteroid’s movement along its orbit, creates curvature in the streaks. Scientists leverage this phenomenon to calculate the distances to the asteroids and approximate the shapes of their orbits.

Most of the asteroids captured reside within the main belt, situated between the orbits of Mars and Jupiter. Their brightness, measured by Hubble’s sensitive cameras, allows for size estimation when compared with their distance. Remarkably, the faintest asteroids in the survey are approximately one forty-millionth the brightness of the faintest star visible to the human eye.

Bruno Merín explains, “Asteroid positions change with time, and therefore you cannot find them just by entering coordinates, because at different times, they might not be there.” Recognizing the challenge of manually sifting through vast archives of asteroid images, astronomers collaborated with over 10,000 citizen-science volunteers to scour the extensive Hubble archives.

In 2019, an international team of astronomers initiated the Hubble Asteroid Hunter project, a citizen-science endeavor aimed at identifying asteroids in archival Hubble data. Developed in partnership with the Zooniverse platform and Google, this initiative leverages the collective efforts of amateur astronomers worldwide to contribute to asteroid research.

This graph is based on Hubble Space Telescope archival data that was used to identify a large unseen population of very small asteroids in their tracks.

The graph presented here is derived from an analysis of Hubble Space Telescope archival data, revealing a previously overlooked population of diminutive asteroids in motion. These asteroids, initially unnoticed, inadvertently disrupted observations of background stars and galaxies within Hubble images. To uncover this hidden bounty, an extensive search effort combed through 37,000 Hubble images spanning nearly two decades. This exhaustive task was made possible through the collaboration of “citizen science” volunteers and artificial intelligence algorithms. The result was the identification of 1,701 asteroid trails belonging to previously undetected asteroids, marking a significant discovery in astronomical research.

A remarkable total of 11,482 citizen-science volunteers contributed nearly 2 million identifications, forming the basis for training an automated algorithm to recognize asteroids through artificial intelligence. This innovative approach holds promise for application in analyzing other astronomical datasets, ushering in new possibilities for discovery. Moving forward, the project aims to delve deeper into the streaks left by these previously unknown asteroids, seeking to characterize their orbits and investigate properties such as rotation periods. Given that many of these asteroid streaks were captured by Hubble years ago, it is currently impractical to track them for orbit determination. The significant findings from this endeavor have been published in the journal Astronomy and Astrophysics.

For those interested in participating in citizen science projects related to NASA, opportunities abound worldwide and are not limited to U.S. citizens or residents. To learn more about how to get involved, visit

Celebrating over three decades of operation, the Hubble Space Telescope remains at the forefront of astronomical exploration, continuously uncovering insights that shape our understanding of the universe. A testament to international collaboration, Hubble is a joint project between NASA and the European Space Agency (ESA). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, oversees the telescope and its mission operations, with support from Lockheed Martin Space in Denver, Colorado. Additionally, the Space Telescope Science Institute (STScI) in Baltimore, Maryland, operated by the Association of Universities for Research in Astronomy, conducts Hubble’s science operations on behalf of NASA.

Surendra Uikey

My name is Surendra Uikey, I am a science blogger, I have been blogging for the past three years, because I love to write, especially on astronomy, and I believe, if you want to learn something, then start learning others, By this it will be, that you learn things in a better way. In 2019, I started, the aim of making was to connect astronomy in simple words to common people.

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