Over 1,000 New Asteroids Discovered Hidden in Hubble Archives

Over 1,700 asteroid traces have been discovered in Hubble data over the previous 20 years, according to researchers. While many of the asteroids have been discovered before, over 1,000 have not. What useful are 1,000 more asteroids? They, like other asteroids, may contain important information about the Solar System's past.

As more telescopes conduct more observations throughout time, the total amount of archived data continues to expand. Sometimes, new analysis methods or renewed efforts from scientists are required to disclose insights hidden in the data. That's what happened with the Hubble Asteroid Hunter project.

In 2019 a group of astronomers launched the Hubble Asteroid Hunter. It's a citizen science project on the Zooniverse platform. Their goal was to comb through Hubble data to find new asteroids.

The astronomers released the results of their project in a new paper titled Hubble Asteroid Hunter I. Identifying asteroid trails in Hubble Space Telescope images. The study is online in the journal Astronomy and Astrophysics. The lead author is Sandor Kruk from the Max Planck Institute for Extraterrestrial Physics.

"One astronomer's trash can be another astronomer's treasure," Kruk stated in a press release.

The information they were looking for had mostly been discarded from other observational projects that weren't focused on asteroids. The data would have looked as "noise" in many situations, thus it was deleted to make the distinct pieces stand out. However, all of this unanalyzed secondary data is still kept and accessible.

"The amount of data in astronomy archives increases exponentially, and we wanted to make use of this amazing data," said Kruk.

The team looked at almost 37,000 Hubble composite pictures. They were collected using the Hubble Space Telescope's Advanced Camera for Surveys and Wide Field Camera 3 between April 30, 2002, and March 14, 2021. Asteroid trails appear as curving streaks in most photographs because they are 30-minute long exposures.

The streaks are at the root of the problem: computers have a hard time detecting them. The Zooniverse platform and citizen scientists can help with this.

"Due to the orbit and motion of Hubble itself, the streaks appear curved in the images, which makes it difficult to classify asteroid trails – or rather it is difficult to tell a computer how to automatically detect them," Sandor Kruk noted.

"Therefore, we needed volunteers to do an initial classification, which we then used to train a machine-learning algorithm."

The volunteers delivered.The photos were classified by 11,482 citizen scientists. Over 2 million people visited Zooniverse's Hubble Asteroid Hunter page, and volunteers submitted 1488 positive classifications in roughly 1% of the photos.

The citizens' efforts trained a machine-learning system to swiftly and reliably search the remaining photos. The program is hosted on Google Cloud, and after trained, it added another 900 detections to the Hubble data, totaling 2487 probable asteroid tracks.

Then came the professional scientists' turn. The findings were discussed by three of the paper's authors, including main author Sandor Kruk. They took out stuff like cosmic rays and other objects, leaving 1701 traces in 1316 Hubble photos. There were 1031 unexplained asteroid tracks, with around one-third of them being known asteroids.

This mosaic consists of 16 different data sets from the NASA/ESA Hubble Space Telescope studied as part of the Asteroid Hunter citizen science project. Each of these datasets was color-assigned based on the time sequence of exposures, whereby the blue tones represent the first exposure that the asteroid was captured in and the red tones represent the last. 

Follow-up observations will establish how many are newly found asteroids and their orbits. Some of 1031's findings are unlikely to be validated, but the remainder will add to our knowledge of the asteroid population in our Solar System.

Because these asteroids are fainter and possibly smaller than most asteroids spotted from the ground, they have avoided detection. This is the first publication to come out of the Hubble Asteroid Hunter project. The authors will utilize the curved form of the asteroid trails in future articles to calculate their orbits and distances.

All asteroids are relics from the early days of the Solar System, usually from before the planets formed. They act as time capsules for the early system, preserving the circumstances there. That is why astronomers are so fascinated by them, and why missions have been launched to retrieve samples from asteroids like Bennu and Ryugu.

"The asteroids are remnants from the formation of our Solar System, which means that we can learn more about the conditions when our planets were born," Kruk explained.

This image from the study is a sky-map of the Solar System Objects (SSOs) identified in the Hubble archived images. The blue stars show the identified, known asteroids. The orange circles show the location of objects for which the team did not find any associations with SSOs. The ecliptic is shown in red. The two gaps in this plot correspond to the Galactic plane, which HST did not observe. 

This type of archive data is increasingly being used by scholars. It is both cost effective and fruitful to examine current photos for fresh findings.

"The use of archival data produced by imaging campaigns whose primary science goals lie outside the Solar System is common practice in asteroid science. Several groups have used various image archives to find and characterize SSOs."

For example, in 2019, astronomers identified nearly 1800 asteroids using historical pictures from exoplanet surveys, with 182 potentially new findings.

Astronomers want to know everything there is to know about the asteroid population in the Solar System because it helps them comprehend the Solar System's past.

"A detailed description of the small bodies in the Solar System puts constraints on the different Solar System formation scenarios, which make concrete predictions on the size and orbit distribution of objects as a function of time," the authors explain. "In particular, both the giant planets' migrations and collisional cascades have effects on the asteroids' size and orbital distributions that could be detectable with specially purposed observational surveys."

However, custom-designed surveys are costly and time-consuming. Proposals for observation face fierce competition from other academics working on different topics.

"We instead decided to produce such a survey from a large archival dataset," the authors write. Other than asteroids, Kruk hinted at anything else in the data. 

"But there were other serendipitous finds in the archival images as well, which we are currently following up." He also stated that their strategy is game-changing and that they want to utilize it again.

"Using such a combination of human and artificial intelligence to scour vast amounts of data is a big game-changer, and we will also use these techniques for other upcoming surveys, such as with the Euclid telescope."

As far as the "… other serendipitous finds…" in the images, Kruk declined to share what those other finds might be. He told Universe Today that the findings are "… not related to unusual asteroids but to other findings in the data. We will report them in follow-up publications and announcements soon, so stay tuned!"