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Asteroids photobombed deep Hubble images

Deep-field of black with tiny galaxies and light-colored arcs.
Asteroids photobombed this long-exposure Hubble Space Telescope image of the very distant galaxy cluster Abell 370. This is one of many Hubble images that inadvertently contained the telltale tracks of asteroids. In all, citizen scientists have located some 1,701 asteroids in Hubble images, including more than 1,000 that were previously unknown. Image via NASA/ ESA/ and B. Sunnquist and J. Mack (STScI).

Asteroids photobombed Hubble images

For nearly three years, citizen scientists have been scouring Hubble Space Telescope images, looking for the telltale arcs of asteroids passing between the telescope and deep space. This month (May 6, 2022), the European Space Agency announced the results of this search, which had been dubbed the Hubble Asteroid Hunter. ESA said the citizen scientists located some 1,701 asteroids in all, including 1,031 that were previously unknown.

First, astronomers identified more than 37,000 composite images that Hubble took between April 2002 and March 2021 with its ACS and WFC3 instruments. Hubble can stare at an object, such as galaxy or nebula, for an average of 30 minutes. Therefore, the closer, moving asteroids show up as curved lines or streaks in front of the background objects.

Next, more than 11,400 volunteers analyzed and classified the images. The volunteers helped spot more than 1,000 asteroid trails, which allowed artificial intelligence to learn what to look for from their discoveries. Eventually, the teamwork of both citizen scientists and AI resulted in the discovery of 1,701 trails in 1,316 Hubble images.

As a bonus, the participants also tagged other objects they saw on the images, such as gravitational lenses, galaxies and nebulae. A forum helped volunteers chat with each other and scientists as they discussed their findings.

16 boxes with colorful asteroid trails from Hubble.
Here are 16 Hubble images showing asteroid trails. Blue hues represent the first exposure while red colors are for the last exposure. Image via NASA/ ESA/ Hubble/ S. Kruk (ESA/ESTEC)/ Hubble Asteroid Hunter citizen science team/ M. Zamani (ESA/Hubble).

Identifying asteroid trails

Known and catalogued asteroids made around one-third of the asteroid trails. That left 1,031 trails from previously unidentified asteroids. These asteroids are probably small and faint, lying farther from Earth and in the main asteroid belt.

Asteroids are small remnants leftover from the formation of the solar system. So, astronomers hope that learning more about these small, distant asteroids will help them have a better understanding of the conditions in the early solar system.

Now astronomers have a new tool for finding asteroids in astronomical archives spanning decades. This is an approach they can use with other datasets.

Bright light near top and fainter with dark lines, with bright arcs over top.
This broken asteroid trail crosses the outskirts of galaxy M91 in Coma Berenices. These 5 trail segments came from individual exposures. Scientists then added them to a cleaned color image of the galaxy. The asteroid enters the image at top center and moves toward the lower left. Because Hubble orbits Earth, it can’t stare at the galaxy nonstop. This introduces the large gaps in the trails you see here. Image via R. Evans and K. Stapelfeldt (Jet Propulsion Laboratory) and NASA/ ESA.

What’s next for the asteroid trails?

Exploring the 1,031 streaks of previously unknown asteroids is the next step in the project. Scientists would like to characterize their orbits and study their properties, such as their sizes and rotation periods. Hubble imaged most of these asteroid streaks many years ago, therefore, they can’t follow them up now to determine their orbits.

What they can do, is use Hubble and the parallax effect. When Hubble orbits Earth, it takes images from each side of its orbit. By knowing the position of Hubble during the observations and measuring the curvature of the streaks, astronomers can determine the distances to the asteroids and estimate the shapes of their orbits.

Parallax is also the reason the asteroid trails looked curved in the images.

Additionally, some of the longer Hubble observations allow the measurement of a light curve for the asteroids. This information can help them measure the rotation periods and infer the shapes of the asteroids.

Teal, dark brown and yellow mottled light with with dots and a sketchy white trail.
The main-belt asteroid 2001 SE101 photobombs the Crab Nebula. Image via ESA/ Hubble/ M. Thévenot (@AstroMelina).

Bottom line: Citizen scientists, as part of the Hubble Asteroid Hunter project, have identified 1,701 asteroid trails, and 1,031 of those streaks are from previously unknown asteroids.

Via ESA

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