Image: Asteroids dance

As if this image of the Hubble Space Telescope was not crowded enough by a myriad of galaxies, nearby asteroids photograph the image, their tracks sometimes mimicking astronomical background phenomena.

The amazing cluster of galaxies Abell 370 contains an amazing assortment of several hundred galaxies tied together by the mutual attraction of gravity. Located about four billion light-years away in the constellation Cetus, the monster of the seas, this huge cluster is a rich mix of varied galactic forms.

Tangled among the galaxies are thin, white streaks that look like curved or S-shaped streaks. These are asteroid streaks that reside, on average, about 260 million kilometers from the Earth – just around the corner from the street in astronomical terms. Traces appear in several Hubble shows that have been combined into one image. Of the 22 total observations of asteroids for this field, five are unique objects. These asteroids are so weak that they have not been identified before.

The asteroid streaks appear curved due to an observation effect called parallax. As Hubble revolves around the Earth, an asteroid will seem to move along an arc compared to background stars and galaxies much farther apart. The movement of the Earth around the Sun, and the movement of asteroids along their orbits, are other factors contributing to the apparent alignment of asteroid trajectories.

All asteroids were found manually, with the majority flashing consecutive exposures to capture the apparent movement of asteroids. Astronomers have found a unique asteroid for every 10 to 20 hours of exposure.

These asteroid streaks should not be confused with the mysterious arches of blue light that are actually distorted images of distant galaxies behind the bunch. Many of these distant galaxies are too weak for Hubble to see directly. Instead, in a dramatic example of "gravitational lenses," the cluster functions as a natural telescope, distorting space and affecting the light traveling through the cluster to the Earth.

The study was part of the Frontier Fields program and the image, assembled from several exposures taken in visible light and infrared, was first published on November 6, 2017.

The position of the field on the sky is close to the ecliptic, the plane of our solar system. This is the area in which most asteroids reside, which is why Hubble astronomers have seen so many passages. Hubble's deep sky observations taken along a line of sight near the plane of our solar system generally record traces of asteroids.

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