“There are a total of 20 objects seen in these fields, corresponding to 7 unique asteroids, most of which are imaged multiple times. Only 2 of them were previously known; the remainder were serendipitously discovered by Hubble. Approximately 10-to-20 hours of observing time leads to the discovery of a new asteroid, telling us something interesting about the density of asteroids at the level that Hubble’s imaging capabilities are sensitive to. As long as you’re observing a target close to the plane of the Solar System’s ecliptic, you’re bound to be polluted by these interlopers.”
There’s an old saying among astronomers: one astronomer’s noise is another astronomer’s data. If you’re trying to view the galactic center, then the interstellar medium is “noise,” but if you’re studying the interstellar medium, then that’s exactly the data you want! Well, a team studying the massive galaxy cluster Abell 370 got to do a very long, deep-exposure image of both that cluster and its parallel field, accumulating a total of nearly 100 hours of observing time. As it so happens, this cluster happens to lie very close to the Solar System’s ecliptic plane, meaning that objects in the asteroid belt occasionally cross through Hubble’s field-of-view. Although it would take tens of millions of Hubble images to cover the entire sky, there are millions of asteroids bright enough for Hubble to see, and a few of them “photobombed” both the galaxy cluster and its parallel field.