We've all been there, ready to take the perfect image, when someone wonders across the field of view. You'd think being located beyond the Earth's atmosphere would make the Hubble Space telescope immune to such frustrations, but you'd be wrong. When NASA's flagship telescope took epic images of distant galaxies, some asteroids got in the way.
Any astronomical image carries the possibility of including asteroids zinging across the field of view, but in most cases, the risk is pretty low. However, when the goal is to capture exceptionally faint objects, astronomers increase the time over which the image is taken. This raises the chance of an asteroid making its presence known.
The images are from the Frontier Fields project to study patches of the sky where vast clusters of galaxies create gravitational lenses, enabling us to capture more distant galaxies than we could otherwise see. These have been contrasted with random areas that don't have clusters large enough to create gravitational lenses. The project has already found the most distant galaxy ever observed.
Both areas, like the famous Hubble Deep Fields, have been given super long exposure times, as Hubble has no danger of clouds or light pollution creeping in to ruin the image. The exposures are not taken all at once, however. Instead, they involve aggregating the light collected over many periods. Hubble is devoting 630 hours of its precious time to the six Frontier Fields and six control areas.
This explains why some asteroid paths seen in the image look similar – they're actually the same asteroid whose path has been replicated by the process of combining multiple images. The 20 lines in the top image were made by just seven asteroids, of which five were previously unknown objects. On average these asteroids are 250 million kilometers (160 million miles) away – while the galaxies are hundreds of millions of light years away. The paths appear curved because Hubble orbits the Earth. The combination of the asteroids' movement through the Solar System, and Hubble's own passage around the Earth, creates a C or S-shaped path.
The image below contains Abell 370, one of the clusters large enough to create a powerful gravitational lens. It has 22 asteroid lines from five previously unknown asteroids. It also includes one (thicker) line which, rather than being an asteroid, is a very distant galaxy whose shape has been distorted by the gravitational lens.
The most distant galaxies in these images look red, because they are moving away from us so rapidly the Doppler shift is visible to the naked eye.