What causes these asteroid trails to be so strangely shaped and repeated? The Phys.org news item Hubble is the ultimate multitasker: Discovering asteroids while it's doing other observations includes the image below.

What causes these asteroid trails to be so strangely shaped and repeated? Usually asteroids appear as short straight segments in long exposures from terrestrial telescopes.




  Some asteroids from within our Solar System have photobombed deep images of the Universe taken by the NASA/ESA Hubble Space Telescope. The asteroidal streaks in this image are created by our virtual neighbours; asteroids in the main asteroid belt between Mars and Jupiter. In the background are thousands of colourful galaxies, some of them billions of light years away. Credit: NASA, ESA, and B. Sunnquist and J. Mack (STScI); CC BY 4.0; Acknowledgment: NASA, ESA, and J. Lotz (STScI) and the HFF Team
What causes these asteroid trails to be so strangely shaped and repeated? 


The linked image is a composite of several long duration exposures captured by the Hubble Advanced Camera for Surveys of the same region of the distant sky. Each individual exposure was about ~2200 seconds long. Nearby objects such as asteroids would have been subject to the proper motion of the asteroid with respect to the Earth and parallax due to the Hubble being in orbit about the Earth during those ~2200 second long exposures.

Each individual streak comprises both proper motion and parallax effects. The parallax effects are what make the streaks have a nonlinear shape. One orbit later (~5725 seconds), another ~2200 second long exposure showed a similar path for the same asteroid, but shifted a bit because of the asteroid's proper motion. This is why the composite image shows twenty streaks (per the linked article; I see sixteen) for only seven asteroids.