Imagine that you’ve put ton of effort into setting up a nice outdoor family portrait photo only to find that, in that one perfect shot where everyone is looking at the camera with eyes open and wide smiles, a cicada flitted in front of the lens and blocked a portion of the image! It’s really not a big deal though because, hopefully, you took a few pictures and can deal with the photobombing bug pretty easily.
Astronomers sometimes face a similar issue on cosmic scales when Hubble accidentally picks up a foreground asteroid tumbling across the field of view of a distant galaxy or nebula. Normally, this is easily calibrated out of the data to the point where it may not even be noticed. However, a team of researchers have combed through Hubble’s data archives to find images just like this in an effort to better characterize the known and unknown asteroids that Hubble has serendipitously imaged over the years. This is no easy task and required using the work of a citizen science project specifically intended to find asteroids in Hubble images to then train a machine learning algorithm to pore over the data looking for space rocks. Over 19 years of Hubble observations, the team found 632 asteroids, 454 of which were previously unknown.
Hubble saw an asteroid, so what?
If asteroids in Hubble images are such a nuisance, then why go to the trouble of finding them in the first place? It turns out that Hubble is uniquely suited to providing more detailed information in a shorter amount of time about asteroids than ground-based observations. Hubble’s orbital motions around the Earth during an observation impart a parallax effect on the relatively nearby solar system objects in the field of view. This is why the asteroid seen in the image at the top of the post follows a curved path. This parallax motion can then be used to estimate distances to these objects in a much shorter amount of time (a single observation!) than traditional ground-based observations. Once a distance is known, it can be combined with the measured brightness to obtain the absolute brightness of the asteroid, which is then used to approximate its size. Some assumptions in this process are inevitable, like assuming the light captured from the asteroids is entirely reflected sunlight. Additionally, the asteroids are assumed to be roughly spherical with a uniformly reflective surface. Synthesizing all of this information, the team was able to produce a randomly sampled statistical study of main belt asteroids using entirely archival data.
How do we show the asteroid in an image?
The fun part for us image processors is then figuring out how to show a sample image demonstrating a photobombing asteroid. This is not the first time we’ve produced an image like this, but this will be the first exploration of the methods used. As mentioned above, these kinds of transient events are usually fully calibrated out of an image because we don’t really want them in the final image. Normally, we would never keep an asteroid in a press release image and all of the tools that we use to produce images are intentionally designed to remove them. Accessing images that show asteroids requires going back to the most raw, uncalibrated state of the data from the MAST archive at STScI. Data in this state show many calibration issues that are removed in pipeline processing like cosmic rays, reflections, and geometrical distortions (artifacts that are detailed in a previous post).
Working with the research team, we found several good candidates of images containing asteroids, and then I extracted the asteroid from the uncalibrated data. I ultimately settled on the image featured at the top of this post, galaxy UGC 12158, because of the aesthetic placement of the asteroid within the spiral arms of the galaxy. Like our previous press image showing asteroids, I wanted to produce a beautiful color image from the data, which would also feature the asteroid. Producing a calibrated color image that also shows the asteroid requires the merging of two different approaches to image processing. The color image must be made using our standard approach, which has been detailed many times over in this blog. The asteroid is isolated from the uncalibrated raw data, and then overlaid and blended onto the clean color image in its exact location.
There is no way to produce a color image containing an asteroid without this overlay method. The reason for this comes back to how color images are produced in the first place. A single, grayscale image taken in a specific filter is combined with images taken in other filters. Color is applied to the data based on the wavelength of light observed. Since the asteroid moves from frame to frame and may or may not be present in each of the filtered images, the resulting image would show up to 3 images of the asteroid in pure red, green, and/or blue when the data are combined in color. The only way around that is to pull the asteroid out of the grayscale data before color is applied and overlay it onto the finished color image.
A final note about distance
Now that we have a better understanding of how this image was put together, I’d like to leave you with one last thought on this matter and that is about the distance scales involved here. As is often the case in astronomy, it is truly mind boggling to consider the distance of the asteroid in this image compared to the distance of the galaxy. This asteroid is likely a main belt asteroid, located somewhere around 2 AU from Earth (remember, “AU” stands for astronomical unit – a standard distance representing the 93 million miles between the Earth and Sun). At 2 AU, the asteroid lies about 186 million miles away. The galaxy UGC 12158, on the other hand, has been measured to a distance of about 400 million light-years (remember a light-year is the distance light travels in 1 year – about 6 trillion miles). At 400 million light-years, the galaxy is about 2.4e21 miles from Earth or 2 sextillion 400 quintillion miles!! Giving some thought to the distances represented in this image really puts it into a different perspective, and that is one of my favorite things about this job.
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