Asteroids in Hubble Images: Finding and Showing the Rocks in the Frame

A family portrait can be ruined by an insect that flies in front of the lens at the perfect moment. Hubble has the same problem on a cosmic scale. Again and again, an asteroid in the solar system drifts through the field while the telescope is staring at a galaxy hundreds of millions of light-years beyond it. Normally the streak is calibrated away and nobody notices. A research team decided to go looking for those streaks on purpose, and in April 2024 one of their finds was turned into a color picture with the asteroid deliberately left in.

632 asteroids in the archive

The team searched 19 years of Hubble observations. Doing that by eye was out of the question, so the work drew on a citizen-science project in which volunteers had flagged asteroid trails in Hubble images. Their classifications were used to train a machine-learning algorithm, which then searched the archive for more. The result was 632 asteroids, 454 of them previously unknown.

Hubble had been catching such objects by chance for decades. An early example, a two-kilometer rock crossing a star field in Centaurus in 1994, is described in Asteroid Trail in Centaurus.

Why a nuisance is useful data

If asteroids in Hubble images are a nuisance, why find them at all? The answer lies in the curve. Hubble orbits Earth, and that motion shifts its point of view during an observation. For a nearby object such as an asteroid, the shift is large enough to bend its apparent path into an arc or an S-shape against the far more distant background. This is parallax, the same effect that makes a nearby post appear to move against distant hills when the observer walks past.

The size of that bend depends on the asteroid's distance, so a single Hubble observation can be enough to estimate how far away it is. A telescope on the ground needs much longer to pin down the same number. With the distance known, the measured brightness gives the asteroid's absolute brightness, and from that its approximate size.

That last step rests on assumptions. The light is taken to be entirely reflected sunlight, and the asteroid is treated as roughly spherical with a surface that reflects evenly. With those caveats, the team produced a randomly sampled statistical survey of main-belt asteroids built entirely from archival data, a population study no one had to schedule telescope time for.

Keeping an asteroid in the picture

Showing one of these asteroids in a polished image means going against the normal workflow. The standard tools for making Hubble pictures are designed to remove transient objects, because they are almost never wanted in the final result. To keep one, the image processors had to return to the raw, uncalibrated frames in the Mikulski Archive for Space Telescopes at STScI. In that state the data still carry everything the pipeline normally removes: cosmic-ray hits, internal reflections and geometric distortion. Those defects are explained in Image Artifacts.

Among several candidates found with the research team, the one chosen was the barred spiral galaxy UGC 12158, because the asteroid's trail falls across the galaxy's spiral arms in a pleasing way. In a raw F475W frame from the Advanced Camera for Surveys, the galaxy is peppered with cosmic-ray marks and the asteroid shows as long streaks across the upper middle of the field.

The finished image needed two methods at once. The galaxy was processed the standard way into a clean color picture. The asteroid was extracted separately from the uncalibrated frames and then laid over the color image at its exact position, blended in so that it sits naturally on top. Because several exposures were combined, the trail appears dashed, one dash per exposure. The released picture is on its ESA/Hubble image page.

Why overlay is the only way

There is no way to produce that picture in a single color step. A Hubble color image is built from grayscale frames through different filters, each frame assigned a color by its wavelength. Between one exposure and the next the asteroid changes position, and some filters may not catch it at all. Stacked directly, it would show up as as many as three separate copies, each one pure red, pure green or pure blue.

In the UGC 12158 data the asteroid appears in the F475W frames, colored blue in the composite, where it barely registers, and in the F606W frames, colored green. The grayscale trail combined from those two filters is what was laid onto the finished color picture. The only route to a natural-looking trail is to take the asteroid out of the gray data before any color is applied, and add it back at the end.

Two distances in one frame

The scale of the picture is the strangest part. The asteroid is probably in the main belt, around 2 astronomical units from Earth, about 186 million miles. UGC 12158 is about 400 million light-years away, which works out to roughly 2.4 × 10²¹ miles, or 2.4 sextillion. The rock and the galaxy share one frame while one is about ten trillion times farther than the other.

The image at the top of this page is an earlier example of the same effect. In the Frontier Fields image of the galaxy cluster Abell 370, about 4 billion light-years away in Cetus, astronomers counted 22 asteroid sightings belonging to five separate objects, all too faint to have been known before. Those were found by hand, mostly by blinking between consecutive exposures, at a rate of about one new asteroid for every 10 to 20 hours of exposure time.

Questions readers ask

How many asteroids were found in Hubble's archive?

The search covered 19 years of Hubble observations and found 632 asteroids, 454 of which had not been known before. Volunteers in a citizen-science project first marked asteroid trails in Hubble images, and their work trained a machine-learning algorithm that searched the rest of the archive.

Why do asteroid trails in Hubble images curve?

Hubble moves around Earth during an observation, so its viewpoint shifts. A nearby asteroid appears to swing against the far more distant background, an effect called parallax. Its path is bent into an arc or an S-shape, and the size of the bend reveals the asteroid's distance.

How was the asteroid kept in the UGC 12158 image?

The galaxy was processed into a normal color image. The asteroid was taken separately from the raw, uncalibrated frames, where calibration had not yet removed it, and overlaid on the color image at its exact position. Combining it directly would have produced separate red, green and blue copies.

How far away is the asteroid compared with the galaxy?

The asteroid is probably around 2 astronomical units from Earth, about 186 million miles. The galaxy UGC 12158 is about 400 million light-years away, roughly 2.4 sextillion miles. The galaxy is about ten trillion times more distant than the asteroid crossing in front of it.