Naked Photo-Bombing Comet 28P/Neujmin captured by the Subaru Telescope (white box and enlarged inset). Image Credits: NAOJ.
A comet slipped unnoticed into images taken by one of the world’s most powerful telescopes. Nearly a decade later, astronomers realized the cosmic photobomb had captured something exceptionally difficult to see: a comet stripped of its familiar glowing shroud.
The accidental appearance of Comet 28P/Neujmin by the Subaru Telescope gave researchers a rare look directly at its solid nucleus. It also showed astronomers something they weren’t expecting.
The surprise came when they examined how that dark surface reflected sunlight. Although the comet 28P looks much like some of the Solar System’s darkest asteroids, its surface behaves strikingly differently — a clue that billions of years of comet activity may have physically reshaped its skin.
The comet nobody was looking for
Subaru Telescope (すばる望遠鏡) is an 8.2-metre (320 in) telescope of the National Astronomical Observatory of Japan, located at the Mauna Kea Observatory on Hawaii.
Subaru is one of the few state-of-the-art telescopes to have been used with the naked eye, but mostly, it’s used to study distant galaxies and stars. No one aimed it at a comet.
Instead, the comet wandered through images collected in 2016 by Subaru’s Hyper Suprime-Cam, or HSC. The telescope’s field of view spans roughly the area of nine full moons, making it particularly good at catching objects that were never intended to be in the picture.
Typical example of a (different) comet, 12P. Image via Wiki Commons.
Researchers later searched publicly available HSC survey data for known Solar System bodies and found 28P hiding among observations originally taken for other science. One Subaru image containing the comet also holds more than 2,000 stars and galaxies, so it’s understandable that the comet just slipped by.
But when they found it, researchers realized the image had extraordinary timing.
×
Thank you! One more thing…
Please check your inbox and confirm your subscription.
When Subaru photographed it, 28P sat more than 10 astronomical units from the Sun — more than 10 times the Earth-Sun distance. That’s farther out than Saturn is from the Sun. At that distance, the comet was so cold that the usual release of gas and dust had essentially shut down. Images showed no detectable coma, the fuzzy cloud that normally hides a comet’s nucleus.
That’s great news because it gives astronomers a chance to look directly at the solid body itself. Even better, Earth, the comet and the Sun happened to be almost perfectly lined up.
Schematic illustration of opposition geometry. Image Credits: NAOJ.
When sunlight strikes an airless object almost directly from behind the observer, the object can suddenly brighten. Shadows between surface grains disappear, while some light can scatter through the material and return toward its source. Astronomers call this the opposition effect, or opposition surge.
It’s like the entire universe conspired to give Subaru a fantastic shot of the comet, and we almost missed it.
It looked like a dark asteroid — until the light hits it
Comets and asteroids are often lumped together as leftover building blocks of the Solar System, but they behave very differently.
Asteroids are generally rocky or metal-rich bodies that remain relatively unchanged as they orbit the Sun. Comets contain more volatile ices, and when they swing close enough to the Sun, those ices vaporize and carry dust into space. This creates the glowing coma and tail that make comets so distinctive. Yet when a comet travels far from the Sun and goes dormant, that difference can become much harder to see. Stripped of its coma, a comet nucleus can look remarkably asteroid-like — which is exactly what made 28P so intriguing.
Measurements across three colors of light showed a reddish surface similar to a D-type asteroid, extremely dark primitive bodies found throughout the outer Solar System.
But 28P did not reflect sunlight like the dark asteroids it resembles. When Earth, the comet and the Sun were almost perfectly aligned, 28P brightened much more sharply than C- and D-type asteroids usually do.
The researchers think this may come from the way light bounces through tiny grains on the comet’s surface and reflects back toward us. That suggests 28P’s surface is built differently, with a different grain size, spacing or texture. Even though its color looks asteroid-like, its structure is different.
A lot of science from a random photobomb
The result raises a tantalizing possibility. A comet and an asteroid might start with similar primitive material but acquire very different surfaces. Each time a comet approaches the Sun, escaping gas and sublimating ice can disturb, remove and rearrange material. Repeated over many orbits, that process could remodel its outermost layers. This is something that’s been theorized, but not conclusively proven.
These comet photobombs could help with that, and there may be many more of them waiting to be uncovered.
“The same archive likely contains many more Solar System objects waiting to be found,” lead author Takafumi Ootsubo of the University of Occupational and Environmental Health said in a Subaru Telescope release. “By applying the same observational approach to many more comets in the future, we hope to uncover how the surface structures of cometary nuclei have evolved over time.,”
In other words, some of astronomy’s next comet discoveries may already have been photographed. Scientists just have to notice what wandered into the frame.
The study can be read for free here.
