I set out to capture the Monkey Head Nebula (NGC 2174 in Orion) with my usual astrophotography setup, aiming for a gorgeous deep-sky image. I took 70 frames at 300 seconds each (about 6 hours total exposure) and stacked them to reveal the nebula's faint details. However, something unexpected showed up: a bright object had drifted across those frames, leaving a streaky trail in the combined image. It turns out I had been photobombed by none other than asteroid Vesta - a hefty rock from the asteroid belt that wandered through my field of view.
Asteroids actually wander into my astrophotos fairly often. Normally, though, they're just faint streaks that don't make it through image stacking. When I stack dozens of long exposures in PixInsight, the software uses pixel-level rejection to clean up things that don't belong - like satellites, planes, cosmic rays, and yes, small asteroids. It's like a smart "cosmic eraser." But Vesta is big and bright! Being one of the brightest asteroids in the sky, the statistical cleanup process could not hide it. Instead of vanishing in the stack, it left a glaring smudge right across my carefully planned nebula shot (see below!).
At first, I wasn't sure what the odd smudge on my stacked image was. It didn't match any celestial object I'd seen before, and it wasn't part of the Monkey Head Nebula. To investigate, I used a feature in my processing software that flips through each individual frame like a stop-motion video (see below). Sure enough, I spotted a "star" creeping across the field, moving in a totally different direction than the background stars. That's when it clicked - I had an asteroid in my shot. The smudge on the stacked image is the software's failed attempt to remove the "unwanted" object. A quick check online for that date, time, and location confirmed it: I had captured Vesta, one of the brightest and largest asteroids in our night sky!
Vesta moves downward near the lower-left corner. The streaks halfway through are passing clouds, and each frame represents a five-minute exposure.
This asteroidal intruder is a fascinating object in its own right, deserving of the attention it grabbed. So, here are some cool facts about Vesta, the photobombing asteroid:
Meet Vesta, the Nebula Photobomber
NASA's Dawn spacecraft captured this full-color view of Vesta during its year-long orbit from 2011 to 2012. Dawn was the first mission to orbit two different bodies in the asteroid belt, visiting Vesta and then the dwarf planet Ceres. (NASA Planetary Data System)
Final Image
What began as a flawed nebula stack became a much more interesting discovery. I removed the asteroid trail from the finished Monkey Head Nebula image below, but the short animation of Vesta crossing the field remains my favorite result from the session.