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Comet 67P/Churyumov-Gerasimenko and Two Bonus Asteroids

2022 February 4

I was processing and analyzing some images of Comet 67P/Churyumov-Gerasimenko from a  January 29th imaging session when the program I was using (Astrometrica) alerted me to the fact that there were two known, but faint, asteroids in the same field of view. This was a real bonus!

Two asteroids join the same field with Comet 67P/Churyumov-Gerasimenko on January 29, 2022. This animation is a screen capture of Astrometrica’s blinking tool. Field of view 20×15 arcminutes. North is up. East is left.[1]

Astrometrica picked up and identified asteroid 33651, also known as 1999 JG84, and asteroid 103516, also known as 2000 BY4.

Astrometrica detected these two small main belt asteroids barely above the noise level. There was just enough signal, however, to measure their magnitudes.  Asteroid 33651 was measured at magnitude 17.0 and asteroid 103516 as magnitude 17.3. JPL’s online Horizons System predicted magnitudes for 33651 as 17.3 and 103516 as 17.0. Comet 67P’s magnitude was measured as 12.5.

I was pretty surprised by the detections of these two asteroids. I image from a heavily light polluted backyard under Bortle 7-8 skies. These two objects were quite faint, and the images were only 30-second exposures.

Neither the JPL’s Small-Body Database, nor the Lowell Observatory’s Asteroid Information Database give sizes for these objects. But, both databases gave absolute magnitudes (H) for  them: 14.6 for asteroid 103516, and 13.5 for asteroid 33651. According to the JPL’s Asteroid Size Estimator, objects with these absolute magnitudes should have diameters in the range of 3-12 km (2-8 miles).

At the time of this imaging session, Asteroid 103516 was 174 million km (108 million miles) from Earth and asteroid 33651 was 263 million km (163 million miles) distant.

All things considered, I am amazed that my small backyard observing rig detected both of these tiny chunks of rock that were well beyond the orbit of Mars.


1. January 29, 2022, 06:28:19-06:58:36 UT. Stack of three images, each a stack of 4 frames at 30 seconds (total 120s). Gain: 250. Telescope: Meade SN-8 (203mm f/4). Camera: ZWO ASI224MC with UV/IR cut filter. Mount: Celestron CGEM.

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