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Welcome! to CACAO!

After a long hiatus, the Amateur Observers' Program is ramping up to support the Comet Wirtanen Observing Campaign. We've learned alot from supporting:

Comet Wirtanen is back! No, really!

On June 1, we reported that Wirtanen had been recovered. It turns out that the object that we reported in the May 8 images was a random asteroid (133655), that had nearly the same expected brightness and proper motion as Wirtanen, but was following ~77 arcsec behind the comet's predicted position.

On Jun 18, 19 and 20, Lori Feaga et al. (UMD) obtained observations of Wirtanen from the DCT. Astrometry from these observations, which were obtained under better conditions (2 airmass during astronomical twilight) than the May data, linked well to measurements from 2014. This confirmed that the June ID was actually Wirtanen, and proved that the May ID was not.

The June 18-20 images were obtained when Wirtanen was at a heliocentric distance of 2.28 AU, geocentric distance of 1.98 AU, and had an r' magnitude of 20.6. It shows a short tail (~5 arcsec) to the SSW (in the anti-solar direction, which also coincides with the direction opposite to the comet's velocity vector).

To re-visit the May 8 observations, we used Wirtanen's new orbit solution to look for the comet in those data, and found that it was close to the newly-predicted positions but barely detectable in individual frames. It became visible when all images were coadded, and is in fact, seen in the coadded image that we presented in our report from June 1.

The new data indicate that Wirtanen is currently 1-1.5 mag fainter than predicted in both May and June. It is not yet clear how this will translate to the brightness around close approach to the Earth.

The updated orbit solution confirms that Wirtanen will make its close approach to the Earth at a distance of 0.0777 AU on December 16, 2018, at 15 UT. Additional astrometry is requested to continue to refine the orbit and constrain the effects of any non-gravitational forces.

In other news, we have added several new pages to the campaign website, including:

  1. A compilation (including links) of observations of Comet Wirtanen that are available in public archives.
  2. A discussion of coma morphology and the role played by collaborations between the professional and amateur communities.
  3. A discussion of our tests of a proxy CN filter (made by Semrock) that we hoped would be useful to the amateur community. (Spoiler, we can't recommend it for the typical amateur, but high-level users may have facilities where it could be valuable).

For more information about the comet, visit the campaign website at
Tony Farnham
for the Wirtanen Campaign

Image/File Submissions

We are now accepting observations (images, sketches, lightcurves,...) of

  • currently visible comets
  • asteroid Ceres
  • asteroid Vesta

  • any of our past targets
  • UMD asteroids

Please visit the LogBook to find out how to submit your observations.

Don't worry, you won't need to know much about astronomy, comets or asteroids. That's what this site is about!

So, let's get started...


Updated: 25-Jul-2018