Comet 2020 F3 (NEOWISE)

The long period comet C/2020 F3 (aka NEOWISE, named after the satellite that first detected it) is one of the most impressive comets in the last few decades.

Although my attempts to see it personally have so far been frustrated by cloud and lack of clear sight lines to the north-east, I have been following the images of it that have appeared in Astronomy Picture Of The Day (APOD). Most if not all of the images posted so far, or presented in pop sci articles online fail to tell you any quantitative details about what you're looking at, so I decided to work out some rough distances and sizes based on the JPL Small Body Database info on NEOWISE, and the image taken from the ISS on July 5th 2020, and the kstars desktop planetarium software.

This image is a cropped, resized and annotated version of the ISS image posted at APOD on July 10, 2020. I've labelled some of the prominent stars, although to a ground-based amateur observer the star Capella, which is hidden behind parts of the ISS off the top left of the image, would be the most most prominent star visible.

From the JPL SBDB we can find that NEOWISE was about 0.3 AU from the Sun on 7/5/2020, and about 1.1 AU (approx 165 million km) from Earth. At a distance of 1.1 AU an angular distance of 1 degree is about 3 million km.

My annotated version of APOD from 07/10/2020, Image Credit: NASA, ISS Expedition 63

The angular distance between Theta Aurigae and Elnath is approximately 10.54 degrees, and the angular distance between Elnath and Kabdhiliinan (Iota Aurigae) is approximately 7.76 degrees based on using a measuring tool in kstars. If we assume the ISS image is not significantly distorted then we can measure the number of pixels between the stars on the image and compare that to the length of the tail in the image to work out its angular size. In fact the image does seem to be be slightly distorted, but taking an average I get that the visible tail in this image is about 1.7 degrees long, so the visible tail is about 5 million km long. Its not every day you can go out and see something 5 million km in size!