Phaethon, the asteroid that is the parent body responsible for the Geminid meteor shower, is making a close approach (relatively speaking) on December 16th. This will be the closest it’s been since 1974 and the closest it will be until 2093. Technically Phaethon is considered a PHA, – potentially hazardous asteroid – it’s highly eccentric orbit brings it to within 6.44 million miles of the Earth on this approach, or 26.8 times the distance from the Earth to the Moon. That, plus the fact it has an absolute magnitude less than or equal to 22 earns it this qualification, although it’s orbit is pretty regular and doesn’t actually pose any risk over the short or long term. It has a diameter of about 3 miles/5 km and an orbital period of 523.5 days. The magnitude at the time I took these images was around 10.7 – it’s been decreasing ever since due to the phase angle changing (the angle between us, the asteroid, and the sun) even though the closest approach wasn’t until 2 days after these images were taken.
Phaethon is kind of a weirdo as far as asteroids or comets are concerned. It has a lot of the behaviors of a comet, like the eccentric orbit, and the ejection of dust particles. It also has the characteristics of an asteroid with the location of its orbit and its composition (most similar to a B-type asteroid). Another issue with Phaethon is that the amount of dust particles that regularly come off the object don’t match the amount of material in the stream that the Geminids come from – the going theory is that there are irregular outbursts that account for the rest of the material. Goldstone Radar Observatory has observations planned through December 21st to study Phaethon and hopefully get as good of a shape model as possible, since they won’t get a chance this good for quite a while. Since Phaethon has properties of both comets and asteroids it falls into the category of “active asteroids”, although it’s been suggested that there is also the possibility that it’s a dead comet nucleus. Astrobites has a great summary about a paper on active asteroids here. One of the primary theories suggested for how Phaethon ejects particles is thermal disintegration – temperature changes can cause particles to become unstable and break off or cause water to take small particles with it as it evaporates.
On this approach Phaethon was quite a fast mover – 30 to 35 arcsec per min. The field of view for my scope/camera is 30 x 23 arcmins, so it moved across the field of view in a little over an hour (depending on the angle I used to frame the shot). Compare that to a main-belt asteroid, where you could take 3 images an hour apart each and you would see movement over a much smaller part of the image.
I imaged this two ways:
- Take 15 second exposures of the the same star field and watch the asteroid move
- Take 15 second exposures of the asteroid, tracking on it so it moves through the stars.
With my Paramount mounts, you have the option to change the tracking rate from the default sidereal rate to one that matches the given solar system target. As long as I have the asteroid info loaded in SkyX, CCDAutopilot pulls the current coordinates at the time of each exposure and sets the location and tracking rate accordingly. I started off with 60 second exposures to test, but Phaethon was moving way too fast and streaked. 15 seconds was the point where I got a good balance on speed and brightness.
- M1 – 10″ R/C with QSI 683wsg
- Baader Luminance filter
- Static image: 53 x 15s exposures
- Moving image: 61 x 15s exposures
Processing was all done in PixInsight – I used the Blink process and a program called ffmpeg to create the animations. Blink can use ffmpeg with any flags/commands/options that you want to use to tweak the output animation, which can be .gif/.avi/.mp4 and probably a bunch of others, but those were the ones I tested. You can also use ffmpeg in a regular command prompt as well if you want to make changes to the video after creating it. It’s pretty flexible. I took an additional set of images last night that tracks the asteroid for about 3.25 hours, and I’m going to use the processing of that data set to create a tutorial for PixInsight/ffmpeg and using Image Containers to apply the same process to multiple images. Last time I tried to do something like this I used ImageMagick and it wasn’t nearly as good as this.
Usually I try to use M2 for asteroid imaging, since that one has the photometric filters and the bigger aperture, but the camera didn’t get back in time and I didn’t want to miss this. In hindsight M1 was the better system since it has a larger field of view.