ISON Image of the Week

Comet ISON Controversy! (Sep 02, 2013)

Has Comet ISON fragmented? Is this really even Comet ISON? Why are there three nuclei and why are they elongated? Good questions - and we have answers! [Click on the image to see a comparison with its "pretty" counterpart image.]
A recent image of Comet ISON, recorded by the Hubble telescope, has surfaced online and caused quite a stir. Far from looking like a comet, we see an almost geometric pattern of streaks of light that look far, far different to the image released by the Hubble team. Understandably, this raises more than a few eyebrows, and in the absence of qualified explanation, lends itself to all manner of interpretation - some quite elaborate! So this week we are going to take a look at the somewhat controversial image you see opposite, and explain exactly what's happening.

It is on a routine basis that we see beautiful astronomical pictures appear online, whether they be from Hubble telescope, Cassini, SDO, or any number of other space-based missions. Often times these images are jaw-droppingly beautiful and colorful, and understandably embraced by the internet community. Many people are happy to enjoy these processed images as they are presented, but others prefer to dig a little deeper and find the original data files from which they were derived. This is fabulous, and to be encouraged and applauded as it certainly educates the public about astronomy and science! But those that choose to dig around like this need to understand exactly what they are seeing when they view "raw" data files, and why they often look so different to the pretty ones that we put online.

The following explanation applies to nearly all astronomical imaging satellites, but here we are just going to use the example of Hubble, since it's the focus of this particular Comet ISON controversy.

When satellites take astronomical images of galaxies, planets, comets, etc, the images they record are very rarely inherently pretty. There is no color, typically no artistic lens flares, the images can be dominated by stray light, noise, cosmic rays, and all manner of other features depending on where in space they are looking. Indeed, many "raw" spacecraft images are just downright ugly! In order to make pretty pictures, scientists often have to take several processing steps to remove instrumental effects, noise, zodiacal light, etc, and then apply appropriate scaling, contrasting and color to the images to return the wonderful pictures we all love to see. The end result is sometimes barely recognizable from the original data.

In the image above (which we recommend you click on to see a comparison with its "pretty" counterpart image), we see something that doesn't look a lot like a comet, and very much not like the beautiful Hubble Heritage image of ISON. We see three distinct nuclei that appear as short streaks oriented in different directions. Why is this?

The Hubble image of Comet ISON is not a single exposure but instead a series exposures taken at different times. Hubble is in orbit around the Earth, so is actually moving through space at a very high velocity. When compiling this image, the Hubble team aligned all of the exposures such that the stars remained fixed in space. This greatly enhances the detail of the stars and galaxies in the field of view and makes the image far prettier to look at. Unfortunately this also means that the comet (which is very much closer to Hubble than it is to the stars) appears in a slightly different location in each of the exposures that are taken, because the Space Telescope never sits still!

Furthermore, the images taken were long exposures -- up to 490-seconds. Any photographer will tell you that a long exposure of a moving bright light will lead to a streak in the image, and that is exactly what we see with the comet. Finally, the streaks are oriented in different directions due to Hubble's elliptical orbit around Earth. Depending on where the telescope is at a given time on this ellipse, it leads to a "streak" that trails in a different direction.

There's a bit more to it than this, but that's the basic overview. This information is not the kind of thing we scientists expect most people to know, so it does not surprise us when there is confusion about images like this. However, wild speculation about UFOs and conspiracies doesn't really help anyone! Instead we urge people to contact us and ask us directly. We are always happy and more than willing to talk about our work. (The trick is getting us to shut up once we've started!)

[Minor update: The Hubble folks also have a similarly worded explanation on their website.

Every week this year we will put up a new image related to Comet C/2012 S1 (ISON). If you have a cool image you'd like us to consider, please send it to, along with a description and any credits you would want applied. We'll contact you if we choose to use your image on the CIOC Website.

See our ISON Image of the Week Archives for earlier picks!