What you might also notice in the PANSTARRS movie is that there’s actually more than one tail on this comet. In fact, there are at least three! The first one is the big dust tail I just spoke about, and by far the dominant feature of the comet. But if you watch the movie again we can see a short spikey tail pointing towards the lower-left of the images. This is very likely to be another dust tail, but one composed of very large and heavy pieces of cometary material that are not so easily influenced by the solar wind or radiation pressure. Instead, they’re big enough that they simply continue to follow the comet in its orbit, rather than being pushed outwards by other forces. Many of the higher-quality images taken from Earth also show this tail.

In addition to those two dust tails, on the right of the comet nucleus we see a thin third tail pointing radially away from the Sun. This is an “ion” or “gas” tail and is the result of the solar wind ionizing the neutral gas in the comet’s coma and releasing a stream of ions pointing directly away from the Sun. Interestingly, these ions can be highly affected by magnetic fields, and it just so happens that the solar outflow in general contains a very complicated and highly variable magnetic field. The result of this is that we frequently see twists and turns in the ion tail of comets, particularly in our STEREO images. Actually, though, PANSTARRS' ion tail is holding very steady. This might be a hint that it is composed of slightly more “exotic” ions than we typically see; perhaps Fe+, but we are going to need to turn to our computer models to figure that out!

Comet PANSTARRS will remain visible in the STEREO-B HI-1 data until around March 19, and all the data will be made freely available online at our SECCHI website and at the main STEREO Science Center site. Unfortunately, the orbit of PANSTARRS means it is going to pass outside the field of view of all the other cameras we have available on both the STEREO spacecraft, and on the Solar and Heliospheric Observatory (SOHO) satellite, so the data returned from STEREO HI-1B images are all we’ll have for this comet. Later this year, though, we could get to see one of the brightest comets in a century – C/2012 S1 (ISON) -- pass through all of our imagers on SOHO and STEREO. We need to wait a few more months before we know for sure how it’s going to perform, but it has the potential to completely overshadow what we’re currently witnessing with Comet PANSTARRS! Stay tuned!